And they took a poll.

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.

That's not the embarrassing part.

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.

What is quantum mechanics really mean?

That's what saying What is the universe?

Really?

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.

And he said, You know what?

What if we just erase those parts from quantum mechanics?

So nothing different happens.

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.

But that's what the math seems to imply.

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

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

Go both ways.

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.

It's probably 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.

You know, the arrow of time.

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.

But is it baffling you?

I think it doesn't make sense.

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.

Just believe them.

They already fit the data.

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 one.

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.

Lee changes.

And that should really bug you.

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

What's the tipping point?

What counts as an observation that causes this split?

There's a very good question.

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.

Light is reflecting off of you.

You're making noise and so forth.

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.

They've gone their separate ways.

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.

That's when the universe splits.

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.

No, it did not.

I'm in the plucky minority on this one, the winner, although again nothing got a majority.

But the plurality 42% of the vote was the Copenhagen interpretation, which is sort of the traditional textbook interpretation, and it's good enough for government work, the Copenhagen interpretation.

That's the one that says that when the cat is in the box and I open it up, and suddenly the state of the cat changes to either being alive or dead, depending on which one I observed.

And if I observe it alive than the part of the cat that was dead ceases to exist, it is erased from the books of reality.

And in that approach, all of these questions we've just been talking about become very, very difficult to answer.

If I open the box.

But I didn't look in, does the that branch of the wave function go away?

What if a virus observes the cat, you know, is the cat observing?

It's all these questions that sort of make our mind's confused.

They're really difficult questions in the Copenhagen interpretation.

In the Everett interpretation, every quantum system is on equal footing.

There's nothing special that causes wave functions to collapse 43% 42% believe it.

And I suspect, you know now I'm I'm going beyond saying true things and saying My personal opinion, which I happen to believe is true.

I think it's just because people haven't thought about it very hard.

Uh, and that's not to say if you think about it very hard, you're led to the many worlds interpretation, but you are lead away from the Copenhagen interpretation when you think about it very hard.

You know, the more people worry about how quantum mechanics really works, the less likely they are to accept the Copenhagen interpretation.

These physicists being so they These are people who do this for their job.

How come they don't think about?

Well, it's the difference between being a race car driver and a mechanic, right?

In order to drive the car really well and really fast, you don't necessarily need to know all of its inner workings.

It might be helpful.

You might make an argument that knowing the best, uh, knowing the car, the best you can makes you a better driver.

But not necessarily right, so working physicists don't always need to understand the interpretation of quantum mechanics.

They just need to know howto work with it, right, how to solve the equations that it gives you.

No one disagrees about what the predictions of quantum mechanics really are, unless you have an interpretation that changes those predictions in some way.

So I think it's embarrassment personally, because why are you doing physics if you're not interested in how the world really works?

Also, a lot of bars on that graph, there's certainly yeah, there's a handful.

Let me just very quickly mention some of the possibilities.

One is something of a Nen genius, but sort almost certainly wrong idea called the G R W theory after the three people who invented whose names I can't pronounce, um, so Gr w says that way.

Functions do collapse, but not when you look at it.

There's just a certain chance per unit time that the wave function of anyone particle will spontaneously collapse randomly and unpredictably.

And then if that particle is part of a bigger system, it brings the whole system along with it.

And so, basically, it's a way that you can enforce big systems to always look classical even when small systems away the rules of quantum mechanics.

It's an ingenious idea.

Another famous one is de Bohm interpretation of quantum mechanics, which actually goes back all the way to Louis Dubreuil, who was one of the founders of quantum mechanics.

And that's a hidden variables theory.

In other words, they say that the wave function is not all the information there is.

You can only make statements and predictions about probabilities because you don't have all the information there is.

Toe have their sneaky variables, but you don't know about people recoil at the idea of that.

But it is a theory that makes sense.

It makes unambiguous predictions, and finally, I'm not.

Maybe not finally.

But one other approach is amore epistemological approach, which means an approach that treats the wave function not as something riel but just as a bookkeeping device for talking about your information, your knowledge about the system.

So it is a purely informational approach to quantum mechanics.

Uh, I'm not an expert in those areas.

I don't see why that would help me understand One way functions collapse, even if you say that it's not riel, but it's certainly one of the things that got votes in that serving.

You think this is embarrassing?

What's it gonna take to fix it?

Are we looking for evidence or proof or a great communicator or evangelist who's gonna make people sing a lot.

Yeah, I need to be clear.

The thing that is embarrassing is not that people disagree with my favorite interpretation, but that we as a community don't agree what it will take.

Well, that's a very good question.

It's gonna I think it depends on what the right answer is.

If the right answer is that we really need a better theory that some new dynamics, like in the GR W theory, is actually involved, then maybe what we need is an experiment.

You know, these air theories that air truly different from conventional quantum mechanics and therefore make experimentally testable predictions, and we're testing them.

Very few people expect to see the quantum mechanics is wrong, but if it is, that would be a huge step forward.

Obviously, if it's not that, then a lot of it is just, you know, convincing a bunch of skeptical, hard nosed physicists that the unpalatable consequences of your favorite interpretation of quantum mechanics need to be taken seriously, whether it's hidden variables or many, many universes out there that you will never see.

Everyone agrees on which direction of time is yesterday, in which direction is tomorrow, for example.

We can remember things that happened yesterday.

None of us can remember things that happen tomorrow, even though things will happen.