Name: 量子力学（恥ずかしい） - 六十の記号 (Quantum Mechanics (an embarrassment) - Sixty Symbols)
Uploaded: 2021-01-14T10:31:41.000Z
Duration: 14 min 7 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

there was this gathering, I think, in 2011 fairly recently of experts in quantum foundations, what quantum mechanics really means.

They took a bullet like 16 different questions to ask the experts, and one of them was simply What is your favorite interpretation of quantum mechanics or approach to thinking about what quantum mechanics really means?

And the answer's was that there wasn't even anyone approach that got 50% of the vote.

The results were all scattered across different possibilities.

The traditional Copenhagen interpretation where the quantum state collapses When you look at it, more modern, many worlds interpretation, where the universe splits in two different possibilities.

Interpretations that add something new to quantum mechanics, like random stochastic changes of the wave function or hidden variables that we can't see.

And I said it was extremely embarrassing to physics that here it is 80 years after we sort of established how quantum mechanics works.

We still don't know what it really means.

Well, it's absolutely great that we have unknown questions.

The embarrassing part is, is particular question has been unanswered for 80 years, with very little by the way of immediately demonstrable progress.

Even though it's such an important question, it seems to me that we have not been trying to answer this question with as much vigor as we should.

I mean, what more important question is there than that?

My favorite interpretation is the many worlds interpretation of quantum mechanics.

This goes back to a guy named Hugh Everett, who was a graduate student in the 19 fifties at Princeton.

And he said, You know, he had this brilliant insight that all the complicated, ill defined parts of quantum mechanics they're all having to do with when you measure something when you observe something.

What if we just erase those parts from quantum mechanics?

There's no special role for observation or anything like that.

The trick is that you, the observer or a quantum mechanical system just like anything else.

So what you need to do, Everett said, was just a trace carefully what the world would look like if there were quantum systems interacting with other quantum systems and What he claimed happened is that every time these systems interact with each other, the world splits, You get one world that looks like one answer was achieved.

Another world, which another answer was achieved in these worlds, will never talk to each other ever again.

It bugs people have all those universes going around.

So is this like a car getting to a T junction?

And instead of turning left or right, it splits into two cars.

It's basically like that, Um, there are justice, money, parts of the universe before and after.

It's not like you're creating more stuff in the universe.

It's kind of like This is just analogy is not exactly like this, but it's like there were exactly two copies of the car all along, but they were precisely the same.

And then they diverged when this quantum event occurred.

So they're sort of all sorts of universe is sitting on top of each other, and they're splitting apart of differentiating as time goes on, Is there a finite number of So this is a very simple kind of basic question to which we don't know the answer.

It could be a finite number or an infinite number.

Uh, I say that because if there were only a finite number of possibilities quantum mechanically, then in an infinitely old universe all of those possibilities would happen over and over again, an infinite number of times.

And this relates back to another important question in cosmology.

Why is the past different from the future?

So I think we need an infinite number of possibilities in quantum mechanics to explain a universe that keeps evolving.

And that's what we seem to observe doesn't make sense to you.

You're telling me, and you know it's baffling May.

Actually, it is an amazing consequence of some simple equations, right?

But that's exactly what we should expect when we do physics that we should follow what the equations tell us.

We should take them seriously up until they are not compatible with the data right until they violate experiment.

So quantum mechanics, in order to fit the data back to the 19 twenties people have come for these dramatically different ideas.

You know, you can't make predictions with 100% certainty and so forth, and they boiled it down to a very simple set of equations.

And whatever it said in the 19 fifties is just take those equations seriously.

Use that to become comfortable with the fact that the world you see is a tiny, tiny slice of the whole world that exists to May.

Equations can have a number at the end of them, or a letter at the end of the more combination of letters and numbers.

What could be on the other side of the equal sign that says to you multiple universes?

What's the result that says, there, What's that equation look like?

Well, there's the equation is the Schrodinger equation.

This is the equation that tells you how the quantum state of the universe changes with time, and the point is that ever it suggested that we don't need two different equations to talk about the change of the evolution of the universe.

We only need the Schrodinger equation in the Copenhagen interpretation.

The thing that we teach our students the thing that's in the textbooks.

There's two different ways that the Quantum state evolves.

It obeys Schrodinger's equation when you're not looking at it, and then when you look at it, it suddenly discontinuous.

That bugs me much more than a whole lot of universities, these splitting events these moments, when the universe splits, what causes that?

What counts as an observation that causes this split?

It is the major, I would say, progress we have made over the past 50 years in quantum mechanics since Everett.

What we understand now is that in order to so, Everett said, it's not enough to think about the quantum system.

You also have to think about the observer.

Well, we've said since then is you also have to think about the entire rest of the universe, okay, and we call this the environment and secretly you are interacting with your environment all the time.

And in quantum mechanics that means you are becoming entangled with the environment, your quantum state is becoming correlated with the quantum state of everything else, and that's a process that takes time, and you can tell exactly how long it takes.

And it's that interacting with the environment that leads to this splitting of universes, if there were no environment than universes, would not split.

It's that if I have an electron here, an electron there, a quantum state where it could be there or there even before it look at it, you could say there's a universe in which it's here and there's a universe in which it's there.

But those universes can interact with each other, right?

I mean, they could interfere with each other in a quantum experiment.

But once I look at it and I see the electron there were there and then all of my degrees of freedom, all of my information get entangled with the environment around me.

Then these two possibilities can't interfere with each other anymore.

De coherence has occurred in the technical jargon, and that's something we think we understand on the basis of equations.

When you and I talk, we talk about this in analogies like turning steering wheel of a car going left and right.

But these aren't the events we're talking about.

Are these events or something much smaller, much more fundamental?

Yeah, the kinds of events that lead the universe to split or simply observations of quantum systems.

And that's a little bad way to put it, because observation makes it sound like the existence of a conscious observer is somehow important to quantum mechanics.

And it's completely not that that's an utterly bogus road to go down.

What an observation simply is is bringing one system into contact with another so that what this system is doing becomes entangled becomes related to what this system is doing.

So if this system is spin up, we're spin down and this system is a little arrow.

Then you bring them in contact and the arrow becomes correlated with what the spin is doing.

You've told me where you stand on this issue.

You favor this, you know, the multiple world thing that didn't win the survey did it.