[Auto-generated transcript] Hello and welcome to the Oxford Playhouse.
My name's Marcus Du Sautoy. I'm the Simonyi professor for the public understanding of science here in Oxford.
And each year we host a lecture here at the Playhouse to celebrate science for the public.
And, uh, it was a lecture first started by my predecessor, Richard Dawkins.
And I just counted up how many we've done. And it turns out we've done 25.
This will be the 25th lecture. So nice to celebrate.
and we're actually going to celebrate, another anniversary because,
this is actually the hundredth anniversary of the discovery of quantum physics.
And this is the quantum physics. Yeah. So I, I felt we should have a lecture, which celebrates, this anniversary and explores,
you know, what do we think about this, theory 100 years later?
Where is it going? Um, obviously a very difficult theory.
So I thought we needed an incredibly, good lecture to come and explore the ideas of quantum physics with us.
And I'm very lucky to have Philip Ball, who's I think he was an editor at nature for many,
many years, has written some fantastic, books, including Beyond Weird.
I think he is I mean, I love reading his books.
I think that he's one of our best, expositor of really difficult scientific ideas isn't scared to push audiences and readers,
into some really, uh, tough, places.
And I think quantum physics is one of those places where you need somebody who really knows what they're talking about.
So, please give a big welcome to this year's Simonyi lecture, Philip Ball.
Thank you, Marcus. Well, I'm very grateful to Marcus for asking me to do me the honour of asking me to to give this lecture.
I first began learning about quantum mechanics about 45 years ago here in Oxford,
where I had the good fortune to be taught by Peter Atkins, um, who was already legendary even at that point.
And I remember that Peter began his first lecture on quantum mechanics by saying that there are,
maybe half a dozen people in the world who understand quantum mechanics, from which I suppose we were to infer that Peter was one of them.
And I'm now here to tell you about it. And I understand that Peter is now in the audience, so no pressure there.
But I'm going to rescue myself by appealing to another authority, some might say in authority without parallel.
And Peter at least will be pleased to hear that that's not God who Einstein was, uh, famously apt to invoke on this subject.
Neither is it Einstein himself who never quite made his peace with quantum mechanics.
Um, the authority I am going for is Richard Feynman, who in the very year that he won the Nobel,
shared the Nobel Prize in physics for his work on an aspect of quantum theory,
said this I think I can safely say that nobody understands quantum mechanics.
Now, quantum mechanics does have a reputation for being impossibly hard, as Marcus just said.
And, um. It's not the mathematics though.
That's the problem. Here is some of the quantum math being done by the pioneer of the subject, uh, Niels Bohr.
And for some of you, sure, this might look pretty tough stuff, but Feynman could do all of this.
Fine. The maths wasn't the problem. The trouble was, that is all he could do.
What he couldn't understand is what the maths was telling us about the nature of the world.
And there's still no consensus about that today. As you've just heard, this year is the 100th anniversary of the first version of quantum mechanics,
which was devised by Werner Heisenberg and his colleagues in 1925 to mark that centenary.
Nature magazine commissioned a survey of what physicists today think about the, uh, meaning of the theory.
And this is the headline. Physicists disagree wildly on what quantum mechanics means about reality.
And, um, one of the key questions in that poll was, uh,
to ask which of the various different interpretations of quantum mechanics the respondents believed, um, this is the result of that poll.
And you can see that there's there's clearly a favourite here.
Um, and I'll come back, uh, later on to what that is.
But its dominance is hardly very convincing, particularly as only half of those who support it, uh, felt at least fairly confident in doing so.
So given all of this, what hope is there that the rest of us are going to understand this 100 year old cornerstone of the framework of modern physics?
Well, I want to suggest to you that we can at least do better than Feynman's admission of bafflement or defeat.
Some might say we don't have all the answers to what quantum mechanics means, but we do have better questions.
We have a clearer sense today than Bore or Feynman had of what's important and what's not.
So we have better questions, and sometimes that's really how science advances.
But in this celebratory year, I want to take, first of all,
to tell you a little bit about what actually happened in 1925, because quantum theory itself began earlier than this.
It was launched in 1900, when Max Planck in Berlin found that he could explain how warm bodies radiate heat by
assuming that the energies of their vibrating atoms could only change in discrete steps,
like the rungs of a ladder, rather than smoothly and continuously, and prank called these discrete little packets of energy quanta.
Now, this is a strange way for energy to behave.
It's a bit like saying that if you pedal your bike harder, your speed will only increase, not gradually,
but in a series of abrupt jumps, because there are some speeds that nature permits and some speed.
The nature doesn't. But Planck didn't worry very much about invoking this rather strange idea,
because he was just using these quanta as a mathematical trick in his equations.
He wasn't positing them as a feature of nature itself.
Five years later, however, Albert Einstein did just that.
He said, we should take these quanta seriously, and he said that we should regard light itself as little packets of energy,
each having an amount of energy proportional to the frequency of the light waves.
This was much more peculiar. Einstein was in effect saying that light was both a wave with a particular
frequency and a wavelength that in the visible spectrum determines the colour.
Both a wave and a stream of these little packets of energy, these little light particles which became known as photons.
And then in 1912, Niels Bohr showed that Einstein's idea could explain why atoms emit and absorb light only at particular frequencies,
but said that these emission frequencies correspond to the,
um, the energy change as the electrons in the atoms hop between the allowed quantum
energy state and those quantum atom explained various experimental observations.
In particular, the. These allowed energy states of electrons seemed to supply an answer to the long standing
puzzle of why different chemical elements show repeating patterns of chemical properties,
as reflected in the periodic table of elements.
But this early quantum theory was really just a very makeshift, um,
and sometimes contradictory mixture of the old classical physics and these new quantum ideas.
And what scientists really wanted was a clear and consistent mathematical theory that
would enable them to calculate quantum properties for genuine quantum mechanics,
to replace the old Newtonian mechanics that we use and continue to use in the everyday world.
So ball gathered together a team of young physicists at his institute in Copenhagen to try to come up.
With such a theory. And one of them was Werner Heisenberg.
Who? In the summer of 1925, took a trip to the little island of Helgoland off the German coast in the North Sea.
Heisenberg was at that time. This was in the summer.
He suffered a terrible bout of hay fever, and it was known that the sparsely vegetated Helgoland could give you a respite from that.
There was very little pollen, uh, on this island. So there he went.
And while he was there, Heisenberg made the conceptual breakthrough that allowed him and his colleagues, Max born,
who was at that stage his actually his boss in getting Gunn and Pascual Jordan to
work out a complete theory of quantum mechanics that was published later that year.
In fact, I have a feeling it may have been in November of 1925 that they published, uh, the Big Paper.
Now, um, so this is Helgoland, uh, where Heisenberg went.
And these photos are actually mine because I went to Helgoland this June to take
part in a centenary conference that brought together really an amazing cast,
all really of the leading experts in the world on quantum mechanics.
So it was a weird and a wonderful meeting.
Um, you see, Helgoland is actually really tiny.
It's basically just a rock in the North Sea. And, um, it doesn't have any big hotels, doesn't have a big fancy conference centre.
So this meeting of all these, you know, Nobel laureates took place in the local town hall.
And, um, someone said that if the ferry that took us there from Hamburg had sunk,
then that would really be the end of quantum mechanics for a generation.
Um, well, and I can tell you that the disagreements about what quantum mechanics means were very evident at this meeting.
So we witnessed Nobel laureates flatly contradicting each other about this on this question.
So let me now tell you what the arguments are about.
And I want to start with some of the things that everyone knows about quantum mechanics.
And when I say everyone, it's everyone in quotes. So if you didn't know any of those things, these things yourself, don't worry all.
But what I mean is that these are notions that you will very quickly encounter the moment
you start looking into what is popularly said and written about quantum mechanics.
So first of all, we're told quantum mechanics is weird.
That things behave differently in the quantum world, and, um, they don't quite make sense.
And here are some of those weird behaviours. So first of all, quantum objects can be both waves and particles.
This is wave particle duality. Quantum objects can be in more than one state or one position at once.
They can be both here and there. Say and this is known as quantum superposition.
You can't simultaneously know exactly two properties of a quantum object.
This is Heisenberg's uncertainty principle. Quantum objects can affect one another instantly over huge distances.
This is so-called spooky action at a distance, and it arises from a phenomenon called quantum entanglement.
You can't measure anything in the quantum world without disturbing it.
Um, and so quantum the the human observer can't be excluded from the theory, and it makes quantum mechanics unavoidably subjective.
And then everything that can possibly happen does happen.
And that there are two separate reasons why this claim sometimes made.
One of them is rooted in Feynman's work itself, which seemed to say that quantum particles,
as they go through the world, they take all possible paths available to them.
The other reason for saying this comes from the controversial many worlds interpretation of quantum mechanics,
which says that every time a quantum object faces a choice of what where it can go or what state it can have,
it takes both choices splitting the entire universe into separate worlds.
Well, now here's the thing. Quantum mechanics itself doesn't say any of these things.
They are attempts to say what quantum mechanics means.
Some of them are misleading. Some of them are plain wrong.
Some of them are just unproven assumptions or into interpretations.
Whatever quantum mechanics does or doesn't mean, I think it's time to change this record.
It's time to stop falling back on these tired old cliches and metaphors that we tell about it,
and to look more carefully and more closely at what quantum mechanics does and doesn't permit us to say.
And the first point to realise is that there's a big difference between the quantum theory, the mathematics,
and the mechanics that scientists use all the time to predict stuff at the quantum
level so that they can make devices like laptops and cell phones and so on.
There's a difference between that and the interpretation of the theory.
And this is what's so hard to grasp, because normally the interpretation of a scientific theory is kind of obvious.
Newtonian mechanics tells us about how things like tennis balls and spaceships move, what path they take through space as forces act on them.
And we don't have to ask, what do you mean by object?
What do you mean by path? It's kind of obvious that's not so for quantum mechanics.
And let me give you a glimpse of why.
To predict what a quantum object will do, how it will behave in place of Newtonian mechanics in place of Newton's equations of motion,
scientists now generally use the equation that was dreamed up by Erwin Schrödinger in late 1925.
He published it in early 1926 to describe the idea that quantum particles can act like waves.
So Schrödinger's equation was really another way to do quantum math from the one that Heisenberg originally proposed.
Now, Heisenberg was very competitive. He didn't like Schrodinger's equation at all, but everyone else did, and everyone else used it,
and they tend to use it now because it's very much easier to use than Heisenberg's method.
But unlike Newton's equations of motion, the Schrodinger equation doesn't give us a trajectory for the quantum particle.
Instead, it uses something called a wave function, and the wave function can be used to figure out what we might find.
Uh, an object doing an object like an electron, where we might find data, what properties it might have.
Um, if we, if we measure them. So a typical, uh, wave function for a particle might, might look something like this.
Um, what does that mean? Well, it's often said or often implied that it means that the particle was kind of smeared out over space.
And it does kind of look that way, doesn't it? But this isn't showing us the density of the particle in space.
It shows us, in effect, what the possible outcomes of measurements of the particles properties,
like position can, could be, along with the relative probability that we'll get that particular outcome.
So the wavefunction doesn't tell us where we'll find the particle.
It tells us the chance that we might find it at a given position in space.
If we look the bigger the amplitude of the wave function at any of these points, then the more likely it is that we'll observe it at that point.
And this is what. So what about quantum mechanics? It seems to point in the wrong direction compared to other scientific theories,
not down towards the system that we're studying, but upwards towards our experience of it.
It says nothing. Or perhaps I should say it says nothing obvious about what the quantum system itself is like.
So in other words, the wavefunction is not a description of a quantum object.
It is a prescription for what to expect when we make measurements on that object.
But it's even more peculiar than this, because the wavefunction doesn't tell us where the particle is likely to be at any instant,
which we then might, you know, verify or disprove by making measurements.
The wave function tells us nothing about the particle itself until we make the measurement.
Strictly speaking, we shouldn't really talk about the particle at all in terms of, uh, a except in terms of the measurements that we make on it.
So in other words, quantum mechanics, um, in quantum mechanics,
it looks as though measurements or observations don't reveal reality, but in a sense produce it.
One way of speaking about that is to say that before we make a measurement, uh, wave function might be this typically spread out broad thing,
but when we make a measurement and find the particle to be in a particular place,
then the wavefunction suddenly switches to being a sharp spike just at that place.
And this change is often called, for obvious reasons, collapse of the wave function.
But we have to be careful about what that means.
What it doesn't mean is that the particle itself goes from being kind of smeared out in space, to being sharply defined.
When we make a measurement of it, all we can say is that before the measurement,
there were different probabilities that a measurement would reveal it in different places.
Whereas after the measurement we can say for sure it's here and it's not there.
Our knowledge is what has changed. And some researchers think that this is really, uh, what quantum mechanics is,
that it's a theory describing how our knowledge about the world changes when we intervene in it,
and that we can deduce anything about what the world was really like before we had that knowledge of it, before we made an observation.
Now, this account of quantum mechanics is more or less the one that was given by Baugh and Heisenberg and their collaborators in, uh, Copenhagen.
And it's known as the Copenhagen Interpretation.
And going back to that, uh, nature poll, that is the one that proved to be, uh, the most popular today.
I'm not saying that that means it's right that the Copenhagen interpretation is right.
In fact, I think we can safely say that the version of it that was promoted by Baugh and Heisenberg
isn't any longer sufficient in itself to fully explain what quantum mechanics means.
But I don't like you that this interpretation has the following virtue that it tells us where our confidence about meaning has to stop.
As it stands, quantum mechanics doesn't permit us to say anything with confidence about reality beyond what we can measure of it.
Now,
one reason why some physicists don't like the Copenhagen interpretation and prefer other ones is this mysterious business of wavefunction collapse.
The problem is that there's absolutely no mathematical prescription within quantum mechanics itself for how collapse happens.
You have to kind of add it in by hand, and that just feels really unsatisfactory.
And in fact, um, some physicists, including Roger Penrose here in Oxford, uh, believe that this is precisely actually what we should do,
that we should add to the Schrodinger equation another term that describes collapse,
and that we need to recognise that collapse is a real physical process that happens and that needs to be put into the theory.
Others prefer the many worlds interpretation in which there is no collapse,
but rather there is this splitting of the entire universe into parallel, mutually inaccessible worlds.
Uh, whenever a classical outcome is, is, is manifest ever manifested from a quantum system?
Well, some physicists find that rather extravagant resolution of the problem.
But the challenge, really, and it's a challenge that still, as yet unmet,
is to find an experimental way to distinguish between these alternative interpretations.
Personally,
I think the wavefunction collapse is is really just an outdated way of talking about the puzzle of what measurement is in quantum mechanics,
and I'll explain later why I think that we can now do without it.
But I just want to point out that, in fact, collapse was never a part of Bohr's interpretation anyway.
It's often assumed many physicists just think it is, because that's what the textbooks seem to say.
But in fact, Bohr didn't like the idea of collapse. He never referred to it himself.
For him, it was just a clumsy way of saying that the quantum world of probabilities and our classical world,
in which we make measurements and get definite outcomes, that these are two different complementary aspects of reality.
Now, just as theirs is popular but misleading idea that quantum objects can be in many places at once.
Sometimes we're told, too, that they can be in two or more states at once.
They can be this and that. And these ideas arise from the possibility that quantum objects can be placed in these so-called superpositions of states.
And I'm going to illustrate, uh, what that really means, uh, using a quantum property called spin.
Um, and this is, uh, so some, uh, particles have spin and it's a little bit like, uh, so they're spinning, but it's not quite that.
It's a special quantum version of that. And for these purposes, you can just think of it as something that makes some particles, like electrons,
act like little magnets with a north and south pole that can be oriented in different directions.
So let's denote those are just up or down. Um, and because you just have these, these this binary, uh, possibility, it's got to be this or that.
Um, then we could use, uh, this property to encode information in binary form.
Um, we could say that, you know, up represents a one, down represents a zero.
And that is the basis of all quantum information technologies today, such as quantum computers,
in which spins or other quantum states are used as quantum bits or qubits to encode information.
But quantum spins can be not just up or down.
That's to say, a qubit encoding either a one or a zero.
They can be in a superposition of up and down states.
So as a spin qubit placed in a superposition like this is often said to be in both states simultaneously a one and a zero simultaneously.
Um, so that's the way it's often talked about, but that's not quite right.
Remember that a wave function only tells us what we can expect when we make a measurement.
And so a better way of saying it in this case, is that a measurement on this qubit could yield an outcome of up.
Or it could yield an outcome of down both all possible outcomes.
And in fact they're the only possible outcomes.
But what the qubit is like before we make that measurement is something that quantum mechanics does not tell us about.
Now, Schrodinger and Einstein didn't like any of this at all.
They the idea that some quantum property was not just unknown in print in practice, but actually unknowable in principle.
Before we look, that just seemed to them to contravene common sense.
Um, that's what prompted Schrodinger in 1935 to come up with his famous thought experiment, in which a quantum event could,
depending on how it comes out, either up or down, say, could either kill or not kill a cat.
And Schrodinger argued that Borje and his Copenhagen crew were basically saying that unless we actually make a measurement on this cat,
unless we actually open the box in which it's kept and look at it, it can be simultaneously both alive and dead.
And that just seemed inherently a nonsensical thing to suppose.
All I'm going to say here about that is that Schrodinger's Cat two is often misrepresented.
For one thing, when you really think it through, this thought experiment doesn't really make sense.
Um, but in any event, the Copenhagen interpretation didn't say that the cat is both alive and dead before you open the box to look at it.
It says that when we open the box, or if we open the box, we might find that the cat is either alive or dead.
It is silent about the state of affairs. Before we do that.
What's more important is Einstein's response, um,
to this matter of how measurement seems or observation seems to conjure reality into concrete existence.
Because in the same year as it's driving his cat, 1935, he and I, Einstein and two junior colleagues, um, Boris Podolsky and Nathan Rosen,
came up with a different thought experiment, which they argued proved that quantum mechanics,
as it then stood, was an incomplete description of reality.
So here is the their paper, um, which they describe this thought experimentally.
It's now known as the EPR experiment.
It's actually the paper is actually pretty hard to follow.
Um, and that's probably because it was written mostly by the Russian Podolsky, whose English wasn't terribly good.
Uh, you can see that from the title, actually. Um, but the experiment was later put into a clearer form by the physicist David Bowen,
and that's the version that most people use to talk about it today.
So in that version, we imagine two quantum particles fired out from a source in opposite directions,
and they have some property that can have only two possible states when it's measured.
So it could be spin, it could be so these particles could be spin up or spin down.
The crucial point is that the way in which they are produced, that means that these two that these properties are correlated in some fashion,
for example, so that if one has a spin pointing up, the other must have a spin pointing down.
We can't specify at the outset which it's going to be, whether it's going to be this way or that way.
But we know for sure that there's going to be this correlation.
So we then know that if we measure the spin of one particle and find, say, that it's up, then we know that the other must be spin down.
And Schrodinger named this, uh, quantum correlation entanglement.
Um, so we say that these two particles are entangled particles.
Now, we can imagine actually this kind of correlation in everyday, um, classical, uh, phenomena.
So say that, uh, we, you and I have a mutual friend and, uh, who buys a pair of gloves and sends us one each.
Weird thing to do, I know, but our friend is a joker. Okay, so he does this.
So if I then open my package and I find that I have the right hand glove, then I know instantly,
without even having to look, that you have received the left hand glove.
But here's the crucial thing. We also would suppose, in that case,
that mine was the right hand glove in the parcel when it was going through the mail and when our friend posted it, and all along.
But according to the Copenhagen interpretation, that is not true for these spins.
Um, that it isn't actually determined until we look at them.
And if that so, then the EPR,
our experiment seemed to be saying that making a measurement on one of these particles instantly fixes what the spin of the other one has to be,
as if there is some kind of communication passing between them.
And this is what Einstein, um, referred to as the kind of spooky action at a distance that standard quantum mechanics seemed to demand.
But Einstein's theory of special relativity had shown that this kind of instantaneous communication,
um, at a distance, is impossible, that nothing no signal, can travel faster than light.
So Einstein argued that quantum mechanics can't be the whole story, that it's incomplete.
And the missing ingredient is often now referred to as a hidden variable.
Some property that, even though we can't ever observe it, somehow fixes a definite orientation for these spins all along.
Well, Niels Bohr, um, came up with a counter argument for why this conclusion of Einstein and colleagues wasn't valid,
and most physicists figured out that May Bohr is probably right, because Bohr is hugely right.
Uh, and, you know, they didn't really look into it very closely.
And the matter just rested there for three decades.
The problem was that it didn't seem possible to distinguish between Boyle's view of events and Einsteins experimentally,
because they both predicted the same measurement outcome, which was just that.
We would see that these two particles have this correlation between them.
So the issue was swept under the rug, um, for several decades.
But then in 1964, uh, the Irish physicist John Bell, whose day job was as a particle physicist at CERN in Geneva,
reformulated the EPR experiment in a way that showed how it could be conducted
for real and how it could distinguish what quantum mechanics alone predicts.
That's both version from what a hidden variables view predicts.
And that's Einstein's version. And those experiments that Bell suggested were first done in the 1970s using lasers to create entangled photons.
And they've been repeated many times since, um, to rule out any possible loopholes in Bell's argument.
In fact, the three recipients of the 2022 Nobel Prize in Physics were the key figures in those studies.
And these so-called Bell tests have always consistently shown that the prediction from quantum mechanics alone,
uh, is right, and that the hidden variables theories cannot be right.
Incidentally, two of these laureates were at the Helgoland meeting, Anton Zeilinger and Alan Asprey.
Uh, John Clowes was meant to be there, but he couldn't come.
Um, and they were two of those that were just flatly disagreeing about what this implied, uh, for what quantum mechanics means.
So what's wrong, then, with the EPR experiment?
What went wrong with with Einstein's reasoning? Well, it relied on the perfectly reasonable assumption of locality that what happens here cannot
affect what happens over here without some way of transmitting the effects between,
uh, the intervening space between them. That's perfectly reasonable.
But as Feynman once said, quanta, at the quantum level, nature is not reasonable.
Once they are entangled, the fact is that, um,
we can't regard these two particles in the EPR experiment any longer as separate entities, even though they look like that.
They look like two particles travelling in space. As far as quantum mechanics is concerned.
Once they are entangled, they are both parts of, in effect, a single object.
Or, to put it in another way, the spinning orientation of one of the particles isn't solely located somehow on that particle in the way that,
you know the redness of a cricket ball is located on the ball.
Um, it is somehow non-local that property only if we accept Einstein's reasonable but wrong assumption of locality in quantum
mechanics do we need to tell the story in terms of there being some kind of instantaneous communication and effect,
um, across the, the, the distance between the particles?
Quantum nonlocality is the alternative to that view.
And Schrodinger argued that this property of quantum entanglement, uh, that was introduced really by the EPR paper.
He said that this is actually the central feature of quantum mechanics.
So it turns out that the big deal about quantum mechanics is not actually really the quantum at all.
It's not really these quanta, these this, this discrete graininess that the theory imposes on the fabric of reality.
Rather, it's this strange interdependence that quantum mechanics seems to impose on its particles.
Perhaps the best way to think about this is that it's a kind of uniquely quantum way of sharing information.
You know, if I want to share some information with you, if I want to tell you what it was like,
how good landed the summer, then I have to use some classical means, some classical channel to do that.
I can tell you about it, or I can send you an email, or I can send you a letter or whatever.
Um, that information can never be conveyed faster than light,
but quantum particles can share mutual information non-local by spreading it between them,
as though the intervening intervening space just wasn't there.
The catch is that this mutual information can never be used to actually send a signal to send a message,
so I can't find out something in one place and somehow use quantum entanglement to instantly communicate it to another place that's not allowed.
Um, that prohibition is fundamental to quantum mechanics, and it's known as the no signalling condition.
And I'm going to come back to that.
But first, I have a little bit more to say about entanglement, because Einstein thought it revealed a failure of quantum mechanics, but in fact,
it supplies one of the reasons why I think that our understanding of the theory today has moved on since the time of Born Einstein.
You see, the problem with the quantum, with the Copenhagen interpretation was that it left this business of, uh, measurement a kind of mystery.
So on the one hand, we have these quantum probabilities of possible outcomes,
and on the other hand, we have definite outcomes of measurements made with classical instruments.
We know with big kit which give us a definite this or that.
The particle is either here or it's there. And Bo insisted that we just have to accept that we need both of these views.
We need the quantum to explain the behaviour of atoms in their particles,
and we need the classical to talk about our own world as we experience it, what we can actually determine to be real.
But this seemed really unsatisfactory.
How can it be that we need these two incompatible frameworks for understanding the world, rather than just a unified view?
And in any case, when then does quantum stop and classical start?
In these days we've been able to see quantum behaviour and some pretty big objects, for example,
in nanoparticles made of thousands and thousands of atoms or in loops of superconducting wire, um, that, uh, figure as big as a bacterium.
And, um, this was actually first shown in the work that won this year's Nobel Prize in Physics.
And these loops of superconducting wire are now used as quantum bits in quantum computers.
So there's no good reason that we have to think that quantum mechanics somehow stops working at some particular size scale.
It just gets harder and harder to see quantum effects that things get bigger.
Well, it actually seems that entanglement holds the key to this so-called measurement problem,
and it's allowed us to start filling in the missing bits.
In the Copenhagen interpretation.
You see, measurement is all about getting information out of a quantum system and into the measuring apparatus or more generally,
out into some big and big sort of messy environment made of lots and lots of atoms.
Entanglement provides the means for doing that, because it enables the quantum system to share information about itself with other particles.
And in fact, um, so here we have the quantum object.
We need to get the information out into its environment and in.
Argument is what let that happen? Because in fact, entanglement happens inevitably whenever a particle interacts with another.
And so it is, um, uh, what allows the quantum system to get information about itself out in the end to the, its environment.
And there's now a pretty well worked out theory for how this happens,
for how information about the properties of a quantum object get broadcast into its environment via entanglement between the two.
And it turns out that in that process, what starts off as quantum behaviour ends up looking like classical behaviour,
the interactions between the quantum object and its environment spread to the quantum mass,
so to speak, spread the quantum mass of the object into the surroundings through entanglement.
And what this does is it produces an imprint of the object in the environment and the imprint of that object in a specific state,
which we can then access.
And for us, these imprint, you know, in regular life, these imprints are often in the form of photons that our eyes can register.
So you're seeing imprints of me. You're not in some sense seeing directly me.
You're saying imprints of me that I'm leaving in my environment via the photons that are scattering from my body to your eyes,
and each of you is seeing a different imprint.
The photons that go into your eyes are different from the photons going into the your your neighbour's eye.
And in this process, the quantum is in in a quantum object, a superposition, say,
get spread and diluted in the environment so that we can't make it out anymore.
And what we're left with is just classical behaviour.
And this process is called decoherence. And it's the key to a theory of quantum measurement.
You could say that that really classical physics is just what quantum physics looks like when you're six feet tall,
and when you experience it via this decoherence.
Now, I don't think we can say yet that this theory of environmental entanglement, if you like, and of decoherence,
I don't think it yet gives us a complete explanation of quantum measurement, but I think it gets us a lot of the way there.
And if you're interested to find out more, I would recommend this book.
It's just very recently been published by one of the pioneers of this aspect of quantum theory, boy, Jack Zurek.
So you can maybe start to see that that at the deepest level,
quantum mechanics today isn't about wave functions and collapse and wave particle duality.
It's the theory of what can and can't be done with information.
And that is precisely where some scientists now think we can get at the essence of what quantum mechanics means,
not from the old ad hoc intuitions about waves and particles and Schrodinger equations and so on.
But by thinking about how information can be encoded, transferred, manipulated and read out in quantum systems.
And if that sounds a bit like computer science, that's no coincidence.
We've all heard, now, I'm sure, about these quantum computers being made by the likes of IBM and Google,
which are expected to be able to do some kinds of computation much faster than our conventional classical computers.
But here's the thing this isn't simply a case of applied science and technology
emerging from pure and fundamental science in quantum information theory.
That interaction goes both ways. So on the one hand,
it's because quantum bits can be entangled so that they share mutual information
much more profoundly and much more efficiently than classical bits can.
It's because of that that quantum computers potentially have this greater computational power,
so that they might be able to solve some problems in minutes.
That would take the best classical computers today longer than the age of the universe.
But the problems faced by engineers of quantum computing problems like how to correct for the random errors that arise,
um, that these problems come about because of fundamental features of quantum theory.
So quantum error correction itself is hard because of a prohibition similar to that no signalling condition.
I mentioned earlier, a prohibition called the no cloning theorem,
which says that you can't make copies of a quantum state that you don't know about because you haven't measured it yet.
And that means you can't make backup copies of your quantum bits.
Um, to in case you get errors in some of them.
So as a result, by addressing problems in quantum information technology,
we're finding out some things about the fundamentals of quantum mechanics itself and acquiring a new language to talk about that.
A language of quantum information.
And in that spirit, I want to offer you a way of thinking about one of the features that's hardest to grasp and to articulate about quantum mechanics,
how it is that the information we obtain from measurements seems to depend on the questions we ask and how we ask them.
That's really what Heisenberg's Uncertainty principle says.
It says that we can only know some things about quantum objects if we accept that that very knowing renders other aspects totally unknowable.
The American physicist John Wheeler, who studied under Baugh and actually had Feynman as a student, actually had voice recognition as well.
Wheeler developed a wonderful metaphor for illustrating this idea of how answers about what's real can
emerge from the questions we ask in a way that's perfectly consistent and rule bound and non-random,
but without requiring there to be a pre-existing truth underneath it all.
So you, I'm sure you know the game of 20 questions.
So one player leaves the room and everyone else agrees on a person, and then the person comes back in,
the questioner comes back in and starts asking questions to find out, to see if they can find out who that person is that we've decided on.
And the questions have to have just a yes or no answer. Okay, just one of their just a spin up or spin down is a quantum game, you know?
So imagine that you are the questioner and you've gone out and then you've come back in and you start asking your questions of,
you know, people in turn. And, um, they give you answers, yes or no, depending on the question.
But you find that after you've asked a few questions, it's taking the answer a longer and longer to figure out whether to say yes or no.
Well, that's kind of weird. But, um, anyway, you keep going and you figure out you're getting close to who it is, and finally you think,
you know and you say it's Richard Feynman, and everyone says, yes, it's Richard Feynman.
And you all laughed and the game is over. But then you say, well, what was going on?
Why was it so difficult for you to, you know, come up with the answer?
Uh, later on in the game and they explain what they really did while you were out of the room.
They didn't decide on Richard Feynman. They didn't decide on any particular person at all.
What they agreed to do was that everyone had to make sure that the answer they gave was consistent with all the previous ones,
in the sense of that they knew it would fit somebody who was well known.
So, you know, to begin with, it's easy. The first question might be, you know, is it male or female?
Because it has to be yes or no. Uh, is it's the person female.
And the answer, uh, you know, the answer I could choose at random.
So they just say no. Okay, fine. But then every subsequent answer has to be consistent with that.
And so of course the options become more and more constrained as the questions proceed.
And it takes longer and longer for the answer to figure out who's going to still fit all of these answers so far.
And everyone was forced, by the nature of the questions asked, to converge on the same final outcome, the same person.
If you had asked different questions, you would have probably ended up with a different final answer.
There never was a preordained answer. You brought it into being and in a way that is fully consistent with the questions you asked.
And what's more, the very notion of there being an answer um, only makes sense when you play the game,
when you ask the questions, if you'd come back in and said, oh, you know what, I can't be bothered.
Just tell me who you thought of. There is no meaningful answer that anyone could have given at that point.
That is what quantum mechanics is like.
It's a theory of what is and what is not knowable, and how these knowns are related and how they depend on the questions we ask.
And I like to think about this as a distinction between a theory of illness and a theory of, if not, quantum mechanics doesn't tell us how a thing is.
It tells us what with calculable probability, what it could be, what the possibilities are along with.
And this is crucial, along with a logic of the relationships between those could.
So if, if this, then it has to be that and so forth.
And what this means is that to truly describe the features of quantum mechanics as far as we currently can,
we should replace all the conventional isms with.
If so, we should say, for example, not here, it is a particle, there it is a wave.
We should say if we measure things like this, the quantum object behaves in a manner we associate with particles.
But if we measure like that, like that, it behaves as if it's a wave.
And we we should say not.
The particle is in two states at once, but rather if we measure it, we will detect this state with probability x and that state with probability y.
This, if nasty, is perplexing because it's not what we've come to associate with science.
We're used to science telling us how things are, and if there are ifs in there, that's usually because we're just because of our own ignorance.
But in quantum mechanics, the ifs a fundamental.
Is there. And is this behind the.
If this. Well that's possible. And simply by admitting as much, we're going beyond the simplistic view of the Copenhagen interpretation,
according to which there's really nothing meaningful to be said beyond the results of observations.
But if there is something like that, if there is a fabric of reality we can talk about,
it's not going to be like the business of everyday life in which objects have intrinsic, non contextual, localised properties.
It will not be a common sense illness.
Quantum mechanics, then maybe the machinery that we humans need.
Here we are at scales pitched midway between the subatomic and the galactic.
At this scale, it's the kind of theory we need to try to compile and to quantify information about a world that has this character,
a world that seems at its finest grain,
incredibly sensitive to the touch, liable to spring off in unpredictable directions,
a world that's not yet quite fully defined, not yet fully real to our classical senses.
Quantum mechanics embodies what we have learned about how to navigate in such a place.
And at any rate, it's vital that we understand that this, if not, doesn't imply that the world, this world,
our world, our home, that it's holding anything back from us, it's hiding reality behind some kind of fuzzy veil.
It's just that classical physics has primed us to expect too much from it.
We've become accustomed to asking questions and getting definite answers.
You know, what colour is it? How heavy is it? How fast is it moving?
Forgetting the almost ludicrous amount that we don't know about everyday objects.
We figured we could go on asking questions like that forever, questions that seem perfectly reasonable and expect to get answers at ever finer scales.
And when we discovered that we can't, we felt shortchanged about nature when we pronounced nature weird.
Well, that won't do anymore. Nature does its best, and we need to adjust our expectations to that.
We need to go beyond weird. Thank you.
Thank you so much, Billy. Uh, we got, uh, chance for some time for questions from you.
Um. Uh, so we've got a roving mic. Um, but I just saw.
You just set me up. This whole lecture was basically so I could tell my favourite quantum joke, if you'll indulge me.
Um. Which if Eisenberg's bombing down the motorway, um.
And gets pulled over by the police, and, uh, the policeman says, do you know how fast you were going?
And, uh, he says, no, but I knew exactly where I was. And then the policeman says, well, you were going 120km an hour and says, oh, now we're lost.
Uh uh, but then there was Schrodinger in the back of the car and, uh, the guys.
Do you mind if I check your boot? And he flips open the boot. Oh, my God, there's a dead cat in here.
The Schrodinger says, well, now it's dead. It's gone. Anyway, so, um, uh, maybe that's a joke.
Doesn't make any sense now, but having that big red cross on it.
Um, anyway, do we have to ask? Put your hands up if you'd like a question, uh, to ask Bill, and, uh, we'll get a mighty you.
Or they've all been bamboozled. Great.
There's a hand up here is going to start us off. So there's a mind for you.
Hi. Um, an elaboration on the Schrodinger. Uh uh, paradoxes.
Weakness, friend, where an observer observes an observer and it's claim.
That's a recent experiment that Harriet Walter established that can be no objective view of reality, uh,
and that different observers will come up with different views, uh, on the on the same physical situation.
Is this a myth, uh, or is this the case?
No, that that is exactly right. That seems to be what it's telling us.
So this experiment, this is actually a much more interesting experiment than Schrodinger's cat,
and it's the one that physicists now spend much more time thinking about or people interested in the foundations of quantum physics,
which is, um, so there was a physicist called Eugene Fechner who came up with this.
So he imagined, um, someone doing an experiment.
And it could be the starting of captivating, could just be measuring a spin in a lab.
So his friend is in the lab doing the experiment,
making a measurement to see if this thing that is in a superposition, to see if they measure up or down.
And, um, they the friend does the measurement and gets a result and posts a little message under the door of the lab saying, I've got a result.
So we know the measurement has been done. But as far as Victor outside the lab is concerned, he doesn't know, uh, what the result is.
So quantum mechanics seems to insist that that being the case, the friend is in a superposition as well of having measured both up or down.
There is no way that Victor can say anything more about the friend than that, um, with the information that he has.
So he has to know that the measurement has been done, but somehow it hasn't.
Somehow it's put the friend in a what seems to be a superposition as well.
And you can imagine an infinite sort of series of the, you know, the boxes where people, you know, don't know.
Vigna kind of tells another friend that it's been done, but, you know, once he knows the result and so on.
So, uh, um, what what when you pursue this, uh, sort of logic through to, uh, its conclusion,
what it seems to indicate is that it's possible in a situation like this for,
uh, one person to have one, uh, set of results, uh,
about what has been done and another person to have another set, and they're incompatible with each other.
Um, but both have to be regarded in some sense as truth that the, the, the actual result, the outcome seems to be observer dependent.
That just seems very strange. But that's what vagueness, friends, seems to imply.
Um, there have been arguments made for, you know, that this we just have to accept this as an element of quantum mechanics, uh, as well.
Um, there are, in fact, on the ferry back from Helgoland, uh, I that I had a long discussion with, uh, a few others about that.
There was someone there who was very,
very keen to see a fitness friend experiment actually done and had ideas about how it could be done using photons.
So you could actually put some of this to the test.
So I'm kind of optimistic that actually we might find a way to carry out a thought experiment like that.
Thank you for the presentation, first of all. Um, my question is a little philosophical.
I just want you to know this. Can I, at any point, can we be really understand the nature of the universe, or should I rather say the universe?
Let us understand its nature itself. Or should you rather say these words will allow us to understand.
Yeah. Uh. Underst understand itself. Like I.
I mean, in, in in the end, this is what it comes down to that people wonder is quantum mechanics something that we can actually get an intuition for?
Or do we have to just resolve ourselves to what seem to be these, these paradoxes, all these things that,
you know, we can't kind of picture we can't picture what quantum nonlocality means.
We can describe it in equations. But how can it be that, you know, the properties of this object are also kind of in this object.
So that is absolutely the question.
You know, can we develop an intuition about this?
Some people have talked about in fact, I think Bob Coke is uh, in, in Oxford two has uh, sort of made steps along this direction.
Some people think if we start teaching quantum ideas really early on in education,
maybe even at primary school, you know, there are ways that you could do this.
Will we be able to develop an intuition at that stage that, you know, we can't easily get once we're set in our ways?
Um, and the other possibility is that because it's now possible to actually do to explore these ideas using quantum computers,
you can you can use them on the cloud.
IBM, for one, makes their quantum a part of their quantum computing resources available on the cloud to schools and others.
So you can, you know, carry out, uh, some of these kind of experiments, basically,
um, will that allow people to develop a better intuition about quantum mechanics?
So I don't know the answer to that.
Um, it's certainly I think I'm too old to, you know, really feel I'm ever going to get an intuition for these things.
And I feel like there are some things you just have to accept. That's the way it is.
But it would be fantastic if there comes a point where, you know,
it does seem possible for us to not just know that this is so because we're told, but to intuit why it should be.
Maybe it's about, uh, um, making everyone mathematicians because I often feel when I'm looking at quantum physics that,
um, uh, you know, the mathematics, as you say, it's sort of you follow the mathematics.
It's it's not that complicated. It's when you then try and translate it into natural language that, um,
and interpret what, what the non computing, uh, measurement means suddenly becomes.
I can't know one thing if I know another. Yeah. And I might, I might upset you here, Markus.
Because there are people.
This may probably not what you're saying, but there are people who say you can't understand quantum, uh, quantum mechanics unless you do the math.
You just have to learn the math.
You just have to be able to solve the Schrodinger equation, and, you know, you're never going to understand it otherwise.
And I feel like that's, um. That's not quite.
I'm not satisfied with that. I think, um, it's true that, you know, you can.
I mean, doing the math is not the same as having an understanding.
And that's why we're still arguing about it, as you say, you know, and I as I say, the math isn't that hard.
You know, you can you can learn it. So I, I don't think that I, I'm not happy with the idea that, oh, you just have to learn the math.
And that's the only way you'll get to understanding quantum mechanics.
I think the math is a way of kind of, you know, not having to look at.
It's the old, the cliche, um, that sort of arose in the, in the 1970s, I guess it was 1980s, actually.
Uh, was the idea that quantum mechanics became about shut up and calculate.
Yeah, stop worrying about these philosophical problems or what seemed like philosophical problems, and just do the math because it works.
Um, and happily, that that situation has changed.
And there are people now, you know,
it's now no longer unfashionable or actually career damaging to ask these sorts of questions seem philosophical and actually,
in some cases, to start to be able to see how you might be able to do experiments to, to, to get at them.
It's probably fair to say that I both understand and don't understand what you're talking about.
Um, but as people as smart as you, or perhaps smarter or certainly smarter than me wrestle with all these, um,
bizarre philosophical paradoxes, what sort of practical value is generated or is expected to be generated?
So the quantum computing, uh, I don't know what that is, but I guess it's probably a computer that's way faster than what we have now.
But how does that make a difference for the average person or humanity or industry or whatever?
Like where where do we end up with after we've had all these, uh, arguments and conversations and, and, uh.
Philosophical discussions. Yeah. Well, it's very good question.
I mean, I should say, first of all, that I wouldn't personally, uh,
demand that there be practical applications of asking these questions in order to make them interesting in their own right.
I think it's, you know, it just feels to me it's really interesting to think, well, what is what is reality really?
That seems like a pretty fundamental question, but it does turn out that, you know,
we do now have this quantum computing industry that's, I think, worth something like 4 billion already around the world.
And there are lots of quantum computing firms, and it's just one of several quantum information technologies.
Quantum cryptography is another using these quantum rules to transmit information in a way that you can't,
uh, possibly, uh, eavesdrop on or you can't eavesdrop on without that being detected.
So it's a much more secure system.
And there's actually now a quantum, uh, internet being built that allows information to be, uh, shared this way in China.
Uh, China is leaping ahead in these areas. Has already got a rudimentary network of that sort.
Um, and yes, quantum computers are for certain sorts of problems, not for all, but for certain sorts of problems.
They should be able to solve them much more quickly.
And in fact, this is something that Feynman it comes from an idea from Feynman himself who figured he he just thought,
well, nature itself is obviously quantum at the fundamental level.
So if we want to simulate nature or if we want to simulate materials or molecules to figure out how they behave,
surely we should do it using quantum principles, not with these classical computers, which where it's really clunky to have to make that translation.
And that is one of the areas that quantum computers are likely to be useful for,
for simulating materials so that we can do the sorts of calculations that might point us towards potentially new,
uh, useful new materials or useful new new drugs.
Um, now, it's it's tempting to overhype that, that kind of potential.
And I think what that the truth is, uh, that we're going to end up I think when this, when this industry matures,
we're going to end up with a hybrid of some things being done classically because classical
computers are fine for them and other things being done quantum mechanically on quantum computers,
because they can do those particular things faster. And we'll never know, because it'll all be done in the cloud,
and we'll never know which parts of our research or whatever we're doing was done by quantum or by classical computing,
where it'll just be, you know, more efficient somehow.
So there are these applications of quantum information technology.
And, um, you know, there are others to do with quantum sensors, very fine measurements of, um,
of gravitational fields, for example, that are already, uh, being used in things like GPS system.
So there are applications that, uh, you know, real world applications that, uh, that are going to be useful.
Um, but yeah, I,
I just reiterate that I think it's really important that we wouldn't make that a criterion for whether we pursue these questions or not.
They seem to me just be intrinsically interesting. Thank you.
I'm neither a physicist or a computer scientist, and you've sort of partly answered the question, I think just then.
But I thought that quantum computer was a thing of beauty, but it kind of didn't look real.
But I wondered if there's an infinite number of if isms.
How do you get an accurate answer out of a quantum computer with an infinite number of observers?
Um, okay. Uh, let me break up the quantum computer because it is a thing of atoms.
It looks like it's something from a doctor or a sci fi film.
Um, dev, I don't know if you saw devs was, you know, they made a big thing of their quantum computer in this series, this Alex Garland series.
But that is what they look like.
The weird thing is that the actual computing bit of this, most of this is wiring and cooling, because you have to cool these things down a lot.
The actual computing bit is really it's like the size of a thumbnail.
Um, uh, because it's all being done on these tiny little superconducting loops.
Or sometimes people just use individual atoms or ions as the qubits, but they are real.
Uh, and they do work and they work as, as predicted.
And they're not yet it's controversial, actually, whether they're yet at the stage of doing anything faster than classical computers,
but they're very good reason to believe that they they will be able to do that.
Um, you know, that, that, that they're getting bigger and bigger and more and more powerful.
Um, so, but. The the the outcomes aren't, um, uh, aren't infinite.
What's really happening? You have a set of maybe these days,
it could be up to about a thousand of these quantum bits that are lots of them are all sort of entangled with each other.
And because, as I say, because of that, they can kind of share information in a very efficient way that allows a computation to be done more quickly.
Um, but, you know, there's it's finite, there's already a finite set of outcomes.
And very often the kinds of problems that they do very well are ones of optimisation, where there's lots of possible answers.
But you want to find the one, the best one.
You want to find the one like in logistics, you want to find the route that you know that is most efficient, that saves you the most time.
That's the kind of, uh, problem that quantum computers are likely to be good at solving.
So, you know, there's a finite set of possibilities, but it's just too big for classical computers that have to work through every single possibility.
And once you've got, you know, 100 different elements, the number of permutations of those is just literally astronomical.
And so it takes forever, you know, literally to find the answer.
Whereas with quantum computers, because of this, extra ways that they can share mutual information, you can do it much more efficiently.
So that's really how it's working. Right. Um, I think we're beginning to run out of time on Beaujolais.
So, uh, because we got to hand over the stage to tonight's, uh, production of Arthur Miller's play.
Um, and Phil has done a fantastic job, I think, of putting us in a kind of, uh,
as our audience member said, a kind of superposition of understanding and not quite understanding.
But I'm. I'm hoping that, uh, you've increased the probability that when we collapse you as an audience as you go out, that you'll be more on the.
I think I understood that, um, than the I'm still lost in this kind of world.
So, um, uh, we're going to come and have a drink in the bar afterwards, so we didn't get the chance to ask a question.
Uh, there'll be a chance to, uh, to meet Bill and, uh, uh, quiz him a little bit more, but,
uh, um, let's just give a big round of applause for, uh, a wonderful talk of the quantum.
Well, thank you. Fielded.