What does a local non-realistic theory look like?

[zonde]

That complaint is close to what we have voiced concerning "retrocausality" in our most recent paper http://www.ijqf.org/archives/2087. Specifically, given that the future is as "real" as the present, then all events are already "there," so in what sense is anything "moving" in either temporal direction, unless you have a metatime? That's why we didn't consider RBW to be a retrocausal interpretation until recent conversations with Wharton revealed a more God's eye view of what retrocausality can mean. So, our adynamical global constraint qualifies as retrocausal not in a metatime or temporal sense, but in a simple methodological sense -- future boundary conditions are needed in the computational process (path integral in our case).

But it's hardly satisfactory that you simply declare that such and such is not there and anything that can lead to it from infinite past and distance or where/when ever is not allowed. No?

 

[DrChinese]

How is that consistent with the evolution of the universe: Originally (or soon after the Big Bang), there is mostly hydrogen, and a 10 or 20 billion years later, the hydrogen has turned into helium, nitrogen, carbon, oxygen, iron, and all the rest of the elements? In thermodynamic terms, that is a huge increase in entropy. How is that compatible with entropy increasing in both directions?

(First, not trying to make any big statement - hopefully nothing controversial here. The only reason for the comment was to indirectly address the thermodynamic arrow of time for non-realistic interpretations which feature a time-symmetric component.)

I would say that if you calculated the number of possible microstates of any closed system at T=0, then the number of possible microstates of that system that could have led to that (say at T=-1 in the past) will be greater. Ditto for T=1 in the future. So entropy increases in both time directions by my measure, and I would think that any quantum level view would support that. What are the histories of any single particle with a specific position at T=0, and what are its future paths?

The reason that it doesn't look that way is that we usually start measuring a specially prepared system at T=0 and never consider how we got there (because the system was not closed in the past).

 

[RUTA]

But it's hardly satisfactory that you simply declare that such and such is not there and anything that can lead to it from infinite past and distance or where/when ever is not allowed. No?

Sorry, I'm not following you. Would you elaborate?

 

[zonde]

Sorry, I'm not following you. Would you elaborate?

Of course. What I said reflected my intuitive understanding of "adynamical global constraint", sorry if it's inaccurate.

 

[stevendaryl]

The reason that it doesn't look that way is that we usually start measuring a specially prepared system at T=0 and never consider how we got there (because the system was not closed in the past).

I know that you're talking about entropy for microscopic evolution, rather than the universe as a whole, but it seems to me that they have to be connected, don't they?

Assuming a Big Crunch at the end of the universe (just because that makes the gross history look a little more time-symmetric), we have one end of history where matter is mostly hydrogen, and we have another end where matter is everything under the sun. So the two ends look very different. (I think the only way that things could be approximately time-symmetric is if the age of the universe, from Big Bang to Big Crunch, were comparable to the Poincare recurrence time for the universe, which is so huge that saying "astronomically huge" is an astronomically huge understatement). So the issue, for a time-symmetric physics, it seems to me, is how to explain why there is (or seems to be--maybe it's an illusion produced by "coarse-graining" somehow, or by the anthropic principle, or whatever) the cosmological arrow of time is aligned with the thermodynamic arrow of time.

If the arrow of time were somehow an illusion, or is a local effect, it's hard to see why vastly separated sections of the universe would have their arrows of time aligned. The argument that every time you measure entropy, you're at a local minimum of entropy, doesn't sound like it would explain why distant stars have their arrows of time aligned, or why they all have their arrows of time aligned with the cosmological arrow of time.

 

[stevendaryl]

Isn't there a problem defining universal entropy increase in GR since there is no unambiguous global definition of energy in general?

I agree, there might not be a well-defined notion of entropy for the universe as a whole. But if you take a subsystem, such as a star, there is definitely a well-defined "arrow of time" for that subsystem: Initially, the star is mostly hydrogen, and then much later the hydrogen is gone, transmuted into heavier elements. The fact to be explained is why the arrows of time for distant stars are pointing in the same direction. That's independent of subtleties of how you define "entropy" for the universe as a whole.

 

[RUTA]

I agree, there might not be a well-defined notion of entropy for the universe as a whole. But if you take a subsystem, such as a star, there is definitely a well-defined "arrow of time" for that subsystem: Initially, the star is mostly hydrogen, and then much later the hydrogen is gone, transmuted into heavier elements. The fact to be explained is why the arrows of time for distant stars are pointing in the same direction. That's independent of subtleties of how you define "entropy" for the universe as a whole.

Right, that sense of an arrow of time is clear.

 

[RUTA]

Of course. What I said reflected my intuitive understanding of "adynamical global constraint", sorry if it's inaccurate.

Another example of an adynamical global constraint is the least action principle applied to the path of a refracted light ray. To use that principle you need to input the emission spacetime event and the future boundary condition (point in spacetime where you want the light ray to terminate). The path in spacetime that obtains is the one that minimizes the total time from emission to termination. In the God's eye perspective (spacetime view), nothing is moving, nothing is happening. The path of extremal action exists "all at once" in spacetime. Now, you can in this case (and in other least action solutions for classical physics) demand the integrand of the action represent a continual process in space as a function of time that is extremal for each infinitesimal forward time lapse. This leads to the corresponding dynamical equation of motion with its view of light moving through space according to the conditions in its immediate spatial vicinity. In this dynamical view, the termination point in spacetime follows inevitably from the properties inherent in the moving object that determine its local interactions as it moves through space (forward in time). These local interactions then dictate where it goes instant by instant. You could change the nature of the interactions along the path and the termination point will change. There's no sense in which the termination point is dictating the path, but rather the converse. There is a problem with this dynamical view in EPR-Bell phenomena (which is the topic of this thread). To overcome this problem, retrocausal accounts say the "termination point/future boundary conditions" (detector settings and outcomes) *must* be input, i.e., there is no forward-time-evolved solution alone (as in classical dynamics) that accounts for the outcomes. If you want a time-evolved solution for EPR-Bell phenomena, you need both forward-time and backward-time evolved components. Did I answer your question?

 

[Jilang]

If the entropy of the system changes do you still have the backwards evolved time component?

 

[julcab12]

If the entropy of the system changes do you still have the backwards evolved time component?

Time is not well defined in QM. We can only assume trajectories moving forward and backward -- retrodiction. If we interpret Time to have that property then the natural and direct consequence is that will be having mixed realities -- time moving forward which we are in and time moving backward that is hidden from us. But we can't say much about it.

http://www.dailymail.co.uk/sciencet...m-experiment-suggests-time-run-backwards.html

 

[Jilang]

Time is not well defined in QM. We can only assume trajectories moving forward and backward -- retrodiction. If we interpret Time to have that property then the natural and direct consequence is that will be having mixed realities -- time moving forward which we are in and time moving backward that is hidden from us. But we can't say much about it.

http://www.dailymail.co.uk/sciencet...m-experiment-suggests-time-run-backwards.html

Indeed. My question was whether is was the change in entropy that separated the classical from the quantum? In QM it seems that there is a reversibility with no change in entropy. Is it when entropy increases there is no going back?

 

[RUTA]

If the entropy of the system changes do you still have the backwards evolved time component?

Yes, the change of entropy due to the exchange of energy (source to sink) isn't affected by the information exchange from the forward and backward-time evolved wavefunction.

 

[DrChinese]

If the arrow of time were somehow an illusion, or is a local effect, it's hard to see why vastly separated sections of the universe would have their arrows of time aligned. The argument that every time you measure entropy, you're at a local minimum of entropy, doesn't sound like it would explain why distant stars have their arrows of time aligned, or why they all have their arrows of time aligned with the cosmological arrow of time.

Sure, I agree with this. But I think it is a bit different question than asking why the elements of the future are participating (if they are) in observations made in the present. In other words, I don't see understanding the arrow of time as an impediment to a time symmetric theory.

I will give you a speculative hypothesis that embodies this. (Yes, I know speculation is bad - but there is no physical evidence pro or con here so I would call it no more speculative than MWI.) Suppose at the big bang, time symmetry was evidenced as follows: half the mass-energy went in one time direction T=+1, and the other half went in the other T=-1. The net "time momentum" is zero. So our observable universe is only one of two halves, and each starts out with a preferred time direction. That would neatly explain the arrow of time, and would be consistent otherwise with what we know. And would provide a basis for an interpretation involving time symmetry.

In RUTA's terms, the block world would be a bit bigger as it extends in a second direction. :)

 

[Jilang]

Yes, the change of entropy due to the exchange of energy (source to sink) isn't affected by the information exchange from the forward and backward-time evolved wavefunction.

Thanks, but I think I didn't express myself properly. If there is a change of entropy between source and sink does that preclude the backward-time evolved wavefunction?

 

[zonde]

Another example of an adynamical global constraint is the least action principle applied to the path of a refracted light ray.

Can't agree with this. I don't see the "global" part in least action principle applied to the path of a refracted light ray.

 

[julcab12]

In QM it seems that there is a reversibility with no change in entropy.

Reversibility has very little meaning in QM since time is just a parameter, in addition to the inherent non-deterministic property of the system(standard). We can assume also that particle does not have entropy.

Is it when entropy increases there is no going back?

Going back in what manner? The increase in entropy makes the state mixed and it is impossible to ever go back to the ordinary notions where things had position and momentum that had exact, well-defined values. Either it goes branching forever or some dynamic is introduced -- bounce/cyclical which is speculative at the moment.

 

[Jilang]

Going back in what manner? The increase in entropy makes the state mixed and it is impossible to ever go back to the ordinary notions where things had position and momentum that had exact, well-defined values. Either it goes branching forever or some dynamic is introduced -- bounce/cyclical which is speculative at the moment.

I was referring to the reverse causality/ backwards time evolution of the wavefunction. If entropy can only increase in either direction would not a change in entropy in the forwards direction put the blockers on a time-reversed component?

 

[Ilja]

Local non-realistic interpretations simply look like the minimal interpretation. They make only the predictions - the probabilities - but do not aim of finding a causal explanation for observable correlations. So, if there is a 100% correlation between A and B, one is not allowed to conclude that A causes B or B causes A or that there has to be a common cause C for A and B. There is such a correlation, fine, so what?

In my opinion, this simply means giving up doing science in this particular direction. It is not very dangerous, because it is restricted to a single domain - the foundations of physics - and the existing theories in this domain are already nice enough that any lack of scientific progress in the direction of the foundations of quantum theory is not necessary. But imagine, say, medicine without the obligation for science to find causes for observed correlations. One observes that smokers often die because of lung cancer. The tobacco lobby could, then, argue: So what? We have no explanation for this? So what, physicists have also no explanation for the violations of Bell's inequalities, the only possible causal explanations would violate relativity, but nobody cares about this and doubts that relativity is wrong. No, they say we should not search for hidden causes. So, let's follow the physicists and give up the search for causal explanations for the correlations between smoking and lung cancer.

 

[Jilang]

So, let's follow the physicists and give up the search for causal explanations for the correlations between smoking and lung cancer.

Ilja, I don't think you need to worry on that score. I think it is pretty much accepted that on an everyday scale we we live a classical world.

 

[Dale]

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