Why Do Electrons Keep Moving and Never Run Out of Energy?（原声文稿）

Transcript of 电子为什么永远转不停？能量从哪来 （中文完整文稿）。

Somewhere inside your body, right now, there are electrons that have existed for billions of years.

They were around before the earth formed, before the sun ignited, and in all that time they have never stopped, never slowed down, and never needed to be recharged. No engine works like that.

A spinning top eventually wobbles and falls, a swinging pendulum gradually comes to rest.

Even the planets lose tiny amounts of orbital energy over immense spans of time.

time. So how can an electron keep moving, apparently forever, without ever running out of energy?

It sounds like a simple question, the kind a curious child might ask.

But in the early 20th century, this exact question pushed physics into a crisis, according to the most successful scientific theories of the time, every atom in the universe should have collapsed almost instantly.

The fact that you exist at all, was by the standards of 1910 physics, impossible.

possible. To understand why, we need to examine two assumptions hidden inside the question.

The first is that motion requires a constant supply of energy. The second is that an electron moves the way a small object moves.

Both assumptions feel natural, and both, in very different ways, turn out to be wrong. Let's begin with the first one.

For nearly 2 ,000 years, people followed an idea associated with Aristotle.

An object keeps moving, only as long as something keeps pushing it. Stop pushing a cart, and it stops. stops. This matches everyday experience so well that it seems obvious.

Yet in the 17th century, Galileo Galilei and later Isaac Newton showed that it is fundamentally mistaken.

Newton's first law of motion states that an object in motion continues moving at the same speed, in the same direction, and less a force acts on it. Motion itself costs nothing. So why does a rolling ball eventually stop?

Because it is constantly losing energy to its surroundings, friction with the ground, collisions with air molecules, and tiny deformations of the ball, all convert its energy of motion into heat, the energy doesn't vanish.

One of the most thoroughly tested principles in all of science, the conservation of energy, tells us that energy is never created or destroyed, it only changes form, a moving object slows down, only when its energy has somewhere to go.

This changes our question in an important way.

The real puzzle is not what keeps the electron moving, nothing needs to, the real puzzle is why the electron never loses its energy.

To answer that, we need to know what an electron is and where it lives.

Thompson studied cathode rays, beams that glow inside sealed glass tubes when a high voltage is applied.

By deflecting these rays with electric and magnetic fields, he showed that they consisted of tiny particles carrying negative charge, far lighter than any atom.

He had discovered the electron.

Since atoms are electrically neutral, Thompson and proposed that electrons were embedded in a diffuse ball of positive charge, like raisins scattered through a pudding. That picture didn't last long.

Beginning in 1909, Hans Geiger and Ernest Marsden, working in Ernest Rutherford's laboratory, fired alpha particles at an extremely thin sheet of gold foil, nearly all of them passed straight through, but a very small fraction bounced off at large angles, and a few even came almost straight back.

Rutherford later compared the surprise to firing an artillery shell at a sheet of tissue paper and having it rebound. In 1911, he explained the result.

Nearly all of an atom's mass and positive charge is concentrated in a tiny dense nucleus, and the rest of the atom is overwhelmingly empty space.

The natural next step was to imagine the atom as a miniature solar system, with electrons orbiting the nucleus the way planets orbit the sun, held in place by electrical attraction instead of gravity. It was an elegant picture. It was also a disaster.

The problem came from James Clerk Maxwell's theory of electromagnetism, which had already explained light itself as a travelling wave of electric and magnetic fields.

One of its central predictions is that an accelerating electric charge emits electromagnetic radiation.

This is not an abstract idea, it's exactly how a radio antenna works, by pushing electrons back and forth, so that they broadcast waves. And here's the crucial detail.

an object moving in a circle is always accelerating because its direction is constantly changing even if its speed is not.

So an orbiting electron should continuously radiate light.

Radiation carries energy away and here at last is the escape route for the electrons energy that friction provides for the rolling ball.

Losing energy, the electron should spiral inward toward the nucleus.

When physicists calculated how long this would take for a hydrogen atom, the answer was roughly a hundred billionth of a second. In that brief instant every atom should have collapsed.

There was a second clue that something was deeply wrong.

A spiraling electron would emit a continuous smear of colors as its orbit shrank, but real atoms don't do that.

When hydrogen gas is heated, it glows only at a few sharply defined colors.

In 1885, a Swiss school teacher named Johann Ballmer had found a simple mathematical formula that predicted the wavelengths of these visible lines without anyone knowing why the formula worked.

In 1913, the Danish physicist Niels Bohr made a radical suggestion.

He proposed that electrons can occupy only certain specific orbits, and that while in those orbits, they simply do not radiate.

Light is emitted only when an electron jumps from one allowed orbit to another, and the energy difference between them determines the colour.

Bohr's model reproduced Balmer's formula with remarkable accuracy.

But Bohr had not explained why those orbits were special, or why an accelerating charge would suddenly stop radiating, he had essentially declared it.

Finding the real reason would require physicists to abandon one of their most basic assumptions, the idea that an electron is a tiny particle traveling along a path at all.

The first crack in that assumption came from a French physicist named Louis de Broglie. In 1924, he proposed something that sounded almost absurd.

If light, long understood as a wave, could also behave like a stream of particles, then perhaps particles like electrons could also behave like waves. He even predicted the wavelength.

It equals Planck's constant, an extremely small number that sets the scale of quantum effects divided by the particle's momentum.

The faster and heavier an object, the shorter its wavelength.

For a baseball, the wavelength is so absurdly tiny that no wave behavior could ever be noticed.

For an electron inside an atom, the wavelength is about the size of the atom itself. Three years later, the idea was confirmed.

Clinton -Davidson and Lester Germer in the United States and independently George Padgett -Thompson in Britain directed beams of electrons at crystals and observed diffraction patterns, the alternating bright and dark regions that appear only when waves overlap and interfere.

In a pleasing historical symmetry, JJ Thompson had shown that the electron is a particle and his son showed that it is also a wave.

This immediately suggested why only certain orbits are allowed. Think of a guitar string. Because it is fixed at both ends, it can vibrate only in patterns that fit its length exactly.

One hump, two humps, three, but never two and a half.

A confined wave naturally produces a set of allowed frequencies.

In the same way, an electron wave, confined near a nucleus, can exist only in patterns that fit. Quantization was no longer a rule imposed from outside.

It emerged from wave behavior itself.

In 1926, the Austrian physicist Erwin Schrödinger wrote down the equation that describes how these matter waves behave.

In plain language, the Schrödinger equation balances an electron's energy of motion against its energy of position in the electric field of the nucleus and asks which wave patterns can exist stably.

For hydrogen, its solutions are not circular paths. They are three -dimensional clouds called orbitals.

The lowest one is perfectly spherical, densest near the nucleus, and fading smoothly outward. There is no track, no circling, no planet -like orbit.

Max Born soon supplied the interpretation.

The intensity of the wave at any point tells us the probability of finding the electron there if we look. Now we reach the heart of the matter.

Each of these allowed patterns has a precise energy, and such a state is called a stationary state.

The wave itself oscillates in a mathematical sense, but the probability cloud, and therefore the distribution of the electron's charge, does not change with time at all.

It doesn't rotate, pulse, or slosh back and forth. And remember the reason a classical electron was doomed.

Radiation is produced by charge that accelerates, by something that changes.

A charge distribution that is perfectly constant in time, has nothing to broadcast. The radio antenna has no oscillating current.

The mechanism that should have drained every atom in a fraction of a second simply has nothing to act on. But this raises a natural question.

If the cloud is perfectly still, is the electron simply sitting at rest?

Here quantum mechanics gives an answer that is just as surprising in the opposite direction.

In 1927, Werner Heisenberg showed that the uncertainty in a particle's position, multiplied by the uncertainty in its momentum, can never fall below a fixed value set by Planck's constant.

movement, squeeze an electron into a smaller region and its momentum necessarily becomes more spread out.

An electron confined to a space about a tenth of a nanometer across cannot have zero momentum, it cannot be still.

This is also why atoms have the size they do, the nucleus pulls the electron inward, lowering its electrical energy, but confining it more tightly raises its kinetic energy.

The ground state is the compromise where the total is lowest, and the resulting radius, called the Bohr radius, is about five hundredths a nanometer. The numbers are striking.

In hydrogen's ground state, the electron's average kinetic energy is about 13 .6 electron volts, while its electrical potential energy is about minus 27 .2.

The total is minus 13 .6, which is exactly the energy needed to pull the electron free.

A typical speed works out to roughly 2200 km per second, about 137th of the speed of light. And here is why this motion never runs down.

The ground state is, by definition, the lowest energy the electron can possibly have.

To lose energy, it would have to drop into a lower state, and there isn't one.

Imagine a ball resting at the bottom of a valley, with one crucial difference.

In the quantum world, even the bottom contains motion. Physicists call this zero -point energy.

The electron isn't burning fuel to keep moving.

It's already at the floor. This also corrects a few common misunderstandings.

The electron's spin measured indirectly in the Stern -Gerlach experiment of 1922, is often pictured as a tiny ball rotating on its axis, it isn't.

Treating it that way would require the electron's surface to move faster than light, spin is an intrinsic form of angular momentum with no true classical equivalent, and electrons do sometimes lose energy.

An electron in a higher, excited state can drop to a lower one by emitting a photon, typically within a few billionths of a second.

Why would a supposedly stationary state decay at all.

The answer comes from quantum electrodynamics, which shows that even empty space contains fluctuations of the electromagnetic field that can nudge an excited electron downward.

These vacuum effects are not just theoretical.

In 1947, Willis Lamb measured a tiny shift in hydrogen's energy levels that only these effects could explain.

Yet in the ground state, there is nowhere lower to go, and so the electron remains, indefinitely. which leaves us with an uncomfortable question.

If electrons move forever without any energy input, haven't we just described perpetual motion? The very thing physics insists is impossible.

The answer lies in what perpetual motion actually means.

The machines that physics rules out are devices that would deliver useful work forever, turning wheels or lighting bulbs without any energy input, that is forbidden by the conservation of energy and by the laws of thermodynamics. An An electron in its ground state does nothing of the sort.

To extract energy from it, the electron would have to fall to a lower state, and as we've seen, no lower state exists.

Its motion is real, but it is locked away, permanently inaccessible.

Zero -point energy itself is no mathematical fiction, it has measurable consequences.

Helium, for example, remains liquid even when cooled toward absolute zero at ordinary pressure, because the zero -point motion of its atoms is too vigorous for them to settle into a solid. But this energy cannot be harvested.

Devices claiming to draw free power from zero -point energy misunderstand the very thing that makes it permanent.

It is the minimum, not a reservoir, explaining why a single atom is stable still leaves a larger puzzle.

Ordinary matter contains enormous numbers of atoms pressing against one another.

Why don't they merge, with electrons crowding ever closer to neighboring nuclei, until everything collapses into a dense mass? The uncertainty principle alone is not enough to prevent it.

The missing ingredient was identified in 1925 by Wolfgang Polly, his exclusion principle states that no two electrons can occupy the same quantum state.

Once the lowest states are filled, additional electrons are forced into higher ones.

This is why atoms have shells, why elements have distinct chemical personalities, and why the periodic table looks the way it does.

It took decades to prove that these principles rules truly guarantee the stability of bulk matter.

In 1967, Freeman Dyson and Andrew Leonard produced a rigorous mathematical demonstration and in 1975, Elliot Leib and Walter Theuring found a far simpler and more elegant proof.

Their conclusion is profound, without the exclusion principle, matter would not merely be different, it could not hold its shape at all.

The solidity of a table is, at its root, a quantum effect. The same principle operates on an astronomical scale.

When a star like the Sun exhausts its nuclear fuel, its core contracts into a white dwarf, an object roughly the size of the Earth but containing a large fraction of a solar mass.

What holds it up against its own gravity is not heat, but electrons refusing to share quantum states. This electron degeneracy pressure has a limit.

In the early 1930s, Subramanian Chandra Sekar showed that above about 1 .4 solar masses, it can no longer resist gravity's pull.

closer to home, a copper wire, offers its own surprise.

Even near absolute zero, its conduction electrons move at around a million meters per second, simply because the exclusion principle forces them into high energy states.

Yet when current flows through the wire, the overall drift of electrons is astonishingly slow, typically a fraction of a millimeter per second. And here, energy really is lost.

Electrons scatter off vibrating atoms and impurities, heating the wire, that is electrical resistance, unless the material becomes a superconductor.

In 1911, Haike Kamilling Ons cooled Mercury to about 4 degrees above absolute zero and found that its electrical resistance vanished.

The explanation, developed by John Bardeen, Leon Cooper and Robert Schrieffer in 1957, is that electrons form linked pairs that move collectively in a single coherent quantum state, one that ordinary scattering cannot disrupt.

Currents set flowing in superconducting rings have been observed to persist for years without measurable decay.

It's the clearest everyday demonstration that motion, free of any way to shed energy, simply continues.

Superconducting magnets now power MRI scanners and steer particle beams at the Large Hadron Collider. Will electrons themselves last forever?

The electron is the lightest particle that carries electric charge, and since charge is conserved, there is nothing lighter for it to decay into. Experiments have tested this directly.

In 2015, the Borexino detector in Italy found no sign of electron decay, and established that the electron's lifetime must exceed about 66 ,000 trillion trillion years, roughly 5 million trillion times the current age of the universe.

That is an experimental limit, not a proof of eternity, but no evidence of decay has ever been found. What remains genuinely uncertain is Supler.

Quantum mechanics predicts the behavior of electrons with extraordinary precision, yet physicists still disagree about what an electron is doing when no one measures it.

In the standard interpretation, asking for its exact path is meaningless.

In Bohmian mechanics, the electron always has a definite position, and in hydrogen's ground state, it is actually motionless, held in place by the structure of its wave.

Other interpretations offer different pictures again.

All of them agree on every measurable prediction, which is precisely why experiments have not settled the question, whether the electron moves in the everyday sense is, at this level, still open. What is not open is the answer to our central question.

Electrons don't run out of energy because they aren't spending any. Motion requires no fuel, only a way to lose it.

In a stationary state, an electron's charge distribution does not change, so it cannot radiate.

Confinement guarantees it can never be still, and the ground state guarantees it can never fall further.

The exclusion principle then turns this stability into the structure of atoms, chemistry, solid matter, and even dead stars.

The restless electron, it turns out, is not resisting the universe's tendency towards stillness. It is already at rest in the only sense nature allows.

The lowest energy the universe permits is not silence, but a motion that can never be taken away. Bye.