The Speed of Light (Finally) Explained in Depth（原声文稿）

Nothing goes faster than light.

It sounds like a record, like the sound barrier, waiting for a better engine. It isn't.

In Star Trek, humanity does it in 2063, in a ship built from an old missile. So why can't we?

By the end, you'll know why nothing can go faster, in four steps.

Protons at the Large Hadron Collider got so close that in a one -second race with a flash of light, they'd lose by under three meters.

and something stranger, you have never once seen the present. So what's in the way?

Start with a rule so ordinary you've never noticed it.

Throw a ball forward from a moving train and its speed adds to the trains.

That works for every ball, bullet, and car you'll ever meet and for a note you toss to a friend across the aisle.

Now send your friend a flash of light instead.

Chase a beam of light at 99 % of its speed.

Simple adding says it should creep ahead of you at 1%.

Instead, it pulls away at the full 299 ,792 ,458 meters per second.

Time a flash of light in empty space, standing still or flying past, and you get that same number. Everyone does, however fast they move.

At 16, Albert Einstein asked himself what would happen if he raced after a beam of light at light speed.

He wrote, 50 years later, that he should then see the light frozen in place and that nothing in experience or in Maxwell's equations allowed such a thing.

Physics had two great ideas that couldn't both be right. "'Speeds add up,' said Newton's mechanics.

"'Light has one fixed speed,' said James Clerk Maxwell's equations of electricity and magnetism, without saying fixed compared to what.

So physicists reached for a fix.

Light needs something to wave in, an invisible medium filling space, the ether.

Light's speed would be fixed relative to the ether, and anyone moving through it would measure something different. instrument. So they went looking for the difference.

Albert Michelson's first try in Germany used an instrument so sensitive that, he wrote, the stamping of the pavement about 100 meters from the observatory made the fringes disappear entirely.

His second try in 1887 sat in a basement in Cleveland, Ohio with the chemist Edward Morley.

A slab of stone floating on a ring of mercury, with two beams of light bouncing back and forth across it.

With the earth racing around the sun, turning the stone through a wind of ether should have shifted their pattern of light by about four tenths of a band.

It shifted by less than two hundredths.

The instrument worked. The expectation was what broke.

Einstein's answer to Newton and Maxwell's standoff was a bet made in 1905.

Keep light speed the same for everyone and give up the simple adding.

He didn't measure it, he assumed it, and asked what else would have to be true.

Every test since, for more than a century, has agreed with him.

Physicists at CERN near Geneva once timed flashes of gamma rays given off by particles racing along at 99 .975 % of light speed.

If light were thrown like a ball, it would have come out at almost twice light speed.

It came out at the speed of light, the same as from a source standing still.

So if you and I fly past each other and time the same flash, we both get the same number. How can we both be right?

Think of the speed of light as an exchange rate, and start with the number itself.

light covers about 186 ,000 miles every second in empty space about 300 ,000 kilometers that's enough to circle the earth seven and a half times in a single second a message of light reaches your friend across the table in a few billionths of a second move your friend to the moon and every hello takes about one and a quarter seconds to arrive.

That's already an awkward phone call.

We know the delay precisely because astronauts left mirrors there, and scientists still bounce lasers off them.

Voyager 1 is now almost a full light day from home.

Susie Dodd, Voyager's project manager, put it this way.

If she sends, good morning Voyager 1 at 8am on a Monday, she gets the answer back back on Wednesday morning at about 8.

And to cross our galaxy, light needs about 100 ,000 years. Here's the odd part.

That number, 299 ,792 ,458 meters per second, has been exact by definition since 1983.

The meter itself is the distance light travels in 1, 299 ,792 ,458th of a second.

So the number really is an exchange rate between space and time.

One second of time is worth about 300 ,000 kilometers of space. Light just happens to travel at that rate.

And everyone, however they move, gets the same exchange rate.

If the rate can't change when you move, what does?

Picture a clock made of two mirrors, with a flash of light bouncing between them. Each bounce is a tick. Now put the clock on a train.

From the platform, you see the flash travel a longer zigzag.

Light speed can't change, so each tick has to take longer.

The moving clock runs slow, and every other clock on board has to agree with it.

So, what gives is time and distance themselves.

Moving clocks tick slower, moving lengths shrink along the direction they travel, and two events that look simultaneous to you don't to someone moving past.

Two lightning bolts hit a railway line at the same moment for someone standing halfway between them.

A passenger on a passing train races toward one flash and away from the other, and light speed is the same for her.

So for her, one bolt struck first. Who is right? Both.

At everyday speeds, the effect is tiny, but clocks can catch it.

Two scientists, Joseph Haefely and Richard Keating, tried in October 1971 by flying four atomic clocks around the world on ordinary airliners, once eastward and once westward.

They had four seats, two for themselves and two for the clocks, strapped in like passengers.

Hafele said they had just plopped them down on the seat and strapped them in.

Back on the ground, with the effects of speed and altitude added together, the clocks disagreed with the ones that had stayed home by tens to hundreds of billionths of a second, as relativity predicted.

GPS satellites live with this every day.

Their clocks lose about seven millionths of a second a day to their speed and gain about 45 from sitting high in Earth's gravity.

So engineers set them to tick slow before launch, on purpose. So time gives a little when you move.

Then why can't your friend just build a faster ship and keep pushing all the way to the speed of light?

Say your friend starts small, with 1 kilogram of anything.

Pushing it to 90 % of light speed takes the energy of a 28 megaton bomb To 99 .99%, about 1 ,500 megatons, some 30 times the biggest bomb ever set off. The speed barely changes.

The bill keeps exploding.

Anything with mass can get closer and closer to the speed of light, but never all the way, because the bill for the final step has no ceiling.

That's what those protons ran into.

Inside the Large Hadron Collider, a proton's energy rose about 15 times between entering the ring and full power.

Its speed changed by two parts in a million. This is nothing like the sound barrier.

That was air piling up in front of a plane, and in 1947, a rocket plane flew through it at about 700 miles an hour.

Out in empty space, there's nothing piling up, and the bill still has no ceiling.

thing. You may have heard that a fast object gets heavier. It doesn't.

Its mass stays the same, and what grows without limit is the energy it takes to speed it up.

The physicist Karl Adler once wrote up the answers he gave his son on this. No!

Well, yes, and actually no, but don't tell your teacher.

Now take the mass away, and nothing is left to slow it down.

A flash of light doesn't speed up to the limit. It leaves the lamp already there. No experiment has ever found any mass in light.

The tightest limit says a photon is at least 500 sextillion times lighter than an electron.

Add any mass at all and the limit becomes a wall you can approach but never touch. So your friend's ship will never get there.

But what if the ship stays home and only the message goes faster?

Say your friend's ship flies away from Earth at 60 % of light speed and it's three light hours out when you press send.

Your message is instant and so is the reply by the ship's clock.

But the ship's now is tilted against yours and the reply lands on Earth one hour and 48 minutes before you pressed send.

You're holding an answer to a message you haven't written.

Now decide not to That's what a faster than light message breaks.

As long as nothing goes faster than light, every observer, however they move, agrees which of two linked events came first.

Your cause always comes before its effect, for everyone.

Let a message beat light, and that guarantee is gone.

Einstein himself spotted the problem, two years after his bet. He admitted it isn't a logical contradiction.

But it clashes so completely with everything we know about cause and effect that physicists still rule such messages out.

A limerick printed in Punch in 1923 put it more simply.

There was a young lady named Bright, whose speed was far faster than light.

She set out one day, in a relative way, and returned on the previous night.

So the speed of light is the speed of cause and effect, the fastest that anything can affect anything else.

Gravity obeys it too, to within a few parts in a quadrillion.

When two neutron stars collided about 130 million light -years away, their gravitational waves and a flash of gamma rays crossed all that distance and arrived within two seconds of each other. Light just happens to travel at the top speed.

For a few months, it looked as if something had beaten it.

A team at Italy's Gran Sasso Laboratory reported that neutrinos sent from CERN, 730 kilometers away, had arrived about 60 billionths of a second early, as if they had beaten light by roughly 18 meters.

In CERN's main auditorium, standing room only and streamed live, the team's physics coordinator, Dario Autiero, closed his talk with these words.

Therefore, we present to you today this discrepancy, this anomaly.

The team asked the world's physicists to check its work.

As its spokesperson, Antonio Hereditato, put it, we could not sweep it under the carpet because that would be dishonest.

The mistake was in the clock chain, and it was two mistakes.

A fiber connector carrying the timing signal underground wasn't seated properly and that made the neutrinos look early.

A clock running slightly fast pushed the other way.

With both corrected, the neutrinos arrival was consistent with the speed of light and four experiments at Gran Sasso agreed. The speed limit held.

Around the world on airliners, relativity shifted clocks by tens or hundreds of billions of a second.

Is there anywhere it makes the difference between arriving and never arriving at all? Picture a rain you can't feel.

Hold out your hand, palm up.

About one muon a second goes through it and has done all your life.

Muons are born high in the atmosphere, typically about 9 miles up, when cosmic rays from space smash into the nuclei of air molecules.

A muon at rest lives on average about 2 .2 millionths of a second, then decays.

Even at nearly the speed of light, that's only enough time to travel about 2 ,200 feet.

So without relativity, only about 1 muon in 10 billion would survive the trip down.

Instead, thousands of them pass through every square meter of ground every minute. it.

If their clocks run slow, the numbers should show it.

Two physicists working at MIT, David Frisch and James Smith, went and counted in 1962.

They set a drum of plastic near the top of Mount Washington in New Hampshire, under iron two and a half feet thick.

Any muon that stopped in the plastic and decayed made a flash, and they counted the flashes.

Then, they took the equipment down to sea level, near Boston, and counted again.

On the mountain, in one hour, they counted 568.

Without relativity, about 27 of those should have been left at sea level. Down by the sea, they counted 412.

Frisch and Smith wrote it in their paper with an exclamation mark, calling them by an older name.

The fact is that we observed 412 instead of 27 mesons left at sea level.

The muons had aged only about a ninth as much as the clocks on the ground. They filmed the whole experiment for students.

From the point of view of a typical muon, nothing slowed down.

Instead, the whole atmosphere is squashed, from 15 kilometers to well under one, and the ground arrives in less than a lifetime.

time. Both views agree on the part that matters.

The muon makes it. So nothing outruns light in empty space.

Then how can astronomers see galaxies receding from us faster than light?

Picture a child running the wrong way on a moving sidewalk.

That's what light from the most distant galaxies is up against, because space itself is stretching.

Far enough away, day, beyond roughly 14 billion light years, the gap between us and a galaxy grows faster than light can cross it.

That breaks no rule, because the galaxy isn't moving through space faster than light. The space between us is growing.

Put a flash of light right beside any galaxy, and the light still pulls ahead at full speed. We can even see some of those galaxies.

Like that child, their light first lost ground.

But the zone around us where Where distances grow slower than light keeps widening, and once it caught up with the light, the light slowly got through.

Webb's most distant galaxies are all in that club. Other loopholes look even easier.

Point a laser at the moon and flick your wrist, and the spot can skate across it faster than light. But the spot isn't a thing.

Each bit of light lands next to the last, and nothing travels along the sweep. Entanglement looks like the perfect loophole.

Measure one of two entangled particles and you get a random result.

Only when the two labs compare notes by ordinary message at light speed or slower do they see that the results match.

The match can't carry your message, so cause and effect stay in order. But here's what still bothers physicists.

They test entangled particles with a kind of game for two.

Particles following secret, pre -written instructions can score at most two.

Entangled particles can score up to about 2 .8, and relativity alone would allow a perfect four without ever letting a message through.

Nobody knows yet why nature stops short. So why 2 .8 and not four?

The moon you see is a second and a quarter old.

The sunlight on your face is a little over 8 minutes old, and the light of most stars? Centuries.

Even your friend across the table is a few billionths of a second in the past, and your brain adds a delay millions of times longer.

You have never seen the present.

Every observer, however fast they move, agrees on that order.

What you see happened before you saw it. The speed of light isn't about light.

It's the speed of cause and effect and it's why everything you see is already the past.