Why Everything You Thought You Knew About Quantum Physics is Different – with Philip Ball

Why Everything You Thought You Knew About Quantum Physics is Different – with Philip Ball

August 30, 2019 0 By Stanley Isaacs


We like to start
here very often. I don’t know whether to reassure
you or to disconcert you. But this is one of the most
popular sayings about quantum mechanics from Richard
Feynman, who said, I think I can safely
say that nobody understands quantum mechanics. Now he said this in 1965,
and that was the year that he shared the Nobel
Prize in physics for his work on quantum mechanics. So at that point, no one alive
knew more than Richard Feynman about quantum mechanics. What hope is there,
then, for the rest of us? Well, quantum mechanics
has this reputation for being impossibly hard. But it’s not the mathematics
that’s the problem. And here’s some of
the mathematics, and it doesn’t look
particularly easy to grasp. But actually, Feynman
was fine with that. He could do the
mathematics just fine. The trouble was,
that’s all he could do. What he couldn’t
understand is what the math meant, what it tells us
about the nature of the world. And Feynman himself didn’t
seem too troubled by that. He said, well, we’ve got a
theory that works and makes amazingly accurate predictions
about how stuff will behave. What more do you want
from a theory than that? Some scientists feel
that same way today, but usually we do want more. We want to know what
scientific theories tell us about what the world is like. And it wasn’t clear
then quite what quantum mechanics was telling us
about what the world was like. And it’s still not clear now. But I want to suggest
that we can do better than Feynman’s admission
of bafflement, or defeat, some might say. We don’t have all the
answers about what quantum mechanics means, but
we do have better questions. We know we have a clearer
sense than we did in the 1960s, or even in the 1980s, of what’s
important and what isn’t. And I want to try to give
you some sense of what I think that is. And let me start with
some of the things that everyone knows
about quantum mechanics. And when I say everyone, I mean
everyone in inverted commas. So if you haven’t seen these
things before, don’t worry. All I mean is that once
you start finding out more about this topic,
perhaps by reading popular accounts of it, then
pretty soon these are notions that you will encounter. And the first of them is that
quantum mechanics is weird. And I want to show you what
some of those weirdnesses are. First one is that
quantum objects can be both waves and particles. And this is called
wave particle duality. The second is that
quantum objects can be in more than one state
at once, or more than one place at once. They can be both here and there. And these are known as
quantum superpositions. Then we hear that you can’t
simultaneously know exactly two properties of a quantum object. And this is Heisenberg’s
uncertainty principle. Quantum objects can affect
each other instantly over huge distances. This is so-called spooky
action at a distance. And we’ll hear more
about it shortly. And it arises from a
phenomenon called entanglement. You can’t measure anything
without disturbing it. And so the human observer can’t
be extracted from the theory. It becomes unavoidably
subjective. And then everything that can
possibly happen does happen. There are two reasons
why this is often said. One of them comes from
Feynman’s work itself, which seems to say that quantum
paths take all possible routes through space. The other comes from the
controversial many worlds interpretation of
quantum mechanics, which says that every time a quantum
system faces a choice of what to do, it takes both choices. OK, now here’s the thing. Quantum mechanics says
none of these things. They’re attempts to
explain or to articulate what quantum mechanics means. Some of them are misleading. Others I think are
just plain wrong. Others are just unproven
interpretations or assumptions. I’m saying that we need to
change the record when we talk about quantum mechanics. We need to stop falling back
on these tired old cliches and metaphors and look
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
quantum theory, the mathematics and the
mechanics that you just glimpsed, which scientists
use daily to make predictions to predict stuff that allows
them to build things like this laptop. So this is stuff
that really works. There’s a big
difference between fact and the interpretation
of the theory. And this is what’s so hard to
grasp about quantum mechanics because normally the
interpretation of a theory is kind of obvious. Newtonian mechanics. This is the old
classical mechanics that tells us how everyday
objects move about and behave. So it tells us how things like
tennis balls and spaceships move. The interpretation
here isn’t difficult. Newtonian mechanics
tells us what paths objects take through
space as forces act on them. And we don’t have to ask
what do you mean by path? What do you mean by object? What do you mean by force? It’s kind of obvious. Well that’s not so
for quantum mechanics. And let me give you
a glimpse of why. To predict what a
quantum object will do, in place of Newton’s
equations of motion, scientists generally
use the equation devised by Erwin
Schrodinger in 1925 to describe the idea that
quantum particles might act as if they were waves. This is the
Schrodinger equation, and it doesn’t tell us what the
trajectory of a particle is. Instead, it gives us something
called a wave function. And the wave
function can be used to figure out where we
might find an object and what properties
it might have, an object like an electron, say. So the typical shape
of a wave function of a particle like
an electron in space might look something like this. OK, so what does this mean? Well, it’s often
said what it means is that the particle is
somehow smeared out over space. And it does kind of look
that way, doesn’t it? But this isn’t
showing the density of the particle or the space. This wave function is a
purely mathematical thing. And what the wave
function lets us deduce is all the possible
outcomes of measurements that we might make on the
particle’s properties, such as its position, along
with the relative probability that we’ll get that
particular result when we make the measurement. So to find out
the position where we would observe this particle,
we simply calculate some number from the wave function, the
value of the wave function at that point in space. And that gives us
the probability that we will see the particle
there if we make a measurement. So the wave function
doesn’t tell us where we will find
this particle. It tells us the chance
that we might find it at a particular
position if we look. And this is what’s so odd
about quantum mechanics, because it seems to point
in the wrong direction, not down towards the thing that
we’re supposed to be studying, but up towards our
experience of it. It says nothing, or
perhaps we should say it says nothing obvious,
about what the quantum system itself is like. In other words,
the wave function is not a description
of the quantum object. It’s a prescription for
what to expect when we make measurements on the object. But it’s even more
peculiar than that because the wave
function doesn’t tell us where the particle is likely to
be at any instant, which we can then try to verify by looking. Rather, what the wave
function tells us– well, it tells us nothing about
where the particle is until we make a measurement. Strictly speaking, we
shouldn’t talk about where the particle is at all. We shouldn’t talk
about a particle at all except in terms of
the measurements that we make on it. Now this account of
quantum mechanics is more or less the one given by
the Danish physicist Niels Bohr and his collaborators
such as Heisenberg. And it’s known today as the
Copenhagen interpretation. Copenhagen was when
Niels Bohr was based. Now I’m not saying that
this interpretation is the right one. But what’s valuable,
I think, about it is 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. And here’s what I mean by that. One way of speaking about
this measurement of a quantum particle says that
before the measurement, the wave function might be
this typical sort of broad, spread out thing. But when we make a
measurement on the particle, suddenly it collapses into this
spike at one particular place because we know, having
made the measurement, where the particle is. Now this is generally
called, for obvious reasons, collapse of the wave function. The problem is that there’s
no real physical prescription for what’s going on here
within quantum theory. You have to sort of put
in this collapse by hand. So that’s a problem. But wave function
collapse doesn’t mean that the particle goes
from being sort of smeared out before we make a measurement
to being sharply defined when we make it. All it says is that before
we make the measurement, there were various
different probabilities that a measurement might
reveal it at particular place, whereas after the
measurement, we know for sure that it’s there. What’s changed is our knowledge. And some researchers
think that this is really what quantum mechanics is,
that it’s a theory describing how our knowledge
of the world changes when we intervene in it. And we can’t deduce
anything from that about what the world was
really like before we had that knowledge about it. So you see, it’s
misleading to talk in this situation about the
particle being in many places at once. The situation tells us only
about the possible outcomes of measurements. It’s the same thing, the
same story with this notion of quantum superpositions. Now it’s often said that the odd
thing about quantum mechanics is not just that they can
be in two places at once, but they can be in
two states at once. And I want to
illustrate what that means by referring to a property
that quantum particles have called spin. And you don’t need
to know anything about exactly what this means
except that for some particles, for an electron, say, the
spin can have two values. And you could think of
them as spin being up or spin being down. And if you make a measurement on
the particle, on the electron, then you’ll find
one or the other. So it’s a binary
property, really. And for that reason,
spins like this can be used to encode
binary information. So you could say
the spin up is a one and the spin down is a zero. And that’s the basis of
the quantum information technologies that we’re
starting to hear about, like quantum computers, in
which spins or other quantum states act as quantum bits,
or qubits, as they’re called. But spins can be not just up or
down, a qubit of one or zero. They can be in a superposition
of up and down states. So what does that mean? Well, it’s often said
that what it means is that the particle,
the electron is both up and down at
once, at the same time. But that’s not right. Remember that the wave
function tells us only what to expect when
we make a measurement. And so in this case,
what it’s saying is that in a
superposition state, a measurement might give
us an up or a down spin. And in fact, those are
the only possible outcomes of a measurement. But what’s the qubit like before
we make that measurement, when it’s in the superposition? Quantum mechanics doesn’t
really tell us that. Well, you see, now I’m
not talking any longer about smeared out particles
or collapsing waves, but about information, and how information
can be encoded in quantum systems, and how we can read
it here by making measurements. This is the perspective that’s
offered by so-called quantum information theory, which is
not just a basis for making these amazing quantum
technologies like quantum computers or quantum
cryptography, which is a way of
encrypting information that it’s impossible to
tamper with, to eavesdrop on, without being detected. It’s not just that. It’s really also a new way of
talking about quantum mechanics itself. Talking about quantum mechanics
in terms of information allows us to see past all the
old-style paraphernalia of wave functions and Schrodinger
equations and quantum jumps, and, I think, to get closer
to the core of what the theory seems to be telling us. And I want to tell you
a story about that, and I’ve got some
props here to help me. Now I hope it will
be illuminating, but at the very
least, I’m fairly sure that it’s the
first time that you will have seen quantum
mechanics discussed with the help of sylvanians. Here they are. So I have two boxes here, A
and B. One belongs to Alice, one belongs to Bob, and
I’ll leave you to figure out which is which. And they are boxes in which they
produce one of these cute toys, either a rabbit or a dog,
when we put coins in. And they will take
either a two-pound coin or a one-pound coin. So we put coin in and we
get one of these toys out. And there are rules
for how that works. And I’m just going to stipulate
what some of the rules are that these boxes
are going to work by. First of all, here’s the boxes. So this is what’s going on. And I’m going to say first
of all that rule number one, if Alice puts a
one-pound coin into her box, it will produce a rabbit. Now I’m going to
add two other rules. If Alice and Bob both put
in two-pound coins, then the boxes between them will
deliver one rabbit and one dog. Doesn’t specify which
way round that would be, but we’ll just get
that combination. Any other combination of coins
than both putting in two pounds will produce either two
rabbits or two dogs. I’m just stipulating
these rules. Now I want to find out what
do the inputs and outputs have to be in order to satisfy them? A pound in Alice’s
produces a rabbit. OK. A pound in Bob’s produces what? Well, let’s think about that. In fact, we kind of have a
lot of these answers already. So we already know a pound in
Alice’s box produces a rabbit. OK. Well, when you
think about it, that means that whatever Bob puts
in, pound or a two-pound has to produce a rabbit
because it could only produce a dog in the case
where both put in two pounds. That’s one of our rules. That’s the second rule. So we’ve almost got
all the rules already. All we need to know
now is what happens when Alice puts in two pounds. OK. Well, we know that if
Alice puts in two pounds and Bob puts in two pounds. We know we have to get
a dog and a rabbit, OK? That’s our third rule. So that means if Alice
puts in two pounds, Bob puts in two
pounds, we get a dog. But that means also that we
get a dog in this case as well. Alice puts in two pounds,
it gives you a dog. OK. The trouble here is that this
doesn’t work because we’re not going to get a dog and a
rabbit in this top case, only in the bottom case where
they both put in two pounds. So that one is wrong. Now so what it
means is we can only satisfy those rules
three times out of four. We get 75% success rate. Maybe we can do better. Well, no matter how you try and
juggle it to see if there are any other combinations that
work, you’ll find it won’t. This is the best you can do. You can only satisfy these
rules three times out of four. OK, but what if
Alice’s and Bob’s boxes could switch their
output depending on what the other one put in? Then it’s a different matter. You know, then we could
say, maybe, Alice’s box gives a dog when Bob
puts in two pounds, but a rabbit when Bob
puts in one pound. OK, well that might work. The thing is, then
we have to know what one has put in before
the other box decides what it’s going to give out. So we need to have
some communication between the boxes. So we need to wipe
them together, and they’ll send a
signal between them. And then we can do better. Well, OK, that’s
fine, but this signal has to travel down the
wire, and it can only do that at the speed of light. That’s fine if they’re here. That takes no time
at all virtually, but it takes some time. And in fact, even at
the speed of light, if Alice’s box is here
and Bob’s box is in, let’s say, Fiji, on the
other side of the world, it takes tenths of a second
for the signal to travel there. So we have to wait that long
before Bob puts in his coin, before Alice puts in her coin,
whichever way round we do it. So we can’t do any better
than this instantaneously, if Bob and Alice put in
their coins instantaneously. So we’re kind of stuck. This communication
won’t work if we’re looking at how to solve this
problem instantaneously. However, these are
classical boxes. Now what happens if
they’re quantum boxes? Well, then we can do
better because it turns out that the rules of
quantum mechanics permit us what looks like
a kind of communication between the boxes that
happens instantly, and which would allow
the boxes to share some information between them
without any physical connection between them. I’m going to say some more
about this quantum effect that allows us to do this. Just take my word
for it at the moment that quantum mechanics
allows us to do it. Well, then we can do better. Then what Bob puts into
his box can instantaneously seem to affect what
Alice puts into her box, and then we can do better. So does that mean, then, that
we can satisfy these rules all the time? Well, actually we can calculate,
using quantum mechanics, how well we can do in that case. And it turns out that if
they’re quantum boxes, we can’t quite get
100% success rate. We can get precisely– well,
not precisely, roughly– 85% success rate
using this quantum what seems like communication
between the boxes. Now what I just told you about,
this mysterious quantum link, is the quantum phenomenon
called entanglement. And I wanted to do
that without any math, without any Schrodinger equation
of wave particle duality, even with anti-particles,
just with sylvanians. Hang on, though. What’s going on here? Because doesn’t Einstein’s
theory of special relativity say you can’t send any
signals faster than light? The speed of light is the
ultimate cosmic speed limit. Well, that’s true. But you see, what’s
going on here is that Alice and
Bob can’t actually verify that they’ve
got this 85% success rate without swapping
information about what their boxes produced. And the only way
they can do that is by communicating with each
other in some normal way, by email, by carrier
pigeon, by letter, whatever. However they do it, they
can’t do it faster than light. And it turns out
that actually this is what special
relativity forbids, that you can’t verify that
you’ve got this success rate faster than light. And what that
effectively means is that Alice and Bob can’t use
this quantum entanglement to send any information to
each other faster than light. And that, it turns out, is
fine with special relativity. Well, entanglement
was discovered in 1935 by Albert Einstein and by
two younger colleagues called Boris Podolsky and Nathan Rosen,
who were, perhaps ironically, trying to show that quantum
mechanics, in their view, had a shortcoming. And so Einstein,
Podolsky and Rosen came up with a
thought experiment that they believed
revealed a deep paradox at the heart of quantum
theory, and which could only be resolved by
adding something more to it. And this thought experiment
was later put in a clearer form by the physicist David
Bohm, and that’s the form I’m going to talk about now. And what Bohm envisaged
was something like this. You have a box
that spits out two particles in
opposite directions, and they are entangled together. The way they’re produced
means that they’re entangled. And what that
means is that there is some relation between
the properties of one and the properties of other. And let’s think about
it in terms of spins. So you can entangle
them in such a way that if the spin of one
of these particles is up, the other one has to be down. So then if we make a
measurement on one of them and we see it has a spin up,
we know that the other one will have a spin down. So they’re correlated. Now perhaps you
can see that this is a little bit like these two
boxes here, but in the sense that the measurement here
is playing the same role as me putting coins in. And what we see,
spin up or spin down, is a binary choice, just like
getting a rabbit and a dog. So this was really an
entanglement experiment. Now actually, this correlation
might sound unremarkable to you because you might say,
well, we could do this with a pair of gloves, let’s
say, a left-handed glove and a right-handed glove. We could send one to Alice
in Melbourne or something, and one to Bob in
Shepherd’s Bush. And then as soon as
Bob opens his parcel and sees that he’s got
a left-handed glove, he knows instantly that Alice
has got a right-handed glove. Instantly he knows that must be
true because they started off as a pair. So what’s the big deal? Well, here’s the big deal. According to the
Copenhagen interpretation, the direction of
these spins up or down for these two
entangled particles, unlike the handedness
of those two gloves, isn’t actually determined
until we observe them, until we make a measurement. And if that’s so,
then this experiment by Einstein, Podolsky
and Rosen seemed to be saying that making a
measurement on one particle somehow instantly
fixes the other, as if the result of
that measurement is being spookily communicated
to the other particle instantly. This is what Einstein called
spooky action at a distance. And again, he said
it can’t be right because special relativity
seems to forbid it. Well, for a long time
no one knew quite how to sort of resolve
this paradox, what the flaw in the reasoning
or what the problem was. Or maybe Einstein and
Podolsky and Rosen were right. No one knew what to
do with it, and it was brushed under the carpet. That changed in 1964,
when an Irish physicist named John Bell, whose job– sort of like John’s, I
guess– was a particle physicist at CERN in Geneva. But in his spare time, he turned
quantum mechanics on its head, and he reformulated the
Einstein-Podolsky-Rosen experiment in a
way that showed how you could make a measurement
to try to figure out what was going on here. And in fact, what he’s
drawn on the blackboard there is basically a
diagram of the experiment that he thought of. And this experiment,
this procedure, it’s kind of slightly analogous
to these black boxes here because what John Bell basically
showed was that if you make some measurements and you find
that there’s a certain amount of correlation– in fact, in his
case, again, 75% correlation, so the rules seem to be
obeyed 75% of the time– then it shows that
you’ve got something like classical
physics, or in fact, something like what Einstein,
Podolsky and Rosen thought was going on, which was
basically saying those spins must have been fixed all along
somehow by some variable that is hidden, that we can’t
see and can’t measure. But if you get a better
correlation between the two spins, if you get this 85% that
quantum mechanics predicts, then Einstein’s
picture doesn’t hold. Quantum mechanics must be right. You must get this strange
what looks like communication. And when these
experiments were done– they were first
done in the 1970s, they were first done kind of
more rigorously in the 1980s, have been done
countless times since– every time they’ve shown
the same clear result, that quantum mechanics is right. You get a better
correlation than any kind of classical physics or any
kind of Einstein-like hidden variables picture can give you. So entanglement really happens. But what was wrong, then,
with Einstein’s reasoning in this experiment? Well, he made the perfectly
reasonable assumption– so reasonable we didn’t even
realise it was an assumption– that we can call
locality, that the idea that the properties of a
particle, of an object, are located on that object. I mean, it just
stands to reason. This box’s blackness
is in the box. What would it
possibly mean to say this box’s blackness is also
kind of partly in this box? But in quantum mechanics, we do
seem to say things like that. It seems that
properties of objects, of quantum objects, when they’re
entangled, can be non-local. And it’s only if we make
this assumption of locality– that everything to
do with this object is fixed here in this location–
it’s only in that assumption that we have to start
thinking about spooky action at a distance and this kind
of affecting this instantly through space. What quantum
mechanics really tells us is that there’s something
else, this thing that is just vaguely called quantum
non-locality, which means that there’s a kind of mixing
of these two things that is very hard to put into
words, but it means that there’s a non-local
influence that means, in effect, we can no longer
think of these two boxes as separate objects. That’s what entanglement means. They’ve somehow become part
of the same quantum entity. So quantum non-locality isn’t
spooky action at a distance. It’s the alternative to
spooky action at a distance. Now Erwin Schrodinger,
when he saw what Einstein, Podolsky and Rosen had said, he
recognised that this phenomenon of entanglement actually was
pretty central to what quantum mechanics was really about. And in fact,
entanglement is what happens all the time when any
quantum particle interacts with any other. They have to become entangled. That is the only
thing that can happen, according to quantum physics. And what this means is that
as a quantum object starts to interact with its
environment, its quantumness, you could say– or you could say if
it’s in a superposition, its superposition starts to
spread into the environment. And it becomes harder
to see the quantumness, that superposition, in the
original object itself. It’s sort of spread out like
an ink drop spreading in water. And so what that
effectively means is that the quantumness
starts to get washed away. This entanglement
leads to a loss. Technically the word
is a decoherence of quantum properties. And it seems to
be that ultimately leads to quantum objects
behaving like classical objects as they start to interact
with their environment. So what that’s really telling
us, and what we can now say, is that there isn’t some
strange situation in which little things like atoms
obey quantum rules, and then, for some reason, big things
like us obey classical rules, and they’re just
different things. Actually, we can
now say that this is what quantum mechanics
looks like when you’re six feet tall, that the
weirdness that we talk about in quantum mechanics is just
the way the world works. And in fact, it’s
kind of us that are weird because by the time
quantum mechanics has become this scale, it kind of
looks different to how it does when you’re talking
about photons and electrons. Why, though, does
quantum mechanics only allow us 85% success? Why doesn’t it allow us 100%? Well, it turns out that
the answer, really, is about how efficiently
these boxes can share that information about,
in this case, what coin was put into them. It’s about the efficiency
of information sharing. If we can make use of
quantum entanglement, then we can improve
the efficiency with which information
is shared between quantum objects like qubits. And this is really how quantum
computing gets its power, by more efficiently
sharing the information among the different
bits of the system than we can use when
we’re using classical bits like little
transistors in laptops. And what it also
tells us is that what makes quantum
mechanics quantum at root doesn’t really
have anything to do with notions of wave functions
and collapse and wave particle duality. It’s really about what can and
can’t be done with information. And let me give you a sense
of where that’s leading us because it’s meant that some
researchers feel that we might be able to reconstruct quantum
mechanics from scratch, getting rid of things like the
Schrodinger equation of waves and particles, but just using
some simple axioms about what is and what isn’t
permitted with information, how it can be encoded and
transferred and shared and read out. And I want to give you just a
flavour of one of these what are now called quantum
reconstructions. This is one– there are
many– this is one suggested in 2009 by Borjvoje
Dakic and Caslav Brukner at the University of Vienna. And they proposed
three what they said were reasonable
axioms from which we might try and construct
quantum mechanics. And here they are. They probably don’t look
that reasonable, or even that, necessarily,
intelligible to you, but I’ll just briefly
say what they mean. Information capacity
was the first one. They said, let’s assume
that all the stuff, all the basic entities,
whatever they are, that make up the
world can encode just one bit of information. They’re like those spins. They can just be up
or down and that’s it. That’s all they can hold. Now they call this
assumption locality. Is a bit confusing
because I’ve just told you about quantum non-locality. But the locality, in this case,
means kind of something a bit different. All it really means
is that there’s nothing hidden behind the
scenes that’s allowing stuff to be done with information. There’s no secret
device underneath here that’s allowing these
boxes to communicate. And lastly, this idea
of reversibility. They said let’s assume
that these bits that can hold just one
bit of information, they can be
converted reversibly. You can go from a one to
zero, from a spin up to a spin down and back again. OK. They said, and they showed,
that with just these three rules about what can be
done with information, you get two possible types
of physics out of them. One is classical physics
and one is quantum physics, with just these rules. What’s more, if you tweak
this third axiom a little bit to say that let’s
assume that in order to do this reversible
sort of flipping of spins, that let’s assume that you
can do it continuously. You can continuously sort of
rotate a spin up to a spin down. If you assume that,
you get quantum rules. If you assume it has to
be just one or the other without this sort of
continuous rotation– so like a flipping a
coin, heads or tails. Once it’s down there, it
goes to heads or tails and you can’t interconvert them,
then you get classical rules. Well, I find that
kind of extraordinary. You can get so much out of
what seems like so little. And the point about these
axioms, about information, is that they can, by themselves,
lead to what looks like quantum behaviour, and all the stuff
that we get out of quantum mechanics like superpositions
and entanglement. And some researchers think that
these reconstructions might lead us to a completely
different perspective on quantum theory, perhaps one
in which the physical meaning of all this seemingly
strange behaviour is clear. Well, that remains to be seen. But what’s already
illuminating is how they focus on this
question of information, on how answers, or
measurement outcomes, are contingent on the
questions we ask just as the outcome of these boxes,
what comes out of these boxes, is contingent on what we put
in, a one-pound or a two-pound. And I think this is
the most productive way to think about
quantum mechanics. And there’s a very nice metaphor
for this perspective that was suggested by John Wheeler. Now John Wheeler, he
studied under Bohm, and he actually had
Feynman as his student. And he had this
wonderful metaphor for how our answers
about reality can emerge from the
questions that we ask, in a way that is
perfectly consistent and rule-bound and
non-random, without requiring any preexisting truth
about how things were. And this is how it goes. It’s based on the
game of 20 Questions. So this is this game I’m sure
you all know, where everyone chooses, let’s say, a person. One person goes out of
the room and everyone else chooses a person. And then this one person has
to come back in and find out who that person is
by asking questions. And they have to be questions
that only have a yes or no answer, binary questions. As you can see, this is
actually a quantum game. OK. So let’s say we
play it like this. Person goes outside. We all decide on a person. Well, we all do our
thing, and then the person comes back and starts
asking questions. And on this occasion, the
person who’s come back in, she starts off in
the normal way. She says is this
person alive or dead? Well, no, what I should
say, is this person dead? And the answer’s yes. . Is this person male? Yes. OK. And so it goes on, except
that the questioner finds that as she asks more
and more questions, it takes longer for
the answer to come. The person she asks sort of has
to think about it for a while before giving the answer, which
is kind of odd because you know, surely it’s either
one thing or the other. Why do you have
to think about it? Anyway, the game goes
on, and eventually she thinks she’s narrowing
in on who it is. And eventually she says, I know. It’s Richard Feynman. And everyone says, yes, it’s
Richard Feynman, and everyone laughs, and the game is over. But then she says,
well, what was going on? Why did it take you
so long each time, when I was asking more
and questions, to answer? And everyone explains that
they’d played the game a bit differently. They decided that they weren’t
going to decide on a person. They were simply
going to make sure that whatever answer
each individual gave when they were asked
was consistent with all the other answers in applying,
at least, to someone, ideally someone famous. So as soon as the first
question, is this person dead, was answered yes, all the
other people’s answers had to be consistent with that. Had to be a dead person
that they were thinking of. Then it had to be a dead male
that they were thinking of, and so on. But the first person
could just equally well have said no to
that first question, and then they would have
converged on someone else, not Richard Feynman. So the options become
more and more constrained as the questions proceed. And it took longer to figure
out who still is going to work? Who’s going to be consistent
with all these answers so far? And everyone was forced by
the nature of the questions to converge on the same person. If you had asked
different questions, you’d have ended up
with a different answer. So context mattered. There never was a
preordained answer. You brought it into
being, and in a way that was fully consistent with
all the questions you’d asked. What’s more, the very notion
of there being an answer only makes sense when
you play the game. It’s meaningless to
ask who the chosen person is in that
situation without asking the questions about them. And quantum mechanics
is a theory a bit like this, I think, of what
is and what isn’t knowable, and how those
knowns are related, and how they emerge from
the questions we ask. And I like to think
of this in terms of a distinction between
a theory of isness and a theory of ifness. Quantum mechanics doesn’t
tell us how a thing is. It tells us what it
could be along with– and this is crucial– along with a logic
of the relationships between those coulds
and the probability that it could be this. So if this, then that. And what this means is
that to truly describe the features of
quantum mechanics, as far as that’s
possible at the moment, I think we should replace
all the conventional isms of quantum mechanics that I kind
of started off at the beginning with ifms. For example, we couldn’t
say here it is a particle, there it is a wave. Rather, we should say
if we measure things like this, then the quantum
object behaves in a manner that we associate
with particles, but if we measure it like
that, behaves in a manner that is like a wave. We shouldn’t say the particle
is in two places at once. We should say if we measure it– if we measure it– we will detect this state with
probability x and this state with probability y. Now this ifness is
kind of 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 that arise, it’s simply because we
don’t know enough. We’re partly ignorant
about those how things are. But in quantum
mechanics, it seems like those ifs are fundamental. Well, OK, but what’s the stuff
that this ifness is all about? Quantum mechanics doesn’t,
obviously, tell us anything about that, and
all we have right now are hints and guesses. And to try to bring
them into sharper focus is a tricky business,
which I think means we have to use sometimes
an almost poetic level of expression, the
kind of thing that will send a lot of physicists
scurrying for cover. Take this attempt, for example,
by the physicist Chris Fuchs. He says “Perhaps the world
is sensitive to our touch. It has a kind of zing that makes
it fly off in ways that were not imaginable classically. The whole structure of quantum
mechanics maybe nothing more than the optimal method
of reasoning and processing information in the light of
such a fundamental, wonderful sensitivity.” And what Fuchs means here
is not the mundane truism that the human observer
disturbs the world. Rather, he’s saying quantum
mechanics may be the machinery that we humans need, at
a scale pitched midway between the subatomic
and the galactic, to try to compile and quantify
information about a world that has this incredibly
sensitive character so it embodies what
we’ve learned about how to navigate in such a place. Well, at any rate,
I think it’s vital that we understand that
this ifness doesn’t imply that the world, our
world, our home, is holding anything
back from us. It’s just that classical
physics has primed us to expect too much from it. We’ve just become accustomed
to asking questions and getting answers, getting
definite answers– what colour is it? How heavy is it? How fast is it moving?– forgetting the almost
ludicrous amount that we don’t know about most
things around us in detail. We figure that we could
just go on forever asking questions and being answered
at ever smaller scales. When we discovered
that we can’t, we felt shortchanged by nature,
and we pronounced it weird. Well, that won’t do anymore. Nature does its
best, and we need to adjust our expectations. We need to go beyond weird. Thank you. [APPLAUSE]