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Quantum mechanics is weird and seems a bit...complicated. But understanding it can help us to understand the universe.

Hosted by: Hank Green

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Sources:
https://upload.wikimedia.org/wikipedia/commons/6/6e/Solvay_conference_1927.jpg
https://journals.aps.org/pr/abstract/10.1103/PhysRev.85.166
https://journals.aps.org/pr/abstract/10.1103/PhysRev.85.180
https://www.cambridge.org/core/books/speakable-and-unspeakable-in-quantum-mechanics/E0D032E7E7EDEF4E4AD09F458F2D9DB7
https://archive.org/stream/TheBornEinsteinLetters/Born-TheBornEinsteinLetters_djvu.txt
https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-reasons-quantum-entanglement-doesnt-allow-faster-than-light-communication/#6bf749a23a1e


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Thanks to Brilliant for sponsoring SciShow. [ ♪INTRO ].

I’ve said it before and I’ll say it again: quantum mechanics is weird. It also hard to make videos about, but we’re doing it!

It’s the physics of the super-tiny, and it’s built around the idea that energy isn’t a smooth spectrum — it can only come in set amounts. But as weird as the implications of that are, it’s stood up to every experimental test thrown at it. And as we’ve learned more about it, we’ve got better at using it to solve problems and invent all kinds of useful things, like lasers and semiconductors.

Although we understand how to use the math behind the theory, understanding what it means has proven to be a difficult challenge. There are a number of different interpretations of quantum mechanics out there, each of which looks at the theory differently. They all make the same predictions about what quantum mechanics looks like on the surface — so, what will actually happen in experiments in the labs — but the math, and the meaning of the math, can look very different.

There’s a lot going on under the hood. One of the more controversial interpretations is called pilot wave theory. At first glance it looks appealing: it allows you to get around the uncertainty and randomness that quantum mechanics is famous for.

But there’s a catch: getting rid of the randomness involves breaking reality in other ways. The conventional interpretation of quantum mechanics is called the Copenhagen interpretation, after the institute where it was devised in the 1920s. It includes a lot of the more well-known ideas around quantum mechanics, like that something can be a particle and a wave at the same time.

It also says that certain things, like which way an electron is spinning, aren’t really set until you observe them — and until you do, the electron is spinning in both directions at once. This concept, that particles are in multiple states at the same time, is known as superposition, and it’s what inspired Erwin Schrödinger’s famous dead-and-alive cat. Schrödinger was one of the pioneers of quantum mechanics, and his cat-in-a-box thought experiment was actually meant to show that some implications of the theory were just … ridiculous.

But it turns out that poor cat was actually a good illustration of superposition — and that, yes, a lot of quantum mechanics makes no sense when you try to apply it to the larger world we’re more familiar with. In the thought experiment, you hide a cat in a box with a flask of deadly poison. But the poison will only be released if a radioactive atom decays, which has, say, a 50% chance of happening.

Radioactive decay is a quantum mechanical process. Whether or not it happens to an individual atom is exactly the kind of random event that the mainstream interpretation of quantum mechanics says is entirely unpredictable. You cannot predict it.

Before you open the box, you have no idea whether or not the atom decayed, and therefore no idea whether the poison was released or not. Normally, if there was a 50/50 chance a cat was alive, you’d say the cat was either alive or dead — you just wouldn’t know which. But here’s where the Copenhagen interpretation is different from regular probability.

Before it’s observed, the atom is in a superposition of decayed and not-decayed — meaning it’s both at the same time. So the cat would be both alive and dead. Once you open the box, you turn the superposition into one state you actually observe — let’s say alive.

No kitties were harmed in the making of this episode. But opening the box doesn’t tell you that the cat was always alive that whole time. Before you opened the box, its true state was the superposition of alive and dead.

Now, superpositions don’t appear in everyday life, but according to the Copenhagen interpretation, on the tiny scale of particles, they’re everywhere, which is obviously very weird. But the idea fits every experiment we’ve ever done, and over time, physicists have come to accept that reality is sometimes … kind of blurred. That’s what pilot wave theory tries to fix.

The theory was first proposed in 1927 by another pioneer of quantum mechanics: Louis de Broglie. It was shelved until the 1950s, when David Bohm rediscovered and improved it. Today it’s also known as the de Broglie-Bohm theory.

It works by distinguishing between particles and waves, instead of treating them as the same thing like the Copenhagen interpretation does. In pilot wave theory, there are still particles and waves, but they exist separately: there’s an equation that gives you a particle’s velocity, and that equation depends on the wave. The wave interacts with the particle by guiding the way it moves — or pilots it, in other words.

That’s where the name “pilot wave” comes from. This wave spans the entire system you’re looking at, whether that’s just a few electrons or the whole universe. Because of this central wave, the properties of matter are set before you observe them, instead of being superimposed.

You may not have all the information you need to figure out what those properties are in advance, but the information is out there. It’s a little bit like flipping a coin, then covering it with your hand. You know it’s either heads or tails, even before you look at it, because it spun a set number of times in the air before it landed.

It was too fast for you to follow the spinning with your eyes, but if you had a high-speed camera or something you’d be able to figure it out. The Copenhagen interpretation says that type of information doesn’t exist for something like radioactive decay, so until you observe it, an atom can be both decayed and not decayed at the same time — and Schrodinger’s cat can be both alive and dead. Pilot wave theory says the information does exist.

Like with the coin, you may not have access to the knowledge that would tell you whether the atom decayed — so on a practical level, we can’t really use the math to figure it out. But the information is out there, and the atom is one or the other, not both. That’s a little closer to how we perceive reality in everyday life, so that is nice.

The problem is, there’s a tradeoff: in a very intrinsic way, pilot wave theory breaks a different, really big rule in physics: locality. Locality is the idea that everything in the universe can only ever affect its immediate surroundings. You can’t interact with something far away without sending some kind of signal to it, and that signal needs to be transmitted through the space between you and that thing.

Most importantly, this means that all signals take time to travel. That’s why you see lightning before you hear thunder. We also know that there should be an upper limit to how fast these signals can move:.

Einstein worked out that the universe’s speed limit is the speed of light. Now, the Copenhagen interpretation actually does violate locality in certain situations. For example, you can generate two electrons in a way that means they must have opposite spins.

Until you actually observe their spins, though, each electron is in superposition, spinning in both directions at the same time. But if you send the two electrons away from each other, wait until they’re really far apart, and observe one right after the other, you’ll always find that they spin in opposite directions. That’s true even if there’s no time for a signal to pass from the first to the second without moving faster than the speed of light.

Einstein really did not like this, but we’ve tested it, and we know it happens. If you’re going by the Copenhagen interpretation, that means somehow the electrons are affecting each other faster than the speed of light. Still, at least in the Copenhagen interpretation violating locality is the exception rather than the rule.

Pilot wave theory, on the other hand, is entirely based on the idea that locality does not need to be a thing, and particles can affect each other instantaneously even if they’re light-years apart. That’s the whole point of the pilot wave. All particles in a system are tied to each other through that one wave, so by extension, all particles in the universe affect each other — without the time delay you’d get if there were signals traveling at the speed of light.

In other words, if you say the information that would tell you whether the cat is alive or dead is embedded in the rest of the particles in the universe, you’re also saying those particles somehow affect each other faster than the speed of light. And unlike the Copenhagen interpretation, it’s not just certain situations that have this problem — it’s everything, everywhere, always. So, uh, some physicists take issue with that.

Still, pilot wave theory is a good reminder that our theories of physics aren’t just guided by what’s really happening, but also by aesthetic and pragmatic choices. If we go with the Copenhagen interpretation, then we need to accept some really weird things, like cats that are simultaneously dead and alive. We also need to accept that some things are just unknowable.

But if we subscribe to pilot wave theory, then we need to accept that some effects may be truly non-local. Both choices have their pros and cons, and for the most part, physicists have decided that Copenhagen makes more sense. Pilot wave theory has seen a bit of a renaissance in recent years, though, thanks to computer simulations.

In the real world we don’t have access to faster-than-light signals, but on a computer, we can pretend that we do. So physicists are using the math behind pilot waves to do quantum mechanics simulations, which could be an improvement on conventional methods for some things. And it’s not just Copenhagen or pilot waves — there are other interpretations of quantum mechanics out there, too.

The history of physics is full of people coming up with weird, impossible-sounding ideas that happen to be right. It’s also full of ideas that happen to be wrong, but are still useful in lots of ways. So even if we never discover whether this theory is actually how the universe works, there’s a lot we can learn just by exploring it to see what it can do.

And if you enjoy exploring theories like this, I think you’ll like the Quantum Computing course on Brilliant.org, where you’ll learn about the laws of quantum mechanics by building your own quantum circuit and racing a classical computer in solving code-breaking puzzles. You can check it out at Brilliant.org/SciShow, and right now, the first 200 people to sign up at that link will get 20% off of an annual premium subscription to Brilliant. So head to Brilliant.org/SciShow to learn more, and know that when you do, you’re also helping to support SciShow, so thanks! [ ♪OUTRO ].