The Time Symmetry Debate in Quantum Theory

[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.