Lecture: Quantum Gravity Breaks Causality -- And You Can Compute With It

31 Mar 2024 ✪ Members first on 30 March 2024 #physics #science #sciencenews

Check out my course about quantum mechanics on Brilliant! First 30 days are free and 20% off the annual premium subscription when you use our link ➜ https://brilliant.org/sabine. If you flip a light switch, the light will turn on. A cause and its effect. Simple enough… until quantum gravity come into play. Once you add quantum gravity, lights can turn on and make switches flip. And some physicists think that this could help build better computers. Why does quantum physics make causality so strange? And how can we use quantum gravity to build faster computers? Let’s have a look. The paper on indefinite causal structures is here: https://arxiv.org/abs/quant-ph/0701019
Summary: Quantum gravity disrupts causality by allowing effects to precede causes, potentially enhancing quantum computing through indefinite causal structures, though its practical application remains distant.

Transcript

0:00

Flip the switch and the light turns on. “Duh,”  you might say. Physicists say “causality”. Lights  

0:06

don’t turn on by themselves and then make  switches flip. And that sounds all well and  

0:12

fine except. Quantum physics screws it up. In quantum physics, lights can turn on and  

0:19

make switches flip, in some sense.  And as if that wasn’t crazy enough,  

0:23

some physicists now say you can actually use that  to build better computers. Let’s have a look.

0:30

Quantum particles can be in two places at  once. We call this a “superposition” of two  

0:35

places. That’s definitely weird, but it gets  weirder. The maths of quantum mechanics says  

0:41

that this shouldn’t just be the case for small  particles. In principle, it could happen for  

0:46

anything, molecules, bananas, cats. This is why Schrödinger was going on  

0:51

about a dead-and-alive cat, because it’s there,  in the mathematics of quantum mechanics. So why  

0:58

don’t we ever see dead-and-alive cats? It’s  because for large objects like bananas or  

1:03

cats these quantum effects disappear very  rapidly, due to tiny interactions with air  

1:09

molecules or the cosmic microwave background.  This is called “decoherence” and is the reason  

1:15

why the quantum-typical behaviour only shows up  for small objects if you measure quickly enough.

1:22

Ok, we only observe it for small  objects, but in quantum physics,  

1:26

things can be in two places at the same time  indeed. And if the thing that’s in two places  

1:32

has a mass, then it has a gravitational pull. And that gravitational pull, according to  

1:37

Einstein, means that space-time itself curves  which then affects the motion of other objects. 

1:43

The often-used analogy is that of a heavy ball  on a rubber sheet seems to attract other balls  

1:49

towards it, even though it’s really just the  curvature of the sheet that makes it happen. 

1:53

But how does space-time curve if a massive  particle is in two places at once? Well, we  

1:59

don’t know. For this we would need what physicists  call a theory of quantum gravity, that describes  

2:06

the quantum properties of space and time. However, most physicists think in any such  

2:12

theory of quantum gravity, space-time itself  would exist in superpositions, so it would do two  

2:18

things at the same time. And if that is right,  that has some very interesting consequences.

2:24

You see the issue is that the curvature of  space-time affects which event can causally  

2:30

influence another event. Let’s take two events  and call them A and B. For these two events to  

2:36

have had any causal relation it must be possible  for some influence to go from one to the other,  

2:42

and that can’t go faster than the speed of light. So if we draw space on the vertical and time  

2:48

on the horizontal axis, and as usual say  that light travels at a 45 degree angle,  

2:54

then for A to be a cause of B, the event  B must be in this area. It’s called the  

2:58

inside of the future light cone, just so  you’ve heard it. If B was down here instead,  

3:03

then it could be a possible cause of A. But  not any event is causally related to any  

3:08

other. Suppose they both sit here next to each  other, then neither can be a cause of the other.

3:14

But here’s the thing. Space-time curvature can  change causal relations. Yes it can. Remember that  

3:21

time near a black hole passes slower. I’ll denote  the passage of time with ticks on the vertical  

3:26

axis. If I put a black hole on the right, then  A can now influence B because time runs so slow  

3:32

at B. If I put the black hole on the left, then B  can influence A. Alright, you might say, so what.

3:38

Well now imagine that the black hole is in  both places at the same time. So which event  

3:44

can influence which one. If A was flipping  a switch and B was the light turning on,  

3:49

then it seems that, depending on how spacetime  is curved, either the switching caused the light  

3:55

or the light caused the switching. This means that if you want to  

3:59

give quantum properties to space-time, as  most physicists think has to be the case,  

4:04

then causal order goes out of the window. And  just in case you think this sounds like nonsense,  

4:10

Sabine probably just made this up  so she has something to talk about,  

4:14

I’ll leave you a reference in the info below.  It’s called an “indefinite causal structure”  

4:20

which you could now say still sounds like  nonsense, but now it’s scientific nonsense.

4:26

Then again some of you might say  this is actually pretty cool,  

4:30

effects coming before their causes. But of  course the fine print is again that this  

4:35

really only happen for teeny tiny things because  these quantum effects are so fragile. Then again,  

4:42

physicists deal with small objects all  the time, don’t they? And that brings up  

4:47

the question of what you can do with it.  You can use this for example to compute.

4:52

Suppose you have a quantum computer and you  have these quantum properties of space-time  

4:57

which scramble up the causal order. You then can  use this to do several computational operations  

5:03

at the same time. Take one number, call it  x, and say you want to add another number,  

5:08

say 3, and then multiply with seven. If  you do it in two different orders then  

5:13

you get both (x+3) times seven and x times  7 plus three, so you have actually done two  

5:20

different calculations. At the same time! If you have an algorithm that uses both  

5:25

operations, you can use that speed up the  calculation. And this is just an example  

5:30

where you change the order of two operations.  The more operations you have the more ways  

5:35

you can exchange them. This means that in  principle, quantum gravity can make quantum  

5:41

computers even faster. And yes there are  some people who have worked out the details.

5:46

So let me be clear that this isn’t something  which is going to happen any time soon, because we  

5:51

haven’t get even measured the quantum properties  of space and time. Though there are several  

5:57

experimental groups working on trying to do this,  as I have discussed in several previous episode.

6:03

But this is one of the reasons why I think  quantum gravity is not just interesting for  

6:08

theoretical reasons or merely philosophy.  It could one day indeed have technological  

6:13

applications. The other conclusion you can  draw from all this is that if quantum gravity  

6:20

gives rise to such indefinite causal orders,  then maybe that means that gravity shouldn’t  

6:25

be quantized in the first place.  I think Albert would prefer this.

6:31

Yes, I've been talking about quantum physics  again. It's without doubt one of the most  

6:36

exciting areas of science at the moment,  though I may be a bit biased. If you want  

6:42

to learn more about how quantum physics works,  you can check out my course on Brilliant.org.  

6:48

It's a beginners course that you can take without  any background knowledge. It'll introduce you to  

6:54

topics such as interference, superpositions  and entanglement, the uncertainty principle,  

6:59

and Bell's theorem. And afterwards, you can  continue learning more about your favorite  

7:04

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7:10

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7:15

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7:21

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7:27

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7:33

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