How does light travel without moving... INCREDIBLE（原声文稿）

Transcript of 光为什么会发光？不动的光子一次讲透 （中文完整文稿）。

Right now, light is hitting your eyes.

Some of it left the screen in front of you a few billionths of a second ago. Some of it left the sun about 8 minutes ago.

And if you step outside tonight and look up, some of it left distant stars before the pyramids were built. All of that light crossed empty space to reach you.

But here's the strange part. Sound needs air.

Ocean waves need water. A guitar note needs a vibrating string. Light needs nothing at all.

It crosses a perfect vacuum where there is literally nothing to shake, nothing to push, nothing to carry it.

So how does something travel when there's nothing there to move, and it gets stranger?

According to Einstein, from Light's own point of view, it may not be traveling at all.

To understand this, we have to go back more than 300 years to an argument between two giants.

Isaac Newton believed Light was made of tiny particles, little bullets shot out from a source. Kristian Huygens, a Dutch scientist, disagreed.

He said light was a wave, like ripples spreading across a pond.

Newton was so famous that for over a century, most scientists sided with him.

Then, in 1801, an English doctor named Thomas Young did a simple experiment.

He shined light through two narrow slits, and looked at the wall behind them.

If light were just particles, you'd expect two bright lines. Instead, Young saw a pattern of many bright and dark stripes.

That pattern is called interference, and it only happens with waves.

When two wave peaks meet, they add up and make brightness.

When a peak meets a valley, they cancel out and make darkness. Light was a wave. Huygens was right. But that created a huge problem. If light is a wave, a wave of what?

Water waves are moving water. Sound waves are moving air.

So scientists assume space must be filled with an invisible substance that light waves travel through. They called it the luminiferous aether.

It had to be everywhere. It had to be perfectly transparent, and it had to be incredibly stiff to carry waves that fast.

Nobody had ever seen it, but almost everyone believed it had to exist.

Meanwhile, people were trying to measure how fast light actually goes.

In 1676, the Danish astronomer Ole Roma noticed something odd about Io, one of Jupiter's moons.

Its eclipses seemed to come early, when Earth was closer to Jupiter, and late when Earth was farther away.

Rumor realized the light from Io simply needed extra time to cross the extra distance. Light wasn't instant, it had a speed.

Later, in 1849, the French physicist, Ypollet Fizot, measured it here on Earth using a spinning -toothed wheel and a mirror several kilometers away.

He got about 313 ,000 km per second, remarkably close to the modern value.

Today, we know the speed of light in a vacuum is exactly 299792458 meters per second.

It's so precisely known that since 1983, we actually use it to define what a meter is. Now comes the moment that changed physics forever.

In the 1800s, Michael Faraday, a self -taught genius who never studied advanced mathematics, was playing with magnets and wires.

He discovered that a moving magnet it, creates an electric current, and that electric currents create magnetic effects.

He imagined invisible lines of force stretching through space around magnets and charges.

Most scientists thought this was just a nice picture, but a young Scottish physicist named James Clark Maxwell took Faraday's idea seriously and turned it into mathematics.

In 1865 Maxwell published a set of equations describing electricity and magnetism, and And hidden inside them was something astonishing.

A changing electric field creates a magnetic field.

And a changing magnetic field creates an electric field. Think about what that means.

If you shake an electric field, it creates a magnetic field.

That magnetic field is changing, so it creates a new electric field a little further along.

That electric field creates another magnetic field, and so on, forever.

The two fields keep regenerating each other, leapfrogging through space.

It's a wave that carries itself.

it doesn't need water or air or ether, it is its own medium.

Then Maxwell calculated how fast this self -sustaining wave should travel.

He used two numbers that had nothing to do with light.

One describes how easily space allows electric fields to form, the other how easily it allows magnetic fields to form.

Those numbers had been measured in laboratories using coils and capacitors.

When Maxwell combined them, the result came out to roughly 300 ,000 kilometers per second.

Maxwell wrote that light must be an electromagnetic disturbance.

In one stroke, he unified electricity, magnetism, and light.

Physicists still call it one of the greatest discoveries in history.

Einstein later kept a picture of Maxwell on his study wall, next to Newton and Faraday.

So here's the first answer to our title.

When light travels, nothing physical moves from one place to another the way a ball does. Think of a stadium wave.

The wave races around the entire stadium, but every single person just stands up and sits down in their own seat. The pattern travels, not the people. Light is similar.

What travels is a pattern of changing electric and magnetic fields. No material object makes the journey.

In that sense, light really does travel without anything moving, but scientists weren't ready to give up the ether.

In 1887, Albert Michelson and Edward Morley built an incredibly sensitive instrument in Cleveland, Ohio. Their idea was simple.

Earth moves around the sun at about 30 km per second.

If space is filled with ether, Earth should be plowing through it, creating an ether wind.

Light moving with the wind should travel at a slightly different speed than light moving across it.

They split a beam of light into two paths at right angles, sent them back and forth, and recombined them to look for the tiny difference. They found nothing. They rotated the device. Nothing. They tried at different times of the year.

Still nothing. Light moved at exactly the same speed in every direction.

It's often called the most famous failed experiment in history, and that failure opened the door to modern physics. Enter Albert Einstein.

At about 16 years old, Einstein had a daydream.

What would happen if he could chase a beam of light at the speed of light.

He would see the wave frozen in place, an electric and magnetic field just sitting there not changing. But Maxwell's equations don't allow that.

A frozen light wave simply cannot exist, something was wrong.

Ten years later, in 1905, Einstein published his special theory of relativity, built on one bold rule.

The speed of light in a vacuum is the same for everyone, no matter how fast they're moving.

If you drive toward a flashlight at half the speed of light, you don't see its beam coming at one and a half times the speed of light. You still measure exactly the same speed.

How is that possible? Because space and time themselves are just.

The faster you move, the slower your clock ticks compared to someone at rest.

This is called time dilation.

And distances in your direction of motion shrink. This is called length contraction. These aren't illusions.

Atomic clocks flown on airplanes have measured it.

GPS satellites must correct for it every single day or your phone's navigation would drift by several kilometers. Now push this idea to the limit.

As anything gets closer to the speed of light its clock runs slower and slower and the distance ahead of it shrinks more and more.

At exactly the speed of light the equations say time would stop completely and the distance in front would shrink to zero.

So in a poetic sense light that left a star billions of years ago and lands in eye was emitted and absorbed in the same instant.

The entire journey across the universe takes zero time and covers zero distance. From light's perspective, it never really moves. Physicists add an important warning here.

Strictly speaking, light has no point of view.

Relativity says you can never build a valid frame of reference that moves at light speed, because nothing with mass can ever get there.

So, what light experiences isn't a question physics can truly answer. But the math clearly shows the trend.

The closer you get to light speed, the closer your experience gets to no time and no distance at all. That's the deeper meaning of our title. Then came another twist.

In 1900, Max Planck found that light energy comes in small packets, and in 1905, Einstein showed that light behaves as particles called photons.

So Newton wasn't completely wrong either.

Light is both a wave and a particle, depending on how you look at it.

Photons have no mass, which is exactly why they can travel at the universe's speed limit – yet they still carry momentum.

That's why in 2010, the Japanese spacecraft IKAROS sailed through space pushed only by sunlight.

Later, Richard Feynman described light with his theory of quantum electrodynamics, one of the most accurately tested theories ever made.

In Feynman's picture, a photon doesn't simply take one path – in a way, it explores every possible path at once.

And those paths add up and cancel out, just like young stripes, until the result looks like a straight line. And there's one more surprise.

Light slows down in glass, to about 200 ,000 km per second, and in diamond to about 124 ,000.

But the photons themselves aren't getting tired.

The light wave makes the electrons in the material vibrate.

Those electrons create their own tiny waves, and the combined wave moves more slowly. Once it exits, it's back to full speed instantly.

In 1999 physicist Lenny Howe slowed light down to about 17 meters per second, slower than a bicycle, by sending it through an ultra -cold cloud of atoms. So how does light travel without moving?

It's a self -regenerating pattern of electric and magnetic fields that needs no medium.

It moves at a speed that every observer in the universe agrees on, and according to relativity, its journey across billions of light years shrinks to nothing at all.

The oldest light we can detect, the cosmic microwave background, left its source about 380 ,000 years after the Big Bang.

It has been crossing the universe for more than 13 billion years.

And yet, in Einstein's equations, for that light, no time has passed.

You're seeing the beginning of the universe in a moment that, for the light itself, never ended.