Electrons Have NO Size! So How Do Physicists Smash Them Together...（原声文稿）

Here's a puzzle that sounds like a riddle. Electrons are among the most common things in the universe.

There are electrons in every atom of your body, in the screen you're watching, in the air you breathe. And as far as anyone can tell, electrons have no size at all.

Not tiny, not microscopic, no size.

Every experiment we've ever done says an electron behaves like a single point, and yet for decades, physicists have been smashing electrons into each other in giant machines.

They've built tunnels kilometers long, packed with magnets, just to make these points collide.

And those collisions have taught us some of the deepest secrets of the universe. So how do you hit something that has no size? How can two points ever meet?

And what does it even mean for something to have no size in the first place?

The answer will change how you think about touching, about collisions, and about what matter really is.

Because it turns out that you have never actually touched anything in your entire life. Let's find out why.

To understand how strange electrons are, let's go on a journey into the very small.

Start with a human hair. It's about a tenth of a millimeter wide.

Now zoom in a hundred times. You're looking at something the size of a small bacterium.

Zoom in another thousand times, and you reach the size of a large molecule. Zoom in ten times more, and you're at the scale of a single atom.

An atom is so small that you could line up millions of them across the full stop at the end of a sentence. But an atom is mostly empty space. space.

At its center is a tiny, heavy core called the nucleus, made of protons and neutrons. And around it are the electrons. How small is the nucleus compared to the whole atom?

If an atom were blown up to the size of a football stadium, the nucleus would be like a single marble sitting in the middle of the pitch. Everything else would be the space where the electrons can be found.

Now, protons and neutrons do have a size, scientists have measured it.

A proton is roughly a millionth of a billionth of a meter across. cross.

That's incredibly small, but it's a real, measurable size.

And when you smash protons hard enough, you find they're made of even smaller pieces inside, called quarks.

So what about the electron?

Scientists have searched for its size for over a century, ever since it was discovered in 1897 by the British physicist JJ Thompson.

They've used more and more powerful experiments to look for any sign that the electron takes up space, or that it's made of smaller parts. And every single time, they've found nothing. No size.

No inner parts. The best experiments tell us that if the electron has any size at all, it must be smaller than about a thousandth of the size of a proton.

That's less than a billionth of a billionth of a meter. As far as we can tell, the electron is a perfect point. It's hard to picture something with no size.

Everything we know in daily life takes up space. A ball, a grain of sand, even a speck of dust. First, you can always imagine cutting it in half.

So what does it mean when physicists say an electron is a point? It means two things.

First, no experiment has ever found an edge to an electron.

Something with a size has a boundary, a surface where it begins and ends.

With an electron, no matter how closely we look, we never find one.

Second, no experiment has ever found anything inside an electron.

Protons have quarks inside them. Atoms have a nucleus and electrons.

electrons, but electrons seem to be made of nothing smaller.

They're what physicists call fundamental particles, the basic building blocks of nature that can't be broken down any further.

Scientists have even tested whether the electron is perfectly round.

If it had any lumps or bumps, or if its electric charge was slightly lopsided, it would show up in extremely sensitive experiments. Teams have looked for this for years.

So far, the electron appears to be perfectly round to an astonishing level of precision. Now, here's an important caveat.

Saying an electron has no size doesn't mean we're certain it's an infinitely small point. It means it's smaller than anything we can measure.

Maybe, one day, an even more powerful experiment will reveal a tiny size or hidden structure. But so far, there's no sign of either. And this is where the puzzle begins.

If electrons are points, two electrons can't really bump into each other like snooker balls. Points don't have surfaces to bump.

So, when physicists say they collide electrons, what exactly is happening?

To answer that, we first need to ask a surprising question. What happens when you touch something? But before we get to that, a quick favour.

If you're enjoying this and you'd like more videos that explain the strangest parts of physics in simple language, consider subscribing and turning on notifications.

It genuinely helps the channel grow, and it means you won't miss the next one. Okay, let's talk about touching. Put your hand on a table.

It feels solid. You'd swear your skin is touching the wood.

But if you could zoom in far enough, you'd see something remarkable. Your hand isn't actually touching the table at all.

Remember, atoms are mostly empty space. The outside of every atom is made of electrons. So the outer surface of your hand is a layer of electrons.

And so is the surface of the table. Electrons carry a negative electric charge. And like charges, push each other away. You've seen this if you've ever tried to push two magnets together the wrong way.

The closer they get, the harder they push back.

So when your hand presses on the table, the electrons in your skin and the electrons in the wood push against each other.

The harder you press, the harder they push back. That pushing is what you feel as touch.

There's also a second effect from the rules of quantum physics. Electrons are very antisocial.

Two electrons can't squeeze into exactly the same state in the same place.

This rule, called the Pali Exclusion Principle, means that atoms resist being pushed into each other.

It's one of the main reasons that solid things feel solid and don't simply pass through each other.

Put these two effects together and you get a surprising result.

When you touch something, your atoms never actually make contact with its atoms.

They get very close, but they're always kept apart by these powerful pushes. What you feel is not contact, it's a force.

So when you sit on a chair, you're actually hovering a tiny distance above it, held up by electric and quantum forces.

When you hold someone's hand, your electrons and their electrons are pushing against each other.

In the everyday sense, you've never truly touched anything in your life. And this is the key to our electron puzzle.

Because if touching is really about forces, then two electrons don't need surfaces to collide.

They just need to get close enough for their forces to act.

So in physics, a collision doesn't mean two surfaces smashing together.

It means two things getting close enough to affect each other, so that they change direction, change speed, or even turn into something new.

Every electron is surrounded by an invisible electric field. You can think of it like a kind of influence spreading out into the space around it.

The field is strongest close to the electron and gets weaker further away.

Any other charged particle that comes near feels that field and gets pushed or pulled.

So when two electrons fly toward each other, they don't need to touch.

As they get closer, their electric fields push against each other harder and harder.

Eventually the push becomes so strong that the electrons swerve away like two magnets repelling. That's a collision. Modern physics describes this even more precisely.

According to quantum physics, the electric force is carried by particles of light, called photons.

When two electrons interact, it's as if they pass a photon between them, a bit like two ice skaters throwing a heavy ball. When one skater throws the ball, they get pushed backwards.

When the other catches it, they get pushed back too. The two skaters move apart without ever touching.

Physicists draw this using simple sketches called Feynman diagrams, named after the American physicist Richard Feynman.

Two lines come in, a wiggly line connects them, and two lines go out. It's a picture of a collision with no contact at all.

The ball -throwing picture isn't perfect.

These exchanged photons aren't quite like normal light, and the ball idea doesn't explain pulling forces very well. But it captures the key point. Particles interact through fields, not by touching. And here's the important thing.

Because the forces reach out into space, an electron has a kind of effective target size, even though it's a point.

The faster the electrons are going, the closer they can get before they swerve, and the more detail their collision can reveal. But there's another twist.

Electrons don't actually travel like tiny bullets at all.

If you watched our video on being in two places at the same time, you'll remember the double slit experiment.

When electrons are fired one at a time at a wall with two slits, each one behaves as if it passes through both slits at once, like a wave.

Then it lands as a single dot on the screen. So, electrons are strange.

When we detect them, they always show up as points, in one exact place.

But when they're traveling, they behave like waves, spread out over space. This gives us a new way to think about collisions.

When two electrons fly toward each other, they aren't two tiny bullets heading along perfect paths. Each one is more like a fuzzy cloud of possibilities.

We can't say exactly where each electron is, or exactly which path it will take.

So when the two clouds overlap, quantum physics can't tell us exactly what will happen in any single collision.

It can only tell us the odds. Maybe there's a high chance the electrons will barely swerve.

A lower chance they'll bounce off at a sharp angle, and an even lower chance they'll do something far more dramatic. This is actually perfect for physicists.

They don't need to aim each electron perfectly at another.

Instead, they fire huge numbers of them, record what happens, and compare the results with the odds predicted by their theories.

And there's another useful thing about the wave nature of electrons.

The faster an electron moves, the shorter its wavelength becomes. Its wave gets more tightly packed. Why does that matter?

Because a wave can only reveal details about as small as its own wavelength. That's why you can't see atoms with ordinary light.

Light waves are too long, about a thousand times bigger than an atom. To see tiny details, you need very short waves.

So by speeding electrons up to almost the speed of light, physicists turn them into incredibly fine probes, with wavelengths small enough to look deep inside atoms, and even inside protons.

In other words, an electron collider isn't just a crash test. It's the world's most powerful microscope.

Imagine there's an object hidden in a dark room and you're not allowed to look at it. How could you work out what it is?

One way is to throw lots of small balls into the room and watch how they bounce.

If most fly straight through, the object is probably small or full of gaps. If they bounce off in all directions, it's probably big and solid.

By studying the pattern of bounces, you can build up a picture of something you can't see. This is exactly how physicists study the tiniest things in the universe.

They can't look at them directly.

Instead, they fire particles at them and study how they bounce. Physicists call this scattering.

The most famous scattering experiment was done over 100 years ago in the laboratory of the New Zealand -born physicist Ernest Rutherford.

In 1909, his assistants Hans Geiger and Ernest Marsden fired tiny particles, called alpha particles, at an incredibly thin sheet of gold foil.

At the time, scientists thought atoms were like soft puddings, with charge spread out evenly, so everyone expected the alpha particles to pass straight through the gold, perhaps swerving very slightly.

Most of them did pass straight through, but a few did something astonishing.

They bounced back, almost the way they came.

them. Rutherford later described it as being as surprising as firing a huge artillery shell at a sheet of tissue paper and having it bounce back and hit you.

The only explanation was that each gold atom had a tiny, heavy, concentrated center. That's how the nucleus was discovered.

Not by seeing it, but by watching how particles bounced off it.

That's the key idea behind every particle collider. Fire small things at a target.

Watch how they bounce. Work backwards to figure out what's inside. And for this kind of work, electrons turned out to be the perfect tool.

Because electrons have no size and no inner parts, they're the cleanest probes imaginable.

When an electron bounces off something, scientists know that everything strange in the result comes from the target, not from the electron itself. So what happens when you fire electrons at a proton?

In the 1960s, scientists in California built one of the most impressive machines of its time.

It was a perfectly straight tunnel, about three kilometers long, – designed to do one thing – speed up electrons to nearly the speed of light.

It was called the Stanford Linear Accelerator, or Slack.

Inside the tunnel, electrons were pushed along by powerful waves of electric energy, a bit like surfers riding a wave.

By the end of the tunnel, they were travelling at over 99 .99999 % of the speed of light.

At the end, the electrons slammed into a target filled with protons, inside hydrogen atoms.

Huge Research detectors measured exactly how the electrons bounced off.

At the time, many scientists thought a proton might be a smooth little ball, with its charge spread out evenly.

If that were true, the electrons would mostly glance off softly, at small angles. But in 1968, the results showed something different.

Far more electrons bounced off at sharp angles than expected.

It was like Rutherford's experiment all over again. Something inside the proton was hard and concentrated.

The pattern of bounces revealed that protons are not solid balls at all. They're made of smaller particles inside. Today we call them quarks.

A proton contains three main quarks, held together by a powerful force.

This discovery was so important that the three physicists who led the experiments, Jerome Friedman, Henry Kendall, and Richard Taylor, won the Nobel Prize in physics in 1990. And notice how it worked.

The electron, a point with no size, was the perfect tool.

Because the electron has no inner structure to complicate things, every sharp bounce told scientists something real about the inside of the proton.

But physicists didn't stop at firing electrons at targets. They wanted something even more powerful.

What if, instead of hitting a target that's sitting still, you smash two beams head -on into each other? That creates a whole new problem.

How do you get two points, racing in opposite directions at nearly the speed of light, to actually meet?

Imagine trying to make two single grains of sand hit each other in mid -air, thrown from opposite ends of a football field. It would be almost impossible.

Now imagine the grains have no size at all. It sounds hopeless, so physicists don't try to hit one electron with another. Instead, they play a numbers game.

First, they don't fire single electrons.

They fire bunches. Each bunch can contain billions of electrons, packed tightly together.

And they fire many bunches, one after another. Second, they squeeze the bunches down to be incredibly thin.

Using powerful magnets that work like lenses, they focus the beams down to sizes far thinner than a human hair.

The tighter the bunches, the more crowded the space where they cross, and the better the chance that some electrons will come close enough to interact. Third, they make the bunches cross again and again.

In a circular collider, the same bunches go round and round the ring, passing through each other thousands of times every second. second. Most electrons miss each time.

But with so many electrons and so many crossings, a steady stream of collisions happens.

Physicists have a special way of measuring how likely a collision is.

They imagine each particle as having an effective target area, called a cross -section.

It's not a real size. It's a measure of how easy it is to make something happen.

The unit they use for it even has a funny name.

It's called a barn, from the old saying about someone so bad at aiming, they couldn't hit the side of a barn.

And here's where the point -like nature of the electron becomes an advantage.

Because electrons are so simple, physicists can calculate the odds of each type of collision with incredible precision, and then check them against what their detectors record.

Every collision is recorded by enormous detectors, built in layers like an onion around the crossing point.

They track the paths of the particles that fly out, measure their energy and identify what they are.

But the most exciting collisions don't just make electrons bounce. They make something entirely new.

In our video on white holes and wormholes, we met the positron, the antimatter twin of the electron.

It's identical to an electron in almost every way, except it carries the opposite electric charge, positive instead of negative.

And when an electron meets a positron, something extraordinary can happen.

They don't just bounce, they can destroy each other completely, turning all of their mass into pure energy. energy.

Physicists call this annihilation. This is E equals MC squared in action, the equation from our scariest equation video.

Mass turns into energy, and because both particles completely vanish, all of their energy is free to become something new.

If the electron and positron are moving slowly, they usually turn into two particles of light, called gamma rays. That's exactly what happens in PET scans in hospitals.

A patient is given a substance that releases positrons.

The positrons meet electrons in the body, annihilate, and the gamma rays that fly out are detected, building up a picture of what's happening inside the body.

But in a collider, the electrons and positrons are moving at nearly the speed of light.

They carry enormous amounts of energy, and when they annihilate, that energy can turn back into mass, creating brand new particles that weren't there before.

The more energy you put in, the heavier the particles you can create.

Particles that are hundreds of thousands of times heavier than the electron itself can pop into existence, live for a tiny fraction of a second and then break apart into other particles that the detectors can track. And here's why electron collisions are so prized.

When you smash protons together, as the Large Hadron Collider does, it's like smashing two bags of marbles. There are quarks and other particles flying everywhere, and it's messy.

But electrons and positrons are points. When they annihilate, the collision is clean.

Scientists know exactly how much energy went in, so they can measure what comes out with incredible precision.

To do this at the very highest energies, physicists built one of the biggest machines in history.

In 1989, a new machine switched on beneath the border between France and Switzerland.

It was called the Large Electron -Positron Collider, or LEP, and it was built by CERN, Europe's great physics laboratory.

LEP was a ring -shaped tunnel, about 27 kilometers around.

That's so big that a train would take several minutes to travel all the way around it.

Inside, electrons raced one way around the ring, while positrons raced the opposite way.

Thousands of magnets bent their paths into a circle and squeezed the beams tight.

At four points around the ring, the beams were steered to cross.

Giant detectors sat at each crossing point, watching for collisions.

One of LEP's biggest achievements was studying a particle called the Zebo -son. It's one of the particles that carries the weak force. force, the force responsible for some kinds of radioactivity.

LEP created millions of Z bosons and measured their properties with astonishing precision.

Those measurements became some of the strongest tests ever made of our best theory of particles, called the Standard Model. And the theory passed.

But LEP also faced a problem that every electron collider has to deal with.

Whenever an electron is forced to travel in a curve, it gives off light and loses energy. This light is called synchrotron radiation.

creation, the lighter the particle and the tighter the curve, the more energy it loses. Electrons are very light, so they lose a lot.

To keep them at high speed, LEP had to keep pumping energy back in, all the way around the ring. This is one of the reasons the ring had to be so enormous.

A bigger ring means gentler curves, and less energy lost.

In 2000, LEP was switched off, but its tunnel wasn't wasted.

It now holds the Large Hadron Collider, which smashes protons instead.

Because protons are about 2000 times heavier than electrons, they lose far less energy as they go round, which lets them reach much higher energies.

It was the LHC that discovered the Higgs boson in 2012.

So what comes next for electron colliders?

The discovery of the Higgs boson in 2012 opened up a whole new set of questions.

Remember from our scariest equation video, the Higgs field gives particles their mass, and it might even hold clues about whether our universe is stable.

To answer those questions, physicists want to study the Higgs boson in extraordinary detail.

The LHC found the Higgs by smashing protons, but proton collisions are messy.

To measure the Higgs with real precision, many physicists believe we'll need to go back to electrons. Clean collisions between points.

One proposal is a new ring at CERN, called the Future Circular Collider.

Its first stage would collide electrons and positrons in a tunnel around 90 kilometers long, more than three times the size of LEP.

It's sometimes described as a Higgs factory because it would produce huge numbers of Higgs bosons to study.

Another idea avoids the synchrotron radiation problem completely.

Instead of a ring, you build a very long straight tunnel with electrons speeding up from one end and positrons from the other, meeting in the middle.

Proposals like the International Linear Collider would squeeze their beams down to just a few billionths of a meter tall at the collision point, so that as many of these size -less particles as possible get the chance to meet.

These machines would cost many billions and take decades to build and none has been fully approved yet.

But they show how far physicists will go to make two points meet. So let's answer the question we started with. If electrons have no size, how do physicists collide them?

First we learned that no size means no experiment has ever found an edge or anything inside an electron. It's smaller than anything we can measure.

Then we discovered that collisions were never about surfaces touching in the first place. You've never truly touched anything.

Your electrons push against other electrons through electric forces and quantum rules, and that's exactly how electrons collide too.

Their fields reach out into space, and when they get close enough, they push each other away or exchange energy without ever making contact.

We learned that electrons are fuzzy waves, so physicists work with odds instead of perfect aim.

They fire billions of electrons in tightly squeezed bunches, again and again, until enough of them meet.

And because electrons are such simple points, they make the cleanest probes in physics.

They revealed quarks hidden inside protons, and when they meet their antimatter twins, they can turn into entirely new particles. Here's a thought to leave you with.

Everything you've ever touched, every hug, every handshake, every step you've taken, has been a meeting of electrons that never quite touched.

The world feels solid because tiny points with no size refuse to get too close to each other.

If you enjoyed this, check out our video on being in two places at once, and let us know in the comments what tiny mystery we should explore next. Thanks for watching, see you next time.