# I Causality in QM?

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1. Nov 15, 2015

### entropy1

I am wondering about the meaning of 'preservation of causality' in quantum mechanics. Is there causality in QM? And does it act back in time? I have some ideas of myself, but want to learn about the general accepted view first.

2. Nov 15, 2015

### Bruno81

In as much as there are 'particles', there's as much causality.

3. Nov 15, 2015

### entropy1

I forgot to give an example: in this publication of Anton Zeilinger et al. he uses entanglement swapping via a Bell-state measurement. It appears that the swap works back in time. It seems to me this could be seen as causality back in time, and thus this might be a violation of causality depending on its definition.

What I want to know is what is ment by 'preservation of causality' in the context of QM. My own view is that there is no causality in any case, only correlations over space and time.

Last edited: Nov 15, 2015
4. Nov 15, 2015

### bhobba

Schroedinger's equation is a partial differential equation hence causal.

Only in some interpretation's is there anything back in time going on in QM.

The issue of causality in QM is purely a matter of what is being talked about. The state is causal - but the results of observations is not necessarily - but we have some interpretations where it is.

Thanks
Bill

5. Nov 15, 2015

### entropy1

Thanks for your reply. I am a layman studying QM on my own. I just got to the Schrödinger equation, so my understanding of it is limited right now. I don't understand what you mean by "Schroedinger's equation is causal" and "The state is causal". Would you like to explain that to me?

6. Nov 15, 2015

### bhobba

Ok. In QM we have this thing called the state - it's a generalisation of the wavefunction you may have heard about. The state and what you are observing determines the probabilities of the observations outcomes. It is a characteristic of QM that all you can predict is the probabilities of the outcomes of observations. If that is inherent in the theory or determined by something else is a matter of interpretation. If it is inherent then its obviously random and not causal. If it is determined by something else and that something else is causal then it may be causal. The theory doesn't say one way or the other.

The state however is determined by a causal equation.

Thanks
Bill

7. Nov 15, 2015

### entropy1

So a 'state' is a (generalized) wavefunction? How can an equation be causal?

For me, the experiment Anton Zeilinger et al. have done shows that due to entanglement swapping (see Fig. 2a for clarity of what I want to discuss here) the cause of a correlation between two particles A(lice) and B(ob) lies in the future (by a Bell measurement by V(ictor)), thereby facilitating retrocausality, if you want to speak of causality in this context of course... You could also speak of correlations and leave the whole thing in the hands of probality. So, indeed, it could be a matter of interpretation; is that what you mean?

8. Nov 15, 2015

### bhobba

If initial conditions determine final conditions its causal.

Correlations do not have to have a cause - they can just be correlations.

Thanks
Bil

9. Nov 15, 2015

### entropy1

But still probabilistic?

Exactly But you could also view them in the light of retrocausality, am I right?

This article of Zeilinger et al. also describes what I mean (see Fig 1).

Last edited: Nov 15, 2015
10. Nov 15, 2015

### bhobba

The state is deterministic, observations are probabilistic.

Yes - that is one interpretation - but a quite uncommon one.

Thanks
Bill

11. Nov 15, 2015

### entropy1

By that, do you mean the evolution of the state??

Why is it uncommon??

12. Nov 15, 2015

### bhobba

Yes.

Its just is. You cant explain taste.

I personally don't like it because its against the spirit of relativity. But that doesn't mean anything.

Thanks
Bill

13. Nov 15, 2015

### entropy1

Thanks.

14. Nov 15, 2015

### Heinera

As far as retrocausality goes, there is nothing in QM that allows you to communicate a message from a future observer to a past observer. For me, that fact settles the question.

15. Nov 15, 2015

### entropy1

But it seems to be possible to force (cause) (V) a correlation (between A en B) from the present (V) to the past (A&B)...??

16. Nov 15, 2015

### bhobba

its possible. None deny that. His issue is the same as mine. It cant be used to send information, so seems strange nature conspired to allow it but prevent that.

Thanks
Bill

Last edited: Nov 15, 2015
17. Nov 15, 2015

### entropy1

If nature would allow that, I think the consequences would probably be enormous, and we would probably live in an entirely different universe!

18. Nov 15, 2015

### entropy1

It just occurs to me, (I don't know very much about it though); if you'd place A en B close together and V far away, and let V 'decide' whether to do a Bell measurement or not, you would have A and B entangled or not! In this scheme, the repeated measuring of entanglement between A en B, yes or no, could in principle send information! But as I understand a Bell measurement requires an 'event-ready' scheme (what is that??), and does not close the fair-sampling loophole!

I can't make out if the random correlation that occurs when there is no entanglement spoils the whole setup though...

I throw this up because I wonder if you guys can tell me why this wouldn't work, so that I gain more knowledge...

UPDATE: In fact, this wouldn't solve anything, because the scheme involves sending entangled photons to V, and V could just as well send a message with photons from V to A&B.

19. Nov 15, 2015

### zonde

No, it doesn't seem so. It is like you find out what type of correlation (out of 4 possibilities) is between A and B. So no forcing or causing.

20. Nov 15, 2015

### entropy1

I think I don't understand what you mean... A and B are initially not correlated at all, right?