Physicist Brian Cox explains quantum physics in 22 minutes

0:00 I think it's important to say that there aren't different rules

0:04 of the game in the subatomic world and the world that we observe.

0:09 This world of common sense, let's say that we perceive,

0:12 and it's pretty well understood I would say, it's well understood.

0:15 How the world that we see emerges from this rather

0:20 strange but well-defined behavior that we see in the subatomic world.

0:30 And it's not only the subatomic world, by the way,

0:32 we have an increasing number of quantum

0:36 technologies that are really based on this behavior.

0:39 The quantum computers being a good example.

0:42 And so you see that this is not just something that you can say, "Well,

0:46 we don't need to think about it really because it's in the world

0:49 of atoms." Because we are using that behavior now in technologies.

0:55 And so it really does become a, an important theory to try to understand.

1:03 [Interviewer] Why is it important that we

1:04 seek to solve the mysteries of quantum physics?

1:10 You could argue that even today,

1:12 the interpretation of what the theory that is telling us

1:17 about the nature of reality itself is not universally agreed upon.

1:23 One of the big changes, actually that's happened,

1:25 in teaching quantum mechanics in universities over

1:28 the last few decades is that it was always,

1:30 when I learned it, so it's about even in the 1990s,

1:34 it was common to teach it historically.

1:37 So you go through the things that we've talked about,

1:39 you go through the photoelectric effect and the stretch of atoms,

1:42 you get to Niels Bohr and his early explanations of how atoms might work,

1:48 following on from Ernest Rutherford in Manchester detecting the atomic nucleus.

1:53 And that, so this idea that an atom is a nucleus, a dense positive charge.

1:58 It wasn't known exactly what the structure was at that time.

2:02 With the electrons, let's say orbiting in inverted commas around it

2:06 so that there's a picture of almost like a solar system,

2:10 but it was known that doesn't work,

2:11 because you have charged particles moving in the vicinity

2:14 of other charged particles and that means that they radiate,

2:17 and they would not be stable.

2:18 And so there's all these things going on, confusion.

2:21 And then Bohr suggests that electrons can only take certain energies,

2:27 which we now call "orbitals" around the atomic nucleus.

2:29 And all this stuff is going on.

2:31 And it used to be that we would teach it like that in university.

2:35 But if you do that, you pick up all the confusion, the confusion,

2:39 decades and decades of confusion that these great physicists felt in trying

2:44 to come to terms with this counterintuitive picture of the world.

2:48 So now, I think it's more common in universities

2:51 to start with the theory as we understand it today,

2:55 and just say, well, this is the way the world works.

2:58 And perhaps the best introduction would be to think of a property of particles

3:05 called "spin." So you might call these things a "qubit." So what is a qubit?

3:11 So you think of a coin which you would toss in the air,

3:15 and it can be either heads or tails.

3:18 So that's a kind of an intuitive picture of the world.

3:21 This thing is either heads or it is tails.

3:25 Now, a quantum coin could indeed have

3:29 the property that it would be heads or tails.

3:32 But the difference between quantum mechanics and classical

3:36 theory is that an object, like a coin, a quantum coin, can also be in what

3:44 we call a "superposition" of heads and tails.

3:48 So that means that it can be in a state where it is,

3:52 let's say, if we observe the thing, we'll talk about that a bit later,

3:57 but it can be in a state where it could be 30% heads and 70% tails,

4:01 or 40% heads and 60% tails or any combination, any mixture of heads and tails.

4:09 And that is a perfectly legitimate description

4:13 of the state of the configuration of this thing.

4:18 Just to give you a sense of a real physical object that would behave that way.

4:22 Particles like electrons, for example, have a property called "spin",

4:25 which can be up or down, it's like heads and tails.

4:30 But that's the key thing that objects like "electrons"

4:34 can not only have definite values of some property,

4:40 some thing that you can measure, but they can be in a mixture of those things.

4:45 And it's not a probability theory in the sense

4:50 that we would usually think of probability theories.

4:52 So usually we'd say, "Well,

4:54 there's a 50% chance it's gonna rain tomorrow." Why do we say that?

4:59 Why we would say such a thing?

5:01 It's because we have incomplete knowledge

5:04 of the system in this case, the weather.

5:06 And so we have incomplete understanding of where,

5:10 I suppose at the most basic level,

5:11 all those water molecules are in the clouds and so on.

5:14 But we don't have enough knowledge to make,

5:17 to precisely calculate what is going to happen

5:21 and so we assign probabilities to it,

5:23 which were, that reflects our ignorance of the situation.

5:27 The key difference in quantum theory

5:30 is that these probabilities are fundamental.

5:34 They're fundamental to the description of nature.

5:38 So it is not the case that if we have an electron in some kind of configuration,

5:45 then our theory predicts probabilities because we don't quite know exactly what,

5:50 how this thing is configured.

5:52 The probabilities are intrinsic to the theory itself.

5:57 And pretty much all of the intellectual challenges and the confusion

6:03 around quantum mechanics comes from that very simple property.

6:07 If you look at pretty much any book on quantum mechanics,

6:10 there is one experiment which you can describe.

6:15 It's a very simple experiment which encapsulates

6:18 all the, I was gonna say weirdness,

6:21 but it's, let's say all the properties of the quantum world.

6:25 It's called the "double-slit experiment." And by the way,

6:28 I'll strongly recommend, I think it's almost universally accepted,

6:31 the best description of the double-slit experiment is freely available.

6:35 It's in the Feynman lectures, volume three, first chapter.

6:39 And I've read quantum mechanics textbooks that say,

6:43 go away and read the first two chapters of the Feynman

6:47 lectures on physics volume three because it's all in there,

6:50 can't be done any better.

6:51 So I could recommend that if you find this interesting,

6:54 the double-slit experiments is,

6:57 essentially, let's say you have something that will emit,

7:01 let's say electrons, so particles, electrons.

7:04 Electron gun that emits particles and then you have a barrier

7:09 that has two slits cut in it and a screen.

7:13 So that's the setup of the experiment.

7:15 So we have something that emits electrons, two slits cuts in a screen,

7:20 and a detector, another screen that the electrons will hit.

7:25 So, and you, so you fire the electrons out.

7:27 So what will that look like?

7:30 Well, if you think of the electrons as just little particles,

7:33 little bullets let's say that are emitted from this gun,

7:36 then you would imagine that the electrons can

7:39 go through one slit or the other one, depending on how they come out.

7:42 And you can imagine that they might get deflected

7:44 around a little bit when they go through the slits.

7:47 But basically on the screen you would expect most

7:50 of the electrons to appear opposite one or the other

7:53 of the slits with maybe a bit of a spread

7:56 because they rattle around a bit when they go through.

7:58 So you get lots here, lots here,

8:01 and pretty much none in the middle, but that's not what you see.

8:05 What you see is a very clear pattern on the screen.

8:10 You see sort of stripes, a stripe on the screen where you get lots of electrons

8:15 and then a stripe where you get very few or none,

8:18 and then another stripe where you get lots and then

8:20 a stripe where you get very few or none,

8:21 and then another stripe, and then very few or none.

8:24 So you get this stripy pattern.

8:27 That pattern is exactly the same pattern that you

8:31 would get if you sent waves through the slips,

8:35 let's say what water waves, any kind of waves,

8:39 then it's easy to understand because what's happening then,

8:42 and this follow, physicists knew this back in the 1700s, right,

8:46 is that you can consider each slit as a source

8:50 of new waves and the waves come out.

8:53 And waves have the property that they can interfere with each other.

8:57 So you can get the peak of one wave arriving at the screen from one slit,

9:03 and a trough of a wave arriving at the screen from another slit.

9:07 And if everything's lined up correctly,

9:09 the peak and the trough cancel out and you get nothing.

9:12 So you get this property where something from each slit,

9:17 it's very easy to understand if it's a big extended wavy thing,

9:21 lines up in such a way that it cancels out,

9:24 and then it could line up in such a way that it reinforces

9:27 and you get a big disturbance there and then it cancels out again,

9:31 and then it reinforces again.

9:32 So you can imagine this stripy pattern on the screen,

9:34 that's what you get with waves.

9:36 The fact that you get it from particles

9:40 is interesting but here's another interesting thing.

9:44 You still get that pattern if you send one particle at a time through the slips.

9:52 So it is, and let me use my language carefully,

9:56 I was gonna say it is as if the electron can somehow explore both paths,

10:02 just like a wave can and then interfere

10:05 with itself to control where it lands on the screen,

10:09 as if is something that people might object to.

10:13 Many physicists would say, "No, it does." So the statement is that the electron

10:21 explores both routes at the same time at once,

10:25 let's say, on its root from the electron gun through the slits to the screen.

10:31 So that's a very strange picture of reality.

10:34 We surely think of particles as following definite paths,

10:39 and it might be that you don't know quite which path it's gonna take,

10:43 but surely you would say it, in reality,

10:46 it will go one route or the other route.

10:49 But that experiment, which I emphasize has been done now many times,

10:55 tells us that nature is not like that.

10:58 It tells us that the electron must in some sense explore

11:06 all roots on the way from the gun to the screen.

11:09 And so the question then becomes, well, what do I mean by in some sense?

11:14 What you'll see in the Feynman lectures is he

11:17 gives you two pictures of how to think about physics.

11:22 One is, he gives you the way that you

11:25 would calculate what you're gonna see on the screen.

11:28 And that's really simple as Feynman points out.

11:31 So what you can do is you can, you can assign,

11:36 calculate what's called a "complex number" for every route that the particle,

11:42 the electron, can take from gun, through the slits, to the screen.

11:46 So every route, every path it can possibly take.

11:49 A complex number, for those that don't know about complex numbers,

11:53 can be pitch as a little clock face.

11:56 So a complex number has a clock hand,

11:59 there's a length of the clock hand and there's the time on the clock face.

12:03 And you can calculate how those clock faces kind of evolve and change,

12:08 and spin around from the moment the electrons

12:11 emitted to the moment it hits the screen.

12:13 And the prescription is very simple.

12:15 You can calculate what those clock faces look like,

12:18 what those complex numbers are,

12:20 and every point on the screen you have to take them all,

12:23 every possible path to that point and just add them up.

12:27 And the length of the clock hand gives

12:30 you the probability to find the electron there.

12:32 It's actually this squared length of the clock hand,

12:34 but whatever, it's the length of the hand.

12:36 That's it.

12:37 So you can see that you have this property, you can have interference,

12:39 you can have one clock arriving at 12 o'clock and one

12:42 clock arriving at six o'clock and they cancel out and so on.

12:45 You just do that, you calculate it and you get the right answer.

12:49 So it's a very, very simple prescription.

12:53 But I suppose the problem comes when you say, well, what does it mean?

12:56 Does it really mean, is it just calculational?

12:59 Is it just mathematics or does it really

13:02 mean that the electron explores every possible route?

13:06 And by that I mean every possible route,

13:10 you might consider it going to the Andromeda Galaxy in bank,

13:13 all right, every possible route on its journey

13:16 from the, from when it's emitted to when it's detected.

13:20 And I think many physicists now would

13:24 say that is a correct description of reality.

13:28 The particle does.

13:29 But I just want to emphasize that the mathematics,

13:35 the way that you make the calculation is pretty simple.

13:39 It's a prescription for making a calculation

13:41 and you get the right answer to the experiment.

13:44 The problem with quantum mechanics, I suppose,

13:48 is what when you try to interpret what

13:50 the calculation means for the nature of reality.

13:54 I think it's important to say that there aren't different rules of the game

13:59 in the subatomic world and the world that we observe this world of common sense,

14:05 let's say that we perceive.

14:07 There aren't different rules and it's pretty well understood I would say,

14:11 it's well understood.

14:12 How the world that we see emerges from this rather

14:19 strange but well-defined behavior that we see in the subatomic world.

14:27 And it's not only the subatomic world, by the way, this is now,

14:31 maybe you could have argued 50 years ago that this is just our philosophy,

14:35 it doesn't really matter, whatever,

14:37 but now we have an increasing number of quantum

14:42 technologies that are really based on this behavior,

14:45 the quantum computers being a good example.

14:48 And so you see that this is not just something that you can say, well,

14:53 we don't need to think about it really because it's

14:55 in the world of atoms and it's all a bit whatever,

14:58 and we, but, and we can just do some calculations.

15:01 Because we are using that behavior now in technologies and so

15:07 it really does become a, an important theory to try to understand.

15:13 Now you go back a few decades then I think you could

15:19 say that the interpretations of quantum

15:22 mechanics are very interesting and very important,

15:28 because we're talking about the nature of reality.

15:30 But you might say, well, it doesn't really matter so much practically, right?

15:35 If that now, I have a lot of colleagues in physics who would,

15:39 I think rightly hate that description because what we're

15:42 trying to do is understand reality, what physics is.

15:45 But now particularly, I think,

15:48 with the possibility of building quantum computers,

15:52 this attempt to understand how large systems

15:55 of quantum mechanical objects behave is becoming extremely important,

15:59 because a quantum computer is a device which is built out of qubits.

16:06 So remember qubit, an example would be an electron,

16:10 which has this property that when you make an observation of the spin,

16:15 let's say of this electronic behaves like a coin,

16:18 it can be up or down, heads or tails, however you wanna describe it.

16:22 But that thing can also exist in a super position,

16:28 a combination of these things.

16:29 And it's a perfectly valid configuration.

16:33 And you use that, that's the kind of property

16:36 that you use in building a quantum computer.

16:40 You can also then ask the question, well,

16:42 what happens if I get another one, a two electrons together?

16:47 These can be in what's called an "entangled state." So

16:51 then you have a much richer structure of this physical system.

16:57 So an example of an entangled state, a very famous thing called a "Bell state",

17:02 would be where you set these things up.

17:05 So the system of these two qubits,

17:08 let's say they're up and down these spins, right?

17:11 So I can have the state up, down plus down, up,

17:16 by that a complete description of this state, up, down, plus down, up.

17:22 What does that mean?

17:23 So let's say I take one of these electrons and I separate them.

17:27 It's a very famous paper written by Einstein Podolsky

17:30 and Rosen that first considered this in the 1930s.

17:34 And they get very upset by the behavior.

17:36 So we've got a system, a state, and it's up,

17:39 down plus down, up, let's say, and I separate these electrons.

17:42 I take one to, I take one to let's

17:46 say to Pluto and leave one on planet Earth, right?

17:51 And then, so what's the description of this thing in that state?

17:55 Well, it's got, in the way I described it, there's a 50,

17:59 50 chance that when I make a measurement

18:01 of this electron that the up or downness of it,

18:05 then there's a 50, 50 chance of either up or down.

18:08 And the same for the other one.

18:10 That's what that state means.

18:12 But if it's up, down plus down, up,

18:16 then it means that if I make a measurement of that one,

18:19 if I say, well, okay, I know what that is,

18:23 it's up, the other one has to be down.

18:26 Up, down or down, up, it can't be up as well.

18:31 Same for the other one.

18:32 If I go to Pluto and have a look at this one and it's down,

18:36 then this one definitely is up, right?

18:40 So this is called "quantum entanglement" and the reason it bothered

18:44 Einstein and others is it would

18:46 appear that something is instantaneously changing,

18:50 and it's not just your knowledge

18:52 of the system that seems instantaneously changing.

18:54 It would seem that the system itself is

18:57 instantly configuring itself when you make some measurement.

19:00 So you might say, well, this is nonsense,

19:02 there must be something else and it turns out that, as far as we can tell

19:07 that, and a noble prize was awarded for some of this researcher a few years ago,

19:11 then, no, there's nothing hidden there.

19:13 This is the way that the system is.

19:15 But you think about that system of two qubits, I think about it.

19:19 So there are, you got these two things, which can be up or down.

19:23 There are four possible combinations of that system.

19:28 You can have up, down, down, up, up,

19:31 up or down, down, four for the two qubit system.

19:35 For a three qubit system, you think about it ups and downs,

19:39 you'll find out there are eight possible combinations, and all mixtures of them.

19:45 For four qubits, then it's two to the power four,

19:49 there's 16 possible combinations.

19:52 We're talking about building quantum computers now in which we have a 100,

19:58 200, 300 qubits, all in principle and tangled together.

20:03 The number of possible descriptions of that system then,

20:07 you have numbers that describe it.

20:09 If there are a 100, it's two to the power of 100,

20:13 different configurations and any mixture of those things

20:17 that are states of the system.

20:19 So pretty soon, two to the power 100, for a 100 qubits.

20:25 What's the number of atoms in the universe?

20:28 If you had two to the 500, you'd far exceed 500 qubits.

20:33 The number of numbers you need to describe that system

20:36 exceeds the number of atoms in the observable universe.

20:40 But it's a thing that we can build, we can build just about,

20:44 we are not far off being able to conceive at least of a network of 500 qubits.

20:50 They're physical things, the kind of like that big, some of them, right?

20:54 You have 500 of them in this room.

20:56 So the power that's hidden in the description of a system like that is immense.

21:04 And the thing that a quantum computer does is it uses that power,

21:09 some of that power, some of that vast

21:13 computational or configurational power of the thing.

21:16 Very, very difficult to do.

21:18 And we've not been able to do it particularly well at the moment.

21:21 You'll read many papers online where obviously Compass like Google, Microsoft,

21:25 IBM are investing a lot of money in these devices because potentially they are,

21:31 that they can carry out computations that no conceivable

21:37 classical computer could make within the lifetime of the universe,

21:42 because of this tremendous freedom in the description

21:46 of the structure of the system.

21:48 So quantum mechanics and quantum entanglement,

21:51 these properties of matter, nature, are becoming very real.

21:56 Because we're getting, we're beginning to be able to access

22:00 this tremendously complicated configuration space to do useful things,

22:06 to make calculations that are useful to us.

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