Undergrad Determinism and causality in QM - a case

[entropy1]

How does any of this relate to "objective" or "subjective"?

Subjectively, we observe a single outcome. In a different thread we observe a different outcome. This difference is a difference in subjective experience, I dare to venture.

 

[PeterDonis]

Subjectively, we observe a single outcome. In a different thread we observe a different outcome. This difference is a difference in subjective experience, I dare to venture.

A difference in subjective experience of what?

Normally, when we talk about "subjective experience", we mean something like: you and I both observe the same objective event or situation, but we have different experiences of it, subjectively.

But in this case, we have two "copies" of the same person experiencing two different outcomes in two different branches. They are not observing "the same objective event or situation" at all; they are each observing different objective events or situations--different branches. So "subjective experience" in the usual sense doesn't even apply: it's not two different people, it's two "copies" of the same person, and they're not observing the same objective thing, they're observing different objective things.

 

[entropy1]

Normally, when we talk about "subjective experience", we mean something like: you and I both observe the same objective event or situation, but we have different experiences of it, subjectively.

My view was that the measurement outcome would be a subjective experience of the measurement made. I don't claim that this subjectivity is bound to a single human observer. All observers in a worldthread agree on the measured outcome. So you say the measured outcome is objective and real, but that all other outcomes in the different worldthreads are too. Perhaps I should use the word "experience" instead of "subjective". Experience is subjective. The significance of that is, I think, that a single measurement spawns several outcomes, so that we have different experiences by different people of the same measurement, objectively as well as subjectively. But the ones in different worldthreads can't compare notes.

it's two "copies" of the same person,

Some "me" could be in thread A while of copy of me is in thread B. But you could symmetrically say some "me" is in thread B while a copy is in thread A. So, they are both "me", just with different experiences. Either that, or I have been replaced by something different altogether, which is what I understand you assert.

 

[PeterDonis]

Some "me" could be in thread A while of copy of me is in thread B. But you could symmetrically say some "me" is in thread B while a copy is in thread A.

No, neither of these is correct. Both "copies" are referring to the same subsystem--the same set of degrees of freedom--and neither one has any more claim than the other to be "the original" vs. "the copy". As you say, the situation between them is symmetric.

Either that, or I have been replaced by something different altogether, which is what I understand you assert.

No. The quantum subsystem representing "you"--the particular set of degrees of freedom that is "you"--has not changed.

However, before measurement there was only one branch, and the quantum subsystem representing "you" had a definite state. After measurement, there are two branches, and the quantum subsystem representing "you" is entangled with the system that was measured, so "you" are no longer in a definite state at all; only the total system, comprising both "you" and the system being measured, has a definite state. The "copies" of "you" in the two branches are "pieces" of the total state, so the difference between them and "you" before measurement is that the "you" before measurement has a definite state, while the "copies" after measurement do not.

 

[PeterDonis]

we have different experiences by different people of the same measurement

No, we don't. We have different experiences by the same person (the same set of degrees of freedom) of the same measurement--different for each "copy" of that person in each branch.

 

[entropy1]

No, we don't. We have different experiences by the same person (the same set of degrees of freedom) of the same measurement--different for each "copy" of that person in each branch.

That is what I am saying.

I thought you were claiming the copies were different persons. Must have misread you.

 

[entropy1]

the difference between them and "you" before measurement is that the "you" before measurement has a definite state, while the "copies" after measurement do not.

Then they better hurry getting a definite state, for there may be another measurement coming.

That is the shortest way to make my point about that in this thread. Having said that, you have the mathematical knowledge that I lack mostly. So maybe we should burry the hatch here.

 

[PeterDonis]

Then they better hurry getting a definite state

They can't. The only way to do that would be to reverse their entanglement with the measured system, and decoherence means that's not possible.

there may be another measurement coming

And that measurement will just add further entanglement to the entanglement that already exists. It won't remove any. Which means it won't put any subsystem back into a definite state.

That is the shortest way to make my point about that in this thread.

And that point is wrong. See above.

 

[entropy1]

How do you explain your remark below then:

the "you" before measurement has a definite state

 

[PeterDonis]

How do you explain your remark below then

We were assuming it. In effect, we were assuming that the measurement we were discussing was the first measurement "you" had ever made.

Of course this assumption is highly unrealistic in practice. This is one reason why it's so difficult to really apply what the MWI actually says. If we take what the MWI says at face value, it means no subsystem of the universe now is in a definite state, taken by itself; only the whole universe, as a single huge joint quantum system, has any definite state at all.

 

[entropy1]

We have different experiences by the same person (the same set of degrees of freedom) of the same measurement--different for each "copy" of that person in each branch.

So you say that all possible experiences are experienced. But still the experience is a series of single experiences (outcomes). So I wondered if the worlds in MWI could reflect possible threads/experiences, but that the observer navigates by adhering to a particular possibility, if you understand what I mean. That way, we have free will as well as determinism (WF). Now this is not mainstream I think, but if you can point out where I go wrong, I would be grateful.

 

[PeterDonis]

I wondered if the worlds in MWI could reflect possible threads/experiences, but that the observer navigates by adhering to a particular possibility

No. that's not what the MWI says. The MWI says that all of the branches are equally real. Trying to pick out any particular branch as somehow being "the one the observer navigates to" contradicts what the MWI says.

this is not mainstream I think

Certainly not. It's personal speculation on your part, which is off limits for PF.

 

[entropy1]

Certainly not. It's personal speculation on your part, which is off limits for PF.

I don't agree. I said in advance that it is probably not mainstream science. I just wanted to know where I was wrong. So one could say that MWI makes a claim about what is real, or to what degree. Which clears the issue up. It also avoids the zombie-problem. I should have thought of that. But perhaps to avoid that, it makes the claim about reality. And of course the math clearly does.

But that does not mean I then adopt MWI.

 

[PeterDonis]

I said in advance that it is probably not mainstream science. I just wanted to know where I was wrong.

None of this contradicts the assertion that your proposal was personal speculation. That assertion by me was not an invitation to argue the point; it was a reminder to you of the forum rules, from a forum moderator.

one could say that MWI makes a claim about what is real, or to what degree.

Yes.

that does not mean I then adopt MWI

As far as your own personal beliefs, independent of this thread, go, that's fine. I'm not a fan of the MWI myself.

But in this particular thread, you said you were assuming the MWI. That means that, for purposes of discussion in this thread, you have to adopt the MWI, even if you don't actually believe it's true. You can't say you assume the MWI for discussion of a given scenario and then not adopt it for discussion of that scenario.

 

[entropy1]

Is it fair to suppose that, in MWI, if a human makes choice A, it is just as much determined than if they make choice B? What I mean is that if the choice is free, by free will, it is nevertheless still compatible with determinism, from the viewpoint of MWI? Or should we say that all choices are made and that there is therefore no real choice, just determinism? This is my key point I think.

 

[mattt]

Is it fair to suppose that, in MWI, if a human makes choice A, it is just as much determined than if they make choice B? What I mean is that if the choice is free, by free will, it is nevertheless still compatible with determinism, from the viewpoint of MWI? Or should we say that all choices are made and that there is therefore no real choice, just determinism? This is my key point I think.

Whatever your conception of "Free Will", it is "compatible" with dBB-QM ("single-world" and deterministic) if and only if it is "compatible" with MW-QM ("many worlds" and deterministic").

What most people conceive as "free will" is incompatible with any deterministic model though...

 

[PeterDonis]

Is it fair to suppose that, in MWI, if a human makes choice A, it is just as much determined than if they make choice B?

Before you can answer any such questions in the context of the MWI, you have to decide whether "making a choice" involves a quantum measurement--something that causes entanglement and decoherence and therefore leads to "branching" of the wave function.

If "making a choice" does not involve a quantum measurement, then as far as the MWI is concerned, it doesn't involve any branching and so "which choice is made" would be deterministic, since the MWI is deterministic and the deterministic result from the MWI is that only one choice is made (no branching).

If "making a choice" does involve a quantum measurement, then it is meaningless to ask whether "which choice is made" is deterministic, because the MWI says that all choices are made; that is the deterministic prediction of the MWI.

 

[PeterDonis]

What most people conceive as "free will" is incompatible with any deterministic model

A compatibilist (of which there are many; we have had some previous PF threads on this) would say this is because most people have not carefully thought through what "free will" actually means. If they did, they would realize that having free will in any meaningful sense requires, if not exact determinism, at least a very, very good approximation to it. Otherwise you can't depend on your free will choice determining what happens.

 

[mattt]

A compatibilist (of which there are many; we have had some previous PF threads on this) would say this is because most people have not carefully thought through what "free will" actually means. If they did, they would realize that having free will in any meaningful sense requires, if not exact determinism, at least a very, very good approximation to it. Otherwise you can't depend on your free will choice determining what happens.

I know what you mean, and I agree with it. It's simply that I don't like to use the term "free will" for that.

I would prefer to use another word for that, given that historically it was conceived with a different meaning (mostly in dualism and similar belief systems) than the better one you have in mind.

 

[entropy1]

Before you can answer any such questions in the context of the MWI, you have to decide whether "making a choice" involves a quantum measurement--something that causes entanglement and decoherence and therefore leads to "branching" of the wave function.

If "making a choice" does not involve a quantum measurement, then as far as the MWI is concerned, it doesn't involve any branching and so "which choice is made" would be deterministic, since the MWI is deterministic and the deterministic result from the MWI is that only one choice is made (no branching).

If "making a choice" does involve a quantum measurement, then it is meaningless to ask whether "which choice is made" is deterministic, because the MWI says that all choices are made; that is the deterministic prediction of the MWI.

Your position is clear. Compatibalist free will works fine with determinism. But if I had the option to choose A and also the option to choose B, I think a quantum measurement must be involved, for that is (in the context of MWI) the only mechanism that makes more than one future possible.

The point I want to make is that in my example choice A and B are both possible, however some choice will be made between them if we view it subjectively. If you insist to view it objectively in the context of MWI, then you can say both choices are made, so really there is no choice made, so free will, if it exist, has to be compatibalist. Do you agree?

 

[PeterDonis]

if I had the option to choose A and also the option to choose B, I think a quantum measurement must be involved

Why? You can exercise such an option by flipping a coin, which is a classical process and doesn't involve any quantum measurement.

that is (in the context of MWI) the only mechanism that makes more than one future possible.

Wrong. The MWI is deterministic; there is only one future--one wave function.

You continue to misunderstand what the MWI says even though you have been corrected multiple times now.

in my example choice A and B are both possible, however some choice will be made between them if we view it subjectively

We've already gone round and round about "subjective". I don't see any point in rehashing that further.

If you insist to view it objectively in the context of MWI

You are the one that brought up the MWI, in the OP of this thread.

you can say both choices are made, so really there is no choice made, so free will, if it exist, has to be compatibalist.

In the context of any deterministic theory, any concept of free will consistent with that theory must be compatibilist, yes.

 

[entropy1]

Before I can even answer this, since we are assuming the MWI is true, you need to give me definitions of "objective" and "subjective" that are well-defined if the MWI is true.

If you say that MWI is deterministic, and that it means that when you measure a quantum property that the quantum gets entangled with the macroscopic state of the measurement device, and you call that strictly objective, then I would note that we always find a single measurement outcome, what I would call subjective (because it is a finding). I won't speculate but please tell me what is wrong with my argument. You tell me the physisist that found three non-commutating outcomes of the same measurement simultaneously. If the result of measuring $(|a \rangle + |b \rangle + |c \rangle)|M_0 \rangle$ is $|a \rangle|M_a \rangle + |b \rangle|M_b \rangle + |c \rangle|M_c \rangle$ (*) then objectively we have three outcomes, but the physisist won't tell his colleague that he found three outcomes, right? It seems to me that three physisists found a single outcome. We call each of the terms of (*) an "outcome".

 

[PeterDonis]

I won't speculate but please tell me what is wrong with my argument.

I already have. Multiple times. I would just be repeating myself if I did it again. You are not even thinking about what the MWI actually says. The MWI explains, explicitly, how each "you" in each individual branch observes a single measurement outcome. It seems like you don't even understand what the MWI actually says, and no matter how many times I explain it, you don't get it.

objectively we have three outcomes, but the physisist won't tell his colleague that he found three outcomes

You are misusing the term "the physicist". After the measurement, there is no "the" physicist. The physical system that you are labeling as "physicist" is now entangled with the measured system and has no definite state by itself. In terms of MWI "branches", each individual branch has a "physicist" in it that observes a single outcome, but there are three of them, not one, so there is no "the" physicist--"the" implies a single one, not three.

I have already explained this multiple times, and you still are not getting it.

 

[entropy1]

I have already explained this multiple times, and you still are not getting it.

If you say so then I guess I have to re-evaluate my thoughts about this issue.

 

[PeterDonis]

I guess I have to re-evaluate my thoughts about this issue.

Yes.

That seems a good note on which to close the thread. After you have re-evaluated and taken some time to consider what is posted here and what is in the literature about the MWI, you can start a new thread if you have further questions.