# Observer superposition in MWI?

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entropy1
In MWI, would you say that a measurement puts the observer in superposition of being in the various worldlines?

If I said "yes" to that, would I be correct?

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Mentor
In MWI, would you say that a measurement puts the observer in superposition of being in the various worldlines?

What worldlines are you talking about?

entropy1
What worldlines are you talking about?
The different worlds containing the different outcomes after Unitary evolution of the wave function?

I see this as a straight forward question.

The follow-up question would be: if an observer Alice gets into superposition that way, and another observer Bob is not observing her, then Alice stays in superposition relative to Bob. But if Bob observes Alice, A certain wordline containing Alice and some outcome she got will be selected according to Bob (collapse). My hypothesis is that a real outcome only gets selected if the observer observes it.

UPDATE: Nevermind, writing this I found an issue with it. But I would still like an anwer on the question in my OP. After that, I am content. (won't bother you guys unnecessarily - finding where you're wrong is satisfactory too )

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Mentor
The different worlds containing the different outcomes after Unitary evolution of the wave function?

I see this as a straight forward question.

The term "worldlines" is normally used in relativity to mean something very different. That's why I was confused.

I would still like an anwer on the question in my OP.

With the clarification above, the answer to your OP question is that it is based on a false premise. The different outcomes are not in different "worlds" in the MWI. (Yes, that means the MWI is misnamed.) There is just one "world", in which the observer is entangled with the observed system. As is true of any entangled state, neither the observer nor the observed system has a well-defined state by itself. Only the full combined entangled state of observer + observed system is well-defined.

entropy1
The term "worldlines" is normally used in relativity to mean something very different. That's why I was confused.
Oh I see haha. Sorry.
With the clarification above, the answer to your OP question is that it is based on a false premise. The different outcomes are not in different "worlds" in the MWI. (Yes, that means the MWI is misnamed.) There is just one "world", in which the observer is entangled with the observed system. As is true of any entangled state, neither the observer nor the observed system has a well-defined state by itself. Only the full combined entangled state of observer + observed system is well-defined.
Ok thanks. That makes the thing less clear to me but that doesn't have to be a bad thing, to the contrary.

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entropy1
Just one thing: is there in MWI no collapse? I know there is not. I am just verifying.

Mentor
is there in MWI no collapse?

That's correct. MWI is a no collapse interpretation.

entropy1
entropy1
With the clarification above, the answer to your OP question is that it is based on a false premise. The different outcomes are not in different "worlds" in the MWI. (Yes, that means the MWI is misnamed.) There is just one "world", in which the observer is entangled with the observed system. As is true of any entangled state, neither the observer nor the observed system has a well-defined state by itself. Only the full combined entangled state of observer + observed system is well-defined.
So that leaves me wondering: do we get an entanglement between a single measurement device and the single thing it measured, or do we have to speak of more than one measurement device, since we get all possible pointer outcomes.

Does the entanglement force us to experience a single outcome that depends on how we got entangled, for instance: if we get outcome A on the measurement, does that mean we got entangled with the particle being in state A? And likewise for the other possible measurement outcomes?

If the particles that make up the measurement setup are the particles that will get entangled with the observed particle, we speak of one machine, and thus of that one machine getting in superposition of outcomes, right?

Could one also say that the particle got entangled with the observer? (I think not) If not, why not?

Thanks.

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Mentor
do we get an entanglement between a single measurement device and the single thing it measured, or do we have to speak of more than one measurement device, since we get all possible pointer outcomes.

This is not a question about physics, it's a question about what words you prefer to use to describe the physics. The physics is the same either way.

Does the entanglement force us to experience a single outcome that depends on how we got entangled, for instance: if we get outcome A on the measurement, does that mean we got entangled with the particle being in state A? And likewise for the other possible measurement outcomes?

"How we got entangled" doesn't make sense as you're using it here. There is only one entanglement process, since it's just unitary evolution of the wave function and there is only one wave function.

the particles that make up the measurement setup are the particles that will get entangled with the observed particle, we speak of one machine, and thus of that one machine getting in superposition of outcomes, right?

What do you mean by "one machine" here?

Could one also say that the particle got entangled with the observer? (I think not)

Why not? The observer is a quantum system too--at least, that's what the MWI says. The MWI says that everything is a quantum system--more precisely, that there is only one quantum system, the universe as a whole, that undergoes unitary evolution, and observers are just particular degrees of freedom within that one quantum system.

entropy1
What do you mean by "one machine" here?
Thanks. Your answer is appreciated. With "one machine" I mean that there is no magical multiplication of the number of particles that makes up the measurement machine.

Mentor
With "one machine" I mean that there is no magical multiplication of the number of particles that makes up the measurement machine.

What does "magical manipulation" mean?

entropy1
What does "magical manipulation" mean?
So I think one could assert that the spawning of a copy of the universe is no multiplication of particles since the "new" universe is effectively isolated from its original. Or perhaps one could not speak of particles as ontological entities. I don't know what you mean actually by that question.

Mentor
the spawning of a copy of the universe

Does not happen. We've already been over this. There is only one "universe"--one quantum system with one wave function that undergoes unitary evolution. Unitary evolution can't "copy" anything.

I don't know what you mean actually by that question.

I thought it was obvious: I don't know what you mean by "magical manipulation", so I was asking you to clarify. Surely you realize that "magical manipulation" is not a standard physics term.

entropy1
Does not happen. We've already been over this. There is only one "universe"--one quantum system with one wave function that undergoes unitary evolution. Unitary evolution can't "copy" anything.
I agree and that is my point in this case.
I thought it was obvious: I don't know what you mean by "magical manipulation", so I was asking you to clarify. Surely you realize that "magical manipulation" is not a standard physics term.
I looked it up: I ment (and wrote): "magical multiplication" (of which I think is implausible ). Perhaps there is a miscommunication.

Mentor
I ment (and wrote): "magical multiplication"

Ah, I misread. There is no "multiplication" of universes period, whether you want to attach the adjective "magical" or not.

I agree

Not entirely. You said:

think one could assert that the spawning of a copy of the universe is no multiplication of particles

You can't assert that "the spawning of a copy of the universe is no multiplication of particles" unless you believe that there is a "spawning of a copy of the universe". But there isn't.

Or, to put it another way, you were basically saying: "the universe gets copied, but particles don't get multiplied". But that doesn't make sense. If the universe does get copied, everything in it has to get copied too. So it has to be both, or neither. In fact it is neither.

entropy1
You can't assert that "the spawning of a copy of the universe is no multiplication of particles" unless you believe that there is a "spawning of a copy of the universe". But there isn't.

Or, to put it another way, you were basically saying: "the universe gets copied, but particles don't get multiplied". But that doesn't make sense. If the universe does get copied, everything in it has to get copied too. So it has to be both, or neither. In fact it is neither.
What does "magical manipulation multiplication" mean?
So I anwered:
So I think one could assert that the spawning of a copy of the universe is no multiplication of particles since the "new" universe is effectively isolated from its original.
by means of response to that, meaning: if you (PD) mean to say that spawning a universe (if we would assert that, which we do not) is no "magical multiplication", then I quess... etc. In fact we are agreeing that there is no magical multiplication.

Pretty complicated miscommunication . In fact, my point is that the measurement machine does not magically multiply itself (into different branches). The particles that make up the machine represent all possible measurement outcomes according to MWI.

Mentor
by means of response to that, meaning: if you (PD) mean to say that spawning a universe

Which makes no sense since I had already said in this thread that there is no such "spawning". Why would you answer a straightforward question by assuming a hypothetical you already know to be false?

In fact, my point is that the measurement machine does not magically multiply itself (into different branches).

And since I've already said that in this thread, a much more easily understandable response would have been to simply quote this that I wrote:

There is just one "world"

And then say "I agree".

The particles that make up the machine represent all possible measurement outcomes according to MWI.

Wrong. A correct statement would be: "the particles that make up the measurement device (I prefer that term to "machine" since the latter does not make clear what you are talking about) are entangled with the particles that make up the measured system". Neither the measurement device particles nor the measured system particles on their own "represent" anything since they are entangled. Only the full entangled system of measurement device plus measured system does.

entropy1
Which makes no sense since I had already said in this thread that there is no such "spawning". Why would you answer a straightforward question by assuming a hypothetical you already know to be false?
It is a miscommunication. You should read my response as: "Well, if you say that, why not say this and this?", not as my personal opinion, which we tripped over because I started the sentence with "So I think...(that if you say that...)". Perhaps it should have read: "So, I then think..." Besides that English is not my natural language. This hypothetical formulating can be confusing.

I see now that you said that, but I did not read it today, I just proceeded in this thread today because I didn't want to start a new one.
Wrong. A correct statement would be: "the particles that make up the measurement device (I prefer that term to "machine" since the latter does not make clear what you are talking about) are entangled with the particles that make up the measured system". Neither the measurement device particles nor the measured system particles on their own "represent" anything since they are entangled. Only the full entangled system of measurement device plus measured system does.
Ah, that makes it even more clear (and you already said it, yes). The rest we agree about.

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entropy1
Wrong. A correct statement would be: "the particles that make up the measurement device (I prefer that term to "machine" since the latter does not make clear what you are talking about) are entangled with the particles that make up the measured system". Neither the measurement device particles nor the measured system particles on their own "represent" anything since they are entangled. Only the full entangled system of measurement device plus measured system does.
So I'm wondering: (If you say that,) is the "whole system" then representing/consisting of all the possible (applicable) measurement outcomes? (Just to be clear on this)

(And because I probably can study (much) easier if I ask these questions beforehand)

Mentor
You should read my response as: "Well, if you say that, why not say this and this?"

And you should not even be asking "if you say that" if I have already said "not that". You can't expect me to read you as asking something I have already contradicted in an earlier post.

I see now that you said that, but I did not read it today, I just proceeded in this thread today because I didn't want to start a new one.

And, as you should now realize, that is a bad idea. We have discussions in threads for a reason. You should not just ignore the information in previous posts before making new posts.

Mentor
is the "whole system" then representing/consisting of all the possible (applicable) measurement outcomes?

Of course, since the wave function contains all possible measurement outcomes.

entropy1
entropy1
And you should not even be asking "if you say that" if I have already said "not that". You can't expect me to read you as asking something I have already contradicted in an earlier post.
As I said, it is a miscommunication, or maybe a little stack of miscommunications.
I started: (a little opaque)
If the particles that make up the measurement setup are the particles that will get entangled with the observed particle, we speak of one machine, and thus of that one machine getting in superposition of outcomes, right?
You responded:
What do you mean by "one machine" here?
I responded:
Thanks. Your answer is appreciated. With "one machine" I mean that there is no magical multiplication of the number of particles that makes up the measurement machine.
What does "magical manipulation multiplication" mean?"
If there is one machine, then I mean by that there is no magical multiplication.
Your response I interpreted as: "Well entropy1, if there is no magical multiplication, what do you mean by it."
So I explained.
So I think one could assert that the spawning of a copy of the universe is no multiplication of particles since the "new" universe is effectively isolated from its original. Or perhaps one could not speak of particles as ontological entities. I don't know what you mean actually by that question.
meaning: "Well, if you put in question that there is no magical multiplication, one could well say that..."
One could erroneously read that "no magical multiplication" is debatable.
So it is a miscommunication and we agree.
And, as you should now realize, that is a bad idea. We have discussions in threads for a reason. You should not just ignore the information in previous posts before making new posts.
Ok.

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Mentor
As I said, it is a miscommunication, or maybe a little stack of miscommunications.

A miscommunication because you failed to read my previous posts in the thread. As I said, that's a bad idea. Particularly with such a short thread as this one, where it shouldn't take long at all to read the previous posts.

I started

I already know what happened. You don't need to keep trying to explain it to me. You just need to read previous posts before making new ones.

entropy1
You just need to read previous posts before making new ones.
I think I actually did read back the thread. I think I was eager and focused formulating the new question. It probably would have been better if I'd started a new thread.

Mentor
It probably would have been better if I started a new thread.

Even then my response to your question would have been the same, basically: "there is no spawning of universes, so your statement doesn't make sense".

I think one could assert that the spawning of a copy of the universe is no multiplication of particles

That is not a hypothetical; it's just a flat statement. A hypothetical would be something like this:

"What if multiple copies of the universe were spawned when a measurement was made? Could we still say there was no multiplication of particles?"

entropy1
Even then my response to your question would have been the same, basically: "there is no spawning of universes, so your statement doesn't make sense".

That is not a hypothetical; it's just a flat statement. A hypothetical would be something like this:

"What if multiple copies of the universe were spawned when a measurement was made? Could we still say there was no multiplication of particles?"
So you're saying it is a question of bad use of the English language? I have said what I meant by that sentence.
"[If you say that then] You could say
A) blablabla
B) blablabla" et cetera.
In fact, it probably is very poor usage of the English language (in the context).

Mentor
So you're saying it is a question of bad use of the English language?

Given your description of what you actually intended to say, yes.

entropy1
Given your description of what you actually intended to say, yes.
I could agree with that.

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entropy1
Thanks. Your answer is appreciated. With "one machine" I mean that there is no magical multiplication of the number of particles that makes up the measurement machine.
Meaning that there is one machine and one universe that don't get multiplicated (magically).
Does not happen. We've already been over this. There is only one "universe"--one quantum system with one wave function that undergoes unitary evolution. Unitary evolution can't "copy" anything.
Of course, since the wave function contains all possible measurement outcomes.
So, my question do the particles of the measurement device represent the outcome, and in fact all possible outcomes simultaneously?

Mentor
do the particles of the measurement device represent the outcome, and in fact all possible outcomes simultaneously?