the most beautiful idea in physics we can’t prove（原声文稿）

Transcript of 物理学最美的想法，却是我们永远证明不了的 （中文完整文稿）。

About two and a half thousand years ago, a Greek man named Pythagoras noticed something strange.

Plug a string, now press it down exactly half way and plug it again.

Same note, just a bit higher.

Cut it to two thirds and you get another note that sounds just right. Simple ratios, clean music.

So Pythagoras made a hypothesis that anybody would definitely make in this situation. Maybe the whole universe works like that.

numbers, harmony, music.

And so for the next 2000 years we thought Pythagoras was smoking that good stuff.

But then in the late 1960s physicists stumbled onto the math and found out something bizarre to say the least.

Pythagoras might have actually been correct.

By the end of this video you'll understand why this idea is either the most important theory ever written or the most beautiful can tell you which one is correct.

So here's the main problem that his community has been stuck on for about a hundred years.

We have two rule books for reality, and both of them are ridiculously good, such that we can't actually tell which one is more accurate than the other.

Rule book one, and this is the one you might have heard of, is Einstein's general relativity.

Gravity isn't a force pulling you down, it's space and time bending around anything with mass.

It predicted the existence of black holes, it predicted ripples in space itself, which we finally detected in 2015.

Your phone's GPS has to correct for these measurements or your map would drift by about 10 kilometers every single day. Rulebook 2. Quantum mechanics.

The rules for the very very very small electrons, quarks and light.

One of its predictions matches experiment to about one part in a trillion, which basically means it's very very very accurate and hence why every chip in the device you're watching this on works because of it.

So the obvious question is what happens when you need bolt at once?

That question takes us right towards the center of a black hole.

The first instance of the Big Bang, an enormous amount of mass crushed into an impossibly small space.

Gravity matters there, but so does quantum mechanics.

Physicists tried to combine the two, and the answer came out as infinity.

Not a number, infinity.

You ask the equations how likely something is, and they'll tell you it's infinity percent. No, no, seriously, I actually mean that.

And a big part of the reason is something we take for granted. Particles are treated as points, zero size.

When you try to calculate gravity between two things with no size at all, the numbers literally run away from you. So what if they aren't points?

Let's zoom in. Past your skin.

Past a cell. Into a single atom.

An atom is almost entirely empty.

If it were a football stadium, the nucleus would be a P on the center spot.

And now let's zoom in even further into a proton, three quarks, and this is roughly where our experiments stop.

String theory says, keep going, way, way, way, way down to something called the Planck length, about 10 to the minus 35 meters.

To put that in perspective, if you blew a single atom up to the size of the entire observable universe, a plank length would be about as tall as a tree.

And down there, string theory says that you find a string of energy vibrating on its own.

This is the picture Michio Kaku builds on in the God equation.

And the easiest way into it is a violin. One string vibrated one way, you get an A. Change how it vibrates, you get a C sharp. The same string, just different notes.

String theory says an electron is one note, a quark is another note, a photon, a particle of light is another.

They aren't fundamentally different things, they're the same thing, vibrating differently.

The particle's mass is how much energy is in the vibration.

Its charge is written into the pattern, Kaku actually has a fascinating way of putting it.

Physics is the set of harmonies you can play on these strings.

Chemistry is the melodies you get when they interact, and the universe, the universe is the symphony. It also fixes our infinity problem.

A string has a size, tiny, but not zero.

You can't crush two strings into a single point, so numbers that used to run off to infinity now settle down.

But honestly, that isn't the strangest part.

The strangest part is something that showed up in the equations themselves.

Let's rewind to 1968, a young physicist at CERN named Gabriele Veneziano is trying to understand the strong force, the thing that glues protons together, and he finds a formula that fits the data almost perfectly. The weird part? It wasn't new.

It was a piece of pure math written by Leonhard Euler more than 200 years earlier, just sitting in plain old textbooks. Nobody understood why it worked, however.

A couple of years later, a few physicists, including Ichiro Nambu, Holger Nielsen and Leonhard Tusken, figured out what the formula was secretly describing. Vibrating strings. Then things went wrong for string theory.

It had started out as a theory of the strong force, the force that holds the nucleus of an them together.

And as that, it lost to a better idea.

And it had an embarrassing habit.

It kept predicting a particle nobody wanted. No mass, a very specific kind of spin.

It showed up in every version of the math, and nobody could get rid of it.

Then, in 1974, two physicists, John Schwartz and Joel Scherck, looked at that unwanted particle and realized what it was.

The Graviton. The particle that would carry gravity. Think about what that means.

For decades people had tried to force gravity into quantum theory and every attempt broke.

And here was a theory where you don't add gravity in. You literally cannot take it out.

Work out the ways the quantum strength can vibrate and one pattern comes out with no mass and a spin of two.

Which is exactly what a graviton, the particle of gravity, should look like.

it falls out of those string equations whether you want it or not.

That's the moment people got hooked. Michio Kakuru was one of them.

In the 1970s he helped create string field theory and in the God Equation he makes the case that strings are our best shot at what Einstein spent his last 30 years chasing.

The one equation that gives you gravity, light, the nuclear forces in all of matter. So So why isn't this just settled? Because there's a catch. Well actually three. Catch one.

Dimensions. For the math to hold together, the strings need ten dimensions. We see four. Up. Down.

Left. Right. Forward.

Back. And time. So where are the other six?

The answer is that they are curled up, unimaginably small.

Picture a garden hole seen from across a field. It looks like a line. One dimension. dimension. Walk up close and you see an ant walking around it.

There was a second dimension the whole time. You were just too far away.

String theory says every point in space hides six of those, curled up far too small to ever notice. Catch two. Too many shapes.

There is a staggering number of ways to curl up those six dimensions. One common estimate is 10 to the power 500.

So each shape gives a different universe, with different particles and different laws.

So critics ask, if the theory allows almost anything, what is it actually predicting?

Catch three, and this is the big one, no evidence. Not yet at least.

Strings are so small that seeing one directly would take a particle accelerator roughly a quadrillion times more powerful than the Large Hadron Collider.

Physicists like Lee Smolin and Peter White have argued for years that theory you can't test isn't really physics yet. And that's a fair point.

The LHC was hoped to find indirect clues, like a new partner particles predicted by an idea called supersymmetry. And so far, nothing came of it.

Gaku's response is that the proof may come sideways.

From dark matter, from gravitational waves, from echoes of the Big Bang.

But that maybe is doing a lot of work, and we still need substantial hard evidence to make it, well, physics.

When Michio Kaku was 8 years old, he saw a photo in the newspaper.

It was of a desk. The man who owned it was Albert Einstein. And he had just passed away. On that desk was an unfinished manuscript.

Einstein had spent the last decades of his life trying to find one equation that explained everything. He just couldn't get there in time.

Kaku had said that that that picture stayed with him.

He wanted to know what was so hard that even Einstein himself couldn't finish it, and he spent his whole career on that question.

So here's where we actually are.

We have an idea that's beautiful, that fixes a problem nothing else has fixed, that hands us gravity for free, and that right now we actually have no way to check if it's actually real or not.

Maybe in 50 years it sits in textbooks next to Newton and Einstein, Maybe it becomes a footnote, a gorgeous piece of math that just isn't how the universe works.

But if you took nothing away from this video, take this away.

Sit with this for a second, because this is probably the best way I can explain how bizarre this theory actually goes.

Every atom in your body, every bit of light hitting your eyes right now, if string theory is right, all of it, you, this screen, the light in between, is the same tiny new thing, vibrating in different ways.

Pythagoras looked at a string and thought he could hear the universe.

Two and a half thousand years later, we still can't say if he was right.

We just know the music hasn't stopped.