No new physics today. Or rather, all of it at once, compressed into twenty questions.

This one is purely for fun. I wanted a way to find out whether these posts are actually landing, or whether you read them, nod, and then it quietly evaporates by Wednesday. That happens to me with things I read too, so there is no judgment in either direction. Half of this is me checking my own work.

Twenty questions, all from posts already sitting in your inbox. Answers and short explanations at the bottom, scoring at the very end.

Do not scroll ahead. You would only be cheating a version of yourself who wanted to feel good about this.

The quiz

1. In 1900, Max Planck was handed a very practical assignment that accidentally started quantum mechanics. What was it?

A. Work out why the sky is blue

B. Measure the speed of light through glass

C. Study light bulbs and figure out how to make them more efficient

D. Improve the accuracy of railway clocks

2. Planck found the assumption that made his numbers fit. What did he think he had actually done?

A. Found a mathematical trick, a bookkeeping device that happened to work

B. Proven light is made of particles

C. Discovered the deepest truth about nature

D. Made an error he would later retract

3. Classical physics says an orbiting electron should radiate away its energy and spiral into the nucleus. How fast?

A. Roughly a million years

B. Roughly one second

C. It never should

D. Roughly 10⁻¹¹ seconds, a hundredth of a billionth of a second

4. Louis de Broglie’s thesis broke physics wide open. How long was it, and what saved it?

A. Two hundred pages, saved by experimental proof

B. Around twenty pages, saved partly because Einstein read it and called it brilliant

C. A hundred pages, saved by Planck’s endorsement

D. Around twenty pages, and it was rejected

5. Davisson and Germer fired electrons through a crystal. What came out the other side and settled the argument?

A. A diffraction pattern, and diffraction is something only waves do

B. Two separate beams

C. Nothing, the electrons were absorbed

D. A single clean spot, exactly as particles should make

6. In 1919 a mostly unknown German mathematician named Theodor Kaluza wrote to Einstein. How long did Einstein sit on the letter?

A. He replied the same day

B. About a week

C. Over two years

D. He never replied at all

7. Kaluza took Einstein’s equations and wrote them out in five dimensions instead of four. The maths split in two. The first piece was gravity, unchanged. What was the second piece?

A. The Schrödinger equation

B. Maxwell’s equations, electromagnetism, uninvited

C. A prediction of dark matter

D. The strong nuclear force

8. Oskar Klein answered the obvious objection: if there is a fifth dimension, where is it? What was his answer?

A. It only existed in the first moments of the universe

B. It sits at the edge of the observable universe

C. It exists only inside black holes

D. It is curled into a loop at every point in space, far too small for any instrument to reach inside

9. Modern physics describes every fundamental force as the curvature of something. There is exactly one force the framework cannot fit. Which one?

A. The weak force

B. Electromagnetism

C. Gravity

D. The strong force

10. I asked you to hold onto one quiet asymmetry between gravity and electric charge. What was it?

A. Charge comes in two flavours and can push or pull. Mass comes in one flavour and only ever attracts

B. Gravity is by far the stronger of the two

C. Charge is quantised and mass is not

D. Gravity travels slower than light, charge does not

11. Einstein called it “the happiest thought of my life.” What was the thought?

A. That nothing can travel faster than light

B. That energy and mass are the same thing

C. That the universe is expanding

D. That a person in free fall does not feel their own weight

12. Seal someone in a windowless box. Which two situations can they never tell apart, no matter what experiment they run?

A. Moving at constant speed and standing still

B. Standing on Earth, and accelerating through deep space at just the right rate

C. Falling and floating

D. Being near a star and being near a black hole

13. For the everyday gravity holding you in your chair right now, which part of the curvature is doing most of the work?

A. The curving of space

B. Both equally

C. The curving of time

D. Neither, gravity is purely Newtonian at that scale

14. Beyond showing the wrong number of dimensions, the rubber sheet picture has one quietly fatal problem. What is it?

A. The dip should point upward

B. It only works for objects that are already in orbit

C. Real sheets are not stretchy enough to model a star

D. The marbles only roll into the dip because real gravity is pulling them down onto the sheet. It uses gravity to explain gravity

15. A flat creature living on the surface of an enormous sphere draws a very large triangle and measures the corners. What does it find?

A. More than 180 degrees, and the bigger the triangle the bigger the overshoot

B. Exactly 180 degrees, always

C. Less than 180 degrees

D. It cannot draw a triangle at all

16. Which ordinary object, probably sitting on a shelf in your old school, is the cleanest picture of curvature there is?

A. A prism

B. A globe, because the lines of longitude set off perfectly parallel and still meet at the pole

C. A pendulum

D. A protractor

17. Who first showed that a creature could measure the curvature of its own surface from the inside, without ever stepping into a higher dimension?

A. Euclid

B. Minkowski

C. Carl Friedrich Gauss

D. Hermann Weyl

18. Why can electric repulsion alone not explain why solid things stay solid?

A. Because electrons carry almost no charge

B. Because electric repulsion only works over long distances

C. Because electrons are electrically neutral in bulk matter

D. Because electrons are slippery. Given only repulsion they would shuffle and rearrange into lower-energy positions, and matter would turn out to be crushable far past anything we see

19. Wolfgang Pauli wrote down the exclusion principle in 1925. How old was he, and what was his reputation?

A. Twenty-five, and already the critic other physicists genuinely feared

B. Forty, and famously gentle with younger colleagues

C. Thirty, and known mainly as a careful experimentalist

D. Twenty-five, and completely unknown

20. Because spin has exactly two values and nothing in between, how many electrons can share a single quantum address?

A. One, and only one

B. Two. One pointing up, one pointing down, and a third has nowhere to go

C. Four

D. As many as you like, provided you push hard enough

Stop. Answers below.

Last chance to commit to your guesses.

Answers

1. C. Light bulbs. He was asked to make them more efficient, started measuring how much light a hot object emits at each wavelength, and the data refused to match the theory. (The Best Story I Heard in 4 Years of Physics)

2. A. A trick. Planck published quantisation and privately thought it was a bookkeeping device his equations needed. He still believed light was a wave. He was wrong, and he did not know it yet.

3. D. About 10⁻¹¹ seconds. Which is why, according to classical physics, the atoms in your hand should have collapsed before you finished reading the sentence.

4. B. Around twenty pages, one idea, and a committee that almost rejected it. Einstein read it and called it brilliant. De Broglie later won the Nobel Prize for those twenty pages.

5. A. A diffraction pattern. Fire particles at a slit and you get a clean spot. Fire waves and you get fringes. The electrons made fringes.

6. C. Over two years. He did not know what to say. (Light Is the Shadow of a Dimension You Can’t See)

7. B. Maxwell’s equations. Kaluza did not put them in and did not assume them. He wrote gravity in five dimensions, pressed go, and electromagnetism fell out on its own.

8. D. Curled up. A tiny hidden circle at every point in your room, smaller than an atom, that you can never get inside. Your fridge magnet is sliding along its contours.

9. C. Gravity. The one that started the whole story is the one that will not fit, and chasing that failure is what sent physicists somewhere very strange.

10. A. Gravity only ever attracts. There is no anti-mass, no negative weight, no gravitational repulsion anywhere in the universe. That is the first hairline crack in Newton’s machine. (Gravity Is Not a Force)

11. D. Free fall. The instant you stop resisting gravity, the feeling of it disappears completely. What kind of force switches off the moment it becomes the only thing acting on you?

12. B. Sitting on a planet and accelerating through empty space. There is no experiment inside that box that can tell them apart. That is the equivalence principle.

13. C. Time. This is the part almost every diagram leaves out, and it is the part doing most of the work for the gravity you feel every single day.

14. D. It uses gravity to explain gravity. Those marbles roll into the dip because a real gravitational field is pulling them down onto the sheet.

15. A. More than 180, and the overshoot grows with the triangle. That creature has never seen a third dimension and has still just proved its world is curved. (What does spacetime curvature look like?)

16. B. A globe. Nobody bent the lines of longitude. Each one is as straight as a line can be on that surface, and they converge anyway.

17. C. Gauss. He proved curvature can be measured from within a surface, with nothing but a tape measure and a protractor. He only ever did two dimensions. The leap to any number at all came later, from one terrifying afternoon.

18. D. They would cooperate. A world held apart by electric charge alone is a world that can be crushed, down to a small fraction of the size ours insists on keeping. Something has to forbid the cooperation. (The Quantum Rule That Holds the World Apart)

19. A. Twenty-five, and already the man who could demolish an hour of work with one sentence from the back of the room. His colleagues half believed that lab equipment failed when he entered the building.

20. B. Two. Spin up and spin down, and the room is full. That single rule about seating is what stops your hand at a table, and out in the dark it is what holds up the crushed remains of a star.

Scoring (Just for fun)

17 to 20. You have read all of it, properly, and remembered it. Reply and tell me, because I want to know how many of you there are.

12 to 16. Solid. You are reading these the way I hoped they would be read.

7 to 11. You are skimming. So am I, with most of my inbox. No shame in it.

Under 7. Either you are new here, or something in the writing is not sticking. If it is the second one, I would honestly like to know which posts slid off, because that is more useful to me than a compliment.

Every question above came from the free half of a post.

The other half is where each of these stories actually finishes. What the Pauli rule does when a star runs out of fuel and starts to collapse. What curvature looks like when an entire galaxy bends into a ring of light and we photograph it. What happens to the idea of a particle when gravity refuses to fit into the picture.

Those are the Thursday pieces, and they are for paid subscribers. £5 a month.

Reply with your score. I will read every one.

In the spring of 1959, two physicists at Harvard aimed a gamma-ray source up the inside of a tower and began hunting for one of the smallest effects anyone had ever chased in a laboratory: a shift of about two parts in a thousand trillion.

Robert Pound and Glen Rebka were not testing whether gravity pulls things. Newton had settled that. They were testing a claim Einstein had made half a century earlier, a claim so strange that most people quietly refuse it the first time they hear it.

Gravity slows time.

Not clocks, as machines. Time itself. Sit lower in a gravitational field and everything that ticks, from atoms to hearts, ticks slower. Einstein put this on paper in 1907, decades before any instrument on Earth could hope to see it.

If you read Gravity Is Not a Force back in June, you saw why: gravity is not a pull but geometry. What I did not show you then is the evidence. The evidence is the good part, because for sixty years it has been creeping closer and closer to your own body.

Start with the size of the claim. The rate difference between two heights comes from one small formula. Take the height h, multiply by gravity g, divide by the speed of light squared:

difference in rate = gh / c²

Put in one meter. You get about 1 part in 10¹⁶: for every meter you climb, time runs faster by one part in ten thousand trillion. The gentlest slope in physics.

Now put in you. Your head sits roughly 1.7 meters above your feet. Run the same arithmetic across a long life:

(1.9 × 10⁻¹⁶) × 80 years ≈ 500 nanoseconds

Stand for eighty years, and your head comes out about half a microsecond older than your feet. Your feet are, permanently, the youngest part of you. They live closest to the slow time.

Half a microsecond is nothing you could ever feel. But the size was never the point. The point is that the number is not zero, and Einstein said exactly how far from zero it would be. Which made it, eventually, something you could check.

The rest of this post is the sixty-year hunt that checked it: a gamma-ray trap inside a Harvard tower, atomic clocks buying their own airline seats, a satellite launched in 1977 with Einstein’s correction switched off on purpose by engineers who did not believe him, the two clocks that caught a footstool’s worth of height, and the experiment that watched time bend across a single millimeter. It ends at the objects where the slowing stops being partial.

Read more

On the morning of May 29, 1919, on a small volcanic island off the west coast of Africa, Arthur Eddington stood under a clouded sky and waited for the universe to cooperate.

He had sailed from England with crates of telescopes and photographic plates for a single reason. A total eclipse was about to pass over the island, and sitting almost directly behind the blacked-out Sun would be the Hyades, a bright cluster of stars. Photograph those stars during totality, compare their positions with photographs of the same patch of sky taken months earlier at night, and you could settle a question that sounds more like philosophy than astronomy.

Does gravity bend light?

The clouds thinned minutes before totality. Eddington exposed sixteen plates, most of which caught clouds. A few caught stars, and when the measuring was finally done back in England, those stars were not where they were supposed to be. Their light had curved on its way past the Sun.

You have probably seen this episode retold a hundred times, always as the moment Einstein became Einstein. The part that usually gets skipped is why the result is strange at all, and the strangeness begins with the equation that ran the universe for two centuries:

F = GMm / r²

The gravitational force between two objects. G is a constant of nature. M is the mass of the big thing, in our case the Sun. Small m is the mass of the object passing by, and r is the distance between them.

A photon, a particle of light, has no mass. Set m = 0, and since anything multiplied by zero is zero:

F = GM(0) / r² = 0

No mass, no force. Newton’s equation seems to say the Sun cannot touch light at all, that starlight should sail past it in a perfectly straight line, and that Eddington’s stars should have sat exactly where the night-time photographs put them.

They didn’t.

So the usual conclusion is that Newton was simply wrong and Einstein fixed him. The real story contains a twist, and the twist is my favorite part of the whole subject.

Ask Newton’s mechanics a slightly different question. Not the force on the photon, but its acceleration. The second law says F = ma, so acceleration is force divided by mass:

a = F / m = 0 / 0

Zero divided by zero. That is not zero; it is undefined. Newtonian physics does not actually predict that light ignores gravity. Handed a massless particle, it refuses to comment.

There is a way to make it talk. Divide the force equation by m before you set anything to zero, and the small mass cancels out of the problem entirely:

a = GM / r²

Look at what survived. The acceleration of a falling object contains the Sun’s mass and the distance, and nothing about the object itself. A cannonball and a pebble fall identically. This is the oldest deep fact in physics, and in 1971 the astronaut David Scott gave it the most beautiful demonstration it will ever get: standing on the Moon, he dropped a hammer and a falcon’s feather side by side, and with no air to interfere they hit the dust at the same instant. “Mr. Galileo was correct,” he said, on camera, in a spacesuit.

Now push the logic. A mountain accelerates toward the Sun at GM/r². So does a grain of dust. Keep shrinking the mass, from a gram to a milligram to a microgram, and the acceleration never changes, because the mass is not in the formula. Why should the final step, from almost nothing to exactly nothing, suddenly behave differently?

In 1801, a German astronomer named Johann von Soldner decided it shouldn’t. He treated light as a stream of very fast particles, let them fall past the Sun the way everything else falls, and computed the bend for a ray grazing the solar edge. In modern numbers, his answer comes out to

θ = 2GM / c²R ≈ 0.87 arcseconds

where R is the Sun’s radius and c is the speed of light. It is an absurdly small angle, roughly a human hair viewed from twelve meters away. Small, but real, and in principle measurable. So the surprise of 1919 was never that light bends. Newton’s physics, taken seriously, already expected a bend. The surprise was hiding in the size of it.

More than a century later, in 1911, Einstein reran essentially the same calculation from his own starting point, knowing nothing of Soldner’s forgotten paper, and landed on essentially the same number. Just under one arcsecond. He was confident enough to push astronomers to test it at the next good eclipse.

A German expedition led by Erwin Freundlich set out for Crimea in the summer of 1914 to do exactly that. While they were traveling, the First World War broke out. Russian authorities detained them as enemy nationals, confiscated the instruments, and the measurement never happened.

That failed expedition might be the luckiest accident in the history of physics. Because Einstein’s 1911 number was wrong.

By November 1915 he had finished general relativity, and when he redid the light-bending calculation with the completed theory, the prediction changed:

θ = 4GM / c²R ≈ 1.75 arcseconds

Exactly double. This was no rounding error and no refinement in the third decimal. The finished theory said Newton’s universe gets the bending of light wrong by a clean factor of two. Had Freundlich measured in 1914, the sky would have contradicted Einstein’s own published prediction, and history might remember him as one more physicist who almost got there.

Instead the test came in 1919, with the right number on the table. Eddington’s plates, for all their clouds and wide error bars (astronomers argued about those plates for decades, and the answer held), pointed at one prediction and not the other. 1.75. Einstein. The Times of London ran the result under the words “Revolution in Science,” and a forty-year-old theorist in Berlin woke up as the most famous scientist alive.

Which brings us to the actual question, the one this whole post has been circling.

Half of that 1.75 you can already explain. It is the falling half: the hammer, the feather, the photon, everything accelerating at GM/r² with the mass canceled out of the equation. That half was sitting in Soldner’s calculation a full century before Einstein.

The other half has no place in Newton’s universe at all. It is not a stronger pull, and it is not some hidden property of light. The extra bending does not happen to the photon.

It happens to the space the photon is traveling through.

That sentence sounds like poetry until you see the geometry, and then it becomes the most concrete thing in the world. It is also the point where the idea of gravity as a force quietly dies, because in the full picture the photon never turns. It travels in a perfectly straight line from its first moment to its last, and what the Sun has bent are the straight lines themselves.

The slowing of time you have just read about is not a curiosity at the edge of gravity. There is a serious argument that it is the whole engine. In that picture nothing pulls the apple down at all: time below it simply runs slower than time above it, and matter drifts toward slow time. The same logic quietly deletes gravity as a force, from your weight to the Moon's orbit. Walking through that argument properly is Thursday's post, and it goes out to paid subscribers. If you have been meaning to upgrade, this is the week I would pick. The free posts ask the questions. The paid ones do the slow work of answering them.

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One admission before I go. I can run the factor-of-two derivation on paper, and I have explained this topic more times than I can count, and I still cannot tell you it feels intuitive. Somewhere between “things fall” and “space bends,” my gut lets go of the handrail and only the mathematics keeps walking. For a long time I assumed that feeling meant I had not understood it yet. Lately I suspect it is what understanding this actually feels like.

A lithium level came back at 1.4 in my psychiatry rotation, and I thought about it way to much at that time.

Not because 1.4 is a catastrophe. It isn’t. We aim for somewhere between 0.6 and 1.2, and 1.4 means you call the patient, ask about the vomiting bug they had over the weekend, ask whether anyone started them on an anti-inflammatory, hold a dose, recheck. Ordinary work.

What stopped me was smaller than that. Lithium is the oldest drug in psychiatry. John Cade published the result in 1949, before the structure of DNA, before anyone had a coherent theory of how a neuron signals. Seventy-seven years later we still prescribe it, we still monitor it every three months, and we still cannot say with confidence what it actually does in there. There are theories. Inositol depletion. Enzyme inhibition. Good theories, argued over for decades, none of them closed.

So the drug is an atom. Three protons, three or four neutrons, an electron shell. Nothing else. And it is the most reliable mood stabiliser we have, and we don’t know why. Trust me, you know many people who are taking lithium, it’s very common.

That gap is where today’s story starts.

Monday I left you holding ten million volts per meter across every membrane in your body, and promised we’d find out whether anything in that dark is computing. Here is the case, both sides of it, as honestly as I can lay it out.

The physicist who got sick

Matthew Fisher is not a fringe figure. He works on quantum phase transitions at UC Santa Barbara, he shared the Oliver E. Buckley Prize in 2015, his father is a well known statistical physicist at Maryland and his brother is at Stanford. This is a physics family, and he is a serious member of it.

In February 1986 he woke up feeling as though he had been drugged. Extra sleep didn’t touch it. Diet, exercise, bloodwork, all normal, all useless. He has described it as a migraine spread over his entire body, every waking minute, and it lasted two full years. It got dark enough that he thought about ending it, and by his own account what kept him going was the birth of his first daughter.

Eventually a psychiatrist put him on a tricyclic. Within three weeks the fog thinned. Within nine months he felt like a different person. He later moved to Prozac and has been adjusting the regimen ever since.

Then he did the thing that makes him a physicist rather than a patient. He went looking for the mechanism, expecting to find one, and discovered that nobody had it. The drugs worked. The explanation was a shrug.

He spent the next five years teaching himself neuroscience and biochemistry, and he started where I started, with lithium, for exactly the reason I gave you above. Every other psychiatric drug is a complicated molecule with a dozen places to hide. Lithium is one atom. If something strange is happening, an atom is the cleanest place to catch it.

Or is it…

The loophole

You already know the objection, because I gave it to you on Monday. In 2000 Max Tegmark calculated how long a quantum state could survive inside a working brain and got somewhere between 10⁻¹³ and 10⁻²⁰ seconds. Thinking happens on the scale of milliseconds. The mismatch runs to ten orders of magnitude and beyond, and for most physicists that closed the file.

Read Tegmark’s paper carefully, though, and you notice what he actually calculated. He looked at a neuron in superposition of firing and not firing. He looked at Penrose’s microtubules. Both of those are stories about electrons and moving charge, and moving charge is loud. It shoves the surrounding water around, and the water shoves back, and the state is gone.

An atomic nucleus is a different object. Tuck a nucleus inside its own electron cloud and it barely converses with the outside world at all. This is not exotic physics. It is the entire reason MRI works: we can address nuclear spins in a living human body and they hold their orientation long enough to be useful.

So Fisher asked a narrower question than anyone had been asking. Not whether the brain is a quantum computer. Whether there is any degree of freedom in the brain that Tegmark’s arithmetic does not reach.

He found one candidate, and only one.

Why it has to be phosphorus

For a nucleus to work as a qubit you want spin one half. Higher spins carry an electric quadrupole moment, which is a handle the environment can grab, and they decohere faster.

Go through the elements your body is actually built from and the list collapses almost immediately. Oxygen-16, spin zero, useless as a qubit. Calcium-40, spin zero. Nitrogen-14, spin one. Carbon-13 is spin one half but only about one carbon atom in ninety is carbon-13. That leaves hydrogen, which is spin one half and is also absolutely everywhere, which makes it noise rather than signal.

And it leaves phosphorus-31. Spin one half. The only stable isotope phosphorus has, so every phosphorus atom in you is one. And phosphorus is not some trace element: it is the backbone of your DNA, the P in ATP, half the surface of every cell membrane you own.

There is a catch, and Fisher is upfront about it. A phosphate ion floating loose in body fluid holds a quantum state for roughly a second before a proton binds to it and wrecks everything. A second is not nothing. It is also not a memory.

Unless you could keep the protons off it.

The cage

Calcium can outcompete a proton for a phosphate. We have known this for a long time, in the most literal way possible, because it is what bone is.

In 1975 a Cornell scientist named Aaron Posner was looking at X-rays of bone mineral and noticed a recurring cluster: nine calcium atoms and six phosphates, packed into a small, almost cubic knot. The structure was named after him. Decades later, when researchers simulated bone growth in artificial body fluid, the same clusters turned up floating free in the liquid.

Look at what that cluster is made of. Calcium, nuclear spin zero. Oxygen, nuclear spin zero. Six phosphorus nuclei sitting on the faces of a cage built entirely out of silence.

Fisher’s estimate for how long a phosphorus spin survives in there is not a second. It is 10⁵ to 10⁶ seconds. A day at the low end. Longer at the high end, and in a later paper he pushed it to three weeks.

The full proposal has more moving parts than I can fit here, but the spine of it runs like this. Your cells constantly split pyrophosphate into two phosphate ions using an enzyme, and Fisher argues the geometry of that reaction leaves the two phosphorus nuclei entangled as they separate. Each one gets absorbed into a Posner cluster, which protects it. The clusters drift, get taken into different neurons, and when two of them bind and break apart they release a burst of calcium. Calcium is the trigger for neurotransmitter release. So if two entangled clusters melt in two distant neurons, those neurons fire in a correlated way that no classical account would predict.

That is a real mechanism. It names molecules, reactions, and an output you could in principle go and measure. Whatever else you think of it, it is not hand-waving.

The experiment that shouldn’t have mattered

A beautiful mechanism is not evidence, and Fisher knew it. So he went looking for something already sitting in the literature that his idea could explain and ordinary chemistry could not.

He found it in 1986, the same year he got sick.

At Cornell, a group led by Jeri Sechzer had given lithium to female rats before and during pregnancy. Not lithium in general. Lithium separated into its two stable isotopes. Lithium-7, which is ninety-two and a half percent of the lithium in the bottle at your pharmacy, and lithium-6, which is the rest.

Chemically these are the same element. Same three protons, same electron shell, same reactions in every way a chemist normally cares about.

Then the pups were born, and the mothers behaved differently depending on which isotope they had been given. The lithium-7 mothers largely ignored their pups and nursed them rarely. The lithium-6 mothers groomed and nursed more than untreated mothers did.

Opposite directions. From one neutron.

For Fisher this was the loose thread. Lithium-6 and lithium-7 have different nuclear spins. If nuclear spin is doing real work inside a mammal, this is what it would look like from the outside.

That is the case for the defence, and I have given it to you at full strength, because what comes next is not kind to it.

Since 2018 four separate groups have stopped arguing about Fisher’s molecule and started measuring it. Not modelling it. Measuring it. Real phosphorus, real magnets, the exact entangled state his proposal requires, and a clock.

Their numbers are below.

So is what happened when Fisher flew to Stanford to run the rat experiment again himself, which is the part I find hardest to write about, because he is the last person on earth who wanted it to come out the way it did.

Read more

A nurse called me at three in the morning about a potassium of 7.1.

If you’ve never worked in a hospital, that number means nothing. Normal sits near 4. At 7, the lab doesn’t wait for anyone to open a screen. They phone the ward directly, because at that level the patient’s heart has started negotiating with an equation, and the equation usually wins.

The ECG at the bedside told the rest. T waves peaked like tents, the complexes already starting to widen. Textbook, in the worst sense of the word.

The first drug we push is calcium, and here is the strange part: calcium removes none of the potassium. The number on the lab report doesn’t move at all. What calcium does is steady the voltage across the heart’s cell membranes, holding the electrical system together long enough for the actual fixes to work. Insulin with glucose, to drive the potassium back into the cells. Dialysis, if it comes to that, to pull it out of the body entirely.

Driving home the next morning, it occurred to me that I’d spent the night doing electrical engineering with a stethoscope around my neck.

The reason is an equation that hangs on no hospital wall I’ve ever seen, and quietly runs every ward anyway. Nernst worked it out in the 1880s, decades before anyone knew what a cell membrane was made of. It says the voltage across a membrane is set by a logarithm: roughly 61.5 millivolts multiplied by the log of a concentration ratio, potassium outside the cell divided by potassium inside.

Healthy numbers are 4 outside and 140 inside. Run the log and you get about minus 95 millivolts. That negative voltage is the tension a resting heart cell lives at. The loaded spring behind every beat.

Now raise the outside potassium to 7. The ratio shrinks, the logarithm collapses toward zero, and the voltage sags. Sodium channels, which need a properly negative resting voltage to reset between beats, stop resetting. Conduction slows and smears. On paper, the sharp spikes of a heartbeat begin melting into a slow sine wave, and a sine wave is the last thing many hearts ever write.

A logarithm can stop a heart. Nothing in that sentence is a metaphor.

That night was cardiology, but nothing about the physics is special to hearts.

You’re built from roughly 37 trillion cells, and essentially every one of them holds a voltage across its membrane, around 70 millivolts in the busiest ones. Seventy millivolts sounds pathetic. A AA battery carries twenty times more.

But voltage was never the impressive quantity. The impressive quantity is field strength, volts per meter, and to get it you divide by distance. The membrane holding those 70 millivolts is about 7 nanometers thick. Do the division and it comes out near ten million volts per meter.

For scale: air itself tears apart at around three million volts per meter. That’s the threshold where electrons get ripped off molecules, the air turns to plasma, and we call the result lightning.

So the field across your membranes runs more than three times past the point where air gives up and becomes a lightning bolt. You maintain it across a wall ten thousand times thinner than a hair, in 37 trillion places at once, powered by whatever you had for lunch.

And your body doesn’t only use this voltage as power. It uses it as language. Your brain is 86 billion of these charged cells wired into a network that runs on roughly 20 watts, the draw of a dim bulb. Stranger still, the voltage seems to carry instructions for building the body itself. In 2012, biologists at Tufts changed the membrane voltage of cells in frog embryos, no gene editing, nothing else, and well-formed eyes grew where no eye belongs, lens and retinal layers included, some of them far down the body. Voltage isn’t only the body’s power grid; it appears to be part of the blueprint.

So the first half of my title costs me nothing. Bioelectric isn’t a claim, it’s a measurement. We monitor it at bedsides and implant its proof in chests.

The word I owe you is quantum.

If you have physics training, your guard just went up, and it should. Quantum states are famously fragile. We build quantum computers inside refrigerators colder than deep space, because a single stray vibration can wreck a calculation. A living body is warm and wet, and it never stops trembling. The standard objection says any delicate quantum state inside you should be destroyed in around a femtosecond, and that objection has a name and a number attached to it. We’ll stand in front of it before the end, I promise. First, though, two facts that are not speculation.

Enzymes cheat. Chemistry, as taught, is a story about climbing: a reaction gathers enough energy to get over a barrier, or it doesn’t happen. But when chemists measured certain enzymes transferring hydrogen atoms, the reactions ran far too fast, and they failed a classical test in a telltale way. Swap ordinary hydrogen for deuterium, its double-mass twin, and classical theory allows the reaction to slow down by a factor of seven at most. In an enzyme from soybeans, it slows down by a factor of about eighty.

A number like that has one honest reading. The proton isn’t going over the barrier at all. Its quantum blur reaches through, and it simply appears on the other side. Tunneling. The same trick that lets the Sun fuse hydrogen, the one I once wrote a whole chapter about, is running your biochemistry while you read this sentence.

Then there’s the robin. Every autumn, European robins cross a continent, steering partly by Earth’s magnetic field, and Earth’s field is absurdly weak. Fifty microtesla, give or take. A cheap fridge magnet is dozens of times stronger. The best-supported explanation for how a small bird reads a field that faint lives in its eye. Blue light strikes a protein called cryptochrome in the retina and knocks loose a pair of electrons whose spins stay correlated, a genuinely quantum object called a radical pair. Earth’s feeble field is enough to tilt how those paired spins evolve, toward one state or another, and the two states end in different chemistry. Different chemistry, different signal, and the bird perceives a direction.

For a long time this was an elegant story in need of a killer experiment. In 2021 it got one. Researchers put the robin’s cryptochrome in a test tube and watched it respond to magnetic fields directly. The same protein taken from chickens and pigeons, birds that never migrate, barely responded at all. Evolution appears to have tuned a quantum sensor, and tuned it hardest in the birds that needed it most.

A compass built from electron spin, working at body temperature, in the rain, in an eye.

So let me draw the line where it honestly sits tonight.

That quantum effects run inside living machinery: settled. That evolution found some of them useful and sharpened them: strongly supported in enzymes, and probably in birds. Neither claim should scandalize a physicist anymore.

The strong claim is the one in my title. That your brain, the 20-watt electrical organ decoding these sentences, doesn’t merely contain quantum events but computes with them. That claim is not settled, and most physicists would bet against it, for one brutal reason. In 2000, Max Tegmark calculated how long a quantum state could actually survive amid the heat and salt of a working neuron. His answer: about 10⁻¹³ seconds, often far worse. Neurons operate on milliseconds. The mismatch is ten orders of magnitude. Publish a number like that, and a field considers the case closed.

Except it didn’t quite close. A physicist who has spent his career on the theory of quantum computation read the same result and asked a sharper question: is there anywhere in the brain that Tegmark’s math can’t reach?

Electron states die fast, yes. But the spin of an atomic nucleus barely talks to the surrounding chaos at all. And he believes he’s found one, a natural qubit, the nuclear spin of an element you swallow with every meal, protected inside a tiny cage-shaped molecule your own biochemistry already builds. If his math holds, that spin could stay quantum not for femtoseconds but for stretches you could measure on a clock.

And buried in the old literature, he found an experiment that should unsettle you. Decades ago, researchers gave lithium to rats, as two different isotopes. The same element, the same chemistry down to every decimal a chemist cares about. The only meaningful difference between the two is the spin of the nucleus. The rats’ behavior, reportedly, differed.

If that result holds up, then something inside a mammal’s skull can feel a nuclear spin.

I don’t know yet whether the title of this post is true. Nobody does, and I’d rather say that plainly than sell you a certainty that doesn’t exist. What I do know is what the night shifts keep teaching me: the border between physics and living things is much thinner than my textbooks let on. About seven nanometers, in most places.

Tonight, somewhere on Earth, a storm will pile up three million volts per meter and the sky will crack open, and everyone underneath will stop to watch. You’ve been holding more than three times that field, in the dark, in every cell, your entire life.

Thursday we find out whether anything in that dark is computing.

Thursday: The Trial of the Quantum Brain. The phosphorus qubit hiding in your own biochemistry, the lithium experiment that shouldn’t have mattered, and Tegmark’s number that was supposed to end the argument. Opening statements are free. The verdict is for paid subscribers.

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There’s a saying that gets passed down to medical students like a family heirloom. Some dean stands up during orientation and tells the new class: half of what we teach you here will turn out to be wrong. The problem is that nobody knows which half.

I don’t remember anymore whether anyone said it at mine. It doesn’t matter, because medicine spends the rest of your training proving it right. Treatments I memorized for exams have since been demoted. Thresholds moved. Diseases got reclassified. The textbooks I studied from have editions the way phones have models, and each edition quietly deletes things the previous one stated as fact.

The saying is usually delivered as a joke about humility. Taken seriously, it’s a bomb. If we can’t tell the right half from the wrong half, what exactly do we mean when we say we know something?

On Monday I argued that your senses never show you reality, only a translation of it. A reader named Randy ( I love all the comments) put it better than I did in the comments: the translation is “optimized for action, not truth.” Which raises the obvious next question. Fine, perception is a translation. But surely the things built above perception, the theories and proofs and diagnoses and laws of physics, can be trusted in a way the raw senses can’t. Reason should succeed where the eyes fail.

Today I want to show you how deep this well actually goes. The branch of philosophy living at the bottom of it is called epistemology, and it circles one question I’ve been chewing on since long before this newsletter existed: how do we know that we know?

Start with the most basic thing you could possibly ask. What is knowledge?

For roughly two thousand four hundred years, philosophy had an answer that looked airtight, and it traces back to Plato. Knowledge is justified true belief. Three conditions. You believe it. It’s actually true. And you have good reasons for believing it, so you’re not just guessing. If it rained last night and you believe it rained because you watched it through your window, congratulations, you know it rained. Belief, truth, justification. All three boxes ticked.

Plato himself, in the dialogue where the idea appears, wasn’t fully convinced by it. The formula outlived his doubts anyway, and for most of Western history that was simply what knowledge meant.

Then in 1963, a philosopher almost nobody had heard of, Edmund Gettier, published a paper three pages long and broke it.

The cleanest version of the problem was actually described by Bertrand Russell years before Gettier formalized it. A man glances up at a clock. It says two o’clock. It really is two o’clock. Clocks are reliable, so his belief is justified. Justified, true, believed. By the classical definition, he knows the time.

Except the clock is dead. It stopped exactly twelve hours earlier, and he happened to glance up during one of the two minutes a day when a dead clock tells the truth.

Something in you refuses to call that knowledge. He was right by pure luck, and luck is precisely what the justification condition was supposed to eliminate. So there is a fourth ingredient, something beyond belief, truth, and reasons, and the genuinely uncomfortable part is this: sixty years later, philosophers still haven’t agreed on what it is. Every proposed patch springs a leak somewhere else. Which leaves us in a strange position. The word every courtroom, exam, and lab report depends on has no accepted definition. We use it fluently. We cannot say what it is.

That’s the small problem.

The big one is older, and it aims at science directly. The engine of all science is a move called induction. The sun has risen every morning in recorded history, therefore it will rise tomorrow. This drug lowered mortality in ten thousand patients, therefore it will lower yours. Every measured constant, every dose curve, every law of physics is a record of the past, extended into the future.

David Hume, writing in the 1740s, asked the question that has never been answered: what justifies that extension?

Try it yourself, and notice that every answer cheats. “Nature is uniform.” How do you know? “Because it always has been.” That is induction testifying on behalf of induction, a witness vouching for himself. There is no independent ground. The one move science cannot function without has no non-circular justification, and Hume understood exactly how radical that was. By his own account he coped with it by having dinner and playing backgammon with his friends, then returning to work the next morning as if the abyss weren’t there.

To be clear, none of this is an argument to stop trusting the sun, or your medications. Planes fly. Antibiotics work. The point is to notice what kind of trust this is, because it is not the kind you thought you had.

And underneath both of these problems sits the oldest one of all. An argument attributed to a Greek skeptic named Agrippa, some two thousand years ago. It is so simple that every child performs it naturally, and so brutal that in two millennia nobody has escaped it. Take anything you believe. Ask why. Then ask why again, and keep going, the way a four year old does, and every chain of justification a human being can construct will end in one of exactly three places.

All three should bother you.

Behind the paywall: the three dead ends, the single tile of certain floor Descartes found at the bottom of them, the move Kant made that connects straight back to Monday’s post, and then the part I care about most, which is how medicine taught me to act on knowledge nobody can prove, and why I’ve made a strange kind of peace with that. If you’ve been thinking about upgrading, it’s free to try.

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Touch is an illusion. Colors are not real. Time is something your brain invented to make sense of the world.

If you spend any time on science social media, you’ve seen these lines so often they’ve probably stopped meaning anything. Half my feed is some version of “you’re not actually sitting on that chair, you’re hovering above it.” And honestly, I judge myself a little for writing about this too.

But I’m doing it anyway, because underneath the clickbait there’s something genuinely useful. Thinking this way lets you see reality a little more naked. The cheap shock question (”is anything even real?”) falls away, and much better questions take its place. What is time, actually? How do we know that we know anything at all?

Let me start with touch, since I’ve covered it before [link: touch article].

A quick recap if you missed it: while you’re holding your phone or computer right now, the atoms in your hand and the atoms in the screen are not touching in any traditional sense. Electric fields push from a distance; no contact is required. The closer you force two atoms together, the harder the electrostatic repulsion fights back, and in the classical picture, making two electrons meet at a single point would take infinite energy. What you call “touch” is your nervous system’s translation of that repulsion.

Notice what this does and doesn’t mean. It doesn’t mean touch is fake. At our scale, “touching” is a perfectly good word for what’s happening. The definition came from perception, and for everyday life, perception is exactly the right tool. The word isn’t wrong; it’s just not fundamental. It’s a translation.

And there is nothing special about touch. Run the same argument through every sense you have.

Sight is nothing but light received by photoreceptors in your eyes. Millions of receptors fire signals toward the brain, and the brain assembles them into the full picture. Color is nothing more than a specific wavelength of light. Red is the wavelength around 700 nm, blue the one near 450 nm. Outside of a brain, the whole concept of color has no meaning; the universe contains only waves of different lengths. If we could see ultraviolet, our brain would presumably paint it as some new color neither of us can imagine. Bees and many birds don’t need to imagine it. They see it every day.

So perception is not reality; it’s a translation of reality. And your brain is not a passive translator. It’s an aggressive one.

The brain does not accept raw, unlabeled input. It is in constant search for meanings, patterns, and definitions, and it forces them onto everything it receives. Psychologists call these the Gestalt principles: the brain fills the gaps. The image below is only a couple of curved pentagons. There is no ball in it. Yet it’s not even possible for your brain to see anything except the ball. Try it.

That’s how weak and strong our brain is...

And it runs deeper than visual tricks. The image formed on your retina is flat, two dimensional, and upside down. You have never once seen that raw feed. Your brain takes an inverted 2D signal and hands you an upright, three dimensional world, so seamlessly that it took humanity most of its history to even notice this was happening.

There are famous experiments with inversion goggles, lenses that flip your vision upside down. Wear them long enough and the brain adapts. After days, subjects could move around and function again, and some reported the world beginning to feel normal. Whether their perception truly flipped back upright, or their brain simply learned to act inside an upside-down picture, is still debated. Which is almost the point: we can’t even be certain what they saw.

The brain will even generate sensation with no input at all. Ask any physician about phantom limb. Amputees feel the missing arm, its position, its itching, sometimes its pain, years after it’s gone. The limb isn’t there. The perception is!

None of this is fringe science, by the way. It’s textbook sensation and perception; I sat through lectures on it in med school. But it carries two consequences that reach far beyond psychology.

First, this perceptive machinery is a blessing and a curse. Pattern-forcing is why we learn absurdly fast: a child needs only a handful of examples to build a working model of an enormously complex universe. But the same ability makes us very prone to biases and false definitions. The most common is confirmation bias, which is basically Gestalt for ideas. The brain fills the gaps in new information with what it already believes, so we don’t really question anything, and everything conveniently confirms what we already know. Now imagine the transition from classical to quantum mechanics. To a mind exposed only to examples of continuity (smooth motion, smooth energy, smooth everything), the possibility of discrete entities was just insane. I’d argue that without confirmation bias, we would have reached quantum mechanics decades earlier.

Second, and this is the uncomfortable one: it becomes very hard to be sure of what we know. If touch is not exactly what it seemed at first glance, what else isn’t? We’ve already been through this once on this newsletter. Mass and energy, two obviously different things, turned out to be the same thing wearing different clothes [link: E=mc² post]. Every time physics matures, another “obvious” perception gets demoted from fundamental truth to convenient translation.

There’s an entire branch of philosophy devoted to this exact problem. It’s called epistemology, and it asks one main question: how do we know that we know? Which brings me to Friday, I’ve read a bit of Epistemology through the years because it’s incredibly fascinating to me and having studied physics and medicine, I have my own understanding on the subject. Full version as always is for paid subscribers so consider it, it’s free to try.

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Hope you’re having a nice week.

Hasan

There is a small violence in fundoscopy (Eyes imaging) that nobody warns you about in training. You dim the room, you get close enough to a stranger’s face that it stops being socially normal, and then you put a light into their eye and ask them please not to blink.

You are looking for the back wall. The optic disc, the vessels branching off it, whatever the years of pressure and sugar have quietly done to them.

What you are doing, if you take the physics seriously for a second, is firing a stream of photons across four different materials in about a tenth of a nanosecond!

They meet the tear film, then the cornea, where the refractive index runs around 1.376 and the light drops immediately to roughly three quarters of its vacuum speed. Then the aqueous humour at 1.336. Then the lens, which is not one material but a gradient, near 1.386 at the cortex and 1.406 at the nucleus. Then the vitreous, back down to 1.336. Then the retina, where the whole trip ends in a molecule bending out of shape.

Monday’s letter was about the fact that light pushes on things at all. Momentum equal to energy divided by the speed of light, mass or no mass, a poppy seed’s worth of force spread across your whole body every daylight hour, real and permanently below your threshold of feeling.

I left something out of it.

That formula, p = E/c, is a vacuum formula. The moment your light enters that cornea it is no longer in a vacuum, and the question of how much momentum it is carrying turns into one of the longest running arguments in the history of the subject. It ran for a hundred and two years. It involved a mathematician who died before the argument he started got properly going, a physicist who spent his career picking fights with Einstein, and roughly a dozen careful experiments that kept contradicting each other without any of them being wrong.

I did not know any of this month ago. I want to walk you through it in the order I found it, because the order is what made it land.

Two men, one year apart

Hermann Minkowski published his version in 1908. If you know the name it is probably from spacetime, the four-dimensional geometry he built out of Einstein’s relativity, the thing that made Einstein’s algebra into a shape. Momentum of light in a medium was a smaller item on a long list.

His answer: the momentum goes up. Multiply the vacuum value by the refractive index.

p = nE/c

And there is something immediately reasonable about that, which you can see without any machinery. A wave that slows down while its frequency stays fixed has to shorten its wavelength. Light entering the cornea at 1.376 has its wavelength squeezed by that same factor. De Broglie’s relation says momentum is Planck’s constant divided by wavelength. Shorter wavelength, larger momentum. The light gets stiffer as it enters you.

Max Abraham published his the following year, 1909. Abraham is a less comfortable figure. He spent years attacking special relativity, backed a theory of the electron that turned out to be wrong, and Einstein wrote about him afterwards with real affection anyway, the way you might about a rival who at least never bored you.

His answer was the opposite. The momentum goes down. Divide by the refractive index.

p = E/(nc)

Notice what this means. For the vitreous humour at 1.336, the two predictions differ by a factor of n squared, which is about 1.79. Nearly double. This is not a rounding disagreement or a second-order correction. Two formulas from two serious people, differing by almost a factor of two, about the momentum of ordinary light inside ordinary water.

And for forty-five years nobody could break the tie, because both arguments are load-bearing.

Why you cannot simply pick one

The strongest case for Abraham is a thought experiment from 1953, by a Hungarian-American physicist named Nándor Balázs, and it is the kind of argument that makes you slightly angry with how clean it is.

Float a transparent block in deep space. Nothing touching it, nothing pulling on it. Send a single pulse of light through it, in one face and out the other, no reflection.

Inside the block the light travels at c/n, slower. So it comes out later than it would have if the block had not been there, and therefore further back than it would otherwise be. The pulse has fallen behind.

But the centre of mass of an isolated system has to keep moving uniformly. That is not a rule about light, it is Newton’s first law with Einstein’s mass-energy equivalence bolted on, and if it fails then everything fails. The light carries energy, energy carries inertia, and the light’s share of the system’s centre of mass has just lagged.

Something has to make up the difference. The only candidate is the block. So while the pulse is inside it, the block must creep forward.

Work out how much momentum the block has to have acquired to creep exactly that far, subtract it from the total, and what remains for the light is E/(nc). Abraham’s value, arrived at using nothing but conservation of momentum and the uniform motion of a centre of mass.

Barnett and Loudon, reviewing the whole mess in 2010, put it plainly: it is difficult to see how any component of that derivation could seriously be open to question.

Now the case for Minkowski, which is just as bad.

Take an atom sitting inside a medium, moving away from a light source, and let it absorb a photon. The absorption only happens if the Doppler-shifted frequency matches the transition. Write down conservation of energy, write down conservation of momentum, solve for the photon’s momentum. Out comes nE/c. Minkowski’s value, from the Doppler effect and two conservation laws.

Or take a slit. Send light through a narrow slit and it diffracts, and the angular width of the central peak is fixed by the uncertainty principle: narrow the slit, spread the momentum sideways. Do the same experiment submerged in a medium and the peak comes out narrower by exactly n. The slit has not changed, so the sideways momentum spread has not changed. The only way the angle shrinks is if the forward momentum grew by n.

So to throw out Abraham you have to throw out Newton’s first law. To throw out Minkowski you have to throw out the uncertainty principle, or the Doppler effect, or conservation of momentum.

Physics had two answers, differing by a factor of two, each resting on something nobody was willing to give up.

So they went and measured it

This is the part that got me, and it is why I wanted to write this one for you rather than post it as a note.

1954. R. V. Jones and J. C. S. Richards hang a mirror in a liquid, shine light on it, measure the pressure. The force scales with the refractive index. Minkowski.

1973. Arthur Ashkin, the same man whose optical tweezers ended Monday’s letter, focuses a pulsed laser on the free surface of pure water with James Dziedzic. Sixty nanosecond pulses, four kilowatts of peak power, a spot about two microns across. The water surface bulges outward, roughly a micron of it, enough to act as a lens. They read it as Minkowski, though this one has been argued over ever since, and Gordon published a challenge to the interpretation the same year.

1975. Walker, Lahoz and Walker measure the force on a barium titanate specimen in crossed fields. Abraham.

1978. Jones again, now with Leslie, better apparatus, dispersive media. Minkowski.

1980. Gibson and colleagues push far-infrared light through germanium and silicon and watch the momentum land on the free charge carriers. Minkowski.

1994. Kristensen and Woerdman measure the torque on an object inside a dielectric, the angular momentum version of the same fight. Minkowski.

2005. Campbell and Ketterle’s group at MIT measure the recoil of atoms inside a Bose-Einstein condensate, which is about as clean as this experiment can be made. Minkowski.

2008. She, Yu and Feng watch light leaving the end face of a silica filament a few hundred nanometres wide and measure which way the fibre bends. Abraham.

Half a century of it. Good groups, improving equipment, no obvious pattern, no experiment that could be dismissed as sloppy, and the answer flipping back and forth like a coin that will not settle.

Somewhere in that list is a pattern.

It has nothing to do with who had better vacuum pumps or which decade you were working in. Sort those experiments by what each one was physically touching, and the disagreement evaporates. Every single one falls into line, including the two that went to Abraham, including the contested one in 1973.

It took a hundred and two years for anyone to sort them that way. And when somebody finally did, the answer was not that Minkowski won, or that Abraham won, or that the truth sat somewhere in the middle at an average of the two.

The answer was that the word momentum has been quietly doing two separate jobs since Newton, and no experiment in the history of physics had needed anyone to notice, because the two jobs give identical answers everywhere except inside a piece of glass.

Below: the sorting rule, the 2010 paper that found it, what the two momenta actually are and why you already know one of them, the calculation where the total comes out as the exact average of Abraham and Minkowski, and what any of this means for the photon crossing your patient’s vitreous while you are squinting at their optic disc.

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There is a kind of afternoon in August here when the sun stops behaving like light and starts behaving like weather. You step out of a building and something arrives on your face and your shoulders, and the most natural word for it is pressure.

That word is doing something slightly dishonest, and I want to spend a Monday on why.

What you are feeling is heat. Your skin absorbs infrared, the temperature of the tissue climbs by a fraction of a degree, and the thermoreceptors sitting underneath start firing. Your brain reads that firing as intensity, and intensity is very easy to mistake for force. It is the same trick your hand plays on you when a metal railing in that sun feels violent to touch and the wooden bench beside it feels fine, even though a thermometer would tell you the two are at exactly the same temperature.

So the pressing sensation is mostly a story your nerves are telling you.

There is a real push underneath it, though. It has been there every second you have ever stood in daylight, it has been measured to several decimal places, and it sits so far below your detection threshold that no human being has ever felt it or ever will.

Light has no mass, and that turns out to be beside the point

Momentum, in the version most of us were handed at school, is mass times velocity. If that is what momentum is, then a photon has none of it, because a photon has no rest mass, and zero multiplied by anything stays zero.

The trouble is that p = mv was never the definition. It is a special case that works beautifully when things are slow and heavy, which happens to describe almost every object you have ever picked up. The fuller relationship is the one relativity gives us:

E² = (pc)² + (mc²)²

Set the mass to zero and the second term simply disappears. What survives is:

E = pc, which rearranges to p = E/c

Momentum equals energy divided by the speed of light, and mass never enters the conversation at all. Its absence does not cancel the momentum. Its absence just deletes a term that was sitting next to it.

I keep running into this pattern and it still gets me. A formula you were given as a definition turns out to be the shadow of a bigger one, and the result you find impossible was hiding inside the approximation the entire time.

How hard, exactly

On a clear day at noon, roughly a thousand watts of sunlight land on every square metre of ground. Divide by the speed of light:

1,000 ÷ 300,000,000 ≈ 3.3 millionths of a pascal

Now stand a square metre of yourself in front of it. That is about three millionths of a newton, which is the weight of a poppy seed. One poppy seed, resting across your whole chest and shoulders, every daylight hour of your life.

You cannot feel it and the reason is anatomical rather than physical. The mechanoreceptors in your skin need force concentrated on a point, and they need orders of magnitude more of it than this, before a single one of them will fire. Sunlight is delivering a smaller load than that, smeared evenly across your entire surface. There is nothing for the nerve to grab onto.

Scale the same arithmetic up and it stops sounding negligible. Earth turns a disc of about 128 trillion square metres toward the Sun, so the total force of sunlight on this planet runs to something close to 600 million newtons, pushing us gently outward, permanently. Which is an enormous number until you put it beside the Sun’s gravitational grip on us and find that light is losing that particular contest by a factor of around sixty trillion.

Nobody was going to notice this by feel. It had to be reasoned into existence first, and then chased.

Kepler noticed something in 1619

He was looking at comets, and he could not get past one detail: the tail never trails behind the comet the way smoke trails behind a train. It points away from the Sun. When the comet swings around and heads back out, the tail goes in front of it. Whatever is making that tail is being pushed, and the push is coming from the direction of the Sun.

Kepler’s guess was sunlight. He wrote it down two and a half centuries before anyone could justify it.

Here is what I did not know until recently, and what makes the story better rather than worse. A comet has two tails, and Kepler was right about one of them.

The dust tail is genuine radiation pressure. Sunlight pushes on grains of released silicate dust, and because those grains stay in their own slightly altered orbits around the Sun, that tail comes out broad, pale and curved, always lagging behind the comet’s path.

The other one, the narrow blue tail that points almost exactly away from the Sun, is not light at all. In 1951 Ludwig Biermann worked out the accelerations in those tails and found them roughly a thousand times too large for sunlight to be responsible. Something else had to be doing it. What he proposed to fill the gap was a continuous stream of charged particles pouring off the Sun, and that proposal is how we came to know about the solar wind.

Kepler pointed at a comet and said the Sun is pushing that. He was correct twice over, and wrong about which mechanism was doing which job, and one of the two answers took another three hundred and thirty years to arrive.

Maxwell predicted it, then refereed the paper that got it wrong

Maxwell’s electromagnetism gave the push a firm theoretical footing in 1873. Light carries energy and momentum, so light presses on whatever it lands on. The number was small but it was no longer a guess.

That same year, the chemist William Crookes built a little glass bulb with four vanes inside, each blackened on one face and silvered on the other, partially evacuated. Put it in sunlight and the vanes spin. Crookes announced he had demonstrated the pressure of light directly, on a tabletop, for anyone to see. Maxwell refereed the paper, and by the accounts I have read he was pleased to see his prediction apparently made visible.

The device is wrong, and it is wrong in a way that is more interesting than if it had simply failed.

A photon absorbed by the black face delivers its momentum, E/c, and stops. A photon striking the silver face bounces back the way it came, which means it has to be turned all the way around, which takes twice the momentum. So radiation pressure is twice as strong on the mirrored side. If light were driving that toy, the black faces would lead and the silver faces would trail.

They do the opposite. Every one of them. Crookes had built a beautiful demonstration of something else entirely.

The actual mechanism is the residual gas in the bulb. The black face runs hotter, and around the edges of each vane, gas creeps from the hot side toward the cold one and shoves the vane the other way. Osborne Reynolds identified the effect. Maxwell refereed Reynolds’ paper too, immediately saw further into it than Reynolds had, and wrote up the mathematics himself in a paper that appeared in the Philosophical Transactions in 1879 and turned out to be the last major work he ever published. He died that November.

There is something I find hard to sit with about that sequence. Maxwell predicted radiation pressure, approved a demonstration of it that was measuring the wrong thing, and then, in the final months of his life, wrote the paper explaining why his own prediction was not what the famous toy was showing.

The real measurement took another twenty years and had to be done in a much harder vacuum, on vanes hanging from fine fibres, with every thermal effect hunted down and eliminated. Pyotr Lebedev did it first in Moscow and reported it in 1900. Ernest Nichols and Gordon Hull did it independently at Dartmouth and published in 1901, and again more precisely in 1903, unaware of Lebedev until afterwards. The push was real, and it matched Maxwell to within the error bars.

Then somebody built a sail

If the force is constant and you have nowhere to be, small stops mattering. There is no fuel to run out of.

JAXA launched IKAROS in May 2010, riding along with the Akatsuki mission to Venus. It unfurled a square of polyimide film fourteen metres on a side, about two hundred square metres of sail, by spinning and letting the weights at the corners pull it open. In July, tracking the craft by Doppler, the team measured the thrust that sunlight alone was producing on it.

1.12 millinewtons. Roughly the weight of a large grain of rice, applied to a 310 kilogram spacecraft.

That works out to an acceleration of about three millionths of a metre per second squared, which is nothing, until you remember it never switches off. Keep pushing at that rate for a year and you have added something like a hundred metres per second to your velocity, without carrying a gram of propellant. In 2019 the Planetary Society’s LightSail 2 became the first small spacecraft to change its own orbit using sunlight and nothing else.

The part I keep coming back to

By 1986, Arthur Ashkin at Bell Labs had worked out how to focus a laser tightly enough that the radiation pressure would hold a small particle at the focus instead of blowing it away. He called them optical tweezers. In 1987 he used them to pick up a tobacco mosaic virus, and then an E. coli.

His first attempts killed the cells. Visible laser light at the powers he needed was absorbed by the water and the cell contents, and the bacteria cooked, and some of them burst. He switched to infrared, which water barely absorbs, and the problem went away. The trapped cells stayed alive. He watched them divide inside the beam.

The forces involved are piconewtons, a millionth of the poppy seed on your shoulders. But an E. coli weighs about a picogram, and against that, a few piconewtons is hundreds of times the bacterium’s own body weight. The force did not get bigger. The thing being held got small enough for the force to matter.

I have been thinking about that ratio for weeks. The same push that cannot make itself known to a single nerve ending in your skin can lift a living cell off the floor of a dish and hold it in mid-water for hours. It is the same equation both times. It is only ever the same equation. Ashkin got half the Nobel Prize for it in 2018, at ninety-six years old.

I will admit the thing I have not resolved. I can do the algebra that says a massless particle carries momentum, and I have done it enough times that it comes out automatically now. That is not the same as understanding it, and I do not think I understand it. When I try to picture what is actually being handed over when a photon strikes a mirror, I get nothing back. Possibly there is no picture and the equation is the whole of it. Possibly wanting a picture is a hangover from being an animal that grew up throwing rocks. I have not decided which of those is the honest answer, or whether they are the same answer wearing different clothes.

Either way, you are being pushed. Slightly. Continuously. Right now, if you are reading this near a window.

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Weird way to start I know, this post is about quantum physics, stay with me.

No doctor loses sleep over this. Of course measurement disturbs the thing being measured. The thermometer steals a little heat. The needle changes the vein. We repeat the reading and move on.

I got the feeling that also a lot of people understand quantum measurements changing reality in the same way. A measurement is after all interference with whatever we’re measuring so of course it will change the result, right? It's funny sometimes. I wish reality conformed to our simple minds lusting for simple answers, but time after time, physics shows me that reality is crazier than fiction. I now wish reality is at least within my logical limit and not beyond it.

On Monday I showed you how this exact intuition ruins the uncertainty principle. Today I want to show you something worse. The measurement problem is not that measuring disturbs the particle. It is sadly that after one hundred years, physics still cannot tell you what a measurement is.

Let me build the problem properly. The setup is simple. We have an electron gun which shoots electrons, a wall with two slits in it, and behind the wall a screen that records where each electron finally lands. We fire the electrons one at a time so they cannot bounce off each other. If electrons are little balls of matter, we expect about half of them to pass through the right slit and the other half through the left slit, which would create two strips on the screen.

That is not what happens. The screen slowly fills with many strips, an interference pattern, the signature of a wave. Each electron leaves the gun as one particle and lands on the screen as one dot, but in between it behaves like a wave that passed through both slits at the same time and interfered with itself. The electron is a wave when we are not looking at it.

So we try to catch it in the act. We place a detector at the slits to see which slit the electron really takes. And the moment we look, the wave dies. Every electron now passes through one slit like a proper little bullet, and the screen shows exactly the two boring strips we expected from the start. Because we measured its position, the electron was forced to become a particle.

This has been observed countless times, with equipment far too precise to blame. The behavior of the electron genuinely depends on whether you are looking at it or not. Everyone summarizes it in the same sentence: measurement collapses the wave function. The smeared cloud of possibilities snaps into one definite outcome the moment a measurement happens.

I will accept the sentence. I only have one small question about it.

What counts as a measurement?

Is it the photon that bounces off the electron? The click of the detector? The computer writing that click into its memory? Or the moment a human being reads the number on the monitor? The equations of quantum mechanics never say. Schrodinger’s equation, the law that rules the wave function, describes a smooth, continuous, fully deterministic evolution. There is no collapse anywhere inside it. Collapse is a rule we glued on afterwards, by hand, because a single outcome is what we always seem to get.

And this glued-on rule has a crack in it that popular explanations politely walk around.

The detector is made of atoms. Atoms obey Schrodinger’s equation, the same one the electron obeys. So take the equation seriously and apply it to the detector itself. It gives a clean, confident answer: the detector does not collapse the electron. It joins it. The mathematics now describes one combined wave function in which the detector registered the left slit and the detector registered the right slit, both branches equally alive, exactly like the electron was in both slits a moment earlier. The wave did not collapse. It grew, and it swallowed the detector.

Then it swallows the computer that records the detector. Then the light leaving that screen, and the air in the room, and everything the light and the air touch next. Follow the chain honestly and it keeps growing, link by link, through every physical object it meets.

There is one object in the room it has not reached yet.

In 1932, John von Neumann, one of the sharpest mathematicians who ever lived, traced this chain link by link to find where the equations allow it to stop. What he found is the reason physicists are still fighting about reality today, and it is exactly where we are going next.

The rest is for paid subscribers: where the chain finally ends, what it did to Schrodinger’s poor cat, and the five escape routes physicists still argue about. One of them involves you splitting.

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When I used to talk about quantum mechanics to my friends, the Heisenberg uncertainty principle was always brought up and everyone had their own opinion on it. They all sensed its existence in their lives. The uncertainty principle, as most people understand it, is an inverse relation between position and speed. If you knew very precisely the position of an object, you would not know its speed precisely. Also, as the measurement of the position becomes more accurate, the measurement of speed becomes less accurate. The opposite is also true where preciseness in the value of speed would sacrifice that of position. This can be represented easily in Heisenberg’s formula:

Δx Δp ≥ ħ/2

It shocked me at first to see that people claimed to understand this fact since I thought it is fairly complicated. The interesting part is that all people have the same wrong intuition about it. They imagine an object moving very fast, and they imagine trying to know the exact position of the object when making a measurement. Even when I imagine their imagination, I can see how they understand that the error in the measurement becomes higher as the speed increases. What about the opposite? If they are very confident in the accuracy of a position, they imagine a slow object where the speed is not clear now. I am not sure if you had the same intuition before reading this, but I hope you can spot how they perceive the principle.

As lovely as this sounds, in which everyone naturally agrees on the same logic, I am sorry to break it to you, but it is not true. The uncertainty principle is a quantum effect and it is defined only with quantum wave functions and nothing else. As we previously explained, a quantum object has a probabilistic distribution of position and momentum (speed). The uncertainty principle is actually:

σx σp ≥ ħ/2

Assume we have the two following graphs of the wave function squared as a function of position. At each position, the y-axis represents the probability of finding the quantum particle at that position. Now let us compare the two: they are centered at the same point where the center represents the mean in this case. The left graph is narrower than the right one; it has a smaller standard deviation. If we were to make a guess on where the particle is, the most likely option is at the mean. Moreover, if we did a measurement to find the position of the particle, the range of possible results is much more concentrated around the mean in the left graph than the right graph. So, we can say that the mean guess has a smaller error in the left graph than the right graph. In the case where the distribution is a probabilistic one, what can we get in the measurements? The standard deviation is exactly a representation of the error in what we measure. As the standard deviation increases, we are less sure if what we measured is true.

What about momentum? Speed is the derivative of position with respect to time, but a speed probabilistic distribution is not the derivative of a position probabilistic distribution. The momentum distribution is the Fourier transform of the position distribution. The Fourier transform of a vertical line is a horizontal line. Consequently, the Fourier transform of a very narrow and peaked distribution (resembling a vertical line) is a very broad and peakless distribution (resembling a horizontal line). So, because of this mathematical relation of position and momentum, the uncertainty principle exists where a limit of the precision in measuring will always be present between the two.

Short one, but it has a huge concept I think. In my eBooks, this was called the most misunderstood law in physics because of how easy it is to misinterpret such words. For thursday, the natural extension for me is to go a bit philosophical to another very badly understood topic: “what is a measurement in quantum physics”. Full version like always is for paid subs.

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A couple of hours’ drive east, at Brookhaven on Long Island, a team led by James Cronin and Val Fitch was watching a subatomic particle do something it was strictly forbidden to do, and rechecking their machine for the fault that had to explain it, because the alternative was unthinkable.

Neither team knew it, but they were holding the two halves of the same story. It is the story of the greatest disaster that ever happened, and of the accounting error that saved you from it. On Monday I showed you the mirror: every particle of matter has an antimatter twin, identical in mass, opposite in charge, and when twin meets twin, both vanish into light. Today I have to show you what that mirror almost did.

The factory only makes pairs

Start with the one thing every experiment on Earth agrees about. Energy can become matter. That is E = mc² read from right to left, and we do it daily in colliders: pour enough energy into a point and particles condense out of it like dew. But they never condense alone. Creation is symmetric. Make an electron and a positron appears with it. Make a proton, and an antiproton is born in the same instant. Across a century of particle physics and trillions upon trillions of collisions, nobody has ever created a piece of matter without its mirror twin. The books always balance.

Now run the Big Bang under that rule.

The early universe was pure energy at temperatures with no name, condensing into particles everywhere at once. Symmetric creation on the grandest scale there will ever be: for every particle of matter, a twin. Then the universe expanded and cooled, creation slowed, and the meetings began. Twin found twin. Annihilation everywhere, an entire cosmos un-happening, matter and antimatter erasing each other back into light within the universe’s first fraction of a second.

Follow the rule to the end and the prediction is clean. Everything meets its twin. Everything cancels. What remains is a universe of light, expanding and cooling forever, perfectly empty of matter. No atoms ever form, no stars ignite, and no one is ever there to notice.

That empty universe is the one our laws predict. It is, by every account we know how to write down, the correct outcome.

It is also, you may have noticed, not what happened.

The audit

So count what actually exists. Count the matter: every galaxy, every star, every atom of you. Then count the light: the sea of ancient photons left over from the early universe. The ratio comes out to more than a billion photons for every single surviving particle of matter.

Read as a receipt, that number tells a precise story. For every billion pairs of twins that met and annihilated, turning themselves into that light, one particle of matter stood alone with no twin to meet. A billion and one against a billion. The billion pairs became an ocean of photons. The one became everything else.

Caption: A billion and one against a billion. The pairs became light. The ones became you. Alt text: Two large numbers, 1,000,000,001 particles of matter in purple against 1,000,000,000 particles of antimatter in gold, above rows of faded annihilating pairs and one bright surviving purple particle labeled the one left over.

You are not made of the universe’s main event. The main event was the annihilation, and the light it produced outnumbers you a billion to one. You are made of the rounding error. Every mountain, every ocean, every person you have ever loved is built from the one-in-a-billion survivors that, for some reason nobody can name, had no partner waiting.

Which brings us back to the hiss in New Jersey.

That ocean of light never went anywhere. It has been traveling for 13.8 billion years while space expanded beneath it, its waves stretched from blinding glare down to faint microwaves, and it arrives today from every direction at once: about 400 ancient photons in every cubic centimeter of space, including the cubic centimeter in front of your eyes right now. That is what the horn in Holmdel was hearing. The hiss Penzias and Wilson could not remove was the oldest light in existence, the surviving glow of the fireball the great annihilation poured its energy into, the very light against which we count the missing antimatter. They had tuned into the receipt. It earned them the Nobel Prize, and it is still falling on your roof tonight.

Caption: The flash never faded. It stretched. Alt text: An annihilation starburst on the left sends a wave whose wavelength visibly stretches as it travels toward a horn antenna labeled Holmdel, New Jersey, 1964, heard as static. Below, a small cube represents one cubic centimeter of space today, containing about 400 photons of the ancient light.

The escape hatch

Now, there is an obvious way out of this whole problem, and I want to close it properly, because it is the first thing everyone thinks of. Maybe nothing went missing. Maybe the antimatter is simply somewhere else. Half the universe minted in matter, half in antimatter, separated early, and somewhere out there are antimatter galaxies with antimatter stars warming antimatter worlds, whose astronomers count their leftover photons and wonder where all the matter went.

It is a lovely idea, and the sky says no. Space is not empty enough to keep the halves apart. Galaxies collide, gas drifts between them, cosmic rays cross the whole galaxy and land in our detectors. Wherever a matter region touched an antimatter region, the border would burn, a permanent front of annihilation blazing in gamma rays. We have mapped the sky in gamma rays for decades, and there are no burning seams. The cosmic rays raining onto Earth from across the galaxy are matter, essentially without exception, and a detector bolted to the International Space Station has spent more than a decade sifting them for a single confirmed nucleus of primordial antimatter. Nothing confirmed yet.

The antimatter is not hiding. It is gone, annihilated down to the last speck. And that leaves exactly one way out, and it is the scandalous one.

If creation is perfectly symmetric, we do not exist. We exist. Therefore, somewhere in the deep laws, the symmetry must fail. The mirror I showed you on Monday, the one physics spent three decades polishing, must have a flaw: some process in nature must treat matter and antimatter differently, must tilt, however slightly, toward us.

For years that idea sat close to heresy, because the mirror had passed every test. Physicists had already caught nature cheating one reflection: in 1957, an experiment led by Chien-Shiung Wu showed the weak force can tell left from right, which was shocking enough. But the repaired, combined mirror, swap every particle for its twin and flip space like a glove, was declared truly fundamental after that. Sacred, even. It had to be.

Which returns us, at last, to Long Island, spring of 1964, and to the particle in Cronin and Fitch’s detector that was doing the forbidden thing. They spent months hunting for the flaw in their own machine, because the only other option was a flaw in the universe.

They published. The flaw was in the universe.

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There is a form I sign in the hospital that I have never once signed without a small private pause. It sends a patient for a PET scan. The instructions we give them sound less like medicine and more like superstition. Nothing to eat after midnight. Sit still in a dim, quiet room for an hour. Try not to talk. Try not to shiver.

And then a nurse injects them with antimatter.

That sentence is not an exaggeration and it is not a metaphor. The syringe contains sugar wearing a radioactive disguise, and over the next couple of hours, inside the patient’s body, it will produce a few trillion particles of genuine antimatter. Each one will live for about a nanosecond, travel roughly a millimeter, meet an ordinary electron, and annihilate. Both particles vanish completely. Where they stood, two flashes of pure light fly apart in opposite directions, and a ring of cameras around the patient is waiting to catch them.

That is the entire scan. The most exotic substance in the universe, the fuel of starship engines and doomsday novels, is a Tuesday morning appointment with a consent form and a cannula.

To explain how that became routine, I need to tell you about the strangest man in physics, and about an equation that refused to behave.

The Brilliancy Of Dirac

In 1928, a twenty-five-year-old at Cambridge named Paul Dirac wrote down an equation. Dirac was famously, almost clinically silent. Niels Bohr called him the strangest man who ever visited his institute, and the physicists around him invented a joke unit called the dirac: one word per hour. What he wanted was an equation for the electron that obeyed both quantum mechanics and Einstein’s relativity at once, something nobody had managed cleanly.

The equation he found did it, beautifully. It predicted the electron’s spin without even being asked. Its predictions matched experiment. Dirac admitted years later that he was too scared at first to test it rigorously, terrified that some ugly fact would kill his beautiful theory. The ugly fact never came.

Something stranger did. For every sensible solution describing an electron, the equation insisted on a second one: a particle exactly like the electron, but with the opposite electrical charge. Dirac tried to ignore the extra solutions. They would not go away. He tried to argue they were protons, since the proton was the only positive particle anyone knew, and predicting a brand-new particle out of pure mathematics felt like arrogance. Colleagues closed that exit. Robert Oppenheimer showed the proton idea would destroy the hydrogen atom, and Hermann Weyl proved the mirror particle had to weigh exactly what an electron weighs.

So in 1931, Dirac surrendered to his own equation. He proposed, in print, a particle no one had ever seen: the anti-electron. Same mass, same spin, opposite charge. And the logic would not stop at electrons. If the electron has a mirror twin, everything does. Every particle of matter in existence should have an antimatter double.

Caption: Same mass, same spin, opposite charge. Predicted on paper in 1931, photographed in 1932. Alt text: A dashed gold mirror line separates a purple electron labeled matter from a gold positron labeled antimatter, with a note that the twins share mass and spin but have opposite charge.

Physics had never seen this before. Until then, theory explained what experiments had already found. Here was a silent man telling the experimentalists what they were going to find, because his algebra left him no other option.

One photograph

The confirmation came within a year, from someone who wasn’t even looking.

At Caltech, a twenty-six-year-old named Carl Anderson was photographing cosmic rays, the high-energy particles that rain onto Earth from space. His instrument was a cloud chamber, a box of vapor sitting in a strong magnetic field, where a passing particle leaves a thin white trail, curved by the field. Negative particles bend one way. Positive particles bend the other.

In August 1932, one photograph stopped him. A single track crossed a lead plate he had placed across the middle of the chamber, and the curve was gentle on one side of the plate, tighter on the other. Tighter meant slower, which meant the particle had lost energy in the lead, which told him its direction of travel: upward. And once he knew the direction, the bend gave him the charge. Positive. But the trail was far too thin and too long for a proton. Whatever drew that line had the mass of an electron and the charge of its opposite.

Caption: One curved line, three deductions, one new half of the universe. Alt text: A stylized cloud chamber with a lead plate across the middle and a gold particle track that curves gently below the plate and tightly above it. Callouts explain that the tighter curve means the particle was moving upward, the bend direction means positive charge, and the thin trail means it has the electron’s mass.

One curved scratch on a photograph, and the universe doubled. Anderson later said he hadn’t set out to test Dirac’s prediction at all; the mirror half of reality simply introduced itself. He named the particle the positron, and both men had Nobel Prizes before either turned thirty-five.

Back to the ward

Now come back to the hospital, nearly a century later.

The tracer we inject is called FDG. It is glucose, ordinary sugar, except chemists have swapped one corner of the molecule for fluorine-18, a radioactive atom that cannot hold itself together. Every 110 minutes, half of whatever remains has decayed. And when a fluorine-18 nucleus decays, one of its protons turns into a neutron and the nucleus fires out Dirac’s particle. A positron. Antimatter, made fresh, inside a human body.

Why sugar? Because cancer is greedy. A tumor burns glucose far faster than the healthy tissue around it, something the German physician Otto Warburg noticed back in the 1920s. Flood the body with disguised sugar and the tumor hoards it the way it hoards everything. The disguise even jams inside the cell, one chemical step into being burned, unable to go further, so it piles up exactly where the hunger is.

This is also why the instructions sound like superstition. The patient fasts so the tumor is starving. They sit still and silent because working muscle takes up sugar too. Talk through the waiting hour and your voice box lights up on the scan. Shiver, and the strange heat-generating fat along your neck and spine glows like a garland. We are not being difficult. We are keeping the body quiet so the disease is the loudest thing in the room.

Then the physics takes over. Each positron born in the tumor travels about a millimeter, the thickness of a fingernail, before it finds an electron. For a fleeting moment the two can even orbit each other, a bizarre half-matter, half-antimatter atom called positronium, with no nucleus at all and a lifetime measured in nanoseconds. Then both vanish. Nothing breaks into fragments the way things usually break. Their entire mass turns into two photons of light, each carrying exactly 511 kiloelectronvolts, which is the mass of one electron expressed as energy, flying away back to back.

Every PET scanner on Earth is tuned to that same number. A ring of detectors surrounds the patient, and whenever two flashes of 511 arrive at the same instant on opposite sides, the machine draws a straight line between them, because the annihilation must have happened somewhere along it. One line tells you almost nothing. A scan collects millions. And millions of lines, laid over each other, cross densely in one place: wherever the sugar pooled.

Caption: Two flashes, one line. Millions of lines, one tumor. Alt text: A ring of PET detectors around a patient outline. Inside, a tracer atom decays, a positron hops a short distance and annihilates with an electron in a starburst, and two wavy gamma rays labeled 511 keV fly in opposite directions to two glowing detectors, with a dashed line of response drawn between them.

The tumor draws its own portrait, in annihilation light, line by line. I have tried to write that sentence in a way that sounds routine, and it keeps refusing.

If the word antimatter has you checking the exits, the numbers are almost comically gentle. Add up every annihilation from an entire scan, trillions of them, and the total energy released is under a single joule, roughly the effort of lifting an apple off a table. The tracer’s radiation dose is real but modest, comparable to what the planet’s natural background gives you over two or three years of simply existing. Annihilation is the most complete energy conversion physics allows, one hundred percent of mass into light, and the one place humanity uses it routinely is to find sick cells early enough to matter.

And here is the part I find quietly wonderful. This is not even the only antimatter in the room. Your own body contains a trace of radioactive potassium, and a few thousand times a day one of those atoms decays by firing out a positron. There are tiny annihilation flashes going off inside you right now, unrecorded. Thunderstorms do it on a grander scale; a NASA space telescope has caught beams of positrons rising off the tops of storms. Antimatter is not exotic. It is merely outnumbered.

Which brings me to the question I have been avoiding, the one Dirac’s mirror forces on you the moment you believe it. If every particle has an antiparticle, then the Big Bang, which minted all the matter there is, should have minted the mirror half in exactly equal measure. Equal amounts, guaranteed to meet, guaranteed to annihilate. The universe should be a thin soup of light and nothing else. No galaxies, no planets, no patients, no you.

And yet.

Count the leftover light in the sky against the matter that survived, and the ledger is off by one part in a billion. For every billion twins that met and vanished, a single particle of matter stood alone, unpartnered, and remained. Everything you have ever touched is built from those leftovers. Why even one survived is one of the deepest unsolved problems in physics, and the leading attempts to answer it are stranger than the problem. That is Thursday’s story.

By the time you read this, the morning’s patient is home. Fluorine-18 halves every 110 minutes, so by midnight the antimatter factory in their blood has essentially run out, and the portrait it drew is waiting in a radiologist’s queue. Dirac’s impossible particle, predicted by a silent man who trusted his algebra over his instincts, spent one morning inside a human being, looking for trouble.

Medicine is full of miracles that arrive disguised as paperwork. This one still gets the pause.

Part 2 lands Thursday: The Universe Should Be Empty. Why anything exists at all, told properly. The first half will be free. The ending is for paid subscribers.

You stand on a metal walkway above a pool of water so clear it barely seems to exist. Twelve meters down, spent fuel rods sit in steel racks, quietly radioactive, hotter than anything you will ever be near. And around them, the water glows. Not a reflection, not a trick of the lighting. The water itself shines with a deep electric blue, as if someone drowned a piece of the sky.

Engineers walk past it every day. Ask them, and most will admit it is the most beautiful thing they have ever seen at work. Very few can tell you what it actually is.

Here is what it is: a shock wave made of light.

Which is a problem, because on Monday we agreed that such a thing should be impossible. Let me remind you why.

A jet flying slower than sound stays comfortably behind the waves it emits. They ripple out ahead of it and warn you it is coming. But push the jet past the speed of sound and the situation breaks. Every new wavefront is now born ahead of where the previous one started, and none of them can escape forward anymore. They stack, one on top of another, into a single wall of compressed air trailing off the jet in a cone. When that wall sweeps over you, you hear the crack of a sonic boom.

So the recipe for a boom is short: the source has to outrun its own waves.

And that is exactly why a boom made of light should not exist. Light in empty space moves at 299,792 kilometers per second, and relativity is brutal about that number. You can accelerate an electron for a billion years and it will creep closer and closer to that speed without ever touching it. Nothing carrying matter, energy, or information gets there. There is no outrunning light, so there should be no boom.

Now come with me to Leningrad, in the early 1930s, into a basement laboratory.

A doctoral student named Pavel Cherenkov had been handed one of the least glamorous projects in physics: expose various liquids to radiation from radium, and carefully measure the faint fluorescent light the liquids give off. Routine work. The kind of thesis that fills a drawer and is never opened again.

The measurements were miserable to perform. The electronic light detectors that fill modern labs did not exist yet, so Cherenkov’s detector was his own eye. Before every session he sat in absolute darkness, often for an hour or more, until his vision grew sensitive enough to register the faintest traces of light. Then, half blind in the dark, he would peer at his samples and judge their brightness by eye. He did this for years.

And week after week, one detail refused to make sense.

There was always a faint blue glow, even when nothing in the liquid should have been able to fluoresce. Pure water glowed. Distilled solvents glowed. He purified his liquids again and again to strip out any contaminant, and the blue light stayed, always the same, no matter what the liquid was.

Fluorescence does not behave that way. Fluorescence depends on the specific molecules doing the glowing, changes from substance to substance, and can be poisoned by chemicals that absorb the energy first. Cherenkov added those chemicals. The blue light ignored them completely.

Then he found something stranger. Fluorescence glows outward in every direction, like a lamp. This light did not. It was directional. It swept forward along the path of the radiation, the way a wake trails a speedboat.

A lamp shines. A wake follows. And a wake is what shock waves look like.

Cherenkov published his measurements, and much of the physics world shrugged. A glow with no substance responsible for it sounded like sloppy experimental work, and by some accounts one prominent Western journal turned his paper away. He went back into the dark room and kept measuring anyway, with nothing but his eyes.

He could not have fully known it yet, but he was staring at the impossible thing: light behaving exactly like a sonic boom. Something in that water was outrunning light.

But nothing outruns light. That is the one law physics never bends.

Unless you go back and read the law again, slowly, and notice the fine print.

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You heard the siren climb to a high pitch as it came toward you, then drop lower as it sped away. That shift has a name, the Doppler effect, and hiding inside it is one of the biggest discoveries in the history of science.

Here is the strange part. The siren never changes its pitch. The ambulance emits the exact same frequency the whole time, whether it is parked or racing past you. So why do your ears hear something different?

Let me put someone in the story and call her Emily, because it is easier than saying “the person standing still” every time.

Frequency, loosely, is how many wave crests reach Emily’s ear each second. When the ambulance drives toward her, it is chasing its own sound. Each new crest leaves from a point slightly closer to her than the one before, so the crests pile up in front, squeezed together. More of them reach her ear per second, and she hears a higher pitch.

When the ambulance drives away, it works in reverse. Each crest leaves from a point slightly farther back, so they spread out. Fewer reach her per second, and the pitch drops.

Nothing about the source changed. The whole effect comes from the motion between Emily and the siren. Hold onto that, because it is about to explain the universe.

A moving source bunches its waves up in front (higher pitch, toward blue) and stretches them out behind (lower pitch, toward red).

Now here is the leap the physicists made. If sound waves get squeezed and stretched by motion, why not light?

Light is a wave too. So swap the ambulance for a star, pouring out light instead of sound. If that star moves toward Emily, its light waves get squeezed toward the blue end of the spectrum. We call that a blue shift. If it moves away, the waves stretch toward the red end. A red shift.

Then someone actually pointed a telescope at the sky and measured it. And what they found was quietly terrifying.

Almost every galaxy we can see is red-shifted. Nearly all of them are moving away from us. And it gets stranger: the farther away a galaxy sits, the more red-shifted it turns out to be. The distant ones are fleeing faster than the near ones.

The same fingerprint of dark lines, shifted toward red when the light comes from a distant galaxy racing away from us.

Picture what that actually means. In every direction you look, galaxies are running away, and the ones deepest in the distance are running the fastest. The space between us and everything else is stretching.

That is how we know the universe is expanding.

One honest note, because the real story is a little stranger than the tidy version. The galaxies are not really flying away through space like shrapnel from a blast. It is closer to say that space itself stretches while the light is still crossing it, pulling the waves longer on the way. The galaxies mostly stay put. The room they sit in is what grows.

So that answers the question in the title. But it leaves one loose thread, and it is the one I keep circling back to.

Sound has a breaking point. When something moves faster than its own sound waves, the waves cannot get out of the way. They pile into a single wall of pressure and you get a sonic boom, that crack you hear when a jet goes supersonic.

So what about light? What happens when something outruns a light wave?

In empty space, nothing can. Light is the speed limit of the universe, and nothing catches it. A boom made of light should be flat-out impossible.

And yet there is a place you can walk right up to and see one. A faint blue glow, sitting at the bottom of nuclear reactor pools, right now, tonight. A shock wave made of light, in a universe that supposedly forbids it.

How that is possible, and why it glows that exact shade of blue, is where we are going on Thursday.

On Monday I left you with your hand resting on a table it never actually touches, floating a few atoms’ width above the surface on nothing but the mutual refusal of two clouds of electrons. That refusal, I said, feels like electricity, because like charges push apart and the push is real. But I also promised you that electricity was not the whole story, and that the thing doing most of the quiet, load-bearing work was not a force at all. It was a rule. Today I want to make good on that, and then I want to take the rule somewhere it has no business going, out to the corpse of a star.

Let me first say plainly why electricity cannot finish the job, because this is the hinge everything turns on.

Electrons repel each other, yes. But electrons are also slippery. Give them nothing but electric repulsion to contend with and they will find ways around it. They will shuffle, redistribute, share the crowded space between two atoms, slide into whatever arrangement lowers the total energy. If repulsion were the only rule in the room, matter would be a negotiable thing. Squeeze it hard enough and the electrons would cooperate, the atoms would fold into one another, and everything solid would turn out to be merely stiff, compressible far past anything we ever see. The arithmetic bears this out. A world held apart by electric charge alone is a world that can be crushed, and crushed to a small fraction of the size ours insists on keeping.

So something forbids the cooperation. Something stands between the electrons and all those tempting lower-energy arrangements and simply says no.

Its name is the Pauli exclusion principle, and the man it is named after was, by wide agreement, the most frightening person in twentieth-century physics.

Wolfgang Pauli was twenty-five when he wrote it down, in 1925, and he was already famous for a particular kind of cruelty. He was the critic other physicists feared, the one who could listen to an hour of work and pronounce that it was not even wrong. A colleague’s theory might be demolished by a single sentence from the back of the room. There was a running joke among his peers, only half a joke, that laboratory equipment would fail the moment Pauli walked into the building, as if reality itself flinched a little in his presence. The rule he gave us is as uncompromising as he was. It does not bargain, and it has no exception anyone has ever found.

Here is what it says, stripped down. Two electrons can never occupy the same quantum state at the same time. Not approximately, not usually, not unless you push hard enough. It is a flat prohibition against two electrons being, in every describable respect, identical.

And this is where Monday’s story quietly folds into this one, because to describe an electron completely you need one more number than you might expect, the one we found together on a cold glass plate in Frankfurt. Its spin. That two-valued spin, up or down and nothing in between, is the last digit of an electron’s identity. Which means that for any single place an electron can be, there are exactly two seats available: spin up, and spin down. Two electrons can share the address, one pointing each way. A third has nowhere to go. The room is full.

Hold onto that picture of an address with exactly two seats and no vacancy, because in a moment I am going to show you how that one rule about seating becomes a physical force, strong enough to stop your hand at a table and, out in the dark, strong enough to hold up the crushed remains of a star.

The rest of this one is for paid subscribers.

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The first thing they teach you on the wards is where to put your hands.

You learn to find a pulse with two fingers laid just so on the wrist. You learn to press an abdomen slowly, watching the face instead of your own hands. You learn the flat of the palm for chest compressions, the exact spot on the sternum, elbows locked, your whole weight coming down again and again on a person whose heart has quit. In a few months you touch more human beings, more intimately, than most people do in a year.

And I have to tell you something that took me an embarrassingly long time to sit with. In the strict, physical, measured-in-a-lab sense of the word, not one of those was contact. I have never touched a patient. I have never touched anyone. You have never touched another person in your life, and you never will.

I know it sounds like a line from a first-year philosophy seminar, the kind of thing that feels profound at midnight and embarrassing by breakfast. But this is not a trick of words. It is one of the most solid, boring, thoroughly confirmed facts in physics, and once you actually see why, the ordinary act of resting your hand on a table stops feeling ordinary.

Let me build it up the honest way.

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Start with the thing everyone reaches for first. Why is a table solid? Easy, you say. It’s made of atoms, atoms are little solid balls, the balls in my hand bump into the balls in the table, and that bumping is what I feel. Reasonable. Wrong, but reasonable.

Here is the problem. Atoms are not little solid balls. They are, to a genuinely disturbing degree, empty. If you took a single atom and blew it up until the nucleus at its center was the size of a marble sitting on the center spot of a football pitch, the outer edge of that atom, where the electrons live, would sit somewhere out past the last row of seats.¹ Everything in between is not stuffing. It is nothing. No material at all.

So if atoms are mostly empty space, and you are made of atoms, and your chair is made of atoms, the real question flips around. It is not “why is the table solid.” It is “why don’t I fall straight through it.” Two clouds of mostly-nothing, pressed together. By all rights you should sink into your seat like a ghost through a wall and keep going.

You don’t. And the reason you don’t has nothing to do with anything bumping into anything.

What you feel is a force, not a surface.

Every atom in your fingertip carries a haze of electrons, and every electron is negatively charged. Every atom in the table carries the same. And there is an iron rule about charge that you already know in your bones from playing with magnets as a kid: like repels like. Push two north poles together and you feel that invisible spring shoving back, harder the closer you press. Electrons do exactly this. As your hand comes down, the electrons in your skin and the electrons in the wood come close enough to shove each other away, hard, long before anything that could be called a surface actually meets.

That shove is the entire sensation of touch. When you feel the table under your palm, you are not feeling the table. You are feeling the electric refusal of its electrons to let yours come any closer. The solidity of the whole world is a force field. Every object you have ever held, you were holding a few atoms’ width away, floating on repulsion, and your nerves reported the push as contact because “push” is the only word they know.

Sit with that for a second, because it reaches everywhere. The handshake. The hand on the shoulder of someone who just got the worst news of their life. A parent lifting a newborn for the first time. Two fingers on a wrist, counting. In every one of those the skin never meets. There is always a gap, tiny, unbridgeable, held open by a force. The most intimate thing we do, and we do it across a distance we can never quite close.

I find that unbearably beautiful and a little sad in the same breath, and I think the sad part is the honest one.

Now. If I stopped here I would be doing the exact thing I promised at the top I wouldn’t, which is hand you a clean story that happens to be incomplete. Because electric repulsion is real, and it is part of the answer, but it is not the answer. It cannot be.

Here is the crack in it. If the only thing keeping atoms apart were electric repulsion, you could beat it. Push hard enough and electrons would find ways to slide past each other, rearrange, share the space, and the whole structure should be squashable, soft, foldable under enough weight. Do the actual arithmetic and matter comes out far more crushable than it is. Whole stars would collapse in seconds. And yet the floor holds a piano, your bones hold your body, diamonds exist. Something is stopping atoms from ever sitting in the same place that is far stronger, and far stranger, than two like charges pushing apart. And here is the part that genuinely stops me. It is not a force at all.

It is a rule. A rule about identity, about what is and is not allowed to be in the same state at the same time. And it traces straight back to something we already watched happen together on a cold glass plate in Frankfurt, back when two physicists were trying to prove Bohr wrong and stumbled into something worse.² That two-valued spin, the one that split a beam of silver into two clean dots and nothing in between, is the real reason your hand stops at the table. Not electricity. Identity.

That is where this stops being a neat fact and becomes one of the deepest, and least talked-about, ideas in all of physics. And it is far too big to bolt onto the end of a Monday.

So here is where we are going.

On Thursday, the full version (paid) picks up on the exact word this one ends on. The rule itself, the one named after the most feared critic physics has ever produced, and why it, not electricity, is the true reason you have never touched anything. And then it opens all the way out. Because the same rule that stops your hand at the table is what holds up a dead star against the crush of its own gravity. A single spoonful of what is left of that star would weigh about as much as a mountain. And when even that rule finally loses, when identity itself isn’t enough to hold the line, the star drops out of the universe entirely and becomes a black hole.

Same rule. Your fingertip, and the last stand of a star. I’ll see you there.

See you Thursday.

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On Monday I left you in that room in Brussels with a promise and a small dishonesty.

The promise was that today I’d tell you what those twenty-nine people were actually arguing about. The dishonesty was that I made it sound like a fair fight. Two great men, two reasonable opinions, take your pick. It wasn’t like that. One of them was defending everything you already believe about how the world works. The other was defending something that sounds, the first time you hear it, completely unhinged.

Here’s the strange part. The one defending common sense was Einstein. And he was wrong.

I’m going to do this in a specific order. First I’ll walk you all the way over to Einstein’s side, until his position feels not just correct but obvious, the only sane thing a person could possibly think. Then I’ll show you the experiment that took that obvious, sane, common-sense picture of reality and quietly ended it.

What the fight was actually about

Strip away the equations and it comes down to two words. Locality and realism.

Realism is the belief that things have definite properties whether or not anyone is looking. The moon is a specific size and in a specific place right now, even though neither of us is checking. A coin in a closed box is already heads or already tails before you open it. Looking doesn’t create the answer. Looking just reveals an answer that was already sitting there.

Locality is the belief that nothing you do in one place can instantly change something in another place. Effects travel. They take time. Push something here and the push spreads outward, and the fastest that spreading can ever go is the speed of light. Nothing reaches across a gap and changes the far side of it in zero seconds.

Look at those two beliefs again and notice they don’t feel like physics. They feel like the definition of a sane person. Things are real when you’re not watching. You can’t touch something without reaching it. Einstein held both, completely, to the end of his life. So do you, whether or not you’ve ever put it into words.

Einstein’s actual weapon

In 1935, ten years before he died, Einstein wrote a paper with two younger colleagues, Podolsky and Rosen. It became the most cited thing he ever produced, which is remarkable for a man who also invented relativity. Everyone just calls it EPR now, after the three surnames.

The paper was not an attack on quantum mechanics’s math. The math worked, and Einstein knew it. It was an attack on the idea that the math was the whole story.

The move was this. Quantum mechanics allows two particles to be created together in a linked state, what we now call entangled. Send them flying apart, one to your left, one to your right, as far as you like. The theory says the two stay correlated: measure one, and you instantly know something about the other, no matter how much space is between them.

Einstein looked at that and saw a trap with only two ways out.

Either measuring the particle on the left physically reached across all that empty space and set the property of the particle on the right, faster than light, instantly, which violates locality and which Einstein considered frankly absurd. He had a name for it. Spooky action at a distance.

Or the far particle already had that property the whole time, sitting there fully formed since the moment the two were born, and quantum mechanics simply doesn’t include it in the description. Which would mean the theory isn’t wrong, just incomplete. There’s more underneath. Hidden machinery the equations don’t mention.

Einstein took the second door without blinking. Of course the properties are already there. Of course there’s hidden machinery. The alternative is spooky nonsense. The dice only look random because we haven’t yet found the thing that loads them.

And sit with that for a second, because he’s clearly right. Isn’t he? The idea that a particle has no real location, no definite properties, nothing settled about it at all until someone measures it, and that measuring it here instantly yanks its partner into agreement across a light-year of empty space, that isn’t science. That’s a magic trick. Einstein is just refusing to be fooled by one. The properties are real. The world is local. We’re missing a piece, and one day someone will find it.

For thirty years, almost everyone who thought about it seriously agreed there was no way to know. It sounded like philosophy. You cannot put “is the coin heads before you open the box” on a lab bench. The whole disagreement looked permanently stuck: two brilliant men, two tastes, and no experiment in the universe that could tell them apart.

The man from Belfast

Then a physicist named John Stewart Bell, born in Belfast, working at CERN, did something nobody had managed in three decades.

Bell’s day job was ordinary particle physics. The EPR question was a thing he worried at in the margins, almost a hobby, the way other people do crosswords. And in 1964 he stopped asking which picture of reality was prettier, Einstein’s or Bohr’s, and asked a completely different question. A measurable one.

If Einstein is right, he said, if every particle really does carry its properties with it from birth, then what would we actually see in the lab? Not in principle. In numbers. On a readout.

When he worked it out, Einstein’s picture and quantum mechanics’s picture did not predict the same numbers. They came apart. Which meant the thirty-year standoff was never really about taste at all. It had a fact underneath it the entire time, and the fact could be measured. You just had to know what to count.

The two machines

Here’s the setup, as clean as I can make it. Forget particles for a moment and picture two machines.

Each machine has a dial with three settings, marked one, two, three, and a single lamp on the front that flashes either red or green when a particle arrives from a source in the middle. That’s it. A dial and a lamp. You set the dial, a particle comes in, the lamp flashes a colour. The two machines can be across the room from each other, or across a country. There’s no wire between them and no way for one to signal the other.

Now run the thing thousands of times. Each run, both dials are set at random, independently, and both lamps flash.

Two facts come out of the data. Both are measured. Both are real. Everything hangs on these two, so hold onto them.

Fact one. Whenever the two dials happen to land on the same setting, the two lamps always flash the same colour. Both on setting one? Both red, or both green, every single time. Both on three? Same colour, always. Never once a mismatch when the settings match. Perfect agreement.

Fact two. Across all the runs, mixing every combination of settings together, the two lamps flash the same colour half the time. Fifty percent.

Look hard at fact one, because fact one is Einstein’s entire case handed to you on a plate. Two machines, no connection, sometimes far enough apart that not even light could carry a message between them in time, and yet when you set them the same they agree without fail, run after run after run. How? There is exactly one sane explanation, and it’s his. Each particle must leave the source already carrying its answers. A little sealed slip of paper: setting one, red. Setting two, green. Setting three, red. The two particles are twins carrying identical slips. That’s the only way they could possibly agree every single time without communicating. The colours were decided at the source, in advance, and carried out to the machines. The answer was there all along.

That is not a loose analogy. That is the whole of Einstein’s position, made physical. The properties are real before measurement. The world is local. The slip exists.

So Bell asked the one question left. Fine: if every particle carries a slip, then fact two is not free to be anything it likes. The slips constrain it. There’s a limit to how those numbers can come out, once you accept the slips are real. Bell sat down and calculated exactly what that limit is. A hard floor. A number that fact two is simply not allowed to fall below, if Einstein is right and the slips exist.

Then people built the machines and measured it.

Not once. Dozens of times, across fifty years, sealing off every possible way the universe might be cheating, one loophole at a time, all the way to a Nobel Prize in 2022.

Reality came back on the wrong side of Bell’s floor.

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Twenty-nine people sat for this photograph in October 1927. Seventeen of them either had already won, or would go on to win, the Nobel Prize.

Read that again. Not twenty-nine good scientists. Seventeen Nobel laureates, in one room, holding still for a single camera, to discuss one topic… what could this topic be?!

Front row, dead center, is Einstein, already at that point the most famous person alive. Beside him sits Marie Curie, the only woman in the frame and the only human in it who had won the Nobel twice, in two different sciences.

Among the others: Bohr, Heisenberg, Schrödinger, Pauli, Dirac, de Broglie, Born, Planck. If you’ve ever taken a physics class, you’ve done homework with half of those surnames printed on the equations. Dirac was twenty-five years old. The man chairing the entire thing, Lorentz, would be dead within a year.

People call this the most intelligent photograph ever taken. That part is fair. What almost nobody mentions is what these people were actually doing in Brussels that week.

They were fighting.

Not politely, and not as a formality. They were split down the middle over a question that sounds like it belongs in a philosophy seminar and turned out to be the foundation of all modern physics: is the universe random all the way down, or does it only look that way because we can’t yet see deep enough?

On one side stood Bohr and the young ones, Heisenberg, Pauli, Born. Their claim was outrageous. They said an electron does not have a definite position until you measure it. Not “we don’t know where it is yet”, it genuinely is not anywhere in particular until the act of measuring forces it to pick. Underneath the math there is no hidden machinery, no secret clockwork ticking away. The randomness is the ground floor. There is no basement.

Einstein found this intolerable. And that was the main reason behind this conference.

If you want to know how absurd this idea is and how revolutionary it was and still is, this picture tells you the whole story.

The funny part for me is that Einstein was the great radical of his age. This was the man who had already thrown out absolute time, curved space with gravity, and melted mass into energy, he had pried physics open with his bare hands. And yet in this room, on this question, he was the conservative. The revolutionary could not stomach the new revolution. He was certain, all the way down, that God does not play dice. That beneath all that quantum randomness there simply had to be a deeper, orderly, lawful reality we hadn’t found yet.

Who was matched to Einstein at the time you might ask? The great Neil Bohr, from what I understood from my professor, Bohr not only debated Einstein intensely, he was the only person ever to even beat Einstein in the debate.

That line “God does not play dice” is the most quoted sentence he ever produced. The usual story has him hurling it across the table at Bohr, who fires back, “Einstein, stop telling God what to do.” Wonderful scene. Also mostly invented. The dice line actually comes from a letter he’d written the year before the conference, and Bohr’s real answer in Brussels was quieter and more cutting than the legend.

But the spirit is exactly right: one man believed reality was solid and knowable, the other believed it dissolved the instant you stopped looking, and neither would give an inch.

So they settled into a strange ritual. Most mornings, over breakfast, Einstein would turn up with a fresh thought experiment, a small, clever trap built to prove the new physics was nonsense. He’d hand it to Bohr. Bohr would vanish into it for the rest of the day. And by dinner he’d be back, having quietly found the flaw.

Einstein kept this up for years, across more than one of these conferences. He never won a single round. And he never, ever stopped.

You would think that settles it. The greatest mind of the century, swinging as hard as he can and missing every time, surely that just means Bohr was right and Einstein was a stubborn old man who couldn’t make peace with the future.

But losing an argument in a room is not the same thing as being wrong. For decades after this photo was taken, the whole dispute looked unsolvable by design, a matter of taste, two giants talking past each other about something no experiment on Earth could test. There was no way to set “is reality real when no one is watching” down on a lab bench.

Until, very quietly, there was…

Someone found a way to take this precise argument, Einstein’s deep, orderly, knowable universe against Bohr’s universe that decides things at the last possible instant, and convert it into a number. An actual number, one you could go out into a laboratory and measure. And when the measurement finally came back, the answer was so strange that settling it earned a Nobel Prize all its own. In 2022.

That is ninety-five years after this photograph was taken.

Einstein died in 1955. He went to his grave certain he was right. He never found out.

My goal today was to set the scene, to let out the fan of me and highlight how crazy. On Thursday I’ll show you exactly what these people were really fighting about, the physics and the philosophy, and then the single most beautiful move in the history of science: how a soft-spoken physicist from Northern Ireland took an unwinnable argument and turned it into an experiment, and which side of that 1927 room the universe actually came down on. It’s not the side you’d bet on.

The full version is for the paid tier. Consider even trying the free trial to check it.

Have a nice Monday!

In 1952, a twenty-two-year-old graduate student sealed a few cheap gases inside a loop of glass tubing and ran electricity through them for a week.

There was nothing alive in that apparatus. No cells, no DNA, nothing living by any definition you care to pick. Just methane, ammonia, hydrogen, and water vapor. The kind of dead, simple chemistry you could scrape off a passing comet. He set two electrodes to throw sparks through the gas, the way lightning once cracked across the sky of the young Earth, and then he left it running and went about his week.

When he came back, the water had turned a murky brown.

He analyzed the sludge. Sitting in it were amino acids. The building blocks of proteins. The exact small molecules that every living thing on this planet, from the bacteria in your gut to the neurons firing as you read this sentence, is assembled out of. They had formed on their own, from dead gas and electricity, at the bottom of a jar that had never once been alive.

His name was Stanley Miller. His professor was Harold Urey. And the experiment they ran in Chicago is still one of the strangest results in all of science, because of what it quietly proves. You do not need life to make the parts of life. You need a few simple ingredients, a jolt of energy, and time. The universe seems to handle the rest on its own.

Now, the exact mix of gases is still argued over to this day, and the early air may well have been gentler than Miller assumed. But here is the thing that survived every revision of the experiment. Change the recipe, and the building blocks still come. They show up in colder versions of the experiment, in simulated volcanoes, even inside meteorites that fell to Earth already carrying amino acids made in the cold of deep space. The universe appears to manufacture the parts of life almost anywhere you give it half a chance.

And here is the part I cannot get over.

Almost a century before that beaker, Charles Darwin had already pictured it. Not in a paper, he was far too careful for that, but in a quiet letter to a friend in 1871. He imagined some warm little pond, with ammonia and phosphoric salts, with light and heat and electricity all present at once, and he wondered whether in a pond like that a protein compound might simply form on its own, ready to begin the long slow climb toward something living. He wrote the words protein compound by hand, in 1871, with no way of knowing that eighty years later two chemists would build his warm little pond out of glass and prove the old man right.

So the building blocks come for free. Wonderful. Except now we have a problem, and it is a far harder one than it first looks.

Because randomness is not supposed to do this.

Hand me a box of letters and shake it, and I do not get a sentence. I get noise. The whole instinct we carry about randomness is that it tears structure down, it does not build it up. Shuffle a deck long enough and you get chaos, not a royal flush lying neatly in order. A famous astronomer once said the odds of life assembling itself by chance were like the odds of a tornado tearing through a junkyard and leaving behind a fully assembled airplane. The numbers are so small they might as well be zero.

By that logic, Miller’s beaker should have made a meaningless brown smear and nothing more. Dead gas, sparked at random, should stay a mess forever.

Instead it produced the alphabet of life, reliably, on a one-week schedule. Run it again and it happens again. Other labs ran it and it happened for them too. Randomness, given a jar and a spark, does not produce noise. It keeps spelling out the same few letters, again and again, the very letters that life happens to be written in.

That should stop you cold.

We are holding two facts that refuse to sit in the same room. Randomness destroys order. And randomness, in that beaker, plainly built it. One of those has to be wrong, or there is something underneath both of them that we have not said out loud yet.

There is something underneath. And it is the same something I was circling on Monday, when I told you that the summation of an enormous number of random events can produce an outcome that looks nothing like random at all. That sentence was not a throwaway line to end the post on. It is the key to this entire thing. It is the reason a dead jar can spell, the reason a warm little pond can begin, and, once you follow it all the way down to the bottom, the reason you are here to read about it.

What randomness is actually doing inside that beaker is not what you think it is doing. And it turns out to be the very same thing it does to a slowly falling population of living creatures, stretched out over a billion years.

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