00:04To answer this question, humans come up with stories to describe the world.
00:09We test our stories and learn what to keep and what to throw away.
00:14But the more we learn, the more complicated and weird our stories become.
00:19Some of them so much so, that it's really hard to know what they're actually about.
00:25Like string theory. A famous, controversial and often misunderstood story about the nature of everything.
00:32Why did we come up with it? And is it correct? Or just an idea we should chuck out?
00:39To understand the true nature of reality, we looked at things up close and were amazed.
00:44Wondrous landscapes in the dust.
00:46Zoos of bizarre creatures.
00:48Complex protein robots.
00:50All of them made from structures of molecules made up of countless, even smaller things, atoms.
00:57We thought they were the final layer of reality, until we smashed them together really hard and discovered things that can't be divided anymore.
01:05Elementary particles.
01:07But now we had a problem.
01:09They're so small that we could no longer look at them.
01:12Think about it. What is seeing?
01:15To see something, we need light, an electromagnetic wave.
01:19This wave hits the surface of the thing and gets reflected back from it into your eye.
01:24The wave carries information from the object that your brain uses to create an image.
01:30So you can't see something without somehow interacting with it.
01:34Seeing is touching. An active process, not a passive one.
01:38This is not a problem with most things.
01:41But particles are very, very, very small.
01:45So small that the electromagnetic waves we use to see aren't too big to touch them.
01:50Visible light just passes over them.
01:53We can try to solve this by creating electromagnetic waves with more and much smaller wavelengths.
01:59But in quantum physics, shorter wavelengths means more energy.
02:02So when we touch a particle with a wave that has a lot of energy, it gets a kick.
02:06By looking at a particle, we change it.
02:09In quantum physics, we cannot know where a particle is and where it's going with absolute precision.
02:14This fact is so important that it has a name.
02:17The Heisenberg Uncertainty Principle, the basis of all quantum physics.
02:22So what does a particle look like then?
02:25What is its nature?
02:27We don't know.
02:29If we look really hard, we can see a blurry sphere of influence, but not the particles themselves.
02:35We just know they exist.
02:38But if that's the case, how can we do any science with them?
02:42We did what humans do and invented a new story, a mathematical fiction.
02:47The story of the point particle.
02:50We decided that we would pretend that a particle is a point in space.
02:54Any electron is a point with a certain electric charge and a certain mass, all indistinguishable from each other.
03:01This way, physicists could define them and calculate all of their interactions.
03:06This can be made precise in quantum field theory, and this solved a lot of problems.
03:11All of the standard model of particle physics is built on it, and it predicts lots of things very well.
03:17Some quantum properties of the electron, for example, have been tested and are accurate up to 0.000000000000002%.
03:28So while particles are not really points, by treating them as if they were, we get a pretty good picture of the universe.
03:35Not only did this idea advance science, it also led to a lot of real-world technology we use every day.
03:42But there's a huge problem, gravity.
03:45In quantum mechanics, all physical forces are carried by certain particles.
03:50But according to Einstein's general relativity, gravity is not a force like the others in the universe.
03:57If the universe is a play, particles are the actors, but gravity is the stage.
04:03To put it simply, gravity is a theory of geometry, the geometry of space-time itself, of distances which we need to describe with absolute precision.
04:13But since there is no way to precisely measure things in the quantum world, our story of gravity doesn't work with our story of quantum physics.
04:21When physicists tried to add gravity to the story by inventing a new particle, their mathematics broke down.
04:28And this is a big problem.
04:31If we could marry gravity to quantum physics and the standard model, we would have the theory of everything.
04:37So very smart people came up with a new story.
04:40They asked, what is more complex than a point, a line, or a string?
04:46String theory was born.
04:48What makes string theory so elegant is that it describes many different elementary particles as different modes of vibration of the string.
04:56Just like a violin string vibrating differently can give you a lot of different notes, a string can give you different particles.
05:03Most importantly, this includes gravity.
05:07String theory promised to unify all fundamental forces of the universe.
05:11This caused enormous excitement and hype.
05:14String theory quickly graduated to a possible theory of everything.
05:18Unfortunately, string theory comes with a lot of strings attached.
05:24Much of the maths involving a consistent string theory does not work in our universe with its three spatial and one temporal dimensions.
05:32String theory requires ten dimensions to work out.
05:36So string theorists did calculations in model universes and then tried to get rid of the six additional dimensions and describe our own universe.
05:45But so far, nobody has succeeded and no prediction of string theory has been proven in an experiment.
05:51So string theory did not reveal the nature of our universe.
05:55One could argue that in this case, string theory really isn't useful at all.
06:00Science is all about experiments and predictions.
06:04If we can't do those, why should we bother with strings?
06:07It really is all about how we use it.
06:10Physics is based on maths.
06:12Two plus two makes four.
06:14This is true no matter how you feel about it.
06:17And the maths in string theory does work out.
06:20That's why string theory is still useful.
06:23Imagine that you want to build a cruise ship, but you only have blueprints for a small rowing boat.
06:28There are plenty of differences.
06:30The engine, the materials, the scale.
06:33But both things are fundamentally the same.
06:36Things that float.
06:39So by studying the rowing boat blueprints, you might still learn something about how to build a cruise ship eventually.
06:46With string theory, we can try to answer some questions about quantum gravity
06:50that have been puzzling physicists for decades,
06:52such as how black holes work or the information paradox.
06:56String theory may point us in the right direction.
06:59When used in this spirit, string theory becomes a precious tool for theoretical physicists
07:04and help them discover new aspects of the quantum world and some beautiful mathematics.
07:09So maybe the story of string theory is not the theory of everything.
07:14But just like the story of the point particle, it may be an extremely useful story.
07:19We don't yet know what the true nature of reality is,
07:22but we'll keep coming up with stories to try and find out
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