The Time Symmetry Debate in Quantum Theory

[DrChinese]

Something like Feynman-Wheeler absorber theory? but that was refuted many years ago, among other things assumed no self-interaction of particles.

There are others:

Relational Blockworld:
http://arxiv.org/abs/0908.4348

Yakir Aharonov and Jeff Tollaksen's take on Time Symmetry:
http://arxiv.org/abs/0706.1232

 

[Jano L.]

but that was refuted many years ago, among other things assumed no self-interaction of particles.

The F-W theory consists of the basic equations (action principle) and a peculiar boundary condition, which is not the only one possible.

Which part do you think was refuted?

 

[audioloop]

I'm not sure if this is what he meant, but there are "time-symmetric" formulations of wave propagation in which the future affects the present in the same way the past does. It's not FTL, in the sense that propagation speed is always <= c, although the propagation can be into the past as well as into the future. This is consistent with SR in the weak sense that there is no violation of lorentz invariance.

http://prl.aps.org/abstract/PRL/v110/i21/e210403
"The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics"
"we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed two-photon state demonstrates that entanglement can be shared between timelike separated quantum systems"the two vector formalism fit neatly with that process.

Can a Future Choice Affect a Past Measurement's Outcome?
http://arxiv.org/ftp/arxiv/papers/1206/1206.6224.pdf

https://www.physicsforums.com/showpost.php?p=4053068&postcount=31
https://www.physicsforums.com/showpost.php?p=4053118&postcount=32
https://www.physicsforums.com/showpost.php?p=4056855&postcount=36-------
but there is the possibility that the photons interchange information at the time of the monogamy creation or like i prefer to say heterogamy (one up, one down) or is an inherent symmetrical process, we need more experimental testing to know.

https://www.physicsforums.com/showpost.php?p=4402245&postcount=135
http://arxiv.org/pdf/1206.4949v2.pdf
"Physical theories are developed to describe phenomena in particular regimes, and generally are valid only within a limited range of scales. For example, general relativity provides an eective description of the Universe at large length scales, and has been tested from the cosmic scale down to distances as small as 10 meters. In contrast, quantum theory provides an eective description of physics at small length scales. Direct tests of quantum theory have been performed at the smallest probeable scales at the Large Hadron Collider, 10^-20 meters, up to that of hundreds of kilometers. Yet, such tests fall short of the scales required to investigate potentially signicant physics that arises at the intersection of quantum and relativistic regimes. We propose to push direct tests of quantum theory to larger and larger length scales, approaching that of the radius of curvature of spacetime, where we begin to probe the interaction between gravity and quantum phenomena"
"The tests have the potential to determine the applicability of quantum theory at larger length scales"

Super Physics Smackdown: Relativity vs Quantum Mechanics...In Space
http://www.technologyreview.com/view/428328/super-physics-smackdown-relativity-v-quantum-mechanicsin-space/#ixzz2UyZfdG1L
.

 

[DevilsAvocado]

Here I go again...
From Age of Entanglement:

"...But what if we let relativity enter the game even deeper? What if the detectors are in relative motion such that each detector in its own reference frame analyzes its photon before the other?...

"...once one assumes that the collapse is a real phenomenon, and once one considers specific models, then the conflict is real and testable"...if both measurements happen before the other, then the quantum correlation should disappear, however large the speed of the spooky action!

"Once the engineering was made feasible, "this experiment was also performed in Geneva in the spring of 1999", reported Gisin. "The two-photon interferences were still visible, independently of the relative velocity between Alice and Bob's reference frames." Alice, in her reference frame, measures her photon first; from Bob's point of view, he has measured his photon first; yet, the correlation is still present..."

The paper is very interesting. Though there seems to be a tiny ‘issue’...

However, it is possible that it is not the detector that triggers the collapse. The photons could take the decision already at the beamsplitter and go out through one output port, like in the Bohm-de-Bloglie pilot wave picture [26] (which much inspired Suarez). With the beam-splitter as choice-device superluminal signaling is not possible (to our knowledge). A corresponding experimental test would be more demanding, a beam-splitter would have to be in motion. A clever way-out could be the use of an acousto-optical modulator representing a beam-splitter moving with the speed of the acoustic wave. We are working on such an experiment.

[my bolding]

My absolutely unscientifically guess is that the “stuff” happens at the polarizer/beam-splitter...

(Does anyone know if they proceeded with that new experiment?)


P.S: I'll get back on PM.

 

[DevilsAvocado]

How does the Bohr model violate the uncertainty principle?

Angular momentum depends on both the radius of the orbit and the velocity of the electron in that orbit. The uncertainty principle stipulates that the radius OR velocity MUST be uncertain = angular momentum can NOT be quantized, because it can NOT be known.

The Bohr model does correctly predict spectral lines for atomic hydrogen.

Yup

That is something at least.

Something is not everything...

I get the impression that we feel very confident we have exhausted all mechanical analogies for things like the double slit experiment when we cannot possibly have.

We shall never ‘give up’, but I think it was Feynman who said that it’s proven that an alternative/succeeding theory has to be at least as ‘weird’ as QM. So, there is not much hope for a classical fruit with a big nut inside...

There are an infinitude of possible mechanical analogies we could use to model a particle like an electron (and all but one will be wrong). I don't know if anyone has seen the experiments with silicon droplets but I saw these a while back and found them very interesting:

Droplets are sweet, but they do nothing. I could put a rubber duck there instead, with same result. Medium is everything... w ∙ a ∙ v ∙ e ∙ s

 

[DevilsAvocado]

There are others:

Relational Blockworld:
http://arxiv.org/abs/0908.4348

Ah! Captain RUTA & RBW! But how do we signal from the future in RBW with 'only' spacetimematter?

“past, present and future are co-constructed as well, there are no dynamical entities or dynamical laws in our fundamental formalism [...] accordingly, all dynamical explanation supervenes on, and is secondary to, non-dynamical topological facts about the graph world”

Yakir Aharonov and Jeff Tollaksen's take on Time Symmetry:
http://arxiv.org/abs/0706.1232

Ouch... but wait... conservation of the CPT symmetry requires time reversal to rename particles as antiparticles and vice versa... tachyonic antitelephone anyone??


(sorry, DrC hysterical lame jokes )

 

[bhobba]

the structure formed by the lattice of events in a real or complex Hilbert space.

The technical meaning of lattice used here is different to what you are interpreting it as. Its an algebraic structure used in Quantum Logic:
https://www.physicsforums.com/newreply.php?do=newreply&p=4403159

Just to elaborate a bit on contextuality being unnatural in the Hilbert Space formalism.

If you choose that as your formalism you would expect the states to tell us something as far as the results of experiment are concerned so you can make predictions. Technically that means defining some kind of measure on the states. If contextuality was true you couldn't do that because it would depend on the basis you expand the state out as. Not only that but modern physics has taught us coordinates (and basis are a generalization of coordinates), being an arbitrary man made thing, are independent of the physics - this is one of key insights of Einstein in GR with his principle of covariance (as Kretchmann pointed out to Einstein, and Einstein eventually accepted, it's a principle devoid of physical content, but is of great heuristic importance - however that is another story).

Victor Stenger wrote a nice book on this a few years ago now:
http://www.colorado.edu/philosophy/vstenger/nothing.html

It quite amusing actually. Some people interpreted this as Stenger was saying the Laws of Nature came from nothing. That wasn't the case at all - they came from symmetries which are hardly nothing. The thing is symmetries are so appealing to our intuition it seems to come from nothing. For example that momentum exists and is conserved in an inertial frame comes from the space translation symmetry of an inertial frame and since that is the definition of an intertal frame you think its pulled out of a hat and comes from nowhere. First definitions contain no physics - the import of an inertial frame is that out there in interstellar space frames exist that are to a very high degree inertial which is an observational matter - the universe doesn't have to be like that - it just is. And secondly you need something to be symmetrical in - in this case its the laws of QM - and their validity is an experimental/observational matter - they may or may not be true. Its just that this symmetry stuff is so appealing and all pervasive in modern physics it seems like magic and beautiful beyond compare - which it is - when you understand it.

Thanks
Bill

 

[bhobba]

Which part do you think was refuted?

I am pretty sure the FW theory has never been refuted - in fact I think that would be pretty hard to do since it was deliberately cooked up to be equivalent to ordinary EM but without fields. The issue with it is no-one has been able to figure out a quantum version - to the best of my knowledge anyway.

Thanks
Bill

 

[DevilsAvocado]

I am pretty sure the FW theory has never been refuted - in fact I think that would be pretty hard to do since it was deliberately cooked up to be equivalent to ordinary EM but without fields. The issue with it is no-one has been able to figure out a quantum version - to the best of my knowledge anyway.

The expansion of the universe is not time symmetric in the thermodynamic limit.
Feynman himself stated that self-interaction is needed to correctly account for the Lamb shift.

 

[Jano L.]

The expansion of the universe is not time symmetric in the thermodynamic limit.

I am not sure what you mean. How do you check whether expansion is time-symmetric? How does it connect to the FW theory?

Feynman himself stated that self-interaction is needed to correctly account for the Lamb shift.

Can you give a reference? People often state many things without convincing arguments. The Lamb shift was measured originally for hydrogen, whose atom consists of two particles. It is hard to show that self-action is necessary when the main forces in play are those of the interaction between different particles, and their effect is hard to evaluate.

The Lamb shift can be explained in other ways, one among which is the interaction of the atom with other particles in the surroundings (their EM field). Self-interaction of one particle on itself is not necessary.

 

[DevilsAvocado]

I am not sure what you mean. How do you check whether expansion is time-symmetric? How does it connect to the FW theory?

The thermodynamic arrow of time + the Second law of thermodynamics are thought to be a consequence of the initial conditions in the early universe. Not possible to ‘rewind’... but to be fair, this is supposed not to be a problem for [any] T-symmetry, because there are no proofs T-symmetry must be broken also at the microscopic level.

How this would play out in the ‘real world’, I have absolutely no clue...

Can you give a reference? People often state many things without convincing arguments.

http://prola.aps.org/pagegif/PR/v76/i6/p769_1/p769

http://prola.aps.org/pagegif/PR/v76/i6/p769_1/p773

3. THE SELF-ENERGY PROBLEM
Having a term representing the mutual interaction of a pair of charges, we must include similar terms to represent the interaction of a charge with itself. For under some circumstances what appears to be two distinct electrons may, according to I, be viewed also as a single electron (namely in case one electron was created in a pair with a positron destined to annihilate the other electron). Thus to the interaction between such electrons must correspond the possibility of the action of an electron on itself.<sup>9

9</sup> These considerations make it appear unlikely that the contention of J.A. Wheeler and R.P. Feynman, Rev. Mod. Phys. 17, 157 (1945), that electrons do not act on themselves, will be a successful concept in quantum electrodynamics.

Sorry to be a ‘party pooper’, but if the [extremely intelligent] author of an idea gives up on it – what have you then...?

 

[stevendaryl]

Sorry to be a ‘party pooper’, but if the [extremely intelligent] author of an idea gives up on it – what have you then...?

Feynmann had plenty to keep his mind occupied. The fact that he gave up on an idea just means that he felt other ideas were a better use of his time.

 

[TrickyDicky]

Feynmann had plenty to keep his mind occupied. The fact that he gave up on an idea just means that he felt other ideas were a better use of his time.

Come on, this comment in response to Devil is meant as a joke, right?

 

[stevendaryl]

Come on, this comment in response to Devil is meant as a joke, right?

No. I don't think that the fact that Feynman gave up on an idea has any implications at all about whether the idea can be made to work.

 

[bhobba]

No. I don't think that the fact that Feynman gave up on an idea has any implications at all about whether the idea can be made to work.

This one has been mentioned in quite a few Feynman biographies such as Genius. He actually stuck with it for quite a while but could never get it to work as a quantum theory and only gave it away with reluctance.

Thanks
Bill

 

[stevendaryl]

No. I don't think that the fact that Feynman gave up on an idea has any implications at all about whether the idea can be made to work.

...also, maybe it's an irrational anti-authoritarian quirk of mine, but I detest when people use "Einstein said" or "Feynmann said" (or in a different forum, "Jefferson said" or "Churchill said") as if that settled the matter. What matters to me is the quality of an argument, not who made it.

 

[stevendaryl]

This one has been mentioned in quite a few Feynman biographies such as Genius. He actually stuck with it for quite a while but could never get it to work as a quantum theory and only gave it away with reluctance.

Thanks
Bill

I think there are two different issues that are getting mixed up. One is the issue of time-symmetric wave propagation, and the second is the issue of eliminating electron self-interaction. I think it's the latter that Feynman gave up on, but I don't see that that has any direct implication for time-symmetric propagation, other than changing the motivation for it.

 

[DevilsAvocado]

Come on, this comment in response to Devil is meant as a joke, right?

Well, it sure made me ROFL!

 

[kith]

Ouch... but wait... conservation of the CPT symmetry requires time reversal to rename particles as antiparticles and vice versa...

This may be connected to the old idea that antiparticles are particles which travel backwards in time. But I'm also curious: do the known time symmetric interpretations work for QFT?

 

[DevilsAvocado]

...also, maybe it's an irrational anti-authoritarian quirk of mine, but I detest when people use "Einstein said" or "Feynmann said" (or in a different forum, "Jefferson said" or "Churchill said") as if that settled the matter. What matters to me is the quality of an argument, not who made it.

I'll give you that, but I think the situation here is more like Feynman said about Feynman...

These considerations make it appear unlikely that the contention of J.A. Wheeler and R.P. Feynman, Rev. Mod. Phys. 17, 157 (1945), that electrons do not act on themselves, will be a successful concept in quantum electrodynamics.

 

[Jano L.]

Self-interaction does play role in Feynman's later ideas, and it seems in his later works he favored it over his earlier theory. But notice the manner he says that and also how he evaluates his new theory in the paper. He is cautious, far from claiming self-action or any part of his other new ideas were proven to be how electrons really behave.

Also, it is worth pointing out that Feynman was not alone in studying the idea that only different particles interact (Tetrode, Frenkel...).

I think today his quantum theory is more developed and better understood mathematically, but the question which part of the formalism corresponds to actual processes and which is just mathematics valued for its ability to predict some subtle experiments, is still very unclear (similarly to the situation in the ordinary quantum theory).

Depending on the field, you will find people whose beliefs range from thinking that electron emitting photon and absorbing it afterwards is a real process that happens and explains the world, to people who will say that such an idea is an ungrounded interpretation of mere perturbation theory to equations nobody has ever seen. I think Feynman himself was aware of this, and when he chooses direct language "as if it really happens" it is rather because this allows him to get to the root of his new mathematics as quick as possible.

So I think it is fair to grant a role to the idea of self-action in the current theories, but we have to be careful saying that opposite ideas were refuted. To my knowledge, there is no direct evidence on self-action. I am not well versed in Feynman's theory, but in other incarnations of the self-action, this can often be substituted by more intelligible concepts (interaction with bath, retardation, ...)

 

[Jano L.]

One is the issue of time-symmetric wave propagation, and the second is the issue of eliminating electron self-interaction. I think it's the latter that Feynman gave up on, but I don't see that that has any direct implication for time-symmetric propagation, other than changing the motivation for it.

Yes, it seems the aspect of time symmetry and the elimination of the fields were more important to him, probably hoping that the infinities can be resolved while self-action will stay.

I think that if we retain self-action in its common forms, the time-symmetry will get spoiled, because the common forms always somehow connect to retarded fields. These seem more important from experience and are asymmetric.

 

[DevilsAvocado]

do the known time symmetric interpretations work for QFT?

That’s a very interesting question. Does anyone have an answer?

Personally it makes me dizzy [common disease of mine] when I try to get T-symmetry + how the ‘everyday in universe’ actually works... glass breaks, eggs becomes omelets, we get older, and the thing expands... How could humans ever exist without the thermodynamic arrow of time? Our brain seems hardwired for past, present and future. Just look at this poor guy with almost no short-term memory (less than 30 sec). He’s basically ‘newborn’ at every instant.

Clive Wearing - The man with no short-term memory
https://www.youtube.com/watch?v=Vwigmktix2Y

 

[stevendaryl]

Yes, it seems the aspect of time symmetry and the elimination of the fields were more important to him, probably hoping that the infinities can be resolved while self-action will stay.

I think that if we retain self-action in its common forms, the time-symmetry will get spoiled, because the common forms always somehow connect to retarded fields. These seem more important from experience and are asymmetric.

Hmm, it never occurred to me that there was anything time-asymmetric about quantum field theory. The use of "retarded" fields is reflected in only using positive frequencies, or how?

 

[Jano L.]

Actually, I was thinking about classical theory in the above post. There, the equations are time-symmetric, but their application requires choice of boundary condition, and these are most often not time-symmetric. The introduction of dissipative self-force was motivated mainly by the idea the radiation was supposed to be retarded and carrying away energy.

In quantum electrodynamics, the fundamentals of the theory seem to be time-symmetric as well but they already seem to imply self-action (electron emits photon and then reabsorbs it). So seems like a somewhat different kind of self-action. The time asymmetry comes in, similarly to classical theory, when one wants to describe some experiment, for example Compton scattering - some conditions on the fields have to be assumed, which are often not symmetric (scattering of plane wave into spherical wave), and also some form of the golden rule is used, which is time-asymmetric.

 

[Ilja]

About time symmetry:

While the Schrödinger equation is time-symmetric, the wave function collapse is not. So, in all interpretations with wave function collapse, QT is not time-symmetric.

In dBB theory, there is no collapse. But the quantum equilibrium is only an equilibrium state. So, once the equilibrium is reached, it will never be left, and the equation is time-symmetric. But if one leaves equilibrium, this is no longer the case - then we have approximation to the equilibrium, as in thermodynamics, thus, no longer a time symmetry.

On the other hand, the fundamental equations of dBB, without any consideration of the development of probability distributions, are again time-symmetric.