History of the Universe and measured outcomes

[entropy1]

This thread is a split-off of this post:

https://www.physicsforums.com/threads/do-macro-objects-get-entangled.946927/page-2#post-5997089

So my issue is this: if, for convenience, we use a Copenhagen interpretation, and we measure an observable WF $\alpha |A \rangle + \beta |B \rangle$, then, if the measured eigenvector will be $|A \rangle$, is it possible that if the measured value had been $|B \rangle$, that the history of the universe would have been different?

@stevendaryl argues in the other thread that if the measument was fully dependent of the history of the universe, this would imply hidden variables, which are not possible according to Bell ineq.

But if it was the other way round, that the measurement outcome determined the history of the universe as much as the history of the universe determined the measurement outcome, it would perhaps not have to be in conflict with Bell anymore? (because there are no hidden variables involved in the usual sense?)

I am not familiar with the relationship between measured outcomes (on a macro-level) and micro-outcomes. That is why I resorted to CI; to approach it more generally is ok with me.

You could also formulate it thus: if I am in a history where I measure outcome $|B \rangle$, how much ('in which ways') would this history differ from a history in which I measure outcome $|A \rangle$?

Pff. I'm glad I have managed to put it into words (somewhat).

 

[entropy1]

I thought of the following: In the case of the history of a single elektron that measures spin up, if the spin would have measured spin down, the history of that elektron would have to have brought that elektron at the same moment at the same detector to measure it spin down, so the history would probably be the same for a spin up measurement as for a spin down measurement.

Unless the elektron was destined to measure spin up. But that would invalidate the point of my issue.

So I probably haven't got a point in the first place.

Any thoughts are still appreciated.

 

[stevendaryl]

@stevendaryl argues in the other thread that if the measument was fully dependent of the history of the universe, this would imply hidden variables, which are not possible according to Bell ineq.

But if it was the other way round, that the measurement outcome determined the history of the universe as much as the history of the universe determined the measurement outcome, it would perhaps not have to be in conflict with Bell anymore? (because there are no hidden variables involved in the usual sense?)

Just for clarification, you mean the history of the universe up until the moment the measurement occurs might determine the result of the measurement? Well, the assumption of locality (that influences can't travel faster than light) would imply that the result can only depend on that part of the universe's history that took place long enough ago that information from that part of the history could reach the measurement event. That would make it a local hidden variable theory of the kind that Bell proved was impossible, I believe.

You could also formulate it thus: if I am in a history where I measure outcome $|B \rangle$, how much ('in which ways') would this history differ from a history in which I measure outcome $|A \rangle$?

Again, are you asking how much different in the past, or how much different in the future?

 

[PeterDonis]

if the measured eigenvector will be $|A \rangle$, is it possible that if the measured value had been $|B \rangle$, that the history of the universe would have been different?

The problem with questions like these is that they are formulated in ordinary language, and ordinary language can be misleading. The sentence you wrote that I just quoted above makes sense in English, but that does not guarantee that it describes a consistent state of affairs. And if it doesn't, then trying to reason about it is pointless.

So the first thing that needs to be done with any question of this sort is to reformulate it in some more precise way (the usual choice is math), so that it can be explicitly checked that the premises you are trying to reason from are consistent. Doing that in this case would require translating phrases like "possible", "if the measured value had been...", and "would have been different". Unless and until such a translation is done and the result checked for consistency, I don't think your question is answerable.

 

[entropy1]

Again, are you asking how much different in the past, or how much different in the future?

I mean in the past, and I don't mean causally per se, I mean that the measurement outcome ("now") and the history before the measurement outcome are related causally as well as retrocausally, so more a kind of correlating.

I realize this may be kind of vague language, as what @PeterDonis is objecting to.

 

[entropy1]

The problem with questions like these is that they are formulated in ordinary language, and ordinary language can be misleading. The sentence you wrote that I just quoted above makes sense in English, but that does not guarantee that it describes a consistent state of affairs. And if it doesn't, then trying to reason about it is pointless.

So the first thing that needs to be done with any question of this sort is to reformulate it in some more precise way (the usual choice is math), so that it can be explicitly checked that the premises you are trying to reason from are consistent. Doing that in this case would require translating phrases like "possible", "if the measured value had been...", and "would have been different". Unless and until such a translation is done and the result checked for consistency, I don't think your question is answerable.

Reasonable point in one way. In another way that can't really be expected of me, given my level of knowledge of the formalism. So then my question has to remain unanswered I guess...

 

[stevendaryl]

I mean in the past, and I don't mean causally per se, I mean that the measurement outcome ("now") and the history before the measurement outcome are related causally as well as retrocausally, so more a kind of correlating.

As far as anybody knows, the exact same history can have different outcomes for a measurement. That's what it means for the outcome to be nondeterministic.

 

[Mentz114]

As far as anybody knows, the exact same history can have different outcomes for a measurement. That's what it means for the outcome to be nondeterministic.

I'm glad you say 'as far as anybody knows' because that issue seems to be undecidable by observation or experiment at present.

 

[entropy1]

As far as anybody knows, the exact same history can have different outcomes for a measurement. That's what it means for the outcome to be nondeterministic.

What I mean is if two different outcomes could have two different histories. My assumption here, I should say, is that history is not necessarily fully determined, but variable, at least in principle.

 

[Demystifier]

So my issue is this: if, for convenience, we use a Copenhagen interpretation, and we measure an observable WF $\alpha |A \rangle + \beta |B \rangle$, then, if the measured eigenvector will be $|A \rangle$, is it possible that if the measured value had been $|B \rangle$, that the history of the universe would have been different?

It isn't possible according to Copenhagen interpretation.

@stevendaryl argues in the other thread that if the measument was fully dependent of the history of the universe, this would imply hidden variables, which are not possible according to Bell ineq.

Hidden variables are not impossible according to the Bell theorem. The theorem only excludes local hidden variables, but nonlocal hidden variables are OK.

But if it was the other way round, that the measurement outcome determined the history of the universe as much as the history of the universe determined the measurement outcome,

See about delayed choice experiments, e.g. here https://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment[/QUOTE][/QUOTE]

 

[Lord Jestocost]

My assumption here, I should say, is that history is not necessarily fully determined, but variable, at least in principle.

Maybe, this quote from Wheeler’s “Law without law” might be of some help:

“It is wrong to think of that past as ‘already existing’ in all detail. The ‘past’ is theory. The past has no existence except as it is recorded in the present. By deciding what questions our quantum registering equipment shall put in the present we have an undeniable choice in what we have the right to say about the past.”

J. A. Wheeler in „Quantum Theory and Measurement“ (edited by John Archibald Wheeler and Wojciech Hubert Zurek), Princeton, New Jersey 1983, page 194

EDIT: I, personally, would supplement Wheeler's quote: "...we have an undeniable choice in what we have the right to say about the past in the context of classical concepts."

 

[entropy1]

I could perhaps illustrate my issue as follows (in 'layman'-language):

Suppose we have two situations:

1. Car X approaches a trafficlight at x₀ at t₀ and the light is red.
2. Car X approaches a trafficlight at x₀ at t₀ and the light is green.

Question: could the history of X (before t₀) have been different in case 1 and case 2? Could we have had different movies?

You could say there is only one unambiguous history. But on what does it depend? If the present is/would be different, why wouldn't history?

 

[PeterDonis]

Question: could the history of X (before t₀) have been different in case 1 and case 2?

We don't know, since we don't have a complete Theory of Everything and so we don't know what we would need to know to answer this question.