Hello and welcome to this series on physics and philosophy from the University of Oxford.
Imagine your favourite football team playing a Premier League final on the same night that your favourite band are headlining at a festival,
and you're lucky enough to be offered tickets for both. The decision is tough, so you decide to flip a coin.
It lands on heads and you go to the match. However, lately you're left wondering What if you chose the concert instead?
What if the coin had landed on tells and perhaps it did?
Then maybe a parallel world in which the coin did land on tells you did go to the constant stead.
The idea of parallel worlds has often been explored in the realm of science fiction.
Today, it seems that the concept may be emerging out of modern physics, too.
I'm anchored to an open and I'm speaking to Dr. David Wallace, tutorial fellow in Philosophy at Balliol College, Oxford.
Dr. Wallace started off as a theoretical physicist with his first dphil thesis on relativistic quantum theory.
However, his interests in the conceptual and foundational aspects of physics soon led him towards a philosophy of physics.
And a second thesis is on quantum theory, according to the average interpretation.
His book, The Emergent Multiverse, was published earlier this year.
In this, he discusses the Everitt interpretation of quantum mechanics, more commonly known as the many worlds theory.
In your book,
you write that the idea of many worlds and parallel universes is an emergent property of the Everett's interpretation of quantum mechanics.
Could you please explain exactly what this interpretation is? Show in quantum mechanics?
What's going on? Essentially, this. This is our best theory of the very small.
It's remarkably effective as a theory, and it seems to say some very strange things about the microscopic world.
So it seems to say that atoms aren't just here or here, but they're somehow here in here at the same time.
Or maybe they're going this way and that way at the same time. And that's weird.
But okay, we can perhaps live with the idea that the microscopic world is weird,
where because of large, slow moving systems, we're not supposed to find the macroscopic intuitive.
But the problem is, what the theory also says is that when you look at the microscopic world,
that kind of neither this nor that, but both at once magnifies up to the level of the device doing the measuring.
So we don't just have atom here and here.
At the same time, we have needed a measurement device pointing this way and pointing that way at the same time.
And if I look at the measurement device and the theory says,
I see this and that at the same time, and that doesn't just seem weird, that seems pathological.
It's not clear we can even make sense of the idea of a needle pointing in two directions at the same time.
Even if we can make sense of it, then it just seems obvious that we don't in fact see needles pointing in two directions at the same time.
So it looks as if something's profoundly wrong with the theory.
And the way this is normally fixed in textbook quantum mechanics is just to introduce a new kind of ad hoc rule that says,
suddenly, when you make a measurement, the rules of quantum mechanics are suspended and we do something different.
But no one's ever thought that's a terribly satisfactory way of setting up a scientific theory.
And an interpretation of quantum mechanics is a slightly awkward term for a way of sorting out this
kind of problem in quantum mechanics and making quantum mechanics a slightly better behaved theory.
So perhaps to give some perspective on how the many worlds theory is playing, there is more or less need to see it in contrast with alternatives.
So the the normal moves other than the many well theory you might take you might call a change the physics move or change the philosophy move.
So change the physics move says look, something's clearly broken.
We have a theory, any theory that predicts that needles are in place at once or people see two things and ones must be wrong.
And so we better we better come up with a new theory that work better than quantum mechanics and doesn't have this problem.
Now, philosophers like to change the physics strategy, but physicists don't like it much because quantum mechanics is a very successful theory and
people are really reluctant to change really successful theories unless there's some concrete,
experimental reason to design the change.
The philosophy theory says, Look,
all of this confusion is coming because you want to think about your scientific theory as a way of understanding the world.
But that's the wrong way of thinking about science and scientific theory is just gadgets to predict what experiments do.
That kind of strategy, if you like, counts,
has changed the philosophy because it's changing the philosophical sense we normally make of scientific theories.
Physicists like to change the philosophy theory because it gets them out of awkward philosophical and conceptual puzzles.
But philosophers don't like it much because it seems to get in the way of what
seems to be a very clear fact about science across the whole of science,
which is that the point of science is to learn how things work. It's not the way we do science so as to understand what experiments say.
We do experiments to understand science. So the many worlds theory tries to kind of avoid that kind of dichotomy in the way it does.
It is basically saying, look, what would it really be like if a needle was here, in here at the same time?
What would it really be like if I saw this result and that was out at the same time?
And the claim is that what it would be like for me to see two results at the same
time is for there to be two mes one seeing one without one seeing the result.
And what it would be for you to hear me telling you one result and hear me telling you the other result at the same time would be for there to be two.
Use one. Hear me tell you one thing and one is we tell you the other thing.
And so the idea is that when you take seriously the idea that somehow we can be
doing two things at the same time and then just dismiss it as obviously silly.
That is, if you think hard about it, rather than just appealing to our intuitions,
which are a really unreliable way of doing science, then you realise that the quantum theory isn't contradictory at all.
It's just telling us something very, very strange about the universe,
which is that quantum mechanical processes cause the universe to branch into many,
many copies so that all these copies of a universe start together at the same time,
for example, with the Big Bang or the new universe has been continuously made all the time.
It's more like the second than the first, but this is kind of the place where my kind of jargon title,
Emerging Multiverse, comes into play at the level of the fundamental physics.
There's just quantum mechanics. At the level of the fundamental physics.
There's no sense of the universe is a totally separate classical, as in definite things happening universes at all.
The description of the world as being made up of classical universes is something
that happens when we look at the theory at a certain level of approximation,
just as our fundamental physics doesn't contain any chairs or tables,
chairs and tables of a kind of structure inside physics that we observe in certain circumstances and certain regimes say,
likewise, the classical universes have that character. But that all being said,
the kind of events that cause the world have the structure of multiple universes
rather than one universe didn't happen all at once at the beginning of time.
They're happening all the time as quantum mechanical.
Multiplicity get magnified up to classical multiplicity.
So every time some process causes a quantum situation that's here and here and here and there,
at the same time, the turn into a macroscopic situation that's here now.
At the same time, that's when a new universe branching happens.
There's not a new law of physics. It's an understanding of what's going on in the existing laws of physics.
So what exactly does it mean for there to be another universe where the alternative happened?
Do these universes ever interact with each other? Could I ever find myself in a parallel universe?
They almost by definition don't interact with each other.
What I mean by that is that the reason in quantum mechanics,
we have took seriously the idea that somehow the atom could be here and there at the same time was because
the the here bit of quantum state of the atom interacts with that bit of the quantum state of the atom.
And that interaction is what physicists call interference.
And it's the phenomenon of interference that fundamentally gives the the experimental results
that tell us that quantum mechanics is just fundamentally different from classical mechanics.
But it turns out to get this kind of interference between the two places that the atoms at the same time is quite a delicate matter.
Dealing with atoms is quite difficult. Doing it with big molecules is very, very difficult.
Doing it with viruses is almost impossible. Not trying doing it with entire humans is going to be completely impossible.
And what normally happens is, unless when you make a measurement of the microscopic world,
unless you control that measurement incredibly carefully, more and more and more and more systems start ending up into place at the same time.
Again, the jargon term for this is entanglement. More and more entanglement happens.
And very, very quickly you get to the stage where there's no prospect whatsoever of interference between the two branches of reality, if you like.
That's the point to which we start using the many world language.
So it's not, if you like, that different bits of reality can't interact with each other.
It's that the the fact that in practice you get to the stage where they're not
going to do that is what makes it sensible to start using the language of words.
So you couldn't ever experimentally verify this theory in the way that you could experimentally verify all the scientific theories.
It kind of depends what you mean by experimentally verified. Get back to my change.
The physics. Does it change the philosophy ideas?
If you think that we have three three ways of doing quantum mechanics, one is just take quantum mechanics.
Literally. That's the many worlds theory. One is change quantum mechanics for a new theory.
And one is rethink the way we think of scientific theories.
Now, you can definitely test the many world theory against and change the physics way of doing quantum mechanics.
The change of physics, ways of doing quantum mechanics normally predict that this kind of two things at the same time goes away at some point,
and that you can in principle look for if you were to ever fail to find these interference phenomena
where two bits of reality do interact with each other that quantum mechanics predicts are there.
If you if you were to ever fail to find those in a situation where the theory predicts that you ought to be able to see them,
that would be a falsification of the of quantum mechanics taken literally.
So if you like, it would be a falsification of the many wells theory. We have to get rid of them anyway else they that happened.
And conversely, every time we make one of those experiments and the result is exactly what quantum mechanics says it should be,
that's the situation where we say, okay, quantum mechanics taken literally many wells theory has passed that test.
Could you ever test quantum mechanics against the change the philosophy idea?
Now you couldn't that that's a very general fact in science.
You can never test any theory against the idea that the theory is wrong, but it still gets the predictions right.
So to take a silly example,
we can test the theory that fossils of dead dinosaurs against the theory that fossils are created by some kind of particular random process.
Because the theory, the fossils of the dinosaurs make lots of predictions about where we'd find fossils and how fossils are structured.
You can't test the theory the fossils are dead dinosaurs against the theory.
The fossils aren't dead dinosaurs, but behave exactly the same way as dead dinosaurs behave.
That's basically why philosophers don't take these take philosophy strategies very seriously.
They seem to, in some sense, cheat.
So it seems that the Money Worlds theory is the only seemingly valid way of looking at the results of quantum mechanics providing us with.
I'm always nervous about saying something quite that strong.
I think the many worlds theory is the only way to make sense of quantum theory as a theory of the world as a whole.
It's always sensible to think, well, maybe quantum mechanics is wrong.
Maybe there's some better theory that we should look for.
And I wouldn't in any way want to denigrate the value of various lots and lots of different projects to try to do that.
But in terms of the quantum mechanics we have.
If you want to understand that theory as a theory of the world, not just a convenient black box that tells us what experimental devices will do then.
Yes, I think the many wells theories, the only way to make sense of it in that sense.
So is it the case that every decision I could possibly make has been made by me in a different universe?
Perhaps it's worth saying a little bit about. What these these words are.
From our perspective, it's not literally true that the thing that makes these wells is something like although in everyday decisions,
it's they're made they're made by a particular physical process.
They're made by this magnifying up to the classical macroscopic level of quantum uncertainties.
However, a great many of the kind of things that determine at level decisions are things that ultimately can be grounded quantum mechanically.
So in particular, something like the weather, whether it's sunny today or raining?
Well, actually, at the moment in the United Kingdom, that seems to be deterministic.
But ideally, whether it's only raining does seem to be determined ultimately by very chaotic phenomena in the atmosphere,
which is to say very great sensitivity of the atmosphere to what?
To earlier conditions. And then ultimately, that sensitivity is powerfully enough to magnify out quantum mechanical variations to this level.
So, yes, there is a branch like this one, but where it's sunny rather have the raining.
And as in your example, you opened the fall of a coin is determined by various issues about how the coin how exactly one
position the coin to start with which in terms you can probably track to quantum mechanical
uncertainties magnify that but there hopefully isn't a branch just like this one in which I
spontaneously decide to do something stupid because hopefully my decision making is determined by.
Some process that isn't just total quantum randomness.
So they philosophers and writers of science fiction have an idea of possible worlds, which include every possibility, as we see it as.
If you like, actors, agents, the many worlds framework isn't quite like that.
It's a bit more tame, a bit more controlled, but it still in practice has a lot in common with it.
I should also say that the ideal of many worlds, although it's been contentious in physics for a long time now, seems to be an idea that.
Seems almost Hayden compared to some of the other aspects of many world stories that have come out of modern physics.
So modern cosmology is filled with the idea that we live in a multiverse,
a set of worlds which are not just emergent in my sense, but are fundamentally present in the physics.
Some of them are in a world in which the laws of physics are supposed to be different.
In many ways, the many worlds in quantum mechanics are quite tame, well controlled, well understood, said Wells.
So perhaps it's not surprising that these days, while I wouldn't say the many worlds theory is a majority view in physics,
it's probably tied for first place among the actual interpretations of quantum mechanics.
The physicists who are interested in this question take seriously. Thank you very much, Dr. Wallace.