Is the past in MWI unambiguous?

[entropy1]

I couldn't match this question with another topic, so I made a new topic.

Suppose we have past H and present P at a certain moment. In present P a measurement is made. For matter of speaking we adopt MWI, so we have the two measurement outcomes A and B diverging in two realities A and B in future F.

Now I only have this question: do future F(A) and future F(B) both have the same past H?

 

[PeterDonis]

do future F(A) and future F(B) both have the same past H?

According to the MWI, yes.

 

[entropy1]

According to the MWI, yes.

Ok, thanks. Are there other interpretations that don't have that?

 

[PeterDonis]

Are there other interpretations that don't have that?

There aren't any other interpretations that have multiple futures to begin with, so I'm not sure what you're asking.

 

[entropy1]

There aren't any other interpretations that have multiple futures to begin with, so I'm not sure what you're asking.

I will have to think about that. Thanks.

 

[PeterDonis]

Moderator's note: Moved thread to QM foundations and interpretations forum.

 

[A. Neumaier]

According to the MWI, yes.

But unitary quantum mechanics is time reversible. MWI is claimed (at least by Michael Price) to be unitary quantum mechanics without anything added. Thus the part should branch like the future!??

 

[WWGD]

I couldn't match this question with another topic, so I made a new topic.

Suppose we have past H and present P at a certain moment. In present P a measurement is made. For matter of speaking we adopt MWI, so we have the two measurement outcomes A and B diverging in two realities A and B in future F.

Now I only have this question: do future F(A) and future F(B) both have the same past H?

Why just two measurement outcomes? Arent there, in the case of continuous variables, infinitely-many outcomes or am I missing something?

 

[entropy1]

Why just two measurement outcomes? Arent there, in the case of continuous variables, infinitely-many outcomes or am I missing something?

I ment to keep the example simple. For instance, a spin measurement.

 

[WWGD]

Still, hope not to go too far OT from this post , but, if worlds with physical existence yet disjoint, i.e., not overlapping physically, are generated at this rate, does this imply an infinite universe , one that is expanding.

 

[PeterDonis]

unitary quantum mechanics is time reversible.

Yes.

Thus the past should branch like the future!??

No. This would be "time symmetric", which is not the same as "time reversible". It's perfectly possible to have a time reversible evolution which is not symmetric in time: a classical example would be an FRW universe that expands forever.

 

[A. Neumaier]

No. This would be "time symmetric", which is not the same as "time reversible". It's perfectly possible to have a time reversible evolution which is not symmetric in time: a classical example would be an FRW universe that expands forever.

Yes. Time reversible means that for every unitary quantum system there is another one in which past and future are exactly interchanged. For this there is no need for symmetry in time.

But MWI treats the two systems in opposite ways - in the first, exactly the same formulas are interpreted as branching, in the second as merging...

 

[PeterDonis]

Time reversible means that for every unitary quantum system there is another one in which past and future are exactly interchanged.

Yes, but these two don't have to be the same solution.

MWI treats the two systems in opposite ways - in the first, exactly the same formulas are interpreted as branching, in the second as merging...

Yes, but there is no solution in which branching/merging occurs the same way in both directions of time. The OP specified that branching occurs to the future (which is the normal way in which the MWI is used); that means no branching occurs to the past, only "un-branching". The time reversed counterpart of this solution, in MWI terms as you are using them, would have merging occurring to the future, but no merging occurring to the past, only "un-merging".

 

[WWGD]

Does the branching apply on different frames of reference? Edit: Is this an issue at all?

 

[PeterDonis]

Does the branching apply on different frames of reference?

It has nothing to do with frames of reference. It's in the unitary dynamics.

 

[A. Neumaier]

Yes, but there is no solution in which branching/merging occurs the same way in both directions of time. The OP specified that branching occurs to the future (which is the normal way in which the MWI is used); that means no branching occurs to the past, only "un-branching". The time reversed counterpart of this solution, in MWI terms as you are using them, would have merging occurring to the future, but no merging occurring to the past, only "un-merging".

Precisely. And since both are possible unitary dynamics for the universe, hence should be interpreted in the same way! Unless the MWI applies only to universes with assumed properties beyond unitary quantum mechanics!

 

[PeterDonis]

since both are possible unitary dynamics for the universe, hence should be interpreted in the same way!

Both are possible unitary dynamics mathematically, but that doesn't mean both are consistent with what we actually observe empirically, just as both an expanding forever FRW spacetime and a collapsing forever spacetime are possible mathematically, but that doesn't mean they both are consistent with what we actually observe empirically. We rule one of the two possible solutions out on the basis of observations.

We observe quantum measurements to have multiple possible results, so I don't think a solution in which only merging occurs to the future and only un-merging occurs to the past is consistent with what we observe.

 

[A. Neumaier]

Both are possible unitary dynamics mathematically, but that doesn't mean both are consistent with what we actually observe empirically

Yes, but the goal of an interpretation is to explain what is actually observed empirically from the theory, not to use the empirical observation as additional input to the theory.

We observe quantum measurements to have multiple possible results, so I don't think a solution in which only merging occurs to the future and only un-merging occurs to the past is consistent with what we observe.

Yes, but for an interpretation that claims to assume nothing but unitarity, this would have to be a deduction rather than an input!

 

[akvadrako]

I would say MWI is time symmetric, it's just that branching coincides with increased entropy. But in a theoretical big crunch scenario, after the mid-point most of the branching would happen in the past.

MWI is just about unitary evolution - if that leads to more branching in the future or the past is something that needs to be determined within the framework.

There is nothing wrong with multiple branches leading to a single state - that happens in double slit experiments and Wigner friend setups.

 

[PeterDonis]

the goal of an interpretation is to explain what is actually observed empirically from the theory, not to use the empirical observation as additional input to the theory

This might be relevant to the question of whether the MWI is a viable interpretation or not.

I don't think it's relevant to the question of what the MWI, warts and all, says about the OP's question, which is the subject of this thread.

 

[entropy1]

Suppose we have a video (as in moving pictures). Then I look at the (a frame of the) video, 'measuring' it. The outcome we call A.

Now, if the measurement result would have been, if could have been, B, wouldn't that mean that the past, that constitutes the video, was different between measurement outcome A and outcome B?

EDIT: I think I was confused: If A and B are purely random outcomes, they don't represent reality, and if the outcome does represent reality, it will necessarily do so.