# I Does realism imply locality or vice versa?

1. May 10, 2017

### Sunny Singh

Through Bell's inequality, we can see that any hidden variable theory of QM will have to satisfy the inequality, but as it doesn't, wave function must be the whole story and so we have to do away with realism. So when a measurement is done on one detector in the EPR experiment, the wave function collapses for both the observers simultaneously and so locality must also be violated. So in this sense, by violation of realism, locality is also violated as a consequence. So i was wondering if they imply each other or i am misunderstanding something here since i have read in some books that they do not necessarily imply each other.

2. May 10, 2017

### Demystifier

Not any hidden variables, just local ones. Non-local hidden variables can violate the inequality and be compatible with QM.

3. May 10, 2017

### Sunny Singh

So if locality is violated, it does not necessarily mean that realism is out as there can be non local hidden variable theories but what if we find out that there are no hidden variables (local or non-local), that realism doesn't exist. Then according to my above logic does it necessarily implies that locality is also violated?
I'm sorry if i am missing some key points as i am very new to Bell's inequality and the concept of hidden variables.

4. May 10, 2017

### m k

An italian team did an entanglement test thru optical wire and got a result.

Means that eighter wire is there or don't.

5. May 10, 2017

### atyy

There are at least two types of locality.

(1) Local realism. Local realism assumes realism. Thus if realism does not hold, then local realism also does not hold.

(2) Signal locality. Signal locality does not assume realism. Thus if realism does not hold, signal locality can still hold.

6. May 11, 2017

### Demystifier

If realism is missing, then locality can be saved.

7. May 11, 2017

### ueit

Can you provide an example of a local, non-realist theory that can explain the observed correlation?

8. May 11, 2017

### Demystifier

Yes, I made such an example by myself:
https://arxiv.org/abs/1112.2034
It is non-realistic in the sense that observed objects are not real, even though the observations are real.

9. May 11, 2017

### Sunny Singh

But sir, in case of the EPR experiment, if realism is missing, how can locality be saved since that would collapse the wave function for both observers and information has to travel instantaneously. Can you please explain this point of yours a bit further?

10. May 11, 2017

### Demystifier

If realism is missing, then wave function is not real. If so, then collapse is also not real, so non-locality is also not real.

11. May 11, 2017

### Staff: Mentor

Mentor's note: A side discussion based on superdeterministic ideas has been moved to another thread, because it is not especially responsive to the original question.

Last edited: May 11, 2017
12. May 11, 2017

### DrChinese

Realism is missing when there are no predetermined outcomes of measurements. Keep in mind that EPR's "perfect correlations" imply predetermination, but Bell implies the opposite.

If the outcomes are NOT predetermined, there are several other possibilities: a) the outcomes are determined at time of measurement, which implies non-local (instantaneous) interactions of some kind; or b) the future affects the past. Please note that for b) you can preserve locality (there would need to be some form of time symmetry). This is done by having the observers become part of the context of the measurement process. There still is no instantaneous effect, although it might look otherwise superficially.

Although this may seem crazy at first: all entangled systems do trace out locally if you allow backward-in-time connections. This is especially notable when you attempt to create a spacetime diagram of entanglement swapping. +/- c is a limiting factor for these setups, and direct FTL action (between Alice and Bob) does not *appear* to describe anything on these.

Of course, whenever you talk about dropping realism vs. dropping locality, you are wading into the interpretations of QM. And there is no experimental evidence to select one of those over another at this time.

13. May 11, 2017

### Xu Shuang

Local realism is not simply a combination of locality and realism, realism alone dictates $(a,b)=f(\hat a , \hat b,{\cal R})$ , in which $\cal R$ denotes "the reality" of the system in question, $(a,b)$ are two measurement results, and $\hat a$ and $\hat b$ denotes how the measurements are carried out for two particles. In comparison, local realism dictates $a=f(\hat a , {\cal R})$ and $b=f(\hat b, {\cal R})$, if the two measurement events are space-like connected. The two equations defines local realism gives Bell inequality, while the equation that defines realism doesn't, hence when Bell inequality is violated, it violates local realism, but not realism alone. Alternatively, if realism is not right to begin with, then local realism cannot be defined, while locality alone can be intact in its phenomenological definition, as in information cannot travel faster than light.

Last edited: May 11, 2017
14. May 11, 2017

### RUTA

15. May 11, 2017

### stevendaryl

Staff Emeritus
Do you have a url to an essay talking about entanglement swapping from the point of view of time symmetry?

16. May 11, 2017

### DrChinese

Good one, lemme see what I have.

17. May 12, 2017

### Denis

The funny thing is that "locality" in this sense - no observable/usable information transfer - remains untouched even if we do not reject realism.

So, we can have realism, together with weak locality. Or simply the same weak locality alone. Or, in other words, we can throw away realism gaining nothing.

18. May 14, 2017

### Simon Phoenix

The short answer to your question is, no, they do not imply each other. The words 'realism' and 'locality' can be confusing and the discussions can get a bit technical with other terms like 'counterfactual definiteness' bandied about. All that is fine, and it's important to be as precise as possible, but I sometimes feel that the jargon obfuscates what is in essence a very simple question: can the world be described by a 'common sense' model?

Of course the devil in the detail here is what is meant by the term 'common sense'. In essence we might describe the world of classical physics pre-QM and pre- special relativity as 'common sense'. In this world an object in motion, such as golf ball, could be (approximately) modelled as a point particle for which we could assign a position and a momentum at some instant in time. We construct the equations of motion that show us how the position and momentum evolve in time and we learn how to do this in high school. The model, of course, is not the physical system itself, but until QM came along nobody had any real problem with thinking of the position and momentum for something like a golf ball as being anything other than a faithful representation of reality - the golf ball really was somewhere and it really was travelling with some specified velocity at some given instant in time.

OK maybe 'nobody' here is too strong - but I think someone who felt that things like position and momentum weren't 'real' things would probably have had a hard time defending their position pre-QM.

So there's this notion that's kind of implicit in classical physics that we can describe things using a set of variables that have some meaning out there in the real world. Special relativity didn't really change this but added an extra feature that one event can only be the cause of another event if there was enough time for a light signal to be transmitted between the events. In other words, if two remote objects interacted with one another that interaction could not occur faster than some minimum time interval, and certainly not instantaneously; so no instantaneous action-at-a-distance.

So the two notions 'realism' and 'locality' are really quite distinct, but both eminently reasonable from the perspective of classical physics. Realism says that things really do have some properties and it doesn't matter whether we measure them or not, those properties exist. Locality says that if one object interacts with another then there's a speed limit imposed on how fast that interaction can get from place to place.

So in a nutshell, can we construct a model of the world from things (properties) which are realistic and local? What Bell showed was that there were certain kinds of experiments we could do and, if we assumed the results could be modelled by some theory that had these 'common sense' properties of realism and locality, then those results had to be constrained to lie within a certain range of values. The amazing thing is that the QM predictions for these experiments can lie outside this range. Bell's inequality is actually nothing at all to do with QM - it is a constraint that classical-like theories which have these 'common sense' properties must satisfy.

Bell's own masterful exposition of all this is still, for me, the best : https://cds.cern.ch/record/142461/files/198009299.pdf

Now of course everything I've written above needs quite a bit of technical 'pinning down' - the intuition needs precise codification and translation into maths, but in essence the underlying problem is simple and ultimately very, very profound. Can we build a kind of clockwork classical model of the world? The answer to that is remarkable; yes we can, but if we want our model to consist of things which have some real objective existence (like position or momentum for example) then the only way to do it is to have these 'realistic' elements connected in some way that violates the bounds of special relativity.

The only successful model I'm aware of that has this property of realism, but is non-local, is the Bohmian version of QM. It still looks nothing at all like traditional classical physics - and I think it's nuts (subjective, non-scientific opinion) but it is at least one counter-example to the supposition that realism and locality imply one another.

Actually in my opinion the issue of 'non-locality' is almost a red herring. It's important to be able to exclude certain kinds of theories as contenders for 'explanations' of QM, but for me the real issue is that even if we admit non-local interactions (instantaneous action at a distance, for example) then the physics we need to explain stuff is still going to look very, very different to traditional classical physics.

19. May 14, 2017

### Denis

The basic question is indeed a very interesting one, not only for scientists, but also for the general public. It would be important to discuss the possibility of interpretations of modern physics as close to "common sense" as possible. Unfortunately, this is almost forbidden to discuss here.

In general, I do not think that a "common sense interpretation" of modern physics would be so very different from traditional classical physics. dBB is the most famous realistic QT interpretation, but not the closest one to common sense, where my favorite is Caticha's entropic dynamics (even if it has a problem with the Wallstrom objection). And interpretations of the Einstein equations of GR much closer to classical "common sense" are also not a big problem.

The use of "locality" is very misleading, because the common sense meaning of this word is very different from the actual use in Bell discussions. The accurate name would be Einstein causality, of Einstein locality. Which connects it to the very special theory it belongs too. A theory which would have a maximum speed of information transfer of 10000000 c would be nonetheless local in the common sense meaning, and not at all excluded by actual observations, and even if our best existing theories are really non-local, like dBB theory, they all may be limits of local theories.

20. May 16, 2017

### facenian

That's not the correct way to see at Bell's inequalities. The violation of Bell' s inequalities(VBI) does not mean that the wave function is the whole story it only means both assumptions, realism and locality can not hold simultineously. Bell stated a theorem that like any other theorem has an hypothesis and a Tesis
H) realism and locality(both valid at the same time)
T) there is an inequality that must be satisfied by any local realistic hidden variable theory
Then he proves VBI by QM so the hypotheis is false which means there are three posibilities
1_ realism is true and locality is violated
2_ realism is false and locality holds
3_ both realism and locality are false

To add to the existing confusion I must say that there are more than one version of Bell's theorem that he himself generalize throuhg out the years. Here we are dealing with his first version published in 1964

This is only true if we accept the Copenhagen interpretation and this we must not do when trying to interpret Bell' theorem

21. May 17, 2017

### Zafa Pi

Would Bohmian mechanics provide an example of non-local hidden variables? If not what would would?

22. May 17, 2017

### Demystifier

Yes. Indeed, this is the best known example of non-local hidden variables.

23. May 17, 2017

### Zafa Pi

So @Demystifier and@DrChinese say locality can be preserved and like the article recommended by @RUTA which says Bell showed nonlocality.
Perhaps one of Demystifier or DrChinese or John Bell could help me out here.

24. May 17, 2017

### Demystifier

Bell proof is a mathematical theorem which, like any other mathematical theorem, rests on certain unproved assumptions. It is always legitimate to question the unproved assumptions, even if the assumptions seem very reasonable. For various ways to save locality by rejecting certain reasonable but unproved assumptions see
https://arxiv.org/abs/1703.08341 Sec. 5.3.

25. May 17, 2017

### Simon Phoenix

I'm just going to re-iterate what Demystifier has said here.

Mathematical theorems are usually of the form of an IF . . .THEN statement. So IF we have $A$ and $B$, THEN $x$ follows.

In the context of Bell's work we have something like (and crudely),

IF (realism, locality) THEN (correlation functions bounded by inequality)

where I'm just focusing on these two properties and excluding things like superdeterminism.

QM predicts correlation functions that don't have to be bounded by this inequality, therefore QM cannot be equivalent to a theory constructed from quantities which have the properties 'realism' AND 'locality'.

It is therefore possible (in principle) to find a theory for which the correlations are not bounded by the inequality by :
(i) dispensing with 'realism' but keeping 'locality'
(ii) dispensing with 'locality' but keeping 'realism'
(iii) dispensing with both 'locality' and 'realism'

It is NOT possible, in principle, to find a theory that agrees with the QM predictions for the correlation functions constructed from variables/quantities which possess BOTH of the properties 'realism' and 'locality'.

Note that we're only talking about the violation of the bound on the correlation functions here - just because we construct some theory (eg by dispensing with 'locality') that has a violation for a particular experiment (eg the singlet state of QM) does not imply that it will also agree with other predictions of QM, or even the predictions of QM for other entangled systems (eg the GHZ state)