If Energy Cannot Be Created...Where Does It Actually Come From?（原声文稿）

While you watch this you are spending energy. Your heart is squeezing roughly once a second. Your lungs are hauling air in and out.

Your brain, which weighs about one and a half kilos, is quietly burning through around a fifth of everything your body uses at rest.

Even sitting perfectly still, you're giving off a gentle, constant warmth.

Add all of that up across a whole day, and a typical adult uses about as much energy as a 100 watt light bulb that's been left on for 24 hours straight. That's a lot of energy, energy. And every last bit of it had to come from somewhere.

Because there's a rule in physics that, as far as anyone has ever tested, has never once been broken here on earth.

Energy doesn't pop into existence. You can't make it out of nothing. So where did yours come from? The obvious answer is food. But that just pushes the question back one step.

Where did the food get it? And where did that come from? And before that. So I decided to treat this like a case. Pick up the trail with the energy inside you right now.

now, follow it backwards one step at a time, and refuse to stop until there's nowhere left to go.

The trail runs through a breakfast plate, a leaf, the heart of the sun, and the first few minutes of the universe, and then it reaches a place where the rule itself starts to crack. Let's open the case.

Before we chase anything, we need to be clear about the rule we're relying on.

In the 1840s, an English brewers son named James Prescott Jewel became obsessed with heat.

He set up an experiment where a falling weight turned a little paddle wheel inside a tank of water.

As the paddles churned, the water warmed up, by a tiny but measurable amount.

What Joule found was that the amount of heat always matched the amount of falling.

Drop the weight twice as far, and you get twice the warming. Motion and heat weren't separate things.

They were the same thing. Swapped from one form to another at a fixed rate.

The story even goes that he took a thermometer on his honeymoon in Switzerland, hoping to show that water at the bottom of a waterfall is slightly warmer than at the top.

Around the same time the German scientists Julius Mayer and Hermann von Helmholtz reached the same conclusion.

Together their work became what we now call the conservation of energy.

In an isolated system, the total amount of energy never goes up and never goes down.

It just changes hands.

Today, the unit we use to measure energy is named after joule.

One food calorie, the kind printed on a packet of biscuits, is about 4 ,200 joules. Here's the easiest way to think about this rule.

Imagine energy is money. In a country where nobody, not even the government, is allowed to print new notes.

Money can move from one account to another.

It can be swapped into a different currency, like euros into dollars. But the total amount of money in the country never changes. Energy works the same way.

Motion, heat, light, electricity, and chemical energy are just different currencies. And the exchange rates between them are fixed. That gives us a powerful tool.

If money can't be printed, then every dollar in your pocket has a history. Somebody paid it to you, somebody paid them.

You can follow the money backwards, transaction by transaction. So that's exactly what we're going to do with your energy. First stop. The last thing you ate.

Let's say the last thing you ate was a bowl of rice and a banana. Simple. Ordinary.

And packed with energy. But where, exactly, is the energy in a grain of rice?

It isn't hot, it isn't moving, it doesn't look like it's doing anything. The energy is hiding in the way its atoms are arranged. Rice is mostly starch, which is just long chains of sugar.

A sugar molecule is made of carbon, hydrogen and oxygen atoms, clipped together in a particular pattern.

And it turns out that this pattern, sitting next to the oxygen in the air, holds more energy than the pattern those same atoms end up in afterwards. Carbon dioxide and water.

When the atoms switch from the first arrangement to the second, the difference is released. That's the same reaction that happens when you set sugar on fire.

The atoms rearrange and the spare energy pours out as heat and light. Your body does exactly the same thing, but far more carefully.

Instead of one big flash, it takes the reaction apart into dozens of tiny steps, inside little structures in your cells called mitochondria.

At each step, it catches a small amount of energy and stores it in a molecule called ATP. In our money picture, a sugar molecule is a big banknote.

Your cells break it into loose change, ATP, which they can spend a little at a time on every heartbeat, every thought, and every twitch of a muscle.

You recycle so much of this loose change that, over a day, the total ATP you make and spend weighs roughly as much as you do.

So the energy inside you came from the arrangement of atoms in your food. Case solved? Not even slightly.

Because now we have to ask who arranged those atoms that way in the first place.

Somebody had to pay to build that sugar, and if you eat meat or eggs, it doesn't change anything. That animal got its energy from what it ate.

Follow almost any food chain on earth back far enough, and it ends in the same place. A plant. A rice plant doesn't eat. It has no mouth and no stomach.

Yet somehow it builds sugar out of almost nothing.

Its ingredients are about as cheap as it gets. Carbon dioxide from the air, water from the soil.

Those are the same spent molecules your body breathes out and sweats away after using up its food. On their own, they hold no spare energy at all.

To turn them back into sugar, the plant has to pay.

And the payment arrives from 150 million kilometers away, sunlight.

Inside each leaf are millions of tiny green structures packed with a pigment called chlorophyll. Chlorophyll is very good at catching red and blue light.

It's bad at catching green, which is why leaves bounce green light back to your eyes.

When chlorophyll catches light, it uses that energy to rip water molecules apart.

The hydrogen is kept for building sugar.

The oxygen is thrown away as waste, out into the air. That waste is the oxygen you're breathing right now.

Then the plant uses the captured energy to stitch carbon dioxide and hydrogen together into sugar. It's the reaction in your mitochondria, run in reverse.

Your cells take sugar apart and release energy.

The leaf puts sugar together and locks energy in. It's not a very efficient deal.

A typical crop locks away only around 1 % of the sunlight that lands on it. But there's so much sunlight hitting Earth that it doesn't matter.

In roughly one hour, the planet receives about as much solar energy as all of humanity uses in an entire year. And it isn't just your food.

Coal is the buried remains of ancient forests, squeezed underground for hundreds of millions of years.

Oil and natural gas are mostly made from tiny ocean organisms that also lived on sunlight.

When you burn petrol, you're releasing sunlight that landed on Earth before the first dinosaurs. wind is the sun too.

It heats some parts of the planet more than others and the air moves to even things out.

Even a hydroelectric dam is solar powered, the sun lifts water into clouds and the rain fills the reservoir high up in the hills.

So almost every trail leads to the same suspect. Almost.

A good investigator checks the cases that don't fit and there are a few kinds of energy on earth that never pass through a leaf or through today's sunshine at all.

First, nuclear power. A nuclear power station runs on uranium.

When a The Uranium atom splits, it releases a huge burst of energy, but Uranium wasn't made on Earth.

It was forged in some of the most violent events in the universe, like exploding stars, and collisions between the incredibly dense remains of dead stars.

That happened long before our solar system existed.

The energy was locked into those heavy atoms back then, and we're only cashing it in now. Second, the heat under your feet. Earth's core is roughly as hot as the surface of the sun.

Part of that heat comes from radioactive atoms deep inside the planet, slowly breaking down.

The rest is left over from when Earth formed, as rocks smashed together and gravity pulled everything inward.

That's the energy behind volcanoes, hot springs and geothermal power plants. It even feeds life.

At the bottom of the ocean, where sunlight never reaches, there are vents gushing hot, mineral -rich water.

Whole communities of bacteria, worms and crabs live there, fueled by chemical energy from inside the earth instead of the sun. Third, the tides.

The moon's gravity tugs the oceans into bulges, and earth spins underneath them.

Tidal power plants capture some of that motion, but that energy has a hidden cost. It's paid for by earth's spin.

The tides act like a very gentle break, so our days are getting longer by a couple of thousands of a second every century.

So these trails really do skip today's sun, but look at where they lead, to exploding stars, to the gravity that pulled the planet together.

To the moon and earth forming out of the same dust cloud, every one of them still goes back to stars and gravity.

So let's go back to the brightest star we know, and look at what's actually happening inside it.

The sunlight that grew your rice left the surface of the sun about 8 minutes before it reached the leaf.

But the surface isn't where its energy was made.

The energy was made deep in the core, and getting out of there is a nightmare.

The inside of the sun is so crowded that energy gets absorbed, sent off in a random direction absorbed again and bounced around over and over.

It can take tens of thousands of years, possibly far longer, for energy made in the core to finally reach the surface, so the sunshine on your skin today began its journey before humans had invented farming. Now let's look inside the core itself. It's about 15 million degrees Celsius.

The pressure is so extreme that atoms are stripped bare.

Hydrogen nuclei, which are just single protons, are slammed into each other again and again.

Then, normally protons push each other away because they all carry a positive charge.

But in the core, a tiny fraction of them collide hard enough to stick.

Through a chain of steps, four hydrogen nuclei end up fused into one helium nucleus. This is nuclear fusion. Here's the strange part.

If you weigh the helium that comes out, it's slightly lighter than the four hydrogen nuclei that went in. About 0 .7 % of the mass has simply gone missing.

It hasn't vanished. It has been turned into energy.

energy. In 1905, Albert Einstein showed that mass and energy are two forms of the same thing.

That's the famous equation E equals mc squared.

The c stands for the speed of light, which is an enormous number, and squaring it makes it far bigger still.

So even a tiny bit of mass is worth a staggering amount of energy.

If you could turn a single gram of mass completely into energy, it would be enough to run an ordinary household for well over a thousand years. Back to our money picture. Mass is the most valuable currency in the universe.

The tiniest coin of it is worth a fortune in heat and light, and the sun is spending it fast.

Every second, it converts about 4 million tons of its own mass into energy. But how did this furnace get lit?

Before the sun existed, there was just a huge cold cloud of gas and dust. Gravity pulled it inward. As it collapsed, it heated up.

up, the way a bicycle pump warms up when you squeeze the air inside. Eventually the centre got hot and crushed enough for fusion to start.

So the sun was switched on by gravity, and it runs on hydrogen.

Which raises the next question, where did all that hydrogen come from?

Stars can build heavier elements out of lighter ones, but no star ever made hydrogen. Hydrogen is the raw material.

To find where it came from, we have to go back almost to very beginning.

About 13 .8 billion years ago, everything in the observable universe was packed into an incredibly hot, dense state. Then it began to expand and cool.

That expansion is what we call the Big Bang.

In the earliest moments, the universe was far too hot for anything like atoms. It was a blazing soup of energy and particles.

But as it spread out and cooled, things began to settle.

Within about the first second, protons and neutrons had formed.

A proton on its own is the nucleus of a hydrogen atom.

For the next few minutes, the whole universe was hot enough to act like the core of a star.

Some of those protons and neutrons fused into helium, then the universe cooled too much for fusion to keep going, and the process froze.

What it left behind was a universe made of roughly three -quarters hydrogen and one -quarter helium, by mass. That's still almost exactly the mix we see in space today.

So the hydrogen burning in the Sun right now was made in the first few minutes of the universe.

universe. And so was the hydrogen inside your rice, inside the water in your cells, and inside you.

A huge share of the atoms in your body are older than the earth, older than the sun, and older than the milky way. Now let's lay out the full trail of the case.

The energy of the early universe got locked into hydrogen, and the pull of gravity.

Gravity squeezed the hydrogen into stars. Stars turned mass into sunlight.

Plants turned sunlight into sugar. You turned sugar into heartbeats and thoughts.

Every step was a transaction, no energy was created, it just kept changing currency.

Which leaves us with one final question, where did the energy of the early universe come from? And here the trail goes cold.

The Big Bang Theory describes how the universe expanded and cooled from that hot, dense state. It doesn't tell us what, if anything, came before it.

If energy can never be created, then it seems like the energy must have simply always been there. But before we accept that, there's something we haven't checked. fact, the rule itself.

Why do we believe energy can't be created in the first place?

For decades after Juul, the conservation of energy was simply something people observed.

Every experiment balanced.

Nobody knew why. The answer came from one of the greatest mathematicians of the 20th century, and most people have never heard her name. Her name was Emmy Noether.

In 1915, she was invited to the University of Göttingen to help untangle the mathematics of Einstein's brand new theory of gravity.

Even so, for years the university wouldn't let a woman teach under her own name. Her lectures were advertised under a male colleague's name instead.

In 1918, she published a result now known as Notha's theorem, and it reveals where the rule we've been following actually comes from.

Her idea, in simple terms, whenever the laws of nature stay the same under some kind of change, something is conserved.

If the laws of physics work the same here as they do a few meters to the left, then one quantity, called momentum, can never be created or destroyed.

If the laws work the same no matter which way you're facing, then spin is conserved.

And if the laws of physics are the same today as they were yesterday, and as they will be tomorrow, then energy is conserved. That last one is the key. To see why, imagine the opposite.

Imagine a world where gravity got a little weaker every night, and stronger again every morning. You could cheat that world. world.

At night, when gravity is weak, you lift a heavy rock onto a high shelf. It's easy, so it costs you very little energy.

Then, in the morning, when gravity is strong again, you let the rock fall onto a generator. It falls harder, so it pays you back more than you spent.

Repeat that every day, and you've built a machine that makes free energy out of nothing.

The only reason you can't do this in our world is that the rules don't change over time.

Gravity at midnight is the same as gravity at noon.

So there's no way to gain the timing, and energy always balances. That changes how we should think about the whole case.

Energy conservation isn't a separate law that someone wrote down for the universe, it's what you get automatically when the rules stay fixed over time.

So the real question isn't whether energy can be created, it's this, does the universe as a whole stay the same over time?

And the answer is no, in 1929 the astronomer Edwin Hubble noticed something odd, almost every distant galaxy is moving away from us, and the further away a galaxy is, the faster it's going.

The reason isn't that we're at the center of some giant explosion, it's that space itself is stretching. Picture a loaf of raisin bread rising in the oven.

Every raisin moves away from every other raisin, not because they're crawling through the dough, but because the dough between them is growing. Now think back to Emmy Noether.

Energy is conserved when the rules and the stage they play out on stay the same over time. But in an expanding universe, the stage itself is changing.

The universe today is not the same as the universe a billion years ago. It's bigger, colder, and more spread out.

So on the scale of the whole universe, the condition that guarantees energy conservation simply isn't met. And we can see the effects.

The oldest light in the universe was released about 380 ,000 years after the Big Bang, when the universe first cooled enough to become see -through.

Back then, that light was a hot glow, like the light from a fire.

fire, but as space stretched, the light waves stretched with it.

Today, the waves are about 1 ,100 times longer.

That ancient fire has faded into faint microwaves, which we can still detect coming from every direction in the sky.

Longer waves carry less energy. Each bit of that light has lost most of its energy. So where did the energy go? Nowhere. It wasn't handed to anything.

There's no account it was paid into. Then there's the opposite case.

space. In 1998, two teams of astronomers discovered that the expansion of the universe is speeding up.

Whatever is driving it has been named dark energy, and it seems to have the same amount of energy packed into every cubic meter of space.

As the universe expands, there are more cubic meters, so the total amount of dark energy keeps growing. Physicists argue about how to describe this.

Some say energy simply isn't conserved for the universe as a whole.

Others say that the total energy of the whole universe isn't something you can even properly define.

But they agree on what matters. Every small, local transaction still balances perfectly. It's only the universe as a whole that doesn't keep a single, tidy set of books.

In our money picture, every shop and every bank still balances its accounts. There's just no central ledger for the entire universe. There's one more suspect in this case.

It's the strangest idea of all, and it's important to be clear. It's an idea, not a proven result. felt. It starts with gravity.

When two objects fall towards each other, they speed up. That motion energy has to come from somewhere.

Physicists account for it by giving gravity a kind of negative energy.

The tighter things are pulled together, the more negative that energy becomes. Now picture a beach. You dig a deep hole and pile all the sand up next to it.

You now have a big, impressive mound, but you haven't created any sand. The pile is matched exactly by the hole. Add them together, and you get zero.

Some Some physicists have suggested the universe might work the same way.

All the matter and energy in it would be the pile, a huge positive amount.

All the gravity holding it together would be the whole, a huge negative amount. And when you add them up, the total might come out to exactly zero.

If that's true, then the universe never needed any energy to begin with.

It could, in principle, have appeared without breaking the rule at all, because the total never changed from nothing.

There's even a hint that points in this direction.

Careful measurements show that, on the very largest scales, space appears to be almost perfectly flat.

In a simplified picture of the expanding universe, flat is exactly what you'd expect if the energy of everything flying apart is perfectly balanced by the pull of gravity holding it back.

The physicist George Gammo loved to tell a story about this idea.

While walking through Princeton with Albert Einstein, he mentioned a colleague's suggestion that a star could form out of nothing, because its negative gravitational energy would cancel out the energy of its mass.

Einstein was so startled that he stopped dead in the middle of the road. Cars had to stop to avoid hitting him.

In 1973, the physicist Edward Tryon took this further and asked, in a scientific paper, whether the entire universe could have started as a tiny, random flicker from nothing. But this is where we have to be honest. Nobody has proven that the universe's total energy is zero.

Working out the total energy of an expanding universe is exactly the kind of calculation physicists argue about.

And even if the total was zero, it still wouldn't explain where the rules themselves came from. It's a fascinating lead. It's not a closed case.

So let's put the whole file together. The energy keeping you alive right now came from your food. It was stored in the way the atoms in that food were arranged.

A plant arranged them, paying for the work with sunlight.

That sunlight was made from mass, converted in the core of the sun, as hydrogen fused into helium, gravity squeezed that hydrogen together and lit the fire.

And the hydrogen itself was made in the first few minutes of the universe, out of the energy of the hot, dense, early universe.

At every single step, nothing was created. The energy only changed currency.

Along the way, though, most of it slipped out of reach.

The rice plant kept only about 1 % of the sunlight that landed on it.

Most of the sun's light streams off into empty space, and never touches anything at all. And your own body turns much of what you eat into plain warmth.

None of that energy disappeared. It just spread out into forms that are much harder to use. Energy never runs out, but useful energy does.

That's a different law of physics, and a whole other case. Then we reach the edge, and the rule itself changed shape.

We learned that energy conservation comes from the laws of nature staying the same over time, on the scale of the whole expanding universe. That isn't quite true.

True, light from the early universe has lost energy to nowhere, dark energy keeps growing, and it's even possible that the total energy of everything adds up to zero.

So where does energy actually come from?

On Earth, and in every experiment we've ever done, the answer is simple. It always comes from somewhere else.

It's handed on, never made.

But for the universe as a whole, the honest answer is that we don't fully know. And that's not a failure.

It's one of the most exciting open questions in all of science.

Maybe someday someone will crack it, the way Jewel cracked heat and Noether cracked the rule itself.

Both of them started with a simple sounding question and refused to stop asking where it led. Think about that the next time you eat something.

That meal is carrying energy that has passed through a plant, through the sun, and back through 13 .8 billion years of history.

You're not making energy, you're borrowing it, for a little while before passing it on. This case stays open. Thank you for watching, see you in the next one.