EPR Experiment and QFT: Are They in Conflict?

In summary: The important question here, is to what extent this is a reasonable thing to do. It's a bit like asking whether the earth is flat or not. You can always use a flat map to describe the surface of the earth, but in some cases (if you're a seaman) you might need to use a globe instead. The lesson from quantum theory is that the world is a bit like a globe, and we have been insisting on using flat maps all the time (because we were used to it and it is easier), but we are now reaching a point where flat maps won't do anymore and we'll have to switch to globes. If you keep using
  • #1
wangyi
56
0
Hi,
I have a question: The EPR experiment told us the world is nonlocal. And as we know, the QFT is a local theory, there is a principle that measurement do not affect each other between spacelike points. Do the two conflict? And can we say "we can not find an experiment for now which does not support the QFT?"
Thank you very much!
wangyi
 
Physics news on Phys.org
  • #2
Well,how can the EPR(-type) experiments fit in with the relativistic quantum mechanics...?I have no idea.Remember that the postulates of nonrelativistic quantum mechanics which lead Einstein,Podolsky & Rosen to formulate that Gedankenexperiment do not take into account the second (and neither the first,actually,but the second is much more relevant) postulate of SR,which was formulated by Einstein himself... Therefore,i think Einstein had something against the whole theory... :rolleyes:

So far,so good with the QFT,at least its SM part.

Da
 
Last edited:
  • #3
wangyi said:
Hi,
I have a question: The EPR experiment told us the world is nonlocal. And as we know, the QFT is a local theory, there is a principle that measurement do not affect each other between spacelike points. Do the two conflict? And can we say "we can not find an experiment for now which does not support the QFT?"
Thank you very much!
wangyi

I think you are mixing two kinds of locality here. QM being non local directly applies to the measurements in QM. In other words, it applies to the observables, eg the EPR paradox. In the QFT-case, locality means that the gauge symmetry needs to be respected at each and every space time coordinate. Don't forget that QFT completely incorporates the principles of QM, as well as those of SR

regards
marlon
 
  • #4
wangyi said:
Hi,
I have a question: The EPR experiment told us the world is nonlocal. And as we know, the QFT is a local theory, there is a principle that measurement do not affect each other between spacelike points. Do the two conflict? And can we say "we can not find an experiment for now which does not support the QFT?"
Thank you very much!
wangyi

As I pointed out in some other thread, EPR results do NOT tell us the world is non-local ! This is only one of the possible explanations, but Bell made *several* assumptions to deduce his inequalities.
The unitary dynamics in QFT is indeed completely local. However, QFT is just an application of quantum theory in general (you know, the Hilbert space, the operators, the states, the superposition principle...).
What people working in elementary particles often forget, is that it is a full-fledged quantum theory in which the superposition principle is universally valid ; usually one works with asymptotic incoming and outgoing momentum states. As such, no "change in basis" is required for the calculation of most quantities, and hence the typical mysteries that are apparent in non-relativistic quantum theory (in the style of the Young slits, EPR and so on) are not made very visible in most treatments on QFT ; the mathematical machinery being already difficult and heavy as it is. But all of these things are also present in QFT.
QFT being an application of quantum theory to a specific model, of course the general conceptual difficulties of quantum theory are the same for QFT as they are for non-relativistic quantum theory ; in particular the resolution of the measurement problem, and all discussion of the projection postulate and so on is just as actual in QFT as it is in non-relativistic QM.
So if you stick to Copenhagen quantum theory, where a measurement induces a collapse of the wavefunction, then this introduces just as much non-locality in QFT as it does in NR QM.

What I tried to argue in the other thread (on entanglement) is that a relative-state interpretation of quantum theory (and thus of QFT) leads naturally to the resolution of the EPR riddle *WITHOUT* violating locality. Of course the price to pay is that you have to accept the conceptual weirdness of a relative-state interpretation. I would like to underline, especially in QFT, the importance of this result. Locality (lorentz invariance) is such a strong guiding principle in all of QFT that it would be very very strange that we would finally introduce a rule (the projection postulate) that bluntly violates it. It simply doesn't make sense that you require lorentz invariance for the lagrangian formulation, that you require space-like field operators to commute etc... and that suddenly, you throw all that in the dustbin and bluntly project the entire field state reaching to the boundaries of the visible universe onto one of its components, simply because you, on earth, did a "measurement".

cheers,
Patrick.
 
  • #5
caribou said:
EPR experiments mean that there is no "local realism". Discussions tend to focus on the "local" bit and miss out on the "realism" bit. Locality can actually taken to be fine and then its realism that goes out the window instead. This is "realism" in the sense of which alternative particle properties can be said to exist at the same time.

Yup ! I'm exactly of the same opinion.

I'm trying to get my head around what a "lack of realism" means in a physical theory. :smile:

I take it as one of the lessons of quantum theory, that has been with it from the very start, and that people have systematically tried to wipe out until it hit us in the face with EPR.
I think the basic lesson from quantum theory is that "observations are relative" and only make sense to ONE observer, in the same way as "time is relative" was the message of relativity.
I have probably been pushing a bit hard here, with consciousness and stuff like that, to illustrate the idea, and I'm affraid this ended up being counter-productive because it was thought to be mixed in with too much mysticism.
Quantum theory, as it stands, allows you to explain YOUR specific observation history. We are used to take it for granted that this observation history is all there is to the world (which is probably the definition of "realism" in the Bell sense), and that anybody else necessarily needs to possesses a similar observation history, because there IS only one observation history, which corresponds to "reality". But we could just as well be on our "personal voyage" through different possibilities, which doesn't need to coincide with "other" voyages by other observers. Only, whenever we CROSS such another observer, we WILL have common observation histories. So the Alice-state that will meet a Bob state will be in agreement with whatever that Bob state has observed.
We get strange results (a la Bell violations) when we take different observation histories, by different observers, together, in the same way as we get strange results when we mix time variables of different observers in relative motion in special relativity.

I think that this is the "lack of realism" content. What our observation history tells us is not "what happened". It is what "we observed what happened" and this is a very personal history. When we encounter another observer, then we should be aware of what he tells us happened to him is not necessarily what "happened" for short. It could be that us "meeting him" is part of our personal history ; and that there are other "hims" which we will NOT encounter. As such, our meeting him has already introduced a certain bias in which histories he's going to tell us. And that bias turns out to be the correlations we find when we compare our observations to his observation history.

It is of course a disturbing idea that all we know, feel and see is just a personal story, and is not a view on the "real world". But this is what quantum mechanics has been yelling at us already for 80 years. Only, we found excuses as for why ELECTRONS "didn't have a position" until we observed them. Or why neutrons didn't have a position (when they diffracted at a crystal lattice). Or why an entire lattice of atoms in a crystal didn't have individual motions but acted as a whole (phonons).
Well, now I think we came to the point where Alice has to accept that Bob didn't have a definite measurement result until she asked him. And vice versa!

cheers,
Patrick.
 
  • #6
I think I have a explanation, and it does not quite related with QFT, accually.
the equation [phi(x),phi(y)]=0 for x,y spacelike only tells that the possibility of measuring the state with phi at x equals to the same measurment on condition that at point y the state has already been measured, it only indicates that the possibility is the same, but not the two measurement unrelated. For example, if we measure a system with J_z=0 using spin along z axis at x and y, the two must get the related(opposite) result, but both of them thinks the possibility is 50%, and no matter the measurment at x has taken place or not, the possibility at y is always 50%, because he doesn't know the information at x.

Is my explanation right? thank you!
 
  • #7
wangyi said:
For example, if we measure a system with J_z=0 using spin along z axis at x and y, the two must get the related(opposite) result, but both of them thinks the possibility is 50%, and no matter the measurment at x has taken place or not, the possibility at y is always 50%, because he doesn't know the information at x.

It is correct that measurements, locally at x, have probabilities which are not altered by whatever is decided to be measured at y. So as you point out, X sees half of the particles spin up, and half of them spin down, no matter what happens at y.
However, the simple case you present (both measure along the z-axis) can also be explained by local realist models: namely for all couples sent out, half of them have a +z spin which is sent off in the x direction, and the corresponding -z spin in the y direction, and the other half have exactly the opposite. Each particle then determines in advance what will be measured at x and what will be measured at y. No mystery at all.
EPR situations are more subtle.

cheers,
patrick.
 
  • #8
Yes, EPR tells us more. I only mean that the causality prinpicle in QFT does not conflict with QM. That is enough for the question i raised in the begining.

regards
wangyi
 

Related to EPR Experiment and QFT: Are They in Conflict?

1. What is the EPR experiment and how does it relate to QFT?

The EPR (Einstein-Podolsky-Rosen) experiment is a thought experiment proposed by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935 to challenge the completeness of quantum mechanics. It involves two entangled particles, where a measurement on one particle can instantaneously affect the state of the other particle, regardless of the distance between them. QFT (Quantum Field Theory) is a theoretical framework that combines quantum mechanics and special relativity to describe the behavior of particles. The EPR experiment highlights the non-locality and interconnectedness of particles, which is a concept that is also present in QFT.

2. What is the conflict between the EPR experiment and QFT?

The conflict between the EPR experiment and QFT lies in the interpretation of quantum mechanics. The EPR experiment challenges the idea of local realism, which states that physical properties exist independently of observation and that there is a limit to the speed at which information can travel. However, QFT allows for the possibility of non-local effects and instantaneous communication between particles, which goes against the principles of local realism.

3. Can the EPR experiment and QFT be reconciled?

While there is no definitive answer, many scientists believe that the EPR experiment and QFT can be reconciled. This may involve modifying our understanding of the relationship between space and time, as well as developing new theories that incorporate both concepts. Some theories, such as quantum entanglement theory, attempt to bridge the gap between the two by proposing that entanglement is a fundamental property of the universe.

4. How does the EPR experiment challenge our understanding of reality?

The EPR experiment challenges our understanding of reality by demonstrating that physical properties may not exist independently of observation and that there may be a fundamental interconnectedness between particles regardless of distance. This goes against our classical understanding of cause and effect and raises questions about the nature of reality and the role of consciousness in shaping it.

5. What are the implications of the EPR experiment and QFT for future research?

The EPR experiment and QFT have significant implications for future research in the field of quantum physics. They suggest that our current understanding of reality may be limited and that there may be underlying principles and mechanisms that we have yet to discover. They also point to the need for further exploration and development of theories that can reconcile the seemingly conflicting concepts of locality and non-locality. Additionally, the EPR experiment and QFT may have practical applications in fields such as quantum computing and communication.

Similar threads

  • Quantum Physics
Replies
1
Views
780
Replies
6
Views
1K
  • Quantum Physics
2
Replies
47
Views
4K
  • Quantum Physics
2
Replies
69
Views
4K
  • Quantum Physics
3
Replies
70
Views
5K
Replies
5
Views
1K
Replies
31
Views
2K
Replies
6
Views
2K
Replies
5
Views
452
  • Quantum Physics
Replies
27
Views
2K
Back
Top