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this is one of more than a thousand episodes you can

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find on BBC sounds and on our website. If

1:25

you scroll down the page for this edition, you can find

1:27

a reading list to go with it. In the end,

1:29

you can see that the German physics student Werner Heisenberg

1:31

effectively created quantum mechanics for which he later won the

1:33

Nobel Prize. He

1:35

made this breakthrough in a paper in

1:38

1925 when he worked backwards from what

1:40

he observed of atoms and their particles

1:42

and did away with the idea of

1:45

continuous orbit, replacing this with equations. As

1:48

we'll hear, this was momentous. And

1:50

from this flowed what's known as his

1:52

uncertainty principle, the idea that, for example,

1:55

you can accurately measure The position

1:57

of an atomic particle or its

1:59

momentum. The notice. When.

2:01

Me to explain and discuss Heisenberg

2:03

on is uncertainty principle are say

2:06

dog a professor of theoretical physics

2:08

at Imperial College London Hurry Cliff

2:10

research fellow in Particle Physicists at

2:12

the University Chemists and Frank Close

2:14

professor emeritus of theoretical Physics and

2:16

fellow emeritus at Exeter Dollars at

2:19

the University was trying to what

2:21

was her in his background as

2:23

suggest who's going to go in

2:25

that direction. While

2:27

he was born in Ninety, no one

2:29

in or Bavaria, his father. Was

2:32

a teacher of classics and Greek

2:34

and I think that the younger

2:36

was very interested in the ideas

2:39

of play so he read Plato

2:41

was while though he was hiking

2:43

in Bavarian mountains and are The

2:45

reason I think that that was

2:47

important for him is that later

2:50

he made a remark which was

2:52

that the smallest units of matter

2:54

are not particles in an ordinary

2:56

sense, but phones ideas only expressed

2:59

in mathematical language. So I think

3:01

it. Was that's classical background from

3:03

his sir his father? That perhaps made

3:05

him look that way but he became

3:08

interested obviously in in math and physics

3:10

and that was what he went to

3:12

study as an undergraduate to to Munich

3:15

and then getting and from nineteen twenty

3:17

to twenty three. I.

3:19

Think the seminal moment for him was

3:21

in Nineteen Twenty Two, he went to

3:24

a lecture given by Niels Bohr. A

3:26

ball was famous for having come up

3:28

with the model of the atom. As

3:31

late as a miniature solar system

3:33

with like a nucleus son in

3:36

the middle and electrons like planets

3:38

willing roundly outside. And this, pete

3:40

the Heisenberg philosophically, because nobody's seen

3:42

these electrons orbiting around so what

3:45

really makes you believe that they

3:47

were there? And so he spent

3:49

a year then working. With.

3:51

ball in copenhagen whipple was based

3:53

on i think it was during

3:56

that year that the mathematical schemes

3:58

that heisenberg developed first began to

4:00

mature based upon what you can

4:02

see. Namely, the one thing we do know

4:04

about atoms is that they emit light. They

4:07

don't emit a whole rainbow, but they

4:09

emit like a barcode of individual lines.

4:12

And why was that? And how could you mathematically

4:14

describe that? And in 1925, he came

4:16

up with the equations that did that. And that is

4:18

what we call now the birth of quantum mechanics. Frank,

4:21

can you tell us the essence of

4:24

quantum theory before Heisenberg stepped

4:26

into the picture? Well, quantum

4:29

theory as such really

4:31

began around 1900. Up

4:33

to that time, nature appeared to

4:35

be continuous. Light,

4:38

there's a whole spectrum of light.

4:40

Motion is a continuous thing.

4:43

But that is how things are

4:46

in the large scale world, which

4:48

we are aware of day to

4:50

day, in which the scientists up to that

4:52

time have been examining. But

4:55

Max Planck, a German physicist, had

4:57

the insight that if he assumed

4:59

that instead of continuous,

5:01

nature was actually discrete, that's

5:03

what the word quantum describes,

5:06

there were certain things that could be

5:08

explained that otherwise made no sense. For

5:11

example, he assumed that electromagnetic waves

5:13

are not a sort of smooth

5:15

legato wave, but more like a

5:19

staccato bunch of what we

5:21

call photons, particles. And

5:23

by making that assumption, he was

5:26

able to explain the way that

5:28

hot bodies radiate light. Without that

5:30

assumption, the classical theory of Maxwell just

5:32

didn't work. And then

5:35

Einstein picked up on this idea

5:38

and supposed that indeed these

5:40

little particles of light existed

5:42

as they crossed space and hit things. And

5:45

with that, he was able to describe

5:47

what happened when light hit metals and

5:49

kicked electrons out, called the photoelectric effect.

5:52

So these were the first two indications that

5:54

if you assume that nature was discrete on

5:57

a small scale, things worked

5:59

well. Then along comes Niels

6:01

Bohr in 1913. Ernest

6:04

Rutherford has discovered the existence

6:07

of the atomic nucleus. The picture

6:09

of the atom that emerges is

6:12

pretty much one that is a good model today.

6:14

As I mentioned earlier, the idea of a miniature

6:16

solar system with the nucleus sun at the middle

6:19

and the electrons, the planets whirling around on the

6:21

outside, that was the

6:23

sort of picture that emerged. The

6:25

problem is that if you use the

6:28

classical theory, the electrons

6:30

whirling around would just spiral into the nucleus in

6:32

a fraction of a second and we wouldn't be

6:34

here. So Bohr made

6:36

the assumption that this discrete idea,

6:38

this quantum idea, applied to electrons

6:40

in atoms, that they couldn't go

6:42

anywhere. They were like on

6:44

rungs of a ladder that they could step

6:46

down. And as you step from

6:49

one rung to the next, a high energy

6:51

rung to a low energy rung, the

6:53

energy difference is radiated as light and that

6:55

is why you see a spectrum

6:57

of lines, a barcode for the

6:59

atoms. And that was Bohr's model.

7:01

But again, it was all ad hoc and

7:05

it worked. But it was a

7:07

bit like imagining back in the

7:09

17th century, people were aware

7:11

that apples fall to earth or

7:14

if you kick something, it moves. But

7:16

the equations, the mechanics, Isaac

7:18

Newton's laws of dynamics had not yet been

7:21

written down and it was a bit like

7:23

that. The idea that nature is discrete on

7:26

the very small scale was

7:28

clearly true, but the equations

7:30

of the quantum mechanics had

7:32

not yet been written down. And that

7:35

is what Heisenberg made the first step in doing. Thank

7:37

you, Faye. Can

7:39

you set the scene for why Heisenberg in 1925,

7:42

why his paper was so distinctive?

7:45

So as Frank described, there was

7:47

an ad hoc model of the

7:50

atom of atoms, but no internal

7:53

dynamics to describe why the model

7:55

had the structure that it does.

7:58

So Heisenberg steps in and makes a

8:00

number of conceptual moves in 1925.

8:04

So he takes on board

8:06

the borion structure of

8:08

states that the

8:10

electron in the atom can be in, and they

8:12

are discrete. So you can

8:15

number them, you can label them,

8:17

one, two, three, four, and they

8:19

are labeled by their energies. So

8:21

there are higher energy states, lower

8:23

energy states. So he takes that

8:25

and doesn't change that particular idea.

8:27

What he does change is that

8:29

he denies the idea that the

8:31

states correspond to the electron having

8:33

particular orbits in space, that the

8:35

electron is going around the nucleus

8:37

at a fixed radius from the

8:39

nucleus and with a fixed periodic

8:41

rotation. So he just

8:43

denies that, he says, forget that idea. As

8:46

Frank said, that's not, he argues,

8:48

that's not an observable thing. We

8:50

cannot experimentally determine where the electron

8:53

is inside the atom. So let's

8:55

just say that that's not

8:57

even speakable. We won't even speak

8:59

about that. We just think of

9:01

these states as being abstract states

9:03

of the electron. So that's the

9:05

first thing. He then centers a

9:08

concept of the transition between

9:10

these atomic states that

9:13

he had already worked on with

9:15

my collaborator, Hendrik Cramer. So the

9:17

transitions are not deterministic. You can't

9:20

predict with certainty when or whether

9:22

an electron in one state will

9:25

transition to another particular state. There's

9:27

just some probability for each possible

9:29

transition. So he takes that and

9:32

the intensities of the

9:34

radiation that the atoms

9:37

emit, that was

9:39

an experimentally determinable quantity that

9:41

experimentalists were measuring in the lab.

9:44

And the probabilities of these

9:46

transitions translate exactly into the

9:48

intensities of the radiation of

9:51

those particular frequencies. So

9:53

if you can calculate the probabilities

9:55

of the transitions between these atomic

9:57

states, then you can predict the...

10:00

The intensities of the particular frequencies of

10:02

light That will be admitted. That.

10:04

All of that was somehow already in

10:06

the literature, but he takes thought and

10:08

he says, okay, I need to predict

10:10

the actual entities of the of these

10:12

atomic states. And I need

10:14

to predict the probabilities. How am

10:17

I going to do that? And

10:19

he works backwards. New said.in, your

10:21

introduction, that's exactly right. He worked

10:23

backwards from the form of the

10:25

measured experimental outcomes of these intensities

10:28

undies, probabilities, and these energies, and

10:30

he asked what would see position

10:32

of the electron has to be

10:34

like in order to give me

10:36

these particular results. These particular experimental

10:39

results and what he discovered was

10:41

that it led him to a

10:43

really. Startling proposal that the

10:45

position of an electron and

10:47

enough and is not given

10:49

by some a number. It's

10:51

not here or there or

10:53

here, but the position is

10:56

represented by a completely new,

10:58

unexpected mathematical and stakeholder matrix.

11:00

This is something very abstract.

11:02

It's not something that can

11:04

be conceptualized as an actual

11:06

place where the electron is

11:08

in space. And he also

11:10

he postulated that the matrix

11:12

that corresponds to. The position

11:14

of an electron satisfies an equation

11:17

of motion. so it's a dynamical

11:19

thing. This is the diaper quantum

11:21

dynamics that Frank described that was

11:23

needed necessary to have to complete

11:25

the quantum formalism to a fully

11:28

fully fledged theory. And the equation

11:30

of Motion said. this matrix position

11:32

or position matrix was the analog

11:34

of Newton's Second law. Says in

11:36

one way Heidelberg was be very

11:38

revolutionary saying that positions are not

11:41

conceptualize a bull as being in

11:43

space Three. Dimensional space that he's

11:45

made to say that on the

11:47

other hand, the equation of motion

11:49

that these mates Aziz obey is

11:51

just a normal, expected, familiar two

11:53

hundred year old. Newtonian.

11:55

Equation of Motion for Evolution as

11:58

of. Position. Thank you Hurry Harry

12:00

Smith can we use develop this paper Hi

12:02

came about and is it doesn't have a

12:04

precious does his hair or thought experiments was

12:06

going on or that the delivery story actually

12:08

around that kind a key insight that leads

12:10

to this paper which is cause bugs getting

12:13

and working with Max born of the time

12:15

but he suffers from terrible hay fever and

12:17

he is so so determine what mad by

12:19

as we go to one place in over

12:21

there aren't any trees which this island called

12:23

Helga land of the German coast in the

12:25

in the north sea see retreats other stays

12:27

in this a loading house essentially and is

12:29

alone. With his thoughts and the wind and the

12:32

city and bits of rock. And it really does

12:34

Hay fever and as it's not a description of

12:36

instead of in the middle of the night he's

12:38

been doing these calculations and he has the Kentucky

12:40

realization that that face been been describing and it

12:43

is lovely. Close memory says i think it's a

12:45

sense or dislike to make or break that. As

12:47

he said of first, I was deeply alarmed. I

12:49

had the feeling that I had gone beyond the

12:52

surface of things and was beginning to see strings

12:54

the beautiful interior and felt dizzy says Israel moment.

12:56

I think this quote romanticized the history of science

12:58

about this. This breakthrough. That he has

13:00

and as I said that the car algebra

13:03

that he discovers or the he finds applies

13:05

to these quantum transitions didn't actually recognizes matrix

13:07

algebra to begin with. It's he has a

13:09

strange rules about how you multiply these different

13:11

what they called amplitudes together in a particular

13:13

Clinton particular rules. and it's when he shares

13:15

his paper with his colleagues. Born in Jordan

13:18

the I think I think is born who

13:20

is a senior and will senior academic and

13:22

getting and who sort of in the back

13:24

of his my things I recognized a strange

13:26

algebraic Laurie I'll have learnt about this years

13:28

ago and it's the. Way makes Aziz most

13:31

matrix makes Cds grids of numbers multiply

13:33

together so it's kind of. He's only

13:35

sort of stumbled upon this algebra. an

13:37

accident is discovered out of the math

13:39

already exists and it's split Very. I'm

13:41

familiar to physicists at the time and

13:43

I think there's this really struggled to

13:45

get ahead around Highs and does Zippers

13:47

because it's so abstract and he used

13:49

the mathematical language that the time is

13:52

ready unfamiliar to people and is actually

13:54

just a year later Another German physicists

13:56

could I when Schrodinger to comes up

13:58

with a different approach to the same

14:00

problem which is based rather than undies

14:02

strain mathematical objects called matrices on take

14:04

much more familiar which is a wave.

14:06

So trading A has this is waived

14:08

description of say an electron around an

14:10

atom and in a wave is something

14:12

that's intuitively much easier to understand such

14:14

as this ever used to dealing with

14:17

the algebra and the science of wife's

14:19

I actually it's later realize that really

14:21

shredding his wife, pitcher and Highs and

14:23

Bugs matrix mechanics or accedes two different

14:25

mathematical ways of elderly describing the same

14:27

saying. But there's a period where trading

14:29

his approach. Is really adopted. Much more

14:31

enthusiastic about the key to because it's

14:33

familiar and this is what he ought.

14:35

Heisenberg and it becomes actually quite a

14:38

sort of a bad tempered add said

14:40

of debate between Frederick Heisenberg as to

14:42

which pitcher is that the correct one

14:44

who wins. Well. In the end actually

14:46

I mean says what's hurting a tries to say is

14:48

that you can think of the electron as a as

14:50

a physical wave so when it when it said as.

14:53

He goes into one of these states

14:55

around the atom. It's a sort of

14:57

physical say that adopts is strange wave

15:00

like structure and more Heisenberg and others

15:02

eventually show is actually this isn't right

15:04

that that the wave isn't really a

15:06

physical thing and it's is later reinterpreted.

15:08

I think by Born On he says

15:10

actually this is not a physical ways

15:12

it's a mathematical objects and what Is

15:15

Rave describes is not. The. Sort

15:17

of the physical nate the electron. But it

15:19

tells you the probability of finding the electron

15:21

at a particular place in space. so it's

15:23

him in some ways is. no, it isn't

15:26

it in the sense concepts. It's not so

15:28

different from Heisenberg. Highs of that has his

15:30

way of representing this information. The grid of

15:32

numbers essentially fruiting her equally has this way.

15:34

But it's not a physical objects. A mathematical

15:37

description of the electron, so nothing is observed

15:39

is everything a thought experiment that. Well.

15:41

I mean. Nothing's. Are observe said

15:44

he spectral lines that Frank talks about. These

15:46

are the key bits of evidence about what's

15:48

going on in the quantum realm. So what

15:50

you do see an experiment cities particular frequencies

15:53

of lights that are absorbed and emitted as

15:55

electrons transition but you never see. ah the

15:57

waves that fertig A and will set you

15:59

know and eyes of bugs Description in a

16:02

way not visualize a bullet holes. So in

16:04

some sense A you have to kind of

16:06

give up on the idea of a mental

16:09

picture of what's happening in that's I suppose

16:11

in some ways was quite revolutionary about Eisenberg

16:13

Saying he says you stick to what you

16:15

can measure and you shouldn't concern yourself with

16:18

kind of it and imagine and imagined picture

16:20

of what's actually going on because only what

16:22

you see in experiments ultimately matters. Did previous

16:25

series in this area just follow after this

16:27

and didn't display say having that Newton said.

16:30

No, I mean I think that there's air,

16:32

sometimes a misunderstanding and that the history of

16:34

science that you have you have one picture

16:36

of the well as a revolution and that

16:38

overtones what does that before? But that's not

16:40

really the way things happen you. You kind

16:42

of realized that acted as a domain in

16:44

which. The. Old. Physics.

16:46

Doesn't work. Buddies. To works

16:49

very well. I'll save you wanted about the

16:51

a Tennis balls going to the news. Laws

16:53

of Motion A perfectly good so that. but

16:55

it in some ways as poses an extension

16:57

of Newtonian mechanics and at it now applies

16:59

the behavior of things that are much smaller

17:01

when you zoom out Newton's Laws to work

17:03

perfectly well. but a breakdown when you get

17:05

down to the scale of atoms and molecules

17:07

is managed to pick up on what? What?

17:09

Harry Saying this but Newton's Laws. Apply.

17:12

Very very well. Two things that

17:14

are big and move around relatively

17:16

slowly. By that I'm in slowly

17:18

compared to the speed of light

17:20

and the tude right revolutions of

17:22

early twentieth century where the Einstein

17:24

os what happens if you go

17:26

to very fast things and made

17:28

the relativistic extension. Of. Newton's Laws

17:31

And now we've got the other

17:33

extreme a very small Things which

17:35

is the quantum extension of Newton's

17:38

laws. So. newton's laws

17:40

off a limousine case of einstein

17:42

at slow speeds and eisenberg at

17:45

large scales can you sum up

17:47

for listeners he stopped of in

17:49

the highest terms by use roomba

17:52

and by other physicists have the

17:54

time and since what his breakthrough

17:56

woman's what he or she broke

17:59

through was different after he'd done it in 1925.

18:04

One of the most remarkable things about his breakthrough was he was only 23 when he

18:06

made it. I mean, that, to

18:08

me, is one of the astonishing things. Harry

18:11

just made the remark about Schrodinger. Not to avoid the

18:13

question, but to put it in a bit of context.

18:16

He came up with this set of

18:18

matrices to describe the quantum world

18:21

of the atom. It

18:23

is pretty difficult to construct the

18:25

matrices. And the fact

18:27

that Schrodinger comes along a year

18:29

later with this wave equation approach.

18:33

And the tools that you need mathematically to deal with

18:35

that are called differential equations. And they're the things that

18:37

all students have learned and are familiar with by

18:39

the time you get to graduate school and meet these

18:41

sort of ideas. So we get

18:44

taught the Schrodinger wave approach. And

18:46

actually, I think one tends to

18:49

use that approach. There's only

18:51

a few occasions I can think of

18:53

where I actually use Heisenberg matrices. And

18:56

because the Schrodinger approach

18:58

became so not

19:01

easy, but relatively speaking, convenient

19:03

to use, that's why

19:05

we tend to think of waves all the

19:07

time. But that quickly gets you into these

19:09

philosophical problems of waves in what? And probabilities,

19:12

and does God play dice with the world,

19:14

and so on and so forth, which may

19:16

be an artifact of the waves, which, in

19:18

Heisenberg's opinion, aren't really there. So

19:21

that's avoiding your question slightly. So what is

19:23

it that really has been done at this

19:25

point? The equations of motion,

19:27

the dynamics that apply to

19:29

the micro world have been identified.

19:31

And you can now apply them to

19:34

the micro world, which you could never do

19:36

before. And one

19:38

of the first things which Heisenberg's

19:40

approach discovers is that

19:43

hydrogen, we talk of the hydrogen atom,

19:45

the simplest thing, a single proton with

19:47

a single electron whirling around the outside.

19:49

But hydrogen tends to exist as a

19:52

molecule of two hydrogen atoms together. Two

19:55

protons, each at atoms length apart,

19:57

but sharing their two electrons. the

20:00

electron swapping backwards and forwards, being

20:02

exchanged between one hydrogen atom and

20:04

the other. And Heisenberg's

20:06

matrix approach turned out

20:08

to have a very

20:10

profound implication, which is

20:12

this, that the electron is a lump of

20:15

charge, but it also acts

20:17

like a little magnet. In

20:19

the jargon we say it spins. It

20:21

can spin up or spin down, like

20:23

a North Pole up or a South

20:26

Pole up. And this is the one

20:28

place where Heisenberg's matrices really come to

20:30

play, and one can illustrate them. A

20:32

simple little column of just two numbers. If you think

20:34

of a ground floor and a first floor,

20:37

if the first floor is occupied, you

20:39

put the one upstairs and the zero

20:41

downstairs. If the ground floor is occupied,

20:43

the one goes downstairs and the zero

20:45

upstairs. Those are the two

20:47

matrix descriptions of an electron

20:50

spinning up or down. And

20:52

the matrix approach of Heisenberg is perfect

20:54

for this. And he

20:56

turns out to predict that there are

20:58

two different forms of molecular hydrogen called

21:01

ortho and para, and

21:04

his matrices predict that one of

21:06

these should be three times more

21:08

common than the other at room

21:10

temperature. And in

21:12

1929, these two forms are

21:14

experimentally discovered and the

21:17

abundance confirmed in line with what

21:19

Heisenberg has predicted. And

21:21

that is mentioned in the citation

21:23

at his Nobel Prize by the

21:25

Stockholm Nobel Academy. I think that

21:28

that was the experimental proof that

21:30

showed that this formalism

21:32

that he'd created was able to

21:35

predict things and explain things which

21:37

had not previously been understood about

21:39

something as fundamental as hydrogen. Quickly

21:42

then, people started applying these

21:44

new techniques to electrons in

21:47

all manner of stuff. Electrons

21:49

in metals, electrons in insulators.

21:52

Why do metals conduct? Why do insulators not

21:54

conduct? These are questions you could not approach

21:56

before. And suddenly, within the space of a

21:58

few years, falling into

22:00

place thanks to this new quantum mechanics.

22:03

And the final thing, I think, in 1928

22:06

was that a Russian theorist

22:09

called George Gamow applied quantum

22:11

ideas to the atomic nucleus.

22:14

That one of the things that had been

22:16

known since 1896 or so

22:18

was that nuclei have a probably

22:21

called radioactivity. They emit this strange

22:23

radiation. And one form is called

22:25

alpha radioactivity. Gamow

22:27

applied quantum mechanics to

22:30

the atomic nucleus and explained

22:32

how alpha radioactivity happens. Previously

22:35

something totally unexplained. Thank you.

22:38

Faye, in what ways was

22:40

his approach revolutionary? It

22:43

was both revolutionary but also, I

22:45

would say, completely embedded in its

22:47

time. No breakthrough is made in

22:49

isolation from everything else.

22:52

Heisenberg was in constant

22:54

communication with colleagues and

22:56

embedded in his intellectual

22:58

milieu. So these

23:01

ideas of atomic states

23:03

and transitions between states, they were

23:06

already there. Another

23:08

aspect of his milieu was the

23:10

movement in philosophy, which people call

23:13

positivism or instrumentalism. And he would

23:15

certainly have been aware of that,

23:17

not least because that tradition is

23:20

generally accepted to have had quite

23:22

a lot of influence on Einstein

23:24

in his development of special relativity.

23:27

So he would have been very

23:29

aware of those ideas that physics

23:31

should deal only with observable quantities.

23:33

So he was primed to accept

23:36

the strangeness of this new

23:38

way of representing position. It was

23:40

so embedded in people's consciousness that

23:42

particles, bodies, every physical entity should

23:45

have a position in space. It

23:47

should have a position in space

23:49

and move around in time.

23:51

But he was just ready to make the leap

23:54

and say, no, we don't have to have such

23:56

a picture. Thank you. Harry, How

23:58

did the uncertainty of the. The

24:00

principal immersion So story was Heisenberg is

24:02

most associated with because it has his

24:04

name attached to and it comes really

24:06

from this matrix description of quantum mechanics

24:08

and it it comes ultimately mathematically from

24:10

the way that matrices multiply with each

24:12

other which is different from ordinary numbers

24:14

if you take two or three numbers

24:16

say one and two or his bad

24:18

example but I wanted to find one

24:20

time to is two and two times

24:22

One is also to doesn't matter which

24:24

order you multiply meat and switch them

24:26

around and it's the same with a

24:28

matrix. that's not true. Sir a

24:31

sudden matrices that describe particular properties of

24:33

an electron. For example to have a

24:35

matrix the describes the position as he

24:37

likes on that say refer to and

24:39

also it's as as corresponding quantity called

24:41

momentum which is essentially related to how

24:43

fast electrons going and multiply by it's

24:46

mass. You have these two matrices and.

24:48

The ordered the you multiplies matrices to get

24:51

a masters say thieves and think of them

24:53

in some ways is representing measuring I'd the

24:55

position or the momentum as the electron. So

24:57

it matters whether you measure the position first

25:00

and then the momentum or the momentum fast

25:02

and then the position and they give you

25:04

different answers and from this mathematically what are

25:06

essentially if he worked with consequences as you

25:08

find that there was a limit to how

25:11

well you can simile Tennessee know the position

25:13

of a quantum particle and it's momentum and

25:15

that is what up there on Santi principal

25:17

states would have. The. Consequences of on

25:19

frank. For. Sound

25:22

and word. m That is

25:24

how the universe is. m.

25:26

Miniseries had this a trade off. You can.

25:29

If. You know the position personally. You can't

25:31

know. Anything. About the momentum and

25:33

vice versa says a trade off on.

25:36

What? You can know on the average

25:38

about both of them. Have enough and

25:40

give an example to p Want to

25:42

thinking well this is the odds that

25:45

visiting the wave on a pond. And

25:47

if I want to know. The.

25:49

Position I just look at

25:51

the where the. That.

25:53

The high spot in the ruthless. But that tells

25:56

me nothing at all about how fast the waves

25:58

moving to get a measurement of it's. I've

26:00

got to watch some ripples pass me

26:03

and the more ripples that pass the more precisely

26:05

I will know the speed of that wave but

26:08

of course the less I know about the position

26:10

because there's been so many waves that have come

26:12

past and vice versa. If I want to measure

26:15

the position precisely I can't know anything at all about

26:17

the speed. So that is

26:19

an example that is familiar

26:22

and now I'm getting into the area

26:24

where I start sort of feeling I'm

26:26

going in a whirlpool around some

26:29

great black hole of ignorance or enlightenment I

26:32

can't be sure which. Waves

26:35

are a very convenient model

26:37

to understand why

26:39

the uncertainty principle can apply to

26:42

things. Is it

26:44

the uncertainty principle that is fundamental and

26:46

waves are a nice model that help us visualise

26:48

it or is it

26:51

waves that are fundamental and the uncertainty

26:53

principle is a consequence of that? I'm

26:56

in the first camp because I think that

26:58

actually the moment you start inventing these waves

27:00

they're a very nice model and we think

27:02

of them like that but

27:04

the moment they start becoming too much reality

27:06

in quotes you get into all of these

27:08

horrible paradoxes about Schrodinger's cat and so forth

27:10

which is not for me I mean I'm

27:12

out of the studio if we're going there.

27:15

So that is the profound nature of it. Where

27:18

it applies what it matters position

27:20

and momentum are

27:22

complementary. Energy and

27:24

time is the other side to

27:27

this. If you know at an instant where

27:29

something is you know nothing at all about

27:31

its energy. If you know its energy precisely

27:33

you know nothing at all precisely about the

27:35

time of the measurement and

27:37

this surprisingly explains why CERN

27:40

is so big. People often say why

27:42

do you need this huge accelerator device

27:44

27 kilometres to measure these

27:46

things that are so small? Answer,

27:49

blame Heisenberg. These

27:51

things are so small you need to

27:53

have incredible precision. In space to resolve

27:55

them and to get that precision you

27:58

have to have extremely high energy. and

28:00

vice versa. There's a good

28:02

joke we get some of it across maybe in

28:05

a bit more easy understandable way which is so

28:07

Heisenberg is driving along in his motorcar and he's

28:09

stopped by a police officer who says do you

28:11

know how fast we're going sir? He says no

28:13

but I knew exactly where I was. If you're

28:15

a physicist that's very funny. But I mean I

28:18

think one of the things we haven't really talked

28:20

about which I think is important about the uncertainty

28:22

principle is it says the thing that's really controversial

28:24

about it is it makes the the observer the

28:26

experiment a part of the system in a way.

28:28

So it matters what you choose to measure determines

28:31

what you will actually observe and that so there's no

28:35

longer this it's sort of there's no longer this

28:37

idea of an experiment as an objective thing that

28:40

just looks at nature as it is. The choices

28:42

you make in your observation determine the results you

28:44

get. So if you choose to measure momentum or

28:46

you choose to measure position you will get you

28:49

will change the system fundamentally and change what you

28:51

see and that is I think

28:53

you know what one of the most difficult ideas

28:55

for people to accept when this is come up

28:57

with in 1927. I think

28:59

Fay alluded to this when you're saying about

29:01

the Heisenberg said nobody's seen

29:03

these electrons in orbits but he did a sort

29:06

of thought experiment about what would you actually have

29:08

to do to detect one of

29:10

these electrons in an orbit. The answer is

29:12

well you'd have to shine light on it

29:14

and then the light would scatter back to you and of

29:17

course in the process because an electron is

29:19

such a tiny thing the action of the

29:21

light that went out hitting it has kicked

29:23

it off somewhere else. So

29:25

you know where it was but not where

29:28

it is sort of thing and this also

29:30

perhaps gives an intuitive feeling for why it

29:32

is that the uncertainty principle doesn't really concern

29:34

us in our day-to-day affairs. It's

29:37

absolutely the essence of the whole thing

29:39

when you're down at the very small

29:41

atomic scale where the little particles are so light

29:44

and photons hit them and kick

29:46

them all over the place but by the time

29:48

you get to macroscopic stuff like us the

29:50

effects are so tiny I can know precisely where

29:52

Melvin is and I can know precisely that you're

29:54

not moving at this moment because you're so wrapped

29:56

by what is going on in the studio. If

30:00

I could somehow shrink you down to the size of the atom,

30:02

I'd be able to know one of those but not both of

30:04

them. I was going

30:06

to ask, what

30:08

value, I mean, this is a lumpen proletarian

30:11

question, I hope you don't mind asking it,

30:13

what value does this have for most

30:15

people? This discovery, people

30:18

think we are better informed in order

30:20

to do what? It's hard

30:22

to overestimate the impact that quantum mechanics

30:25

has had, scientifically and

30:27

technologically, it affects our daily

30:29

lives. So from the

30:32

predictions of the abundances of the

30:34

light elements produced in the first

30:36

few minutes after the Big Bang,

30:38

through the standard model of particle

30:40

physics that's tested and explored at

30:42

CERN, to the behaviour

30:44

of semiconductor materials that are

30:46

in the chips of all

30:49

of our phones. So it has had a

30:51

huge impact on all of science,

30:53

much technology that we

30:56

all use. So you simply can't overestimate

30:58

just how successful it has been. And

31:01

yet, it's such an interesting

31:03

situation whereby, I know I

31:05

may drive Frank out of

31:07

the studio in a moment,

31:09

but I would claim that

31:11

we cannot deduce, we cannot

31:13

recover classical physics from quantum

31:15

physics. It is not possible to

31:18

take quantum physics as Heisenberg set

31:20

it out, and from

31:22

that deduce, recover, have classical

31:24

physics, the physics that describes

31:27

the behaviour of macroscopic objects,

31:29

the things in this room, from

31:31

that quantum world. But from the

31:33

rules of quantum theory as Heisenberg

31:35

laid them down, because he was

31:38

very clear that the theory is

31:40

empty unless there's an observer. If

31:43

there's an observer observing the system,

31:45

then the theory, the quantum mechanical

31:47

dynamics and the rules of prediction

31:49

give you predictions about the results

31:51

of observations that the observer makes

31:53

on the system. If there's

31:56

no observer external to the system,

31:58

then you cannot make an observer.

32:00

predictions. People call this the

32:02

Heisenberg cut. It's necessary. You have

32:04

to divide the world, the whole

32:06

world, into two pieces. One, the

32:08

quantum system to which the quantum

32:10

dynamics applies. There are

32:13

quantum states. There are these

32:15

matrices that correspond to, I won't say

32:17

describe, they correspond to positions

32:19

of particles. And all

32:21

of that takes place in this abstract mathematical space,

32:24

which is L-squared. Then the rest

32:26

of the world on the other side

32:29

of the Heisenberg cut is the classical

32:31

world, the world of observers, of experiments,

32:33

of apparatuses. And

32:36

we can make definite statements about the

32:38

outcomes of the measurements that we make.

32:41

And there's this mysterious and absolutely

32:43

not set out in any axiomatic

32:46

way interaction between the two. Of

32:48

course, one of the heuristics of

32:50

quantum theory is that when you

32:52

interact with, when you measure, when

32:54

you observe the quantum system, then

32:57

your decision about what to observe and

32:59

what to measure, how you set up

33:01

the apparatus determines what the possible outcomes

33:03

of your of your experiment are going

33:05

to be. The actual

33:07

interaction is not described by the

33:09

theory. My students will say, oh

33:12

yeah, well, what's the measurement? And

33:14

I say, right question, wrong place.

33:18

Let me teach you the rules of quantum

33:20

mechanics as laid down by Heisenberg. Sorry, when

33:22

you say wrong place, you mean they should go and answer

33:24

in the philosophy department? No, no, no, no, no. I mean,

33:26

wrong place and time. I said, come and ask me later.

33:30

We have to get through this material.

33:32

Come and talk to me later. So

33:34

it's the question that springs to everyone's

33:36

mind when you first learn it, because

33:39

it's central. The concept of measurement is

33:41

central observation. It's central. And without it,

33:43

Heisenberg's rules simply do not work. They're

33:45

just empty. They don't say anything. And

33:48

so we're left with this puzzle.

33:51

It's fascinating, a fascinating situation for

33:53

us all to be in as

33:55

theorists. We have this phenomenally successful

33:57

theory. Einstein called it our most

34:00

successful physical theory. I don't deny

34:02

that, don't disagree. But on the

34:04

other hand, it leaves us without

34:06

any picture at all of

34:08

a quantum system as it is

34:11

in space. Okay, Frank, do

34:13

you want to... I'm a Luddite physicist. To me,

34:15

it is all very interesting,

34:17

but I remember a

34:19

cartoon I saw many years ago of somebody

34:21

in an elevator who looked washed out and

34:24

somebody said that that smithers, he

34:26

was doing great and then he started worrying

34:28

about quantum mechanics. Strangely,

34:32

the rules, if you apply them, they

34:34

make predictions. I mean, that is indeed the test

34:36

for me. If your theory

34:38

makes a prediction that can be tested and experiment

34:41

either confirms or denies, then you

34:43

know what you're doing. I'm also amazed

34:45

that, I mean, as you alluded to,

34:47

this last six months I've

34:49

benefited from magnetic resonance imaging, polytron

34:51

emission tomography. They are things that

34:54

the quantum world has led to

34:56

as tools that we now use in the macro

34:58

world. So without quantum mechanics, a lot of the

35:00

things we're taking for granted today would never have

35:03

been invented. Whatever the philosophy

35:05

behind it, I don't know. Unfortunately, we have

35:07

a limited time. We don't have the five

35:09

hours I really deeply like at this moment.

35:11

I really would. I'm joking. It's absolutely fascinating

35:13

for somebody who gave up physics at the

35:16

age of 14, always asked to

35:18

give up physics at the age of 14

35:20

in order to take a blood in. Imagine

35:22

that. Harry, I'm going to switch now. We're

35:24

talking about theories, but what about the man?

35:27

One of the things about the man is

35:30

that the politics of the 1930s affected

35:32

him and his position

35:34

with regard to Germany. Yeah,

35:36

so Heisenberg lived through a very

35:39

turbulent period in German history. So when he's a

35:41

young man just going into university, Germany is coming

35:43

out of the humiliation of the First World War.

35:45

There are armed groups on the street and Heisenberg,

35:47

I think, is dismayed by what's happened to his

35:50

country. He is a sort of patriotic German. But

35:52

then as you get into the 30s, the rise

35:54

of Nazism, antisemitism becomes

35:56

increasingly prevalent in the

35:58

academic world. world and there

36:00

are certain physicists, German physicist Philip Leonard is

36:03

one example, Johannes Stark, who are deeply opposed

36:05

to what they see as Jewish physics. So

36:07

this is the sort of ideas proposed

36:10

by Einstein around relativity, but also by

36:12

extension quantum theory as well. So

36:15

Heisenberg's reaction, I think, to this at first is

36:18

his view is that somehow physics should be separate

36:20

from politics, that actually politics is beneath the dignity

36:22

of a sort of an academic aristocrat, that this

36:25

is a sort of essentially the ultimate ivory tower,

36:27

that he shouldn't have to deal with this. And

36:30

his reaction to sort of what's going on around him is

36:32

I think quite, he doesn't

36:34

come out of it sort of smelling of

36:36

roses. He's not an enthusiastic supporter of the

36:38

Nazis, although when they do come to power,

36:40

he does express some sympathy with some of

36:42

the things they're trying to do as sort

36:44

of national revival, because he wants Germany to

36:46

sort of be on the up. But at

36:48

the same time, he does spend a lot

36:50

of effort trying to prevent the dismissal of

36:52

his Jewish colleagues from their university positions, and

36:54

also to persuade his colleagues not to leave

36:56

Germany. And it's clear that in the 30s,

36:58

his key objective is really

37:00

to preserve German physics, so that when

37:03

the Nazis go, there will be still

37:05

physics of high quality in Germany, that's

37:07

his primary focus. But as a result,

37:09

he does accommodate to a large extent

37:11

with the regime that he finds himself

37:14

living under. How does this affect his

37:16

reputation in the world of

37:18

physics? Well, it's interesting, because Heisenberg actually comes

37:20

under quite virulent attack by people

37:22

like Stark. So I think it's in 1937,

37:26

Stark is writing articles in

37:28

Nazi publications attacking Heisenberg as a

37:30

sort of, he's not

37:32

Jewish himself, but saying he's a sort of

37:35

supporter of Jewish physics. And this leads to

37:37

an SS investigation into Heisenberg. So he's actually

37:39

put under investigation by the SS. He

37:43

becomes very desperate, he's interrogated in Berlin in

37:45

the SS headquarters. He ends

37:47

up writing to Himmler directly pleading

37:49

his case, eventually he's exonerated after

37:51

quite a sort of traumatic investigation,

37:54

at which point he is kind of keen to

37:56

prove his usefulness to the regime.

37:58

And this extends into the the Second World

38:00

War when he becomes involved in the

38:02

German nuclear research as well. So I

38:04

think it definitely harms his reputation. He's

38:06

given many opportunities actually to leave Germany.

38:08

So he's offered positions in

38:10

America, for example, at Columbia University. And many of

38:12

his colleagues are perplexed as to why

38:15

he refuses to leave Germany, given the sort

38:17

of pressures he's come under personally. I think

38:19

a lot of his colleagues view him quite

38:21

critically for not having taken a more courageous

38:23

stand against what was going on in his

38:25

country at the time. I'll play. What is

38:28

continuing impact? So at

38:30

the moment I'm working on a project on

38:32

quantum field theory. So one

38:34

of the things that I think we've

38:37

alluded to already is that the principles

38:39

of quantum mechanics, which were essentially discovered

38:41

in 1925 by

38:43

Heisenberg and then separately in a sort of

38:45

different form by Schrodinger, those

38:47

principles were very swiftly formalized and

38:50

then it was realized that they

38:52

could be applied to

38:54

any physical system whatsoever, so

38:57

long as the classical form of the

38:59

theory obeyed Newton's laws. You

39:02

could do this process which

39:04

people call quantization. So you take

39:06

the classical form of the theory,

39:08

Newton's laws, and you turn

39:10

the handle almost, Heisenberg's writing a

39:12

handle, and you produce the quantum theory.

39:14

And that this was so universal that

39:17

there was, I think people thought, by

39:19

the time there's no limit to what

39:21

we can apply this to. It's

39:23

so universal we can apply it

39:25

to any system at all. And

39:27

one of the systems that people

39:29

applied it to was field theory.

39:31

So electromagnetism was a field theory,

39:34

the fields of Faraday and Maxwell.

39:36

And so quantum field theory is

39:38

a quantum theory that is completely

39:40

in accord with those axioms that

39:42

were essentially laid out by Heisenberg

39:44

in 1925. So I'm doing

39:46

a project on quantum field theory and

39:48

we say the word Heisenberg probably about

39:50

20 times a day. So there's the

39:52

Heisenberg operators, there's the Heisenberg

39:55

equation, there's the Heisenberg picture. Although

39:58

on the other hand... My

40:01

research area is quantum gravity,

40:03

so I would like to

40:05

understand how quantum matter can

40:07

be compatible with our understanding

40:10

of space-time as Einstein laid

40:12

it out in general relativity. Gravity

40:15

does not fit into this

40:17

paradigm. You cannot take general relativity,

40:19

turn the Heisenberg-Schrodinger handle and produce a

40:22

quantum theory for gravity that works.

40:24

And the reason is that the theory

40:26

of gravity that we have, our best

40:28

theory of gravity, is a theory

40:30

of space-time itself. So

40:32

in order for there to be a quantum

40:35

theory of gravity, there has to be a

40:37

quantum theory of space-time. So space-time itself must

40:40

be part of the quantum system

40:42

and that raises this issue which

40:44

I've mentioned that

40:47

in order for the Heisenberg-Schrodinger rules

40:49

to apply, you need something outside

40:51

the system to do the observing.

40:54

But if your system is space-time

40:56

itself, then kind of by definition

40:58

there's nothing outside it because everything that happens

41:00

happens in space and time. So how

41:03

could you make that work? So the

41:05

struggles in producing a theory of quantum

41:07

gravity are as much conceptual as they

41:10

are technical and this

41:12

idea of the necessity of there

41:14

being an observer and hanging everything,

41:16

all your meaning on the results

41:18

of measurements and observations is actually

41:21

a barrier now to making progress

41:23

in quantum gravity in my view.

41:25

Finally Frank, what Faye was just

41:27

saying, the universe,

41:30

why are we here at all? And

41:32

I don't

41:34

mean here to dare, the whole thing she's saying.

41:37

And it has been suggested semi-seriously

41:39

or maybe even seriously that

41:42

the universe's existence is itself an

41:45

example of Heisenberg's uncertainty

41:47

principle, the idea that you can

41:50

overdraw the energy accounts by a

41:52

small amount for a small

41:54

amount of time so long as the product

41:56

of the two is constrained by

41:58

this quantum uncertainty. And

42:00

one of the surprising things is that

42:02

the universe itself, because there's

42:04

a lot of gravity around, when you're

42:07

in a gravitational field you have negative

42:09

potential energy. There's a lot of positive

42:11

energy around in all of our MC

42:14

squares and so forth. It is possible

42:16

that the sum total energy of the

42:18

whole gravitational universe is nothing. In

42:21

which case Heisenberg says you can borrow

42:23

that nothing forever. And

42:25

so the universe could be

42:28

a quantum fluctuation satisfying Heisenberg's

42:30

uncertainty principle. The

42:32

problem, or a problem with that

42:34

is, so where was that encoded?

42:38

Who or what encoded that principle

42:40

that enabled a universe to erupt

42:42

out of nothing with a total

42:45

energy balance which Heisenberg, millennia, eons

42:47

later, would formulate? Answers on a

42:49

postcard. Well thank you

42:52

all very much, Shops, exhilarating. Thanks

42:54

Frank, Frank Close, Thay Dauker and

42:56

Harry Cliff and our studio engineer,

42:58

Never Mysterion. Next week, uprising

43:01

in Algeria in 1871 against the rule

43:03

of France, when that country

43:05

was reeling from the parish commune and

43:07

the loss of Al-Saslaren to the new

43:09

Germany. That's a mccranny

43:11

rebuilt. Thanks for listening. And

43:14

the In Our Time podcast gets some extra

43:16

time now with a few minutes of bonus

43:18

material from Melvin and his guests. That

43:21

was terrific. I'm afraid I'm going to ask you to do some more

43:23

of it. When

43:27

you're asking about how the

43:29

Heisenberg uncertainty principle gets used, I

43:31

know that you're always loving these

43:33

amazingly small things or huge numbers

43:35

and things that you keep totting

43:37

around. In particle

43:39

physics all the time, this complementarity

43:42

between energy uncertainty and time

43:44

uncertainty is key. I

43:48

mean time uncertainty, most of the particles

43:50

that we find are unstable. They live

43:52

for 10 to the minus

43:54

24 seconds. That's the time

43:56

it takes light to cross one tenth

43:58

the size of a particle. atomic nucleus, that small

44:00

amount of time. But Heisenberg

44:03

tells us that if you

44:05

try to measure the mass of that particle, its

44:08

m phi squared, its energy will be

44:10

uncertain because of the limited

44:13

time. And we can measure that uncertainty

44:15

in energy and that's the way that we do it. At

44:18

CERN they discovered the Z boson

44:20

years ago by tuning the

44:22

beams to start making it and the beams

44:25

showing nothing, then gradually they built up to

44:27

a peak and then down the other side. So there

44:30

was a spread in the energy

44:32

and from Heisenberg that spread in

44:34

energy was interpreted as the lifetime

44:36

of the particle. So we can

44:38

measure times of that minute

44:41

amount by using Heisenberg and I think it's

44:43

fair to say those are the smallest measures

44:46

of time that we actually do measure in

44:48

practice and it's using Heisenberg to do it.

44:50

Harry? And one of the things we

44:53

didn't talk about I think is I don't

44:55

know if we emphasise enough how

44:57

revolutionary the new quantum mechanics was

44:59

because there's the sort of famous

45:01

debates between Bohr and Einstein and

45:04

also Einstein and Heisenberg. So Einstein

45:07

really reacted very strongly against Heisenberg sort

45:09

of really pragmatic, we only worry about

45:11

what you can measure and

45:13

Einstein really wanted to hang on to this idea

45:15

of a mental picture of what was going on

45:18

which Heisenberg sort of approached and I but also

45:20

this fact of the universe being probabilistic and not

45:22

being deterministic. I mean we did talk about it

45:24

I think but maybe that sort of should be

45:26

emphasised a bit and just how radical that was.

45:29

It's a complete break with how you think about

45:31

the world in the 19th century where if

45:33

you know the positions of all the atoms in this room and

45:35

how fast they're going you can predict arbitrarily

45:37

far in the future what's going to happen and

45:39

quantum mechanics says no you're not allowed to know

45:42

that. You can only say what the probability of

45:44

certain outcomes is in the future. Who

45:46

says that's the only thing you can say? Well

45:49

I mean that's sort of fundamental to quantum mechanics.

45:51

So now even if you know Everything

45:55

you can know about a system because of

45:57

the uncertainty principle, because of quantum. The

46:00

mechanics. Or you can save

46:02

from this position in time is that there

46:04

are certain possible outcomes and we can assign

46:06

from ability to those outcomes. but we can't

46:09

say which outcome we're gonna rise up. and

46:11

that is that is the sort. Ospreys, The

46:13

philosophical of the really big change that comes

46:15

in with quantum mechanics meant to know what's

46:17

gonna happen to one particles in the future.

46:20

the universe. He got to know two things

46:22

precisely, both where it is now and how

46:24

fast is is moving them, and Heisenberg says

46:26

you can't know both of those. So that's

46:28

the uncertainties that you're. Bringing. In

46:30

so in principle to say you can't know

46:33

but both of those together or you can't

46:35

node both of those told a you can't

46:37

know both of them to perfect precision. The

46:39

trade off on the amount of trade off

46:42

is controlled by the size of the says

46:44

that the discrete quantum and he fights. If

46:46

you know say the the position of a

46:48

lecture on exactly you know nothing about his

46:50

momentum at all. See lose all information about

46:53

how fast as going and correspond the other

46:55

way around. So if you know exactly how

46:57

fast is going, you have no idea where

46:59

it is. As I wasn't a Frank's to

47:02

talked about, this had this idea of a

47:04

wave. so if even a wave met in

47:06

a wave on the surface a pond that

47:08

goes on forever that mathematically has a well

47:10

defined momentum So you can express that as

47:12

a pure status momentum as I so as

47:14

you know either minutes of that way personally.

47:16

but a wave has infinite an extent has

47:18

no position is everywhere so you can't say

47:20

where it is equally if you want to

47:22

localize that wave. one of the ways you

47:24

can do is by adding up lots of

47:26

different waves with different frequencies in such a

47:29

way to the peaks and. Troughs Adult from Cancel

47:31

Out That You Get you went up with a

47:33

spike at one location. see got now a location

47:35

but I made up of a huge number of

47:37

different ways. a different frequencies and you don't know

47:39

immense Ms anymore so that that's the sort of

47:41

that added a how and shooter that is. That's

47:43

one way of thinking about going my way. Think

47:45

with the listeners, invite them now. Less

47:48

draw away on a piece of paper. a

47:50

series of dots. so please put down the

47:52

first. And what's the wavelength

47:54

of that wave? No. idea

47:56

at all put some more dot scene

47:59

and i can now begin get an

48:01

idea of what the wavelength is but

48:03

you spread those dots over a range.

48:06

So there's a trade-off between position and

48:08

wavelength. That's much better than

48:10

what I said actually. What

48:13

you said is the mathematical realisation of that.

48:15

It's called Fourier analysis. It's got a long

48:17

history back into the centuries.

48:20

What Harry and Frank

48:22

are describing is indeed the mathematical

48:24

reasons for the theorem which

48:27

we call the uncertainty principle, the

48:29

uncertainty relation. What's

48:31

really revolutionary about it is that it

48:34

doesn't refer to the position

48:36

or the momentum of

48:38

the particle in itself because that you

48:41

have to deny. So what's

48:43

really revolutionary is that you say it doesn't

48:45

have a position, it doesn't have

48:47

a momentum and this is

48:49

just an uncertainty which expresses

48:52

it. Experimentally it

48:55

takes the form of statistical uncertainty

48:58

about sequences of measurements. You do

49:00

many, many repeated measurements

49:02

of the same thing over and over

49:04

on an identically prepared quantum

49:06

system unless the uncertainty

49:09

relation relates to the statistics

49:11

of those measurements. It doesn't

49:13

say actually in the axiomatic

49:16

formulation, it doesn't say anything about

49:20

the position or the momentum of

49:22

the particle. That's what's so fascinating

49:24

about the 1927 paper about the

49:27

uncertainty relation or uncertainty principle. Heisenberg

49:30

is having this debate with himself in

49:32

print on the page. So he's

49:35

debating Schrodinger. So as Harry

49:37

said there was this

49:39

discussion, debate argument between Heisenberg and

49:42

Schrodinger at the time and you

49:44

can see that in the 1927

49:46

paper. He's responding to Schrodinger

49:49

in print in the paper but

49:51

he's also responding and debating with

49:53

himself and he's trying to

49:56

hold through to this idea that you should

49:58

not ever talk about a particle. particles

50:00

having a position and a momentum.

50:02

But then he uses that picture

50:05

to describe this so-called Heisenberg microscope,

50:08

whereby you imagine that the particle

50:10

does have a position, you're trying

50:12

to locate it by shining light

50:14

on it, blah blah. But

50:17

that whole Heisenberg microscope argument relies

50:19

on you having this picture of

50:21

it being having a position

50:23

and momentum, which you're supposed to

50:25

deny. So it's simply, he's struggling,

50:27

and you can see it is

50:29

totally fascinating, struggling with himself about

50:32

wanting not to talk about position

50:34

and momentum and space, but then

50:36

needing to talk about position and

50:39

momentum and space in order

50:41

to give people a heuristic

50:43

way of understanding the uncertainty

50:45

relation. I think that's really what

50:47

I was alluding to when we

50:49

had this debate in the, during

50:52

the programme when I said

50:54

I'm a low diet physicist, that if

50:56

I start asking myself, trying

50:59

to understand what's really going on

51:01

there, I end up in a fog. If

51:04

I use the rules that have been

51:06

developed, they work. And

51:09

in that sense, I'm in the latter camp, like

51:12

an engineer, I would use the rules and

51:14

let others worry about why they are.

51:16

I think it also is

51:19

interesting what you said about Heisenberg's debate, the

51:22

gap between the micro world and the

51:25

last cut. Whether

51:27

you look at Heisenberg's or Schrodinger

51:29

and Weis and things, you

51:31

end up, there's a problem. It's the

51:34

Heisenberg cut, or it is the Schrodinger's

51:36

cut. Whichever way you look at it, there's

51:39

this dichotomy, how do you get from

51:41

the quantum world to the macro world?

51:44

So I take what you say perfectly,

51:47

Frank and I, in

51:49

your lab, I completely agree that

51:52

as a practical matter, the

51:54

rules of quantum theory work for

51:56

making predictions about the

51:59

results of your experiments. But what's

52:01

practical depends on what you're trying to do.

52:03

So if you're trying to find

52:06

a theory of quantum gravity in which

52:08

the whole universe is quantum, then

52:11

as a practical matter, the

52:14

rules of quantum theory as laid down by

52:16

Heisenberg will not work. They

52:18

cannot because there's no external observer. So

52:20

you have to, you're forced as a

52:22

practical matter, if you can

52:24

think of trying to find a theory

52:26

of quantum gravity as a practical matter. As

52:29

a practical matter, as a working scientist, you have

52:31

to go beyond it. How are you doing? Are

52:34

you coming? I'm an experimental physicist. I'm sitting slightly out

52:36

of my depth here, to be honest. But I mean,

52:38

one thing I was going to ask Fay about this,

52:40

actually. So with quantum gravity, as I understand it, my

52:42

very limited understanding, you're talking about effects

52:44

that only really manifest themselves at extremely

52:46

high energies, extremely short distances in terms

52:49

of. So the question is, in

52:51

terms of the practical things you would observe in an

52:53

experiment, what is a

52:55

theory of quantum gravity actually trying to solve? Because

52:58

at the moment, as far as

53:00

I said, you're much more expert than me. But as

53:02

far as I know, there aren't really any experimental inconsistencies with

53:04

the two theories we have for the universe

53:06

at the moment, quantum mechanics and general relativity.

53:08

They cover everything we've ever seen perfectly well.

53:10

And it's more a sort of, we're worrying

53:13

about things that might happen in very extreme

53:15

conditions that we haven't yet observed. So I

53:17

mean, I suppose if we're going to take

53:19

that practical approach that Frank takes,

53:22

so quantum gravity, what are the problems where

53:24

you think, why do we need such a

53:26

theory? And is it just, is it more

53:28

philosophical and kind of we would like a

53:30

quantum theory of gravity because we think these

53:33

two things should be reconciled. But what's the

53:35

actual practical problem we're trying to address? Well,

53:37

I'm sorry to take, I'm on Fay's side

53:39

here. Yeah, OK. But this is exactly analogous

53:41

to what Einstein was worrying about in 1900,

53:44

about what happens if he

53:46

lives in the light wave. And by doing

53:48

those thought experiments, found the contradictions and moved

53:51

forward. And likewise, the questions about the imagining

53:54

what happens if you do an experiment

53:56

where you're making quantum black holes fluctuate

53:58

out. you can

54:00

imagine that, although we can't do it in

54:03

the laboratory, we can imagine that

54:05

and we can show that quantum

54:07

theory as at present formulated and

54:09

general relativity cannot live

54:11

together and something has to give. Is that a

54:13

fair statement or good thing? Yes, absolutely. But I

54:16

think there's more than just an intellectual

54:19

problem there of trying to bring

54:21

together two theories which at the

54:23

moment are in contradiction in certain

54:26

extreme regimes. So for example,

54:29

what people call the standard model

54:31

of cosmology today is

54:33

a very simple model, but there

54:35

are parameters in the

54:37

model that so thus

54:40

far are just phenomenological parameters. You just

54:42

choose them to fit the data. And

54:45

two things about those parameters.

54:47

One is that they're starting to

54:50

be real tension between our observations

54:53

and the standard model.

54:55

So there's something called the Hubble

54:57

tension, which is measurements

55:00

of the Hubble constant or

55:02

Hubble parameter today from

55:05

late time measurements of observations

55:07

of, for example, supernovae and

55:10

early time measurements of, for example,

55:12

the cosmic microwave background radiation. So

55:14

those two measurements of this Hubble

55:16

parameter today are in tension. And

55:19

some people, increasing number of people, believe that

55:22

that tension is a real tension.

55:25

You know, it's something which we can now say,

55:27

yes, there's definitely a discrepancy between the model and

55:29

our observations. So that's the first thing. The second

55:31

thing is some of those parameters

55:34

are very strange. So for

55:36

example, at the Big Bang,

55:38

the hot, dense state that the universe

55:41

began in, we believe, space

55:43

is very, very, very, very,

55:45

very, very, very flat. You

55:49

would expect it on

55:51

dimensional grounds to be very

55:53

curved, that everything is

55:55

of the scale of what people call the Planck

55:58

scale, that there should be, there is curvature

56:00

on the Planck scale in time,

56:02

but in space it's super super

56:04

super fun. As far as we

56:06

know it's consistent with being perfectly

56:08

flat, but there's a bound

56:10

on how much curvature there can be in

56:13

space. It's very strange and

56:15

that is what people call a fine-tuning problem. Why

56:17

should it be so flat then? So

56:19

those initial conditions of the

56:21

universe, so the dynamics of the universe we

56:24

believe is general relativity and other classical

56:26

theories, but the initial conditions

56:29

are unexplained. Why is the world, the

56:31

universe, the way it is? Why did

56:34

the cosmos start out the way it did?

56:37

And we expect, we believe, we

56:39

hope that a theory of quantum gravity

56:42

would tell us why the

56:44

universe started out the way it did, because in

56:46

some sense it would tell us what happened before the Big

56:49

Bang. So what led up to the Big

56:51

Bang? That would be the deep quantum regime in which

56:53

there's no classical space-time at all, we

56:55

would have the full quantum theory of space-time

56:58

which was probably looks like nothing

57:00

that we have now, and

57:02

that would then give us those initial

57:04

conditions. So isn't that, it is a

57:06

scientific problem today, not just

57:08

an intellectual exercise that may bear fruit

57:11

in the future. The fact that we can

57:13

ask such questions actually is the result of the

57:15

things that Heisenberg did and have been developed from

57:17

that. I mean the

57:19

fact that such questions can be asked and

57:22

tackled scientifically today are

57:25

a legacy of the things that Heisenberg did

57:27

in 1925 and grew from it. And

57:31

also actually moving away from science

57:33

into technology, the big breakthroughs people

57:37

are expecting technologically are a lot of them are

57:39

quantum-related, so quantum computing which has become increasingly

57:42

a kind of growth area. A lot of my PhD

57:44

students who work in experimental physics are now going off

57:46

to, one of my

57:48

old students is now writing software for quantum

57:51

computers as his job and this has become

57:53

an area that's potentially going to have revolutionary

57:56

impact on the way we live, particularly coupled with AI

57:58

and other So,

58:01

you know, the future I think is going to be quantum as

58:03

well. So the legacy of

58:05

Heisenberg isn't just scientific, it's

58:07

also now, well, it has had a huge technological impact

58:09

already, but it's going to continue to do so in

58:11

the future as well. Thank

58:13

you very, very much. Long time, here comes

58:15

our producer man, Simon. Chief, thank

58:18

you. You have to go, thank you. I've got

58:20

to go. Three-two, thank you very much. In Our

58:22

Time with Melvin Bragg is produced by Simon Tillotson.

58:33

Hello, I'm Greg Jenner. I'm the host of

58:35

You're Dead To Me on BBC Sounds. We

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And we are back for a seventh series

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