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

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]