Time Symmetric Quantum Mechanics

dm4b
I've been seeing more and more papers that seem to suggest Time Symmetric Quantum Mechanics (TSQM) is becoming the more parsimonious explanation to some newer experiments.

For those unfamiliar with this formulation, it's a two-state-vector formulation, with one of the state vectors propagating backwards in time from the future (don't worry, causality is preserved!)

Here's one, although from 2012:

http://arxiv.org/abs/1206.6224

An EPR experiment is studied where each particle undergoes a few weak measurements of different spin-orientations, whose outcomes are individually recorded. Then the particle undergoes a strong measurement along a spin orientation freely chosen at the last moment. Bell-inequality violation is expected between the two strong measurements. At the same time, agreement is expected between all same-spin measurements, whether weak or strong. A contradiction thereby ensues: i) A weak measurement cannot determine the outcome of a successive strong one; ii) Bell's theorem forbids spin values to exist prior to the final choice of the spin-orientation to be measured; and iii) Indeed no disentanglement is inflicted by the weak measurements; yet iv) The weak measurements' outcome agrees with those of the strong ones. The only reasonable resolution seems to be that of the Two-State-Vector Formalism, namely that the weak measurement's outcomes anticipate the experimenter's future choice, even before the experimenter themselves knows what their choice is going to be. Causal loops are avoided by this anticipation remaining encrypted until the final outcomes enable to decipher it.

So, is this formulation being taken more seriously these days? The recent QM interpretation polls sure didn't seem to indicate that it is, despite these recent experiments.

dm4b
More from the Intro. Sounds like a rather Lorentzian/Minkowskian way of viewing things ;-)

Bell's theorem [ 1] has dealt the final blow on all attempts to explain the EPR correlations [ 2] by invoking previously existing local hidden variables. While the EPR spin outcomes depend on the particular combination of spin-orientations chosen for each pair of measurements, Bell proved that the correlations between them are cosine-like and nonlinear (Eq. (1) hence these combinations cannot all co-exist in advance. Consequently, nonlocal effects between the two particles have been commonly accepted as the only remaining explanation.

It is possible, however, to explain the results without appeal to nonlocality, by allowing hidden variables to operate according to the Two-State Vector Formalism (TSVF). The hidden variable would then be the future state-vector, affecting weak measurements at present. Then, what appears to be nonlocal in space turns out to be perfectly local in spacetime.

Quantumental
Huw Price writes quite a lot about this, here is his most recent paper: http://arxiv.org/abs/1307.7744

huelsnitz
I have not had time to read about Time Sym QM, yet. But it sounds similar to the old Transactional Interpretation. Is anyone here familiar with that? How does Time Symmetric QM differ from the Transactional Interpretation?

Warren

dm4b
Huw Price writes quite a lot about this, here is his most recent paper: http://arxiv.org/abs/1307.7744

That was a very cool article, thanks for sharing.

I'm really quite surprised by the lack of interest in this flavor of QM. With it's increasing utility and elegance with explaining some recent experiments, plus the mind-blowing physical interpretation, it's hard to believe more folks aren't interested in exploring this interpretaion/formulation.

With that said, it does sound like support is growing for it.

Staff Emeritus
Huw Price writes quite a lot about this, here is his most recent paper: http://arxiv.org/abs/1307.7744

Very nice paper, but what I would like to see, if even for a toy example, is how complex amplitudes--the sort of mathematical tools used in standard quantum mechanics--can be understood in terms of retrocausality. Cramer's Transactional Interpretation sort of seems to be close to doing this, but I don't see how it relates to Huw's concept of retrocausality.

Why does the whole mechanism of Hilbert spaces and Hermitian operators and so forth work so well?

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dm4b
Very nice paper, but what I would like to see, if even for a toy example, is how complex amplitudes--the sort of mathematical tools used in standard quantum mechanics--can be understood in terms of retrocausality.

I think you'll get what you're looking for here:

http://arxiv.org/abs/0706.1232

and here:

http://arxiv.org/abs/quant-ph/0105101

the_pulp
That was a very cool article, thanks for sharing.

I'm really quite surprised by the lack of interest in this flavor of QM. With it's increasing utility and elegance with explaining some recent experiments, plus the mind-blowing physical interpretation, it's hard to believe more folks aren't interested in exploring this interpretaion/formulation.

With that said, it does sound like support is growing for it.

Yes it surprises me too. Recently Wikipedia included it in the "traditional" interpretations of QM http://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics. Given its characteristics described in that page (locality, determinism, non contextuality, unique history, etc) I smell that this interpretation is somekind of direction of the next theory of "super super tiny particle physics". But I am clearly not the one to state this kind of propositions.

Thanks!

Gold Member
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Jilang
Physicists ask photons 'Where have you been?
http://arxiv.org/abs/1304.7469
http://prl.aps.org/accepted/27074Y6bS8a10b4901dc7d435d32e59308c3919be

In praise of weakness
http://physicsworld.com/cws/article/indepth/2013/mar/07/in-praise-of-weakness

This is a great experiment and if this is true it answers the question from my last my thread. Why is the probability the amplitude multiplied by its complex conjugate ... Maybe the joint probability of the wavefunction meeting itself coming the other way through time is the reality as we perceive it?

1 person
audioloop
This is a great experiment and if this is true it answers the question from my last my thread. Why is the probability the amplitude multiplied by its complex conjugate ... Maybe the joint probability of the wavefunction meeting itself coming the other way through time is the reality as we perceive it?

excuse me, which thread ?

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point of encounter of the two vectors ? -> reality ?

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Jilang
excuse me, which thread ?

------
point of encounter of the two vectors ? -> reality ?

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Why is probability amplititude squared? Last post was on the 26th. Sorry I don't know how you make a link to it.

audioloop
Why is probability amplititude squared? Last post was on the 26th. Sorry I don't know how you make a link to it.

https://www.physicsforums.com/showpost.php?p=4583901&postcount=26

I find this really fascinating. The Schroedinger Equation is a diffusion equation with an imaginary diffusion coefficient (or real diffusion in imaginary time?) Is that just a coincidence or is there some underlying process driving it? Why would it become more uncertain over time?

two vectors in time, one forward and one backward.

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Relational BlockWorld:

http://arxiv.org/abs/0903.2642

Cramer's:

http://www.npl.washington.edu/npl/int_rep/gat_80/ [Broken]

Thanks for the advert, Doc Here is our most recent work on RBW (to appear in IOP book on quantum spacetime): http://arxiv-web3.library.cornell.edu/abs/0908.4348v11. It's a paper with a long history, as is evident by the "0908" prefix. This version was posted on 18 Nov 2013 and will likely be the last, since it's now scheduled for publication.

RBW is a time-symmetric interpretation of QM in the trivial sense that it's a blockworld interpretation. However, there are no quantum entities (wave function or otherwise) moving through the experimental equipment to 'cause' detector outcomes. So, RBW isn't really cast in the spirit of TSQM where paths through spacetime connect detector outcomes with emission events allowing one to tell 'causal' stories (if you allow for the future to 'cause' events in the past). We think of TSQM as a dynamical interpretation, since it tells stories using worldlines. RBW on the other hand is an adynamical interpretation, since the fundamental rule isn't about interacting, time-evolved things, but a self-consistency criterion applied to the action which characterizes the spatiotemporal configuration as a whole (to include outcomes). The idea of finding a rule for the construct of the action is certainly not new, but we are looking at it in an entirely new way. Our view is very similar to http://www.mdpi.com/2073-8994/3/3/524 that I posted earlier (Wharton et al). We differ from Wharton et al in that RBW allows relationships to exist between events that aren't connected by a contiguous mediating entity in spacetime. We used this idea to explain the Union2 supernova data without accelerated expansion or dark energy Classical & Quantum Gravity 29 055015 (2012) http://arxiv.org/abs/1110.3973, so it's not without empirical consequence.

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audioloop
RBW is a time-symmetric interpretation of QM in the trivial sense that it's a blockworld interpretation. However, there are no quantum entities (wave function or otherwise) moving through the experimental equipment to 'cause' detector outcomes. So, RBW isn't really cast in the spirit of TSQM where paths through spacetime connect detector outcomes with emission events allowing one to tell 'causal' stories (if you allow for the future to 'cause' events in the past). We think of TSQM as a dynamical interpretation, since it tells stories using worldlines. RBW on the other hand is an adynamical interpretation, since the fundamental rule isn't about interacting, time-evolved things, but a self-consistency criterion applied to the action which characterizes the spatiotemporal configuration as a whole (to include outcomes). The idea of finding a rule for the construct of the action is certainly not new, but we are looking at it in an entirely new way. Our view is very similar to http://www.mdpi.com/2073-8994/3/3/524 that I posted earlier (Wharton et al). We differ from Wharton et al in that RBW allows relationships to exist between events that aren't connected by a contiguous mediating entity in spacetime.

then is acausal and static.
and how can exist actions in an acausal and static framework ?

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then is acausal and static.
and how can exist actions in an acausal and static framework ?

The action is calculated over spacetime to include emission and reception events, so it's a "blockworld" quantity by design. Whether or not you choose to call the events contained in a particular action "causal" is up to you, e.g., in TSQM future events can 'cause' past events. This is precisely the point Wharton et al argue for in the paper I cited earlier. Here is an excerpt from the second page:

"In outline, our proposal is based on the hypothesis that the symmetries of the path integral should be mirrored in the symmetries of any ontology underlying quantum theory. We shall call this the Feynman Integral Symmetry Hypothesis (FISH), to be precisely defined in Section 2. As we shall show, FISH’s effect is not only to constrain potential ontologies by excluding models whose ontologies do not share the symmetries of the path integral but also to allow us to construct ontological models of specific experiments, when we already have a model of some other experiment, to which the given experiment bears an appropriate path integral symmetry. (It is true that some of the models constructed in this way are counterintuitive, by ordinary standards. However, the suggestion we are exploring is that symmetries of the path integral might be a better guide to the nature of the quantum world than are the intuitions these models seem to offend.)"

They then show how the action for an experiment with innocuously time-like related events is equivalent via a symmetry transformation to the action for an experiment with space-like related events that violate the Bell inequality. Thus, per FISH, the ontology of the two experiments is the same and TSQM is an interpretation for which that holds, i.e., the present and past exist on equal ontological footing with the future and 'causal' relations need not be strictly ordered. The belief that causal relations are necessarily ordered such that "the past causes the future" is a bias associated with our temporal perception, but that perception is not consistent with the blockworld reality physics is giving us. So, the ontological picture per TSQM is that the microworld is like the macroworld with causal paths in spacetime, but these paths have no necessary causal ordering.

RBW takes this idea one step further by dispensing with microworld causal paths altogether. Instead of decomposing macroscopic worldtubes via microscopic worldlines, we say worldtubes arise from amalgams of space, time and source (in parlance of QFT). A rule for the construct of the graphical action is then applied to these elements of "spacetimesource" and this rule guarantees the elements will underwrite the causal picture of worldtubes in spacetime. To see how this is done, read the Introduction of the IOP paper I cited supra, http://arxiv.org/abs/0908.4348. This paper contains figures to help visualize the idea, too.