Where Do Photons Actually Go When You Turn Off The Light... (SHOCKING!)（原声文稿）

Transcript of 关灯后光去哪了？光子的一生彻底讲清 （中文完整文稿）。

You flip off the light and the room goes dark instantly. It's something you've done thousands of times without a second thought. But think about it for a moment.

A second ago, that room was flooded with light.

Trillions upon trillions of tiny particles of light, called photons, were streaming out of the bulb, bouncing off the walls, the ceiling, your bed, your face. The room was packed with them. Then you flipped the switch, and they all seemed to vanish. So where did they go? Did they fall to the floor?

Did they get sucked back into the bulb? Are they still hiding in the corners of the room, or did they simply stop existing?

The real answer is surprising.

Some of those photons disappeared in less than a millionth of a second, and turned into something you can feel.

Some ended up inside your eyes, some slipped out through your window, and are still travelling right now, racing out into space, and might keep going for billions of years.

And the darkness that's left behind?

It isn't actually empty at all, let's follow the light and find out where it really goes.

Before we follow the light, let's remind ourselves what it actually is.

Light is made of tiny packets of energy called photons. Each photon is like a single drop of light. It has no mass. It carries a small amount of energy.

And as we saw in our video on where light gets its speed, it's always moving at the speed of light, about 300 ,000 km every second, from the moment it's created. Photons are a bit strange.

In some experiments, they behave like tiny particles, arriving one at a time, like raindrops.

In others, they behave like waves, spreading out and overlapping, like ripples on a pond. Physicists think of them as both, depending on how you look.

For this video, it's easiest to picture them as tiny glowing raindrops, flying in straight lines. So where do photons come from? Let's look inside a modern LED light bulb.

An LED contains a tiny chip made of special materials.

When electricity flows through it, electrons are pushed across a boundary inside the chip.

As each electron settles into a lower energy spot, it has some energy left over, and it gets rid of that extra energy by releasing a single photon. Old -fashioned bulbs work differently.

They pass electricity through a very thin wire, called a filament, until it gets so hot it glows.

The atoms in the hot wire are jiggling so violently that they constantly shake off photons. The same way a hot stove glows red. Either way, the key idea is the same.

Photons aren't stored inside the bulb bulb waiting to come out. They're made, fresh, from energy.

Electricity goes in, and photons are created on the spot, already racing away at the speed of light.

That's important, because it means a photon's life has a clear beginning.

And as we'll see, it also has a clear end.

But first, let's get a sense of just how many of them were filling your room.

An ordinary household LED bulb uses about 10 watts of electricity.

Only part of that energy comes out as visible light, but even so, the number of photons it produces is staggering.

Every single second, a bulb like that sends out roughly 10 billion billion photons. That's a one followed by 19 zeros, every second.

To put that in perspective, if every one of those photons were a grain of sand, a single second of light from your bulb would be enough fine sand to fill thousands of Olympic swimming pools. And the next second, thousands more.

So with that many photons photons pouring out, you might expect your room to be absolutely stuffed with them.

But here's the surprising thing, they don't stay around very long.

If you could freeze time and count the photons floating in your room at one single instant, you'd find far fewer than you might expect. Maybe around a trillion.

That still sounds huge, but it's millions of times smaller than the number your bulb makes every second. Why the difference? Because each photon lives an incredibly short life. life.

Your bulb is like a tap pouring water into a bucket with a huge hole in the bottom. Water rushes in, but it drains out just as quickly. The bucket never fills up very much. The light in your room is a constant flow, not a stored supply.

Photons are being made, and then destroyed, over and over, trillions of times a second.

The brightness you see is just the balance between the two, which means that when you switch off the light, you're turning off the tap.

And the question becomes, how quickly does the bucket drain? And where does the water go? But before we find out, a quick favor.

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 flip the switch.

Let's slow time down enormously, and watch what happens the instant you switch off the light.

First, the electricity stops flowing into the bulb.

In an LED, that means the electrons stop crossing the boundary inside the chip, so no new photons are created.

The tap has been turned off.

The bulb fades out very quickly, in a tiny fraction of a second.

But the photons that were already released don't know the switch has been flipped. They're already flying through the room. Nothing can call them back.

They just keep going in a straight line, at the speed of light, until they hit something.

And in a room, they don't have to go far. A typical room is just a few meters across.

At the speed of light, a photon crosses a room that size in about 10 billionths of a second. That's 10 nanoseconds.

To give you a sense of how short that is, a single beat of a hummingbird's wings takes millions of times longer.

In the time it takes you to blink, light could cross your room tens of millions of times.

So almost instantly after you flip the switch, every photon in your room reaches a surface.

Her wall, the ceiling, the floor, your bed, your skin, your eyes.

and when a photon hits a surface one of two things can happen.

It can bounce off or it can be swallowed up.

If it bounces it carries on its journey heading off in a new direction until it hits something else.

If it's swallowed its journey ends right there.

So to find out where the photons go we need to follow both paths. Let's start with the bounces.

When a photon hits a surface whether it bounces or gets swallowed depends on what the surface is made of and on the color of the light. That's actually what color is.

A white wall looks white because it bounces back most of the light that hits it, all colors together.

A black sofa looks black because it swallows almost all of the light, bouncing back very little.

And a red cushion looks red because it bounces back mostly red light, while swallowing the blues and greens.

So every time you see a color, you're seeing the photons that survive the bounce, the ones that weren't swallowed.

A typical white wall bounces back around 80 % of the light that hits it. That sounds like a lot.

And it means that after you flip the switch, many photons don't just hit one wall and stop. They bounce. Then they bounce again, and again. But each bounce is a gamble.

On every bounce off a white wall, there's about a 1 in 5 chance the photon gets swallowed. Off darker surfaces, the odds are much worse.

So with every bounce, fewer and fewer photons survive. After 10 bounces, only about 1 in 10 is left. After 20 bounces, only about 1 in 100.

After 50 bounces, only about 1 in 100 ,000. And remember how fast this is happening.

Each trip across the room takes about 10 billionths of a second, so even 50 bounces take only about half a millionth of a second.

In other words, within about a millionth of a second after you switch off the light, almost every photon in your room has been swallowed. That's why the room goes dark instantly.

Your eyes, which take around a tenth of a second to react, could never catch the fading light.

As far as you're concerned, the darkness arrives at the very same moment you flip the switch.

So almost all the light ends up being swallowed. But what does that actually mean? Where does a photon go when it's swallowed?

When a photon is swallowed by a surface, physicists say it's been absorbed.

And here's the most surprising part of the whole story. When a photon is absorbed, it doesn't go anywhere. It stops existing. That sounds impossible. How can something just stop existing?

Let's zoom in and see. The wall, like everything, is made of atoms. And each atom has electrons around it.

When a photon arrives, it can hand all of its energy to one of those electrons or to the atoms themselves.

At that moment, the photon is gone. Not hidden, not stored, gone.

All that's left is its energy, now given to the material it hit.

This surprises a lot of people because we're used to things being conserved. If you push a ball into a box, it's still in the box. But photons don't work like balls. The number of photons in the universe isn't fixed.

They can be created out of energy, like in your light bulb, and they can be destroyed, turning back into energy.

What's always kept is the energy itself, not the photons. So where does that energy go? Mostly, it becomes heat.

Remember from our entropy video that heat is really just tiny particles jiggling around.

When a photon is absorbed, its energy usually makes the atoms in the wall jiggle a little bit faster.

And faster jiggling means a slightly higher temperature.

So every time you switch on a lamp, the light warms up your walls, your furniture, and you. Just a tiny bit. That's why a black car gets so hot on a sunny day. It absorbs almost all the sunlight that hits it, turning it into heat.

A white car bounces more of it away, so it stays cooler. So here's our first big answer.

When you turn off the light, almost all of the photons in your room end up absorbed by the walls and objects around you. They vanish, and leave behind a tiny bit of warmth.

earth, but a few of them end up somewhere much more interesting, inside you.

Some of the photons in your room never hit a wall, instead they fly straight into your eyes and those are the only photons you ever actually see.

That's worth thinking about.

You never see a wall directly, you only see the photons that bounced off it and happen to land in your eye.

Every image you've ever seen, every face, every sunset, every word on a page has been a pattern of photons arriving at the back of your eyes.

At the back of each eye is a thin layer of cells called the retina.

It contains millions of tiny light -sensitive cells called rods and cones.

Cones help you see color in bright light. Rods are extremely sensitive and help you see in dim light.

Inside these cells are special molecules that change shape when they're hit by a photon.

When a photon arrives, it's absorbed, just like at the wall.

But instead of only making atoms jiggle, its energy flips one of these molecules into a new shape.

That tiny change sets off a chain reaction inside the cell, sending an electrical signal along the nerves to your brain, so the photons that reach your eyes don't bounce back out, they're absorbed and destroyed, and their energy becomes a signal.

In a way, every photon you see gives up its life so that you can see it.

Rods are so sensitive that experiments have shown they can respond to a single photon.

In a dark enough room, after your eyes have adjusted, people can sometimes detect flashes made of just a handful of photons.

Now, you might have noticed that after you switch off a bright light, you sometimes still see a faint glowing shape for a moment. That's called an after -image, but it isn't made of leftover photons. The photons are long gone.

It's your eyes' cells, recovering from being so heavily stimulated, still sending signals to your brain for a short while.

So the darkness after you flip the switch really is dark. But is it really empty? Not quite. Here's something remarkable.

Even after you switch off the light, your room is still full of photons. You just can't see them.

Remember, when the photons from your lamp were absorbed, they made the atoms in your walls jiggle a little faster. And jiggling atoms do something interesting. They give off light of their own. Anything that has a temperature is constantly giving off photons.

Very hot things, like a stove ring or the filament in an old bulb, give off visible light. That's why they glow red, orange or white.

But cooler things, like your walls, your furniture and your body, give off a different kind of light called infrared.

Infrared light is just like visible light, except its waves are longer and its photons carry less energy. Our eyes can't detect it, but we can feel it as warmth.

When you hold your hands near a radiator or a campfire, much of the warmth you feel is infrared light hitting your skin.

So in a dark room, every surface is glowing with infrared photons, all the time.

room. The walls are glowing, the floor is glowing, and so are you.

In fact, your skin, at a little over 30 degrees Celsius, is constantly giving off hundreds of watts of infrared light.

You don't cool down to room temperature because the walls are glowing right back at you, giving you almost as much as you give off. This is exactly how night vision thermal cameras work.

They don't need any light at all. They see the infrared glow that every warm object gives off.

That's why firefighters can use them to find people in smoke -filled rooms, and why rescue teams can spot people at night from a helicopter. And here's the connection to our story.

The energy from your lamp's photons didn't just disappear into the walls, it turned into heat, and that heat is now being given off again as infrared photons.

In a sense, your lamp's light hasn't fully gone. It has been transformed into a different kind of light, one your eyes can't see.

The darkness in your room isn't empty, it's filled with an invisible glow.

There's one kind of material that seems to break the rule that light vanishes instantly.

You probably had some as a child, glow in the dark stars on the ceiling, or a watch with glowing hands, or a toy that shines softly after the lights go out.

So do these things somehow catch photons and store them, then let them out later? Not quite. But they do something almost as clever.

Glow in the dark materials contain special substances, often crystals made with a material called strontium aluminate.

When light hits them, the photons are absorbed, just like in a wall.

But instead of their energy turning straight into heat, it pushes electrons in the crystal up into higher energy positions. And here's the trick.

In these materials, the electrons get stuck.

They fall into little traps, a bit like a ball landing in a small dip halfway up a hill.

They can't easily fall back down.

Over time, the gentle jiggling of the atoms around them gives the trapped electrons a nudge, and one by one they escape from their traps.

As each one falls back down to its normal position, it releases its extra energy as a brand new photon. That's the glow.

So the photons you see glowing from the stars on your ceiling aren't the same photons from your lamp. Those were destroyed when they were absorbed.

These are new photons, made from energy that was stored in the material and released slowly, over minutes or even hours.

It's like the difference between catching rain in a bucket and using rain to fill up a water tower that slowly drains out through a tap. tap.

The water that comes out of the tap is new, but it was made possible by the rain that fell earlier. So even here, photons don't really linger.

Their energy does, and the material turns that energy back into light, slowly, long after the lamp has gone out. So far we've followed the photons that stayed in the room. But what about the ones that got away?

If your room has a window, some of the photons from your lamp never hit a wall at all. They fly straight through the glass and out into the night.

Glass is transparent because it lets most visible photons pass straight through without absorbing them. Once outside, the photons keep going in a straight line.

Many of them hit something nearby, a tree, a neighbor's wall, the road.

Some get scattered by tiny particles of dust and water in the air.

That's part of why the night sky above a city has a faint orange glow called light pollution.

It's the light from millions of lamps and street lights, bouncing off the air above us.

But some of those photons are aimed upward inward, with nothing in their way.

They pass through the atmosphere, getting thinner and thinner, and within a fraction of a millisecond, they're out in space.

In fact, this is how astronauts and satellites can photograph Earth at night.

When you look at those famous images of glowing cities spread across the continents, you're seeing photons that escaped from street lights, homes and windows and traveled up into space.

Some of the light from your lamp, on some night, might have been part of an image like that.

And once a photon makes it into space, something remarkable happens.

There's almost nothing out there for it to hit.

Space is incredibly empty, so the photon just keeps going. It passes the moon in just over a second.

It crosses the orbit of Mars in a few minutes. Within hours it's passed the outer planets.

And within about a day, it passes Voyager 1, the most distant spacecraft humans have ever sent.

In a single day, the light from your lamp can travel further than anything we've ever built has managed in nearly 50 years. And it doesn't stop there.

As we saw in our light -year video, the space between the stars is unimaginably empty.

A photon heading out of the solar system could travel for years, centuries or even millions of years without hitting a single thing.

After about four years, your photon would pass the distance of the nearest star.

After a hundred thousand years, if it were heading across our galaxy, it could cross the entire Milky Way.

And if it slips out of the galaxy into the vast gaps between galaxies, it could keep traveling for billions of years. Photons don't get tired, they don't wear out.

In empty space, a photon will keep going until something finally absorbs it.

That might be a cloud of gas, a planet, a star, or, very rarely, the eye or telescope of someone far away.

We know this happens because we see it every day.

When you look at a distant star, you're catching photons that left it years, centuries or thousands of years ago.

When astronomers look at the most distant galaxies, they're catching photons that have been traveling for over 13 billion years. And the oldest light of all fills the whole universe.

Shortly after the Big Bang, the universe was filled with a hot glowing gas.

When it cooled enough, about 380 ,000 years after the beginning, light could finally travel freely.

Those photons have been traveling ever since. Today, they're everywhere.

A faint glow from every direction in the sky, called the cosmic microwave background.

Some of those photons are passing through your room right right now.

But there's one thing that does change for a photon on a long journey.

The universe is expanding, and as space stretches, it stretches the light traveling through it too.

The photon's wave gets longer, and it loses energy.

Visible light slowly shifts toward red, then infrared, then microwaves. That's called redshift.

So if a photon from your lamp escaped into deep space and traveled for billions of years, it would slowly fade into a longer, weaker wave.

But it would still be out there, crossing the cosmos, carrying a tiny trace of the moment you switched on a light.

As a child, you might have wondered if you could trap light in a jar and keep it for later. Close the lid while the lamp is on, then open it in the dark.

It never works, because the inside of the jar absorbs the photons within a tiny fraction of a second, just like the walls of your room.

But scientists have found clever ways to hold on to light, at least for a while. One way is with mirrors. mirrors.

The best mirrors used in labs can reflect more than 99 .999 % of the light that hits them.

Put two of them facing each other, and a photon can bounce back and forth between them hundreds of thousands of times before it's finally absorbed.

That can keep light trapped for around a thousandth of a second, which is a very long time for a photon. Another way is even stranger.

In our video on stopping time, we met the physicist Lene Hao, whose team slowed light down to the speed of a bicycle inside an ultra -cold cloud of atoms.

In 2001, her team went further and brought a pulse of light to a complete stop, held it, and then released it again.

What really happens is that the light's information is copied into the atoms, almost like writing it down, and then later used to recreate the light.

Using a similar idea, a team in China reported in 2021 that they had stored light's information inside a special crystal for about an hour. These experiments aren't just curiosities.

They could help build future quantum computers and ultra -secure communication networks, where information carried by light needs to be stored and released on command.

So let's follow the energy from your lamp all the way to the end. Electricity flows into the bulb and becomes photons.

Within about a millionth of a second of switching off, almost all of them are absorbed by your walls, your furniture, and you. Their energy becomes heat.

That heat is given off again as invisible infrared photons.

And those photons are absorbed and given off again, passing the energy around, until eventually some of it leaks out of your house, warming the air outside.

And what happens to the heat of the whole earth? Our planet is constantly giving off infrared light into space.

In our entropy video, we saw that the earth receives energy from the sun as bright, concentrated sunlight, and sends it back out as many more low -energy infrared photons, spreading in every direction.

So, in the end, the energy from your lamp joins that same flow.

A small part of it eventually leaves the Earth as a faint infrared glow, heading out into the universe. Here's a thought to leave you with.

Every time you switch off a light, you're watching a tiny version of the universe's own story.

Light is created, it travels, it bounces, it's absorbed, and its energy spreads out.

Most of it becomes heat, warming the world around you.

A few photons make it into your eyes and let you see.

and a lucky few slip out into space where they may keep traveling for billions of years.

The light doesn't really vanish, it just changes form and carries on.

If you enjoyed this, check out our video on where light gets its speed from.

And next time you switch off the lights, think about the photons racing out of your window into the night. Thanks for watching, see you next time.