Support for retrocausal quantum theory, in which the future influences the past

#1
https://m.phys.org/news/2017-07-physicis...uture.html

EXCERPT: Although there are many counterintuitive ideas in quantum theory, the idea that influences can travel backwards in time (from the future to the past) is generally not one of them. However, recently some physicists have been looking into this idea, called "retrocausality," because it can potentially resolve some long-standing puzzles in quantum physics. In particular, if retrocausality is allowed, then the famous Bell tests can be interpreted as evidence for retrocausality and not for action-at-a-distance—a result that Einstein and others skeptical of that "spooky" property may have appreciated. In a new paper published in Proceedings of The Royal Society A, physicists Matthew S. Leifer at Chapman University and Matthew F. Pusey at the Perimeter Institute for Theoretical Physics have lent new theoretical support for the argument that, if certain reasonable-sounding assumptions are made, then quantum theory must be retrocausal....

MORE: https://m.phys.org/news/2017-07-physicis...uture.html
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#2
(Jul 6, 2017 12:13 AM)C C Wrote: EXCERPT: Although there are many counterintuitive ideas in quantum theory, the idea that influences can travel backwards in time (from the future to the past) is generally not one of them. However, recently some physicists have been looking into this idea, called "retrocausality," because it can potentially resolve some long-standing puzzles in quantum physics.

My philosophical intuition agrees very strongly with that. I'll go so far as to speculate that there might be a unified theory of quantum theory and time lurking in those kind of ideas.

Why does causality seem to propagate only in one temporal direction, from past to future? What's the source of that fundamental asymmetry?

Imagine science-fiction's time-travel paradoxes. It seems to me that if causality can propagate backwards, we might end up with a whole lot of superimposed possibility states instead of one coherent reality. (You never traveled into the past/you did and you killed your parents before you were born... future states are determined by past states/past states are determined by future states... causal loops) instead of one coherent reality.

The time-travel paradoxes remind me of a macro-version of the superimposed states that quantum theory imagines on the micro-scale.

Now imagine that the ground-state of the universe is time-symmetrical, where causality propagates as easily into the past as into the future. It seems to me that no coherent reality could form in that kind of universe and this ground-state would be the equivalent of ancient philosophy's primordial chaos.

Now imagine some kind of physical event that causes all the causal chains in its vicinity to propagate away from it, like a giant splash in the fabric of being with ripples moving away in ever larger circles. Now think of the Big Bang, which creates a past-future gradient out of which a consistent physical reality successively crystalizes, in the form of sub-atomic particles, as atoms, as molecules, as larger physical structures (like planets, life and us).

It's possible to speculate that perhaps the reason for quantum weirdness is that retrocausality can still happen for short temporal distances on the microscale. But on the macroscale, the overwhelming temporal current stirred up by the Big Bang dominates.

One reason why I like this speculation is that it accounts for the passage of time and for our intuition that the future consists of a whole host of unrealized possibilities, different ways that things can turn out, not all of them consistent with one another. Perhaps the present moment ('Now') is the wave that the Big Bang has created spreading its time-asymmetry ever outwards into  time-symmetric spacetime.

So perhaps things like us exist like surfers on the splash kicked up by the Big Bang, what we call the present moment, in-between the unformed chaos of future possibility and being frozen in amber in the determined past where whatever happened, happened.

It's just a science-fiction speculation and no doubt there are logical problems with it. I make no claims about it being consistent with contemporary theoretical physics. But I like it. It describes how I personally conceive of reality much of the time.
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#3
The problem with this...from reading the actual paper...is that physics is not ontology nor is the assumption of realism necessarily justified.

"If we do not believe that the for-
wards  evolving  state  vector  is  a  state  of  reality,  than
neither  Price's  aregument  nor  the  two  state  vector  for-
malism, provides evidence that we should view the back-
wards  evolving  state  vector  as  a  state  of  reality.   The
ψ-epistemic view provides natural explanations for many
quantum  phenomena  that  are  otherwise  puzzling,  such
as the indistinguishability of quantum states and the no-
cloning theorem, which have been discussed in detail else-
where." - https://arxiv.org/pdf/1607.07871.pdf

"Probably the most important issue not discussed so far in
this review is the question of whether the reality of the
quantum state can be established experimentally. When
it comes to experiments in the foundations of quantum
theory, tests of Bell’s Theorem are somewhat of a gold
standard. Violation of a Bell inequality rules out a wide
class of local realistic theories independently of the de-
tails of quantum theory. Consequently, the experimenter
does not have to assume much about how their experi-
ment is represented within quantum theory. From their
observed statistics they simply get a yes/no answer as to
whether local hidden variable theories are viable, modulo
the known loopholes. In modern parlance, tests of Bell’s
Theorem are device independent[102].
In contrast, a test of the reality of the quantum state
would not be device independent simply because the
“quantum state” is the thing we are testing the reality
of, and that is a theory dependent notion. Consequently,
one has to assume that our quantum theoretical descrip-
tion of the way that our preparation devices work is more
or less accurate, in the sense that they are approximately
preparing the quantum states the theory says they are, in
order to test the existing
ψ-ontology results. Therefore,
it is desirable to have a more theory independent notion
of whether a given set of observed statistics imply that
the “probabilistic state”, i.e. some theory-independent
generalization of the quantum state, must be real. It is
not obvious whether this can be done, but if it can then
experimental tests of
ψ-ontology results would become
much more interesting.
...
This is still quite far from establishing
the reality of the quantum state, for which one would
want to test many pairs of quantum states with a variety
of different inner products, and to show that the overlap
ratios are consistently close to zero. Recall that, due to
the fact that part of the indistinguishability of quantum
states may be explained by the coarse-grained nature of
measurements in an ontological model, it is diffcult to
attach significance to overlap ratios that are significantly
larger than zero. Further theoretical work is required to
derive feasible experiments that can achieve much lower
ratios."
- https://arxiv.org/pdf/1409.1570.pdf
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#4
(Jul 14, 2017 05:44 PM)Yazata Wrote: It's possible to speculate that perhaps the reason for quantum weirdness is that retrocausality can still happen for short temporal distances on the microscale. But on the macroscale, the overwhelming temporal current stirred up by the Big Bang dominates.

Some Russian physicists claim to have experimentally detected retrocausality on the microscale. Or at least modeled it on a computer. Or something...

https://www.nature.com/articles/s41598-019-40765-6

They write:

Uncovering the origin of the "arrow of time" remains a fundamental scientific challenge. Within the framework of statistical physics, this problem was inextricably associated with the Second Law of Thermodynamics, which declares that entropy growth proceeds from the system's entanglement with the environment. This poses a question of whether it is possible to develop protocols for circumventing the irreversibility of time and if so to practically implement these protocols....  Using this algorithm on an IBM quantum computer enables us to experimentally demonistrate a backwards time dynamics for an electron scattered on a two-level impurity.

I continue to like the idea that retrocausality might actually happen for very short spatial-temporal intervals on the microscale. But I'm a bit skeptical that these guys have actually captured it happening or how it could happen (or whatever they did).

Of course I'm not a physicist (not even close) so I will have to wait for professional commentary on this from the physicists and the philosophers of physics.
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#5
They seem to only be talking about the thermodynamic arrow or time (which is one of many). And it's rather trivial that there has to be some kind of localized, temporary violation of the second law for order to develop.
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