Quantum Worlds

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And I'm Robin Aince.

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Hello, I'm Robin Ince.

And actually, those are probably the last words I'm going to say for the whole of this show.

Yeah, because today's show is all about quantum mechanics.

And Robin actually, well, he doesn't have a clue.

No, I don't.

Though apparently, in another world, I made some different choices when I was younger.

And I do have a very deep understanding of quantum mechanics, wave functions, Hermitian operators, and Hilbert space.

Hermitian operators, Robin, what are they then?

I don't know.

Though I do think that very briefly, I took a quantum leap.

And when I first said Hermitian operators, I was someone who knew what they were.

But now I've made a quantum leap back again, like Deepak says.

And again, I'm ignorant.

Anyway.

Today, we, by which I mean me and the panel, will be discussing quantum mechanics.

What is quantum theory?

What, if anything, does it tell us about the nature of the physical world?

And could it shed light upon deep questions about the nature of space and time and the origin or otherwise of the universe?

Today we are joined by three wave functions that have, with all possibility, collapsed into our...

Not right.

Right.

I know it's technically wrong, but you can teach them.

And there's money in that.

There'll probably be another book contract if you're lucky.

And

anyway, our guests are.

Hello.

I'm Jamal Khalili.

I'm a professor of physics at the University of Surrey.

And what I find most intriguing about quantum mechanics is that after almost 100 years of this most powerful theory in all of science, we still can't agree on what it means.

I'm Sean Carroll.

I'm a physicist at Caltech in Pasadena, California.

What I find most intriguing about quantum mechanics is that I wrote a whole book explaining it in perfectly understandable terms.

And Robin was just telling me five minutes ago that he read the book and is now saying he knows nothing about quantum mechanics.

Which reflects the fact that not only people on the street, but even physicists, as Jim is alluding to, have failed in their job of understanding quantum mechanics, despite it being the most important thing we know about the universe.

I'm Katie Brand.

I'm a writer and comedian.

And the thing I find most intriguing and exciting about quantum mechanics is that in a many worlds theory, it means there must be another universe in which Piers Morgan is functionally illiterate and therefore can never write another word again.

And this is our panel!

Katie, if you read Sean's Book, You Front, there's bad news on that Piers Morgan possibility.

Sean, we will start with you, and I know this is the idea of this big, this may be the only question that gets asked all night.

This might just be your show, because to give people some sense of what is quantum mechanics.

Yeah.

Of course, we argue about what is quantum mechanics.

We don't know the final answer.

But functionally, the reason why quantum mechanics is hard is because it says that what you see when you look at things is fundamentally different than what they are when you're not looking at them.

And this is a feature of quantum mechanics that is not shared by any other theory.

An electron, what we call a particle.

You can see little pictures of an electron moving through a chamber, and there's a line as if it's a particle, and we call it a particle, and then they say, but when you're not looking at it, it is a cloud that spreads out all throughout space.

That's the fundamental mystery of quantum mechanics.

Can I ask a a question?

Can she ask a question?

Yeah, is that allowed?

Just something I just.

Does this apply to all electrons of everything, or is this just sort of special, magical, crazy electrons?

This applies not only to all electrons, but to all of everything.

In fact, the reason why Robin was getting in trouble by saying there are three different wave functions is that there is only one wave function of the entire universe.

And I told him that.

I know you told him.

And we are all entangled within that wave function, which makes us only see a tiny, tiny sliver of reality.

See, that is why today's show is brought to you by Codeine.

Because this is

such a...

It's such a beautiful.

And I did, I loved your book because the word glimmers of light where you just started.

But it is, in terms of its practicality and its pragmatic idea, and it just, because it's this kind of idea that it's a universe of cheeky particles.

I see it as more of a kind of slapstick thing that, you know, you're looking at it, it's going, I'm past, I'm fine, I'm just over here, I'm just over here, and then you look away, go, they're not looking, I'm everywhere, they're looking again, I'm just over here.

And that's that hard, you know, my three Stooges version of quantum mechanics, I hope it helps.

But it's like the Laurel and Hardy thing where they're going in and out of doors on a train station, like that.

Yes, that's what I see.

Yeah, the double piano experiment.

One is going up a hill, one is not going up a hill.

That's a whole...

But this is why it is so hard for even professional physicists to really quite come to terms with it, because if you see a particle there and then the equations are telling you, but no, it's really a spread out wave when you're not looking, everyone in the world will think but it's really a particle.

Like I I I don't really believe this.

And there's this huge mental block to accepting what the scientific evidence is trying to tell you.

Well Jim, I think i if everyone's heard something about quantum mechanics, it's probably Schrödinger's cat, which is a an idea which I think does encapsulate a lot of the strangeness, a lot of the subjects we want to talk about.

So could you give us the not the one minute, but the three or four minute introduction to Schrödinger's experience?

Yeah, so I mean, quantum mechanics is interesting because unlike the other great theory in physics, the 20th century, Einstein's theory of relativity, which we attribute quite rightly to Einstein, one scientist, quantum mechanics was developed by lots of physicists.

mainly in the 1920s.

And Erwin Schrödinger, Austrian physicist, was one of the people who contributed.

He came up with an equation, which is named after him.

Schrödinger's equation is the thing that you learn when you study physics or chemistry at university.

Solving that equation tells you about how atoms behave and how electrons move within atoms and so on.

But Schrödinger, despite being this, you know, one of the founders of quantum mechanics, was one of the people, actually along with Einstein as well, who was uneasy about what it told us about the universe.

And in the mid-1930s, he came up with this, what's called a thought experiment of putting a cat in a box.

So, you know, this is popular culture now.

We talk about cats being dead and alive and it's cruelty to cats.

Schrödinger never actually put a cat in a box.

It was something that he came up with to try and show just how crazy quantum mechanics was and his discomfort with it.

So you say if you have a put a cat in a box together with some poison in a jar which is triggered by the release of a radioactive atom.

So an atom that spits out, its nucleus spits out a particle, an alpha particle.

That particle triggers a Geiger counter, which then triggers a hammer that hits the glass, releases the poison, and kills the cat.

Right.

Now, he said, what if you had that box closed for, let's say, an hour, within that hour, there's a 50-50 chance that that atom would have spat out this alpha particle.

That means there's a 50-50 chance the cat would be killed by the time you open the box an hour later.

He said, but because the atom and the radioactive particle are described by quantum mechanics, you cannot say at any one time whether it has spat out the particle or not.

You have to describe it as this wave function, this mathematical abstract entity which says the particle has both been emitted and not emitted at the same time.

And he said, well, okay, while we're quite happy to talk about things like that happening down at the level of atoms, we're talking about cats here.

And if the fate of the cat is now intertwined with the atom, then the cat must also be dead and alive at the same time until you open the box.

And he was highlighting how uncomfortable he was with quantum mechanics, because that's crazy.

You never open the box and see dead and alive cats at the same time.

So that led to this whole philosophy of people arguing over decades: is it the person, the experimenter, who opens the box that forces the cat to decide whether it's dead or alive?

This is called a quantum measurement.

You know, do you, how can you qualify to make a quantum measurement?

Do you have to have a PhD?

Do you have to wear a lab coat?

You know, Can the cat decide for itself?

What if you stick

Schrodinger in the box and see how he likes it?

You open the box, oh, I'm feeling a bit half dead and half alive.

Thank you very much.

You've collapsed me to the live state.

And the thing about quantum mechanics is that we still can't decide for sure exactly the narrative, the story that goes along with the maths that predicts cats should be dead and alive at the same time.

Surely all this could be solved by just not putting a lid on the box, though.

And then you can just see.

Well, yes, yes.

Oh, the cat's fine.

Oh, no, no, it's dead.

Yeah, you would be able to see it.

Then it would either be dead or alive.

But the thing is, you think, well, okay, so you don't know if it's dead or alive.

Well, that's just your ignorance.

It's like not knowing what you've got for Christmas until you unwrap your present.

But quantum mechanics says it's more than that.

It's not just the state of our knowledge.

The cat is either dead or alive, we don't know.

Strictly speaking, it says it is both dead and alive until you open the box.

And we're going to talk about different ways of getting around this issue.

As you've just found out, also about what Jim gave his children for Christmas.

50% of them got a live pet, 50% got a dead pet.

He doesn't have favorites, it's just the way it worked out.

This is actually an opportunity for Robin to do an impression because that is called, it's what's called a linear superposition.

And everybody accepts the facts that the description of an atomic nucleus that decays will be, it will be what's called a linear supersonic, both, as Jim said, decayed and not decayed until you observe it.

Well, maybe not.

We're going to talk about that, but linear superposition.

But it's Robin's favorite Brian Blessed impression.

Oh, yeah, when we had Brian Blessed on, and he said, I'm sure many of you know about this, but he said, I have to say Gordon's alive, right?

And we said, no, you can't just say Gordon's alive.

So we wrote him a little script, which was Schrodinger's Flash Gordon.

And so the first one on the show, he just went, Gordon's alive and dirt.

He's in a super position.

And

what was lovely about it was that break-in.

He's in a super position.

Actually, What kind of position?

Super!

Absolutely super.

That was.

But I know, you know, that idea is, it's such an,

but I also love the rhythm in which you told the story of, you know, split out the particle that broke.

It sounded like you were doing the Was an Old Woman Who Swallowed a Fly.

Yeah, she swallowed Pavlov's dog to catch Schrödinger's cat.

I mean, I'm not thinking about super position in a different light.

It's like a really excellent position.

Sean, what are the different interpretations of what Jim just described?

I suppose at some level, every undergraduate physicist accepts the fact that you can have this description of

a nucleus, but then it sounds absurd when you extend that description to a cat.

Yeah, and this was exactly Schrodinger's point.

I mean, his point was not to say, look how weird quantum mechanics is.

His point was to say, surely you don't believe that cats are neither alive or dead.

They're in superpositions of both.

But those are the rules that we teach our undergraduates, that things are in superpositions until we measure them.

And then all the questions that Jim asks, what do you mean measure?

Like, who counts as a measurement?

What if I just glance at it?

Does that really count?

None of those are answered by the traditional textbook way of presenting quantum mechanics.

So happily, we have better.

positions now that we can take.

My favorite one, which I'm sure we'll get to, is called the many worlds interpretation of quantum mechanics.

Hugh Everett, who's a graduate student, invented it.

There's a long tradition in quantum mechanics of inventing a sensible way of thinking about it and then being kicked out of the field.

So Everett left physics after doing this.

He said, what happens is the two possibilities, you open the box and the cat's alive, or you open the box and you see the cat dead, they're both real, but they're real in two separate universes.

When that quantum mechanical system becomes entangled with the rest of the world, the world branches into two possibilities.

And in fact, that's what Schrodinger's equation says happens.

It's not a new thing that Everett invented.

It's right there in the equation.

It's just hard to believe because you never feel like you've branched or you've appeared into two different worlds.

So that's where the philosophy comes in.

Other people like Roger Penrose have said, well, when the system becomes big enough, its wave function just can't take it anymore.

It collapses somewhere.

So a nucleus can be in a superposition, but a cat just can't be.

Other people have said, well, there's a wave function, but there's more.

to reality than that.

David Bohm famously said, there's hidden variables that we don't know about that actually, the cat is either only alive or dead.

We just have no way of predicting it.

So then we kicked him out.

He moved to Brazil.

You know, it's a long story.

But these are not different interpretations.

These are different physical theories with potentially different experimental consequences.

I think that the idea that we should think of these as interpretations is an old-fashioned one, and we can do better now.

We should just

a definition, because we've used the word wave function several times.

So could you define wave function?

So, I mean, it's what the thing really is.

It's not that weird.

I like to think about a pen.

We're doing another visual aid on radio again.

Sorry about that.

But I can hold a pen vertically.

No one's surprised about that.

I can hold it horizontally.

No one's surprised about that.

I can also hold it at any angle.

Again, no one's surprised about that.

That's not the surprising thing.

The surprising thing is that imagine a pen that when you looked at it, it was only ever seen to be either horizontal or vertical.

That would be weird, right?

And that's the world that quantum mechanics gives us.

The set of possibilities of the pen being at any angle, that's what the wave function describes.

The cat being alive is like a horizontal pen, the cat being dead is like a vertical pen.

The wave function is just the collection of all these different possible measurement outcomes in some combination.

Katie, what's your.

I felt a genuine sort of ripple of terror when you said my name there.

You're literally just processing this.

Does the cat know it's dead?

That's what I want to know.

But that's

what is your general idea.

You know, this is not a subject, like me, it's not something you're immersed in.

When you start to hear these ideas, I think it's one of the hardest things.

It is so counter-instinctual.

It is dealing in a scale that.

So, in the first 15 minutes of this show, can you take us through your emotional?

Yes, intimidation, terror, a sort of glib euphoria at the idea that I might have solved this by by the end of the recording, the crushing knowledge that I won't.

I don't know, I just sort of feel like I'm trying to grapple with things that I understand and I'm sort of the thing that I've come up with is calories

in the sense that I feel like calories are different whether they're observed or unobserved.

They behave differently in different circumstances.

So for example, anything you eat at a wedding doesn't have any calories in it.

Anything you eat in the company of someone who's slightly bigger than than you doesn't have any calories in it.

Anything you eat on a plane, if you eat crisps and you're really, really hungry and you haven't had any tea, they don't have any calories in them.

Whereas, you know, in normal life, they do.

So I think that's the only that's the only way I can.

Or bank statements.

I feel like whenever I get a bank statement, I feel like before I open this,

you know,

I could be, or I could be massively overdrawn again, you know, like the moment of not opening it.

And usually I don't open them for that that very reason.

And that's why I believe I'm much richer than I am and constantly on the run.

So I'm, yeah, I'm sort of grappling with sort of small banal things that I could perhaps sort of relate this to.

But, you know, I do find this really genuinely fascinating.

And I've talked before on this show about my atrocious science teaching at school and particularly my physics teacher, Mr.

Potiphar, who kind of sort of smelt this interesting sort of mingling of vodka and extra strong mints.

Can I just check?

Is he likely to be listening?

Is he still around?

I think it's highly unlikely that he's still around.

Or maybe he is.

I mean,

if there is a Schrödinger's Potiphar, it will be the only contribution to physics the man has ever made.

What I like is that in one world, he's having the extra strong mints to cover up the taste of the smell of vodka.

In the other, he's trying to cover up the extra strong mints by drinking a load of vodka.

So there's very different takes there on that.

Yes, and I still don't know which one I was in.

I find it's a really fun.

It's so totally fascinating, as I'm sure everyone here feels, and we're just trying to grapple to understand it.

And what it always boils down to with me is: does it mean time travel's possible or not?

Does it mean there's parallel universes or not?

Because those two things I find very, very exciting.

I felt a sort of frisson of activity over there.

The first thing you said, actually, it's an interesting question, which is you said, does the cat know?

Well, I was jotting down things about what is reality.

I know that's a big part of this.

And are we the only people looking?

Does it all come down?

Is there someone else looking?

Is there some other element that's looking?

And the reality of the actual objective reality of the cat for the cat itself.

So I'm glad you thought that was interesting.

And now please tell me why.

Because the cat could be, the cat, it doesn't have to be a cat.

It could be you, as you said.

It could be Schrödinger.

So

can we explore that a bit?

If it's a person in the box, let's say, what is the person experiencing?

I mean, when I was taught quantum mechanics, and maybe I should say something about the standard view, view, it's called the Copenhagen interpretation.

The idea was that, you know, if a measurement, which is, you know, how you decide whether or not the cat is dead or alive, if a measurement is made and recorded such that at some other point that you can go and check, then that's it.

That has so-called collapsed the state of the cat.

So it is either dead or alive.

So the cat can measure itself.

That's still taught in undergrad.

Yeah, I mean, someone in a box, you could say, make a little mark every time you are aware that you're alive.

Yeah.

and then you could somehow, then when you open the lid, if you're not going to be able to get it.

You can go back and figure out exactly when it comes to

the problem.

Have we solved it then?

Pretty much.

The problem is that

we were talking about this earlier: that we are of a generation, Sean Brian and I, of a generation where we were taught quantum mechanics by people who were taught quantum mechanics by the masters.

We were probably second or third generation.

And the masters, the people who really laid out the maths of the theory weren't people like Schrodinger.

They were people like Niels Bohr, the Danish physicist, and Werner Heisenberg,

Wolfgang Pauli.

And their view was that all this chat about what exactly is going on is a waste of time.

That it's, you know, if you're going to worry about how the cat can be dead and alive at the same time, go and do philosophy.

You know, we're physicists.

But it developed into this...

what's called an instrumentalist view, that all we care about is the results of measurements, making predictions.

And quantum mechanics mechanics is brilliant at that.

Without quantum mechanics, we wouldn't have most of physics and chemistry, we wouldn't have learnt so much about the universe.

So it works, the maths works.

But physicists until recently haven't really been that interested in what it all means.

It's only now that we're starting to say, hang on a minute, enough's enough.

We've had it for nearly a century.

It's about time we made a decision about what's going on.

So, Sean's point about what is the story, you know, are there many worlds or not?

Is there something else going on?

We should be able to come up with a way of testing what is the correct picture of reality.

Because that's what I was interested in.

In one of the things you say in your book, which is for so long, quantum mechanics was considered, and perhaps to some extent even now, a bit of an embarrassment to study as a professional physicist.

Oh, yeah.

That for a very long period, it's like, you know, the whole shut up and calculate thing, which is a misquote as well.

I can't remember if it's in your book or not, where is it David Merman?

Merman.

Merman, yeah.

He actually said, shut up and calculate, but I won't shut up, which is a totally different, you know, kind of statement.

But that moment, and you talk about, I think, trying to get a grant, and someone's saying, Don't mention the quantum mechanics stuff.

So, why?

Why?

I mean, you talk about philosophers seem to engage with this, physicists, very wary of it.

I think there's a lot of reasons.

I mean, one was just the immense personal charisma of Niels Bohr.

Like, he was the godfather of quantum mechanics, and he declared it finished.

And people, even people like Einstein and Schrodinger said, wait, well, that doesn't sound right.

And they lost the PR battle very thoroughly.

But there's a bunch of things going on.

If you think about when were these discussions being had, the 1930s and 40s.

And guess what?

Physicists came up with other things to be interested in in those years, right?

And they're developed, and the center of gravity of physics moved from Europe to the United States.

Europeans, bless their hearts, like to think about philosophical topics.

Americans just want to build stuff, right?

And it hurt the science because

this attitude developed that there are scientific questions where we can write down the equations and make predictions, and that's good and solid and respectable and true.

And then there's this, but what does it mean, which is kind of fuzzy, and you're talking about your feelings, and we shouldn't let that into the scientific discourse.

And that has meant that 90 years later, we still don't know what we're talking about when we talk about quantum mechanics.

We should, I think, drill down into this.

You mentioned the many worlds interpretation.

So we've mentioned this idea of

led by Niels Bohr, the Copenhagen interpretation, that there is a reality there, which is the reality that we understand and perceive, really, that we're comfortable with.

And then, there's, as you mentioned, this other interpretation where if it were, let's say it were me in the box, then there would be, in reality,

two versions of me.

The universe would have split, and there would be one in which I was dead, I wasn't there anymore, and there's one in which I would be alive.

And that is a So the question is, could you elaborate on that?

Because it sounds so bizarre that there are multiple copies of us,

not only in some sense, but really in this description of the universe.

Yeah, it's really true.

And Schrodinger's equation is perfectly clear on what happens.

And no one disagrees on what the equation says.

The question is, are you willing to consider yourself?

as a quantum mechanical system.

You know, you say, sure, the little particle is a quantum mechanical system.

Maybe the cat, I don't know, is that classical or quantum, but surely I obey the rules of classical mechanics.

And Everett, after a sherry-fueled night of speculation, said, look, I'm made of atoms.

Atoms obey the rules of quantum mechanics.

I obey the rules of quantum mechanics.

So if the atom can be in a superposition of I decayed and I didn't decay, or if the cat can be in a superposition of I'm alive or I'm dead, then I, Hugh Everett, and by extension every other person, can be in a superposition of, I saw the atom decay and I didn't see the atom decay.

The only question is, can you believe it?

Can you bring yourself to accept that many, many, many, many times per second, the entire universe is duplicating into multiple copies where things are just a little bit different.

Does this go out and out and out, like, to the point where you could argue that the universe both exists and doesn't exist at the same time?

Nope.

You can argue that there's more than one universe that exists.

They all exist.

That's the thing.

It's all there.

It exists.

Deal with it.

Okay.

Yes.

But

we should discuss this because it does sound unbelievably profligate sort of

interpretation of a theory that, in order to make the mathematics, keep the mathematics simple, let's say, keep the theory as mathematically elegant and concise as it could be, you allow the existence, literal existence of essentially

near enough, an infinite number of universes, two trillion galaxies and billions of copies of each of us, countless billions.

That does sound

unusual.

It certainly is unusual, yes.

But again, as you seem to admit,

we're kind of a physicist brawl, by the way.

Yeah,

this is like a bar brawl.

We'll be bringing out the the blackboard soon.

I happen to share this view, actually.

Yeah, it's actually a very good question.

We shouldn't make fun of it.

But the math says this is true.

The simplest, prettiest, most austere, pure, unadulterated version of the math says, yeah, billions, maybe an infinite number of universes.

Deal with it, right?

And so you have a choice.

You can say, well, that math is really gorgeous and compelling, and it explains what I see.

Therefore, even though it implies the existence of many, many copies of myself just slightly different, I'm going to believe it because what I want out of physics or out of science is the simplest understanding of the underlying laws of nature.

Or you can say, no, no, no.

I mean, come on, catch your breath.

There's stuff around us.

There's a table.

There's water.

There's this stuff.

That's what matters to me, not pretty math.

What I want is the version of physics that sticks closest to what I see.

I would like physics that explains one copy of all this stuff.

So, do you ugly up the math to stick closer to what you see, or do you accept that what you see is a tiny sliver of reality to stick with prettier math?

I'm on the side of prettier math.

That's

my point.

Well, because of the society we live in, we'll just put it to a vote, and whatever the audience decides will be

a referendum, you would say.

That goes so well.

Well, members of the jury, the case against many worlds theory.

And Sean is very persuasive, and I've read Sean's book, and I have to say, I'm not as averse to many worlds as I was before reading Sean's book.

It's the truth that is persuasive.

I am merely its mouthpiece.

First of all, I mean, I think we have to.

We're a little bit jump away from a kind of credit card hotline where you can donate

to the work of the great physicist, and your bountiful donation will be revisited on you a million times more in the other year.

Well, this is why Jeremy Kyle's been cancelled, and we thought, well, ITV, what what about a more evidence-based feud show?

I'm going to ask Katie because the one-minute discourse on truth in a minute, but yes, no, I'm going to say many.

They'll cut all my bit out to you.

Don't worry about it.

No, it's true.

I mean, but the maths doesn't tell us that there are many worlds.

It's true, the simplest interpretation of the maths.

This is the most elegant, simple way of explaining, giving a narrative, a story to what's going on.

But, you know, if you're going for the simplest explanation explanation of reality, you just say, well, God made it that way.

End of discussion.

So it's not always the simplest that is the correct one.

So we're still struggling to find that experiment that can discriminate.

Because nature has to work one way or the other.

Either there are many universes or there aren't.

We have to be clever enough to find a way of picking out the correct way nature behaves.

Welcome to Infinite Monkey Roast Battle.

I'm Jimmy Carr.

This is.

I was, for someone who isn't gifted at maths or went to a convent school, for example, and wasn't taught any maths till they were about eight, for example, who just did art and Jesus.

When you say, like, oh, the maths is really pretty, so therefore that compels me to believe in the many worlds theory, or to say, oh, well, there's something in me because I'm ignorant of these things.

You sort of want to go, well, just make some more maths then, or get some more maths, or get better at maths, or do different maths.

Like, what is it?

I know how I sound, but I am a broken Britain.

But what is it about this maths that is so compelling and pretty and beautiful?

If you're able to verbalize that, I know it's obviously you want to write an equation and say, look how beautiful that is, but it'll just look like any other equation to me.

Sure.

So, to someone like me, what is it that's so compelling about this maths that leads you to such an extraordinary conclusion?

Yeah, so I mean, if you were Isaac Newton, or back in Isaac Newton's day, and you said, Well, what is the story about physics?

How does it work?

You would say, Well, there is the universe, the state of the universe.

There's a bunch of particles, and I could tell you the position of every particle and the velocity of every particle.

And then there are the laws of physics, and the laws of physics chunk the universe forward in time.

And they say, if you start here, here's where the bowling ball flies, etc.

If you go with quantum mechanics, all that's replaced, but the same basic pattern is there.

There's a state of the universe.

It's the wave function that we talked about, right?

And then there's an equation that says how it goes.

There's the Schrodinger equation, which we already talked about.

And then the point is that many worlds, Hugh Everett's great insight is simply the words, and that's all.

He stops there.

He says, if you just buy into that, everything else follows as long as you're willing to accept these multiple universes.

Every other version of quantum mechanics still has a wave function and Schrodinger's equation, and then a bunch of extra stuff.

And the extra stuff is designed to get rid of the other worlds, because we don't want to deal with them.

Because it's sort of philosophically jarring.

Yeah, exactly.

I mean as the philosopher David Lewis once said, I do not know how to refute an incredulous stare.

And most of the objections to many worlds are less like, I don't get that, I don't believe it, right?

It's not like, here's where it disagrees with experiment, here's why it's inconsistent, it's just like, eh, all those worlds.

And that's not really an argument.

And so

the reason why it's fairly uncontroversial to say that the math of many worlds is prettier is that literally it's the same math as everybody else, but everybody else adds worse things to sort of break away the other universe.

It's clunky.

It clearly shows that.

Just to avoid many worlds, which feels a bit radical.

Yeah.

Can we,

if it's possible, dig even deeper into this subject?

Because if this thing is all there is, it is the deepest description of reality, then the things that we're very familiar with, in particular space and time, must in some sense emerge from this theory.

They're not fundamental in such approaches.

So, could you describe?

This is now cutting-edge physics, but how we're beginning to try and extract these notions of just simple things like space and ask the question: what is time?

This question that's been asked forever, I suppose.

How are we beginning to approach that?

Yeah, you know, the best, most highly trained physicists in the world still, even though they understand quantum mechanics is right, there's a classical intuition that it's hard to shake.

So, typically, when we build a quantum mechanical model of some feature of the world, we start with a classical model and we quantize it.

This is what we're taught to do.

So here's space, here are glasses and pens and tables and things.

We're going to quantize all of that.

But presumably, reality doesn't do that.

Reality just is quantum mechanical from the start.

So what we should be saying is, well, what is reality?

Quantum mechanics says it's a wave function.

And then you say, well, why do wave functions look like tables and glasses and bottles and all that that stuff?

So here's a simple idea.

You notice that two things bump into each other when they're at the same point in space.

And when they're not, they don't.

It's a feature of reality.

So let's wind that one backwards and say, when two things bump into each other, we define that to be they're at the same point in space.

And so you ask, in the quantum mechanical wave function, what does it mean to bump into something?

And can we build that up into a picture of a three-dimensional space?

And the answer is very plausibly yes, but this is exactly what we're trying to do these days.

See, we're very near the end of.

Sorry, Jim, did you want to?

Oh, I was going to say one of the, you know, people probably are aware that the holy grail of physics is to find this so-called theory of everything, which brings together quantum mechanics with Einstein's theory of relativity.

Einstein's theory of relativity tells us about the meaning of space and time.

Quantum mechanics talks about the world of the very small.

And it's because we've failed to combine those two theories together, looking for what's called a theory of quantum gravity, that has led now to people thinking, well, maybe we have to rethink things from scratch.

Which one of those theories is the more fundamental?

And I guess probably most theoretical physicists would now say quantum mechanics is, and therefore Einstein's theory, heaven forbid, Einstein's theory of relativity may have to be modified to fit into quantum mechanics.

And that's why these ideas about the foundations of quantum mechanics, the sort of stuff we're talking about, what it all means, is actually more important than just philosophy, that it actually might have a bearing on finding this theory of everything that we've been looking for for so long.

Sorry, I was going to say that's exactly right.

And despite the fact that most physicists think that quantum mechanics is more fundamental than space-time and general relativity, when it comes to reconciling them, they say let's quantize gravity, which means let's start with Einstein's general relativity and try to make it look more quantum.

What we should be doing is trying to find general relativity, gravity, space-time within quantum mechanics.

And we've just started to try to do that and it seems kind of promising.

What would such a theory tell us about the, I suppose, the deepest question of all, which is

why is there anything at all?

How did the universe begin?

Did the universe have a beginning?

If we were to achieve such a synthesis, what would it be able to tell us, be capable of telling us about those questions, about origins, for example?

Can I just say that I find it delightful that when we just got the signal signal saying time's up, that you decided to ask a question which could simply be summed up as, why is the something rather than nothing?

Which has ended up being pretty, but it's a good question.

Yeah, and I said it in the introduction, so we've got to get to it.

Otherwise, people will feel short-changed.

You've obviously never listened to this show before.

My God, the tangents we go off to.

If we really knew how gravity came from quantum mechanics, we would be able to answer the question of whether there was a beginning or not.

There's no time ever in the history of physics, past, present, or future, when physics will tell you why the universe exists rather than not.

If you develop a theory of everything, and I'm assuming it's going to be a combination of you and just one of us.

Yes.

No, rock or five.

Sorry, sorry, rubbing accident.

I've been doing this show for 120 episodes.

I've been listening.

If there was, you know, if you were to be able to develop this sort of theory of everything,

what would it mean for just sort of us normal kind of muggles?

What would you do with it?

How would life change?

How would you change, or how would everything change?

I'd feel very satisfied.

Does that count?

We used to do theology at university.

I did.

And so,

if, for example, it is shown, so through doing physics, that the universe is eternal, for example, does not have a beginning in time, what would that mean?

I mean, you asked the question, but what would it mean to you?

What do you think it would mean to people who think about religion religion if you showed that the universe, in fact, is eternal?

As in, there was no point at which the universe didn't exist and therefore there can't really be a creator.

Yeah, well,

does that follow?

That's my question.

Well, I mean, I'm not now religious, and in fact, I became an atheist on a pilgrimage to Rome.

Yes, and I did meet the Pope at the end, and it felt like the closer I got to the Pope,

the more atheist I became.

So it was embarrassing when I did finally meet him.

But so I can't answer from a personal point of view because I would just find it very exciting because I'm now an atheist and I would just find it fascinating because I find all of this fascinating.

But I do think, you know, a lot of Christians have tried to combine science or the idea of evolution with the idea of guided evolution and that you know all of these scientific discoveries have been kind of folded into religion for the more open-minded or intellectual perhaps, people who have religious beliefs.

So, personally, I would think that what would happen, I'm sorry to disappoint anyone who's a very militant atheist, is that religious people would simply find a way to fold that into their existing story.

And I think probably a lot of humans would be comforted by that.

But I think it's hard to know because what I'm trying to understand is how radical would it be for humans, just sort of on a lay person's level, like me, if you did discover the theory of everything, or would it just be sort of felt to be

a kind of

another layer of very complicated physics that most people don't really engage with?

Or would it be sort of front-page news, like as if an alien had landed in someone's back garden in Richmond?

Well, I think a theory of everything, we don't know.

Physics for hundreds of years has been on this quest to unify, to simplify, to find overarching descriptions of nature, of reality.

And we don't know, you know, so when Faraday and Maxwell in the 19th century combined electricity and magnetism, they couldn't predict that it was going to lead to the electric motor and radios and T Vs and computers and so on.

So we don't know, in our advancing knowledge of how the world works, what practical applications it would lead to.

The people working on this theory of everything are not looking to take out patents on it because it's going to be useful.

They just want to know the deepest secrets of nature.

I also think there's a thing when you say front page news, when we look back, Einstein was front page news.

And there were in the 20th century, in the first half, science often was front front-page news.

And there's still science going on that should be front-page news.

But unfortunately, our media has decided that that is not as immediately kind of tacky and sticky.

So we have that, it's all out there.

So I think that's part of it as well: is to say there are brilliant and wonderful ideas, and this is far more interesting than your particular attack on, you know, Harry and Megan or whatever it might be.

But it's also interesting, isn't it, how Harry and Megan can be both royal and not royal.

I think, Katie, I think I love your question because as soon as you said it, you know, what are the implications?

My first answer is there aren't any.

No frying pans, no better iPhones, no cure for malaria.

But then when you develop it in the question, but like, how does it change how we think about our role in the universe?

That's exactly where the implications are.

And it's a set of ideas that physicists, again, are a little bit afraid of touching too closely.

And it's certainly not the case that some scientific advance is going to convince everyone to disbelieve in God.

But as science moves forward, like ever for the last 500 years, the role, the set of jobs for God to have has sort of gotten smaller.

And everyone has a tipping point where they go, well, okay, then I don't need it at all.

And mine was just outside the valve.

You're right, very close.

We need a lot of tipping.

But that leaves all this work that God had done for us in giving meaning and morality and purpose to our lives.

Physics doesn't help with that.

And it's not going to help with that.

And so suddenly you're thrown into this situation where you sink or swim.

And I think that the swimming is realizing that purpose and meaning in our lives is something that is up to us to invent, up to us to give to life.

It's not handed from outside in any way.

I think this is the sensible conclusion to draw from progress in science over the last 500 years.

And again, it's a matter of facing up to those consequences.

We asked the audience a question, and that was, Schrödinger put a cat in a box for his famous thought experiment.

What would you put in a box to be simultaneously dead and alive, and why?

And one of my more specific questions.

I've got here Pete Burns, because you spin my right round, baby, right round,

both up and down, I presume.

Yeah, so there we are.

That's listen, granddad.

Because it says, spin me right round, baby.

It says spin you.

Yes.

Yes, Grandad.

Yeah, you're quoting the song.

I'm reading what was sent in.

I respect our listeners even when they forget what.

You spin me white.

I like the idea of you going, listen, granddad, you don't know dead or alive.

Many people who bought that dead or alive single are granddads now.

They are.

Here's one: predictably a strawberry to finally settle the argument.

It is both dead and alive, but this is linked, that's from Sarah, linked to the next one, which says a raspberry to keep all the strawberries company.

Hashtag inclusivity.

Inclusivity of fruit.

This one is my career, a spectacular failure that gave me a good living.

Everyone involved with the movie Cats.

And if you need to ask why, you obviously haven't seen it.

So thank you very much to our panel, Katie, Jim and Sean.

Next week we return finally away from this philosophical whimsy that we've been dealing with to real science because we are going to ask, what is that strange light in the sky?

Is it a Chinese lantern?

Is it Jupiter?

Or is it an advanced extraterrestrial civilization that's traveled light years across the galaxy in search of another intelligent civilization?

And when they get here, they decide that rather than have a conversation, they just find a loner, give them a quick rectal examination, and then go home.

What we're asking is: is the universe just jam-packed full of alien proctologists?

That's science, Brian.

Good night.

In the infinite monkey cage.

Without your trousers.

In the infinite monkey cage.

Till now, nice again.

Hope you've enjoyed listening to that podcast.

If you haven't, don't worry about it.

In many worlds theory, it means there's lots of worlds where you absolutely loved what you just listened to.

But if you really didn't enjoy it and you'd like to listen to something else, it wasn't your taste, you're not interested in science and logic and rational thought, then you can find many other podcasts on BBC Sounds.

You can download those and you don't believe it.

Not enjoy them either.

Dream World.

You sound miserable.