What Is an Element of Reality?

[vanesch]

However, your interpretation of this result hangs on a crucial premise: namely, "no signalling" is what relativity *really* requires. That is not obvious. Surely relativity requires something *at least as strong as* "no signalling" but many people believe it requires something more, something stronger.

I agree with you that "relativity would look nicer" with Bell Locality. But Bell Locality is like diamonds for the girl. If you can have it, why not.
However, info locality is much more essential, because it leads to a paradox. Not a theoretical, unesthetic one, no, a real one.

Make a device that, upon receiving A, sends out not-A (you know, with a 74 series inverter). I hope I don't have to explain you in detail (because it is a bit long and technical) that if you have an FTL phone, you can build 2 of them, put one in a spaceship etc... and phone back to your past. So you can build it such that your device receives its output at 12PM and will send it at 12:03PM. You monitor with an oscilloscope: what will you see ?
Will you receive A or not-A at 12 PM ? Will you send out then not-A or A at 12:03PM ?

See, information locality is the bare necessity. So we cannot let it go.
Maybe you require more, but you require AT LEAST this. So this should be AT LEAST a great principle.

And this puts my equivalence: Bell locality <==> info locality and determinism
in another light. I would like to have Bell locality. But we know we can't have it. I desperately NEED info locality. So what's the deal ? Let go determinism !

It would be frankly bizarre if what it imposed on that structure was somehow intimately bound up with human activities like "signalling" and building telephones and whatnot. It seems like the requirements of relativity ought to be more fundamental -- it ought to forbid any kind of causal influence outside the light cone, even if (for whatever reason) it is one that can never be used by humans to transmit information. That just makes sense. And as soon as you start thinking that way, you will come to believe, like Bell, that "Bell Locality" is what relativity *really* requires, not merely "no signalling."

As I said, all this is nice. But we CAN'T have it. So do we throw out determinism, or do we throw out relativity ?

Note that info-locality has nothing to do with humans per se. It means that one can conceive physical structures leading to a paradox, such as my inverter gate. If it needs a certain sophistication to arrive at such paradoxal physical situations doesn't matter. It is as if a physical theory didn't allow for poles of 25.3 meter because then they would also be only 3 m long, and you were arguing that that is not a problem, because poles of 25.3 meter are not found in nature, you need humans to think of that. I'd say that your theory then has a very serious problem.

 

[ttn]

The superposition principle says:
if L1 is a "complete state of nature" and if L2 is "a complete state of nature", then there are infinitely many other "complete states of nature" described by a L1 + b L2. If I understand Bohm a bit, this only applies to the "wave function" part, but not to the "guiding wave" part, no ?

Yes, that's right. The wf would be a superposition, but the particle would be definitely in one or the other localized (in config space) "part" of the wf.

The amount of solispsim in MWI is in fact rather modest, you know. It only relates to what *you* observe, ...

That is the funniest thing I have read for a long time!

and honestly, you should acknowledge that that is a very private affair. It doesn't deny the existence of others either. Only, the person you talked to yesterday is not "the same" as the one you're talking to today, but a clone with exactly the same memory and physical body, which has a new "I experience" (while the "I experience" of the person you saw yesterday is now somewhere else, forever separated: admit the romantic drama in all this ).

Maybe you could explain how you understand QM's predictions for measurement probabilities in MWI. For example, if I prepare a bunch of spin 1/2 particle in the +z state and then measure their y-spin one after the other, how come I typically find about half +y and half -y?

When you say that there is no measurement problem in Bohm, how does this happen then ? Because there IS an objective difference between Copenhagen QM and MWI for instance: it is the physical process that determines measurement. In Copenhagen QM, this system CANNOT be considered to be in a superposition,and in MWI that's what you do. So there is a difference _in principle_ because, with enough care and technology, you COULD make the measurement instrument interfere with itself in MWI, and not in Copenhagen. So on which side does Bohm then flip ? Could I, or could I not, in principle, make a measurement instrument interfere with itself ?
When is a physical process a measurement ? I don't know enough about Bohm to realize this.

Yes, according to Bohm, in principle you could get something macro to interfere. The other branches of the wf continue to exist, only they are "empty", not containing the "particle" (i.e., actual configuration space point) and hence not (presently) affecting the particle's trajectory. But in principle, if you could arrange for one of those empty lumps of wf to overlap with the active lump, it would affect the particle, i.e., you'd see interference.



cheers,
Patrick.[/QUOTE]

 

[ttn]

I agree with you that "relativity would look nicer" with Bell Locality. But Bell Locality is like diamonds for the girl. If you can have it, why not.
However, info locality is much more essential, because it leads to a paradox. Not a theoretical, unesthetic one, no, a real one.

Make a device that, upon receiving A, sends out not-A (you know, with a 74 series inverter). I hope I don't have to explain you in detail (because it is a bit long and technical) that if you have an FTL phone, you can build 2 of them, put one in a spaceship etc... and phone back to your past. So you can build it such that your device receives its output at 12PM and will send it at 12:03PM. You monitor with an oscilloscope: what will you see ?
Will you receive A or not-A at 12 PM ? Will you send out then not-A or A at 12:03PM ?

See, information locality is the bare necessity. So we cannot let it go.
Maybe you require more, but you require AT LEAST this. So this should be AT LEAST a great principle.

Sure, but info locality is so superficial, you can keep it while having a theory whose guts is rather blatantly at odds with relativity. I say: if you're stuck with something which is fundamentally opposed to the principle of relativity (but rather magically obeys signal locality nevertheless) you should seriously consider that the principle of relativity is wrong or doesn't mean what we thought it meant or something. In effect, that *is* what you're doing when you admit that you have to give up Bell Locality (which "would look nicer") and comfort yourself with mere info locality. You're saying: I thought relativity actually prohibited nonlocal action at a distance, but evidently it doesn't. It merely prohibits something very high-level, something almost suspiciously human. Anyway, my point is just: don't pretent this is the same relativity you wanted or thought you had before. The fact that Bell Locality is violated means relativity is "less right" than we expected; we should face up to that squarely and not sweep it under the rug.

And this puts my equivalence: Bell locality <==> info locality and determinism
in another light. I would like to have Bell locality. But we know we can't have it. I desperately NEED info locality. So what's the deal ? Let go determinism !

I see. But I interpret it differently: I would like to have Bell locality, because I see relativity as requiring it. But we know we can't have it. So, too bad for relativity. (And note: you only "desperately NEED info locality" for relativistic reasons. If we've been thinking about relativity incorrectly all this time, e.g., maybe there really is a preferred frame in which collapse occurs or whatever, then that provides a whole slew of totally different possible ways of eluding the causal paradoxes you claim to NEED info locality for!)

As I said, all this is nice. But we CAN'T have it. So do we throw out determinism, or do we throw out relativity ?

Bell stated that, for him, the upshot of all this was that we needed to seriously rethink the idea of fundamental lorentz invariance. I think that means he wanted to "throw out relativity". I personally feel the same way. But I feel this way based on the reasoning above, not merely based on some obsessive attachment to deterministic theories.

 

[vanesch]

Maybe you could explain how you understand QM's predictions for measurement probabilities in MWI. For example, if I prepare a bunch of spin 1/2 particle in the +z state and then measure their y-spin one after the other, how come I typically find about half +y and half -y?

I'll do it for 3 particles, ok ? And let us say that it is Alice who does the measurements, then comes to tell me.

Beginstate:
|ignorant-me> |ignorant-Alice>|1z+> |2z+> |3z+>

Alice "measures" first particle:

(u = sqrt(2))

|ignorant-me> (1/u|Alice+>|1x+>+1/u|Alice->|1x->) |2z+> |3z+>

Both Alice states measure second particle:

|ignorant-me> (1/2|Alice++>|1x+>|2x+>+1/2|Alice+->|1x+>|2x->+
1/2|Alice-+>|1x->|2+>+1/2|Alice-->|1x->|2->) |3z+>

The 4 Alice states measure the third particle:
|ignorant-me>
(1/2u |Alice+++>|1x+>|2x+>|3x+> + 1/2u|Alice+-+>|1x+>|2x->|3x+> +
1/2u|Alice-++>|1x->|2x+>|3x+> + 1/2u|Alice--+>|1x->|2x->|3x+> +
1/2u |Alice++->|1x+>|2x+>|3x-> + 1/2u|Alice+-->|1x+>|2x->|3x+> +
1/2u|Alice-+->|1x->|2+>|3z+> + 1/2u|Alice--->|1x->|2->|3x+>)

Now, Alice comes to me and tells me her result. In doing so, I become entangled with her because of her telling me (which is a physical interaction):

1/2u |me+++>|Alice+++>|1x+>|2x+>|3x+> + 1/2u|me+-+>|Alice+-+>|1x+>|2x->|3x+> +
1/2u|me-++>|Alice-++>|1x->|2x+>|3x+> + 1/2u|me--+>|Alice--+>|1x->|2x->|3x+> +
1/2u|me++->|Alice++->|1x+>|2x+>|3x-> + 1/2u|me+-->|Alice+-->|1x+>|2x->|3x+> +
1/2u|me-+->|Alice-+->|1x->|2+>|3z+> + 1/2u|me--->|Alice--->|1x->|2->|3x+>

But now the "Born rule of my consciousness" has to be applied which means that the "I experience" in my "|ignorant-me>" state has to choose between all these different entangled me-states, in order to pick one, with probability equal to the coefficient ||^2 (as the Born rule prescribes). Here, all have probability 1/8.

So my "I experience" will go into one and exactly one of these "body states", say, the third one (with probability 1/8). All the other "body states" will acquire a new "I experience" as if it were an exact copy of myself at that moment (except for the different information from Alice), and I will never hear of them again.

So now, my continued I experience has seen a transition from a body in the "ignorant me" state into the state |me-++> and is in a product state with an Alice body state which is also in an |Alice-++> state (so we will both remember that the first particle was seen in the x- state, the second in an x+ state and the third one in an x+ state).

For all what matters now, we only need to work with this branch which corresponds to my "I experience". So it is *as if* I applied a projection postulate into the state |me-++>|Alice-++>|1x->|2x+>|3x+>.

Note that in all interactions of alice and the particles, there has only been a "local" interaction: when Alice was measuring particle 1, only those parts of the state description which had to do with Alice and with particle 1 were affected, the rest remained in a product state.

You can also analyse this way, EPR and quantum eraser experiments ; I put some of my posts on that in my journal.

From Alice's point of view, of course, each time she did a measurement, she became entangled, so up to the point where she has a consciousness (that's not sure of course, how can I know) her "I" experience had to choose. These choices do not have to be the same as mine. So it is very well possible that the "I experience" of the ignorant Alice, which I knew well, ended up in |Alice+-->. But I will not know, whatever I do with Alice, because the Alice state I can still interact with, namely |Alice-++>, will have EXACTLY the same bodystates, ideas, passions, in the past, and she will have a new "I experience", a clone, say, of the Alice I knew.

cheers,
Patrick.

 

[vanesch]

And note: you only "desperately NEED info locality" for relativistic reasons. If we've been thinking about relativity incorrectly all this time, e.g., maybe there really is a preferred frame in which collapse occurs or whatever, then that provides a whole slew of totally different possible ways of eluding the causal paradoxes you claim to NEED info locality for!

Well, I would think that relativity (SR) was experimentally verified to the extend that there had to be no doubt about these paradoxes, no ? I would think that the aspects you need to get to that paradox are not so very complicated. Once time dilatation and so on have been verified (and I would think they are), I'd think it is obvious that FTL signalling comes down to signalling in the past, no? I'm not talking here about "the spacetime structure of the universe" or something, just about almost feasible experiments ?
So whatever you will replace SR with, you'll be stuck with that paradox (in the same way as we will be stuck with Aspect-like results) ?

cheers,
Patrick.

EDIT: an important remark: in all this stuff, I try to stick to SR and QM predictions. If you want to propose a new *physics* theory, then this changes the tone of the discussion, because then the first thing to do is not to sit down and talk, but to propose discriminating experiments.
So once you talk about a "preferred frame" the best thing to do is to design an experiment that could show us that preferred frame, not to talk about how this could be nice if it were true.

 

[ttn]

Well, I would think that relativity (SR) was experimentally verified to the extend that there had to be no doubt about these paradoxes, no ?

Sure, it was completely reasonable to think that -- up until the point it became clear that Bell Locality was violated in nature!

I would think that the aspects you need to get to that paradox are not so very complicated. Once time dilatation and so on have been verified (and I would think they are), I'd think it is obvious that FTL signalling comes down to signalling in the past, no? I'm not talking here about "the spacetime structure of the universe" or something, just about almost feasible experiments ?

Actually, no. There exist possible theories with more spacetime structure than relativity (e.g., a preferred frame/foliation) but which also contain time dilation and all the other usual relativistic effects and in fact agree completely with relativity at the level of actual observation. The most obvious example is the old Lorentz type theory, which Bell wrote about in his beautiful paper "How to Teach Special Relativity." In a theory like this, you can have FTL signalling without any causal paradoxes.

So whatever you will replace SR with, you'll be stuck with that paradox (in the same way as we will be stuck with Aspect-like results) ?

Not true.

EDIT: an important remark: in all this stuff, I try to stick to SR and QM predictions. If you want to propose a new *physics* theory, then this changes the tone of the discussion, because then the first thing to do is not to sit down and talk, but to propose discriminating experiments.
So once you talk about a "preferred frame" the best thing to do is to design an experiment that could show us that preferred frame, not to talk about how this could be nice if it were true.

Yes, I agree, it would be good to do this. Of course, you already know it's going to be hard -- all the pre-Bell stuff that people (justifiably) took as strong evidence against the existence of a preferred frame, presumably means that if there is a preferred frame, it's well hidden. After Bell, one can say that it is hidden by quantum randomness/uncertainty, which is an interesting twist. So perhaps if Bohm's theory is true and someday some clever person figures a way around the uncertainty principle, we could use Bohm's nonlocal correlations to actually detect the preferred frame (in which presumably those nonlocal "signals" propagate). This has been pointed out many times, but, as you suggest, it's all talk until there's some serious suggestion for how to actually do it.

 

[ttn]

But now the "Born rule of my consciousness" has to be applied which means...

Yes, yes. But I was asking specifically about *probabilities*. You mentioned the Born rule here, which usually goes something like this: "the probability of a particular outcome is equal to ..." You are of course talking about something slightly different, and that's what I want to understand clearly. Is there a probability involved at all? It seems there isn't since, with certainty, all possible "outcomes" (or "experiences" if you want to talk in terms of your own solipsistic consciousness...) in fact occur. So there is nothing for probability to mean. Perhaps you want to say that the Born rule probabilities give the probability that "the current you" will find yourself in the next moment experiencing some particular thing. But I don't see how that makes any sense. The current you will, with certainty, find yourself in the next moment experience *all* the possible things, period. What is the probability supposed to be a probability *for*??

that the "I experience" in my "|ignorant-me>" state has to choose between all these different entangled me-states, in order to pick one, with probability equal to the coefficient ||^2 (as the Born rule prescribes). Here, all have probability 1/8.

It's the "in order to pick one" part that I don't understand. In what sense exactly is one picked? It seems to me that the rest of your story commits you to the notion that they're all picked, i.e., there's no "picking" going on at all, i.e., there's nothing for the Born rule probabilities to be probabilities *of*.

So my "I experience" will go into one and exactly one of these "body states", say, the third one (with probability 1/8). All the other "body states" will acquire a new "I experience" as if it were an exact copy of myself at that moment (except for the different information from Alice), and I will never hear of them again.

This seems to trade on equivocations about the meaning of "I". If "all the other 'body states' ... acquire a new 'I experience'" in exactly the same way as one another, how do you pick one out uniquely as "the real later you"? Aren't they all equally real, equal claimants to the title of "the real later you"? After all, it's essential to the whole MWI perspective that there is no collapse, i.e., that all branches of the wf continue to really exist, period. So what are the Born rule probabilities supposed to be probabilities for? The whole evolutionary story told by the theory is entirely, 100% deterministic.

Looked at another way, in standard QM, Born's rule only applies at the moment that the wave function collapses. You want to get rid of the collapse and have only the unitary evolution. So don't you get rid of any talk of Born rule probabilities in the process? It seems so, but if that's right, then I don't see how MWI can constitute a valid interpretation of QM, an essential part of which is *predictions for relative frequencies of various outcomes*.

Perhaps I'm attacking a bit of a straw man at this point, and should give you a chance to clarify your view before I say anything more. Hopefully my comments will at least give you a sense of the direction of my attack so you can defend yourself appropriately.

 

[vanesch]

So don't you get rid of any talk of Born rule probabilities in the process? It seems so, but if that's right, then I don't see how MWI can constitute a valid interpretation of QM, an essential part of which is *predictions for relative frequencies of various outcomes*.

Well, I'm not a real MWI-er. What you write (get rid of the Born rule) is indeed part of the program of MWI, and I think I have found a mathematical proof that this cannot work, which I hope to publish soon.
I think that there is some real need for the Born rule. On the other hand, I want to get rid of the projection postulate, and up to there, I follow MWI.

Imagine yourself, and imagine that we cut your body in 2 pieces, and restituate the missing half so that we now have 2 exact copies of your original body. (I think that this cannot really be done, but no matter).
When you guys both wake up after the operation, one of you will be in the left bed, and the other one in the right. But I can imagine that your *I experience* now is in one of both. Surely you don't experience a "body doubling" do you ? If I put a needle in one of the new bodies, YOU will feel pain in one case, and nothing at all in the other case, because it is in "the other copy's body" that I pick a needle.
Why are you in "the left bed" and not "in the right bed" ?

Next, imagine that I can scan your body up to subatomic scale and reproduce it completely. I now have an exact copy of your body, and that new body remembers exactly everything you know, etc... but I can assume that you still are yourself, and that, if I didn't even tell you that I made a copy of you, you wouldn't even know. Your *I* experience is still in your old body, but your new copy has a new *I* experience, with exactly the same memories as you do. If I put a needle in the copy's body, you won't feel anything.

Well, I postulate that exactly the same happens with these entangled states of your body: that ONE of these states inherits your "I" experience as a continuation of what happened before, and that the others are "new copies", but starting out with (almost, because of the different observations) identical personalities, mindsets, memories etc... To all external observers, you cannot make any difference between the you with your "old I experience" and the new copies because they behave in exactly the same way.
But to you, as a subjective experience, it makes of course all the difference in the world, if you are in one copy or in another.
And I claim that the fundamental physics of "I experiences" is the Born rule, which decides probabilistically in which of the copies your subjective I experience will now be.

There. You don't believe a word of it, do you

cheers,
Patrick.

 

[vanesch]

Imagine yourself, and imagine that we cut your body in 2 pieces, and restituate the missing half so that we now have 2 exact copies of your original body. (I think that this cannot really be done, but no matter).
When you guys both wake up after the operation, one of you will be in the left bed, and the other one in the right. But I can imagine that your *I experience* now is in one of both. Surely you don't experience a "body doubling" do you ? If I put a needle in one of the new bodies, YOU will feel pain in one case, and nothing at all in the other case, because it is in "the other copy's body" that I pick a needle.
Why are you in "the left bed" and not "in the right bed" ?

To make it more dramatic, imagine I'm explaining you the procedure, and I let you choose which of the two copies, the one in the left bed, or the one in the right bed, will be tortured atrociously, while the other copy will be made rich.

Don't you think it is 1 chance out of 2 that you will be lucky and 1 out of two that you will doom yourself ?

cheers,
Patrick.

 

[ttn]

Imagine yourself, and imagine that we cut your body in 2 pieces, and restituate the missing half so that we now have 2 exact copies of your original body. (I think that this cannot really be done, but no matter).
When you guys both wake up after the operation, one of you will be in the left bed, and the other one in the right. But I can imagine that your *I experience* now is in one of both. Surely you don't experience a "body doubling" do you ? If I put a needle in one of the new bodies, YOU will feel pain in one case, and nothing at all in the other case, because it is in "the other copy's body" that I pick a needle.
Why are you in "the left bed" and not "in the right bed" ?

OK, good example. Can we give names to the 3 people involved here, just to make it easier to refer to them? Say Albert is the guy that walks into the doctor's office, and Bob and Bill are the two guys on the two beds at the end of the day. Fair? If I read you correctly, you want to say that one of the two, either Bob or Bill, is somehow "more really the same person as" Albert. Yes? But I don't understand what could possibly underwrite that claim. Presumably if Bill, say, inherits all of Albert's memories from before the operation, then, by symmetry so does Bob. So, sure, if you ask Bob: "are you Albert?" he'll say "You bet I am! I remember being at home this morning eating breakfast and my wife called me Albert, etc..." But if you ask Bill the same question, he'll give the same response. Right?

So there seems no basis (either in terms of bodies or minds) to pick one or the other of the two copies (Bob or Bill) as "special", as "really Albert".

Next, imagine that I can scan your body up to subatomic scale and reproduce it completely. I now have an exact copy of your body, and that new body remembers exactly everything you know, etc... but I can assume that you still are yourself, and that, if I didn't even tell you that I made a copy of you, you wouldn't even know.

OK...

Your *I* experience is still in your old body, but your new copy has a new *I* experience, with exactly the same memories as you do. If I put a needle in the copy's body, you won't feel anything.

Yes, that's my understanding. But I think you run the risk of getting confused when you refer to putting a needle "in the copy's body." For I see no objective basis for claiming that one is *really* the copy. There are just two people who are like identical twins (in terms of sharing body structure, etc., but having their own individual consciousnesses) but who happen to share a memory history. Won't each legitimately regard the other one as "a copy of me", and legitimately believe themself to be "the real Albert."

Well, I postulate that exactly the same happens with these entangled states of your body: that ONE of these states inherits your "I" experience as a continuation of what happened before, and that the others are "new copies", but starting out with (almost, because of the different observations) identical personalities, mindsets, memories etc...

That makes no sense to me. If they all inherit my memories, then what picks out "ONE" as special, as the one that "inherits my 'I experience' as a continuation of what happened before"? They *all* do that!

To all external observers, you cannot make any difference between the you with your "old I experience" and the new copies because they behave in exactly the same way.
But to you, as a subjective experience, it makes of course all the difference in the world, if you are in one copy or in another.

The word "you" in this last sentence seems ambiguous. Are you referring to Albert here, or Bob, or Bill, or what? Surely it matters to Bob that he is a real person with memories and a mind and all that. But it matters equally much to Bill. Does it matter to Bob that he isn't Bill, that, for him, Bill is some other person? Sure. But it is symmetrical: it matters equally much to Bill that he isn't Bob, etc...

When you write that it matters "if you are in one copy or in another", I can only assume that the "you" is referring to Albert. It matters to Albert that one of the copies is "really him" and the other one isn't. Well, I don't think that's true; I don't think there's any way it could possibly be true since there is, by construction, no difference between Bob and Bill.


Now, people usually want to try to get out of this problem by saying that the evolution along the branching structure picks out one of the branches as "special" -- e.g., there really is some fact of the matter as to whether Bob or Bill is "the real Albert", the continuation-forward-in-time of Albert's "I experience". That solves this problem, but at the price of bringing back in everything that is wrong with Copenhagen. Because then you just have some extra law (Born's rule) for picking one of the branches at random as "real" and then you throw the others away as "empty" or "unreal" or "full of mere mindless hulks" or whatever. Well, you might as well just collapse wave functions and forget all the (then pointless) "many worlds" stuff...

 

[ttn]

To make it more dramatic, imagine I'm explaining you the procedure, and I let you choose which of the two copies, the one in the left bed, or the one in the right bed, will be tortured atrociously, while the other copy will be made rich.

Don't you think it is 1 chance out of 2 that you will be lucky and 1 out of two that you will doom yourself ?

Ah, good, this makes my objection clearer. You're asking "me" which of the two guys in the bed to torture. But who is "me"? There is no presently-existing third person who represents the "real me". There are just the two guys in the bed. If you ask Bill, I'm sure he'd prefer you torture Bob. And if you ask Bob, he'll likely prefer you torture Bill.

I think there is a 100% chance that one of the guys will end up happy and the other in pain. There is a 100% chance that "you will be lucky" -- for some value of "you" (either you=Bob or you=Bill). Talking about a probability of 50% that "you will be lucky" seems to presuppose that one of the two guys is "really me", and I don't know how you can claim any such thing without bringing in some new dynamics (other than the unitary evolution) that will confer privileged status on one of the two copies.

So much for getting rid of the projection postulate...

 

[vanesch]

I will try to give another example that should hopefully illustrate better what I'm at.

Imagine the following situation:
Somebody proposes the following deal:
You come to his office, and - staying awake and everything - your body will be scanned by a zargon ray scanner, which allows one to make an exact copy of you, with all of your memory content and everything. That copy will then be tortured slowly to death, and you will get a lot of money for it. Do you accept the deal or not ?

I can tell you, I do.

Now, somebody else proposes you another deal:
You will again be scanned, but now *YOU* will be tortured to death, while your copy gets a lot of money. Will you accept the deal ?

I can tell you, I won't.

Nevertheless, AFTER the copying, both you and the copy in the lab next doors are *identical*. But one of them is you, and the other one is a copy, and your *I* experience stayed (I assume) in your body.

To prove this, a final deal:

A copy will again be made, but this time, both of you will walk into a room with 500 different doors. The first one who will go through the 32nd door will get the money, and the other one will be tortured. Do you accept ?

I won't because of course my copy will know the deal too. So there will be a fight to get through this door and it is not sure I'm going to win against my copy and I don't like money that much that I want to risk to be tortured (but I don't give a damn about my copy).

Well, it is the same with the Born rule. I will be the one, selected randomly via the Born rule, in one of the states, and the others will be copies.

Does this clarify more what I want to say ?

cheers,
patrick.

EDIT: to add something here: in all these examples, the copy will of course know it is the copy, because it has had a strange experience: at one moment it was standing in the doctor's office in the scanner, and suddenly it blinked its eyes in a humming cylinder. Of course, the first thing is just a memory of something it never did.
But we could add more drama by making the inside of the scanner and of the humming "building" cylinder look identical, and placed symmetrically in a room. In that case, both the original you and your copy will have an identical experience.
You could then even accept the third deal: your copy is the one who will start to count the doors on the wrong side (and will think of you the same). Surprise when your copy will open the bad door, thinking he will be rich.
But, but... you might be a bit scared during that period that, well, maybe you are not the original, after all...

 

[ttn]

I will try to give another example that should hopefully illustrate better what I'm at.

Imagine the following situation:
Somebody proposes the following deal:
You come to his office, and - staying awake and everything - your body will be scanned by a zargon ray scanner, which allows one to make an exact copy of you, with all of your memory content and everything. That copy will then be tortured slowly to death, and you will get a lot of money for it. Do you accept the deal or not ?

I can tell you, I do.

Now, somebody else proposes you another deal:
You will again be scanned, but now *YOU* will be tortured to death, while your copy gets a lot of money. Will you accept the deal ?

I can tell you, I won't.

Nevertheless, AFTER the copying, both you and the copy in the lab next doors are *identical*. But one of them is you, and the other one is a copy, and your *I* experience stayed (I assume) in your body.

A nice, disturbing example.

But isn't it equivalent to the Albert/Bob/Bill case, except now you're saying: Bob and Bill are laying on the two beds, the doctor comes in and sends Bob out of the room for a minute, at which point he offers Bill lots of money if he'll consent to Bob getting tortured. Well, sure, I suppose Bill will consent to this. As would Bob, had it been Bill that was sent out into the hallway. It's like I said earlier: by construction there seems to be nothing to pick out one of the two as "really the same person as" the original guy, Albert.

Now you seem to want to get around this by assuming the operation (zargon ray scanner or whatever ) is such that the copy is produced in a different place than the original, the next room, say. And that does indeed suggest a way to identify "the real Albert" -- if Bob is the one in the next room over, then he's the copy and Bill is Albert.

That's fine as far as it goes. But then I think you're going to have a problem mapping that onto the quantum/MWI context. Because there, if I understand correctly, there is not supposed to be anyone of the copies that is in any way dynamically special. It's not like when the universal wave function splits, one of the new branches "exists in the next room over", whatever the heck that would mean. They all exist in the same (huge) configuration space of the universe. So either the analogy breaks down, or you bring in something to break the symmetry and pick one of the branches (and that will end up being equivalent to the collapse postulate, with all the badness that entails ).

To prove this, a final deal:

A copy will again be made, but this time, both of you will walk into a room with 500 different doors. The first one who will go through the 32nd door will get the money, and the other one will be tortured. Do you accept ?

I won't because of course my copy will know the deal too. So there will be a fight to get through this door and it is not sure I'm going to win against my copy and I don't like money that much that I want to risk to be tortured (but I don't give a damn about my copy).

Well, it is the same with the Born rule. I will be the one, selected randomly via the Born rule, in one of the states, and the others will be copies.

I don't follow you here. What is it that picks you out as, well, you? Is it that the copy was zargon-generated in the next room over? If so, then it's the same problem as above. But if you're intending to gloss over that (or have something less obviously asymmetrical, like the earlier example of cutting Albert in half down the middle and zargon-regenerating the two missing halves) then I see no basis at all for identifying one of the two as "really you".


Here's my problem in a nutshell. When the wf "branches", either one of the several branches is picked out as somehow special, or they're all equivalent. If they're all equivalent, there's no meaning to talking about probabilities and hence no place for the Born rule. If one of the branches is picked out as special, you could probably (ha ha) have a stochastic law so that which branch is picked is governed by the Born rule. But then I fail to see how the theory is any better than Copenhagen -- it will suffer from the same measurement problem, etc...

 

[vanesch]

Now you seem to want to get around this by assuming the operation (zargon ray scanner or whatever ) is such that the copy is produced in a different place than the original, the next room, say. And that does indeed suggest a way to identify "the real Albert" -- if Bob is the one in the next room over, then he's the copy and Bill is Albert.

In the Bob and Bill case, the doctor will not be able to know which one actually has the "continuation of I experience" and which one only thinks he has it, but whose *I experience* just came into being. There is no way to know this from the outside, or from any physical aspect because both will behave in exactly the same way, in all possible circumstances. But - it is strange that I cannot make you see this ; it is almost as if I'm talking to a copy of you - can't you conceive that, if YOU were undergoing this operation, your experiences are as follows:
you come in the doctor's office, you lie down on the operation table, you sleep under anesthesy, and... you wake up in the LEFT bed, with a thing which terribly looks like yourself in the bed to the right of you ? And that thing looks at you, and tells you: Hi, I'm Albert ! And you now realize that there is NO WAY IN THE WORLD you are going to be able to convince the doctor that you are the real Albert ?
You will tell me that the "guy in the right bed" will of course have exactly the same reaction. But that's not the point. That only makes both indistinguishable from the outside. Your subjective experience, I hope, will continue to exist in ONE of the copies.

If we go back to the other story (with the zargon scanner), I took that story not for the doctor to be able to make the difference, or an external observer to make the difference, but to try to make you see where your "continued I experience" attaches to one and only one body, and one could imagine that a zargon scanner doesn't affect you that much that your "I experience" flips over to the copy: you remain "yourself", in that your experience is: you walk into the doctor's office, stand on a metal plate, you hear "zmmmm" while you're still looking at the doctor, you step down, and get a big cheque, while you hear screaming in the room next door. In the same way you would have gone to the dentist, opened your mouth, had an intervention, and went home again. Imagine now a weird dentist who installed a zargon scanner on his chair and makes copies of you without you even noticing. It doesn't make a difference for you, does it ?

But now, let us imagine that there has been a technical improvement in zargon ray scanners, with zargon II rays.
Again, you go as usual to your dentist, go lie on the chair, open your mouth... and suddenly, you find yourself in a strange room, you look out of the window and... you see yourself walking home from the dentists, next door ! zargon II rays transport ALSO your *I experience* ! But for the dentist, nothing unusual, he knows that his copies are screaming, and try to convince him that they are the real you ; he already had that with his old zargon scanner.

BTW, for a "beam me up Scotty" transporter, I'd prefer a zargon II scanner with build-in meat grinder !

cheers,
Patrick.

 

[ttn]

In the Bob and Bill case, the doctor will not be able to know which one actually has the "continuation of I experience" and which one only thinks he has it, but whose *I experience* just came into being.

Yes, the doctor won't be able to tell. What's not clear to me is whether there is any fact of the matter at all. (I am thinking here of the original chainsaw-in-half-and-regenerate case, not the zargon-ray-in-the-next-room case.) If there is a perfect symmetry -- Bob and Bill each get exactly half of Albert's brain, etc. -- how is there any basis *in fact* (not merely in some particular person's knowledge) for saying that one of the half-copies is the "real" forward-continuation of Albert, but the other is a mere copy? Any fact which you point to as arguing for, say, Bill as the forward-continuation of Albert, I will say: yes, but that same fact applies equally to Bob, so where's the difference?

it is strange that I cannot make you see this ; it is almost as if I'm talking to a copy of you

I am admittedly a bit of a mindless hulk...

- can't you conceive that, if YOU were undergoing this operation, your experiences are as follows:
you come in the doctor's office, you lie down on the operation table, you sleep under anesthesy, and... you wake up in the LEFT bed, with a thing which terribly looks like yourself in the bed to the right of you ? And that thing looks at you, and tells you: Hi, I'm Albert ! And you now realize that there is NO WAY IN THE WORLD you are going to be able to convince the doctor that you are the real Albert ?

Yes, I think that is exactly what "I" would experience. But you are forgetting that this other guy would experience something very similar, except he'd wake up with "me" on his left. My point is, not only could I never convince the doctor that I'm the real Albert -- I could never convince the guy in the other bed that I'm the real Albert. Any argument I tried to make for that claim ("but I remember coming in here this morning and my wife said 'goodbye Albert' before I left home!") wouldn't work, because the other guy would be able to make precisely the same argument.

My point is that the natural grammar here is misleading. You write: "you lie down on the table, you sleep under anesthesy, and... you wake up...". I think it is grammar, and grammar only, that makes you believe there is some natural or obvious connection between the person referred to by "you" in the first two occurrences of that word, and the third occurrence of it. Of course, Bill will believe that he is Albert -- he has a more or less continuous memory of Albert evolving continuously forward in time *into him*. So he naturally believes he is Albert. But Bob has exactly that same continuous sense. So literally , when you say "...and then *you* wake up" I don't know who/what you are referring to by the word "you". The grammar suggests you are, in some sense, referring to the same thing that word meant earlier in the sentence (Albert), and if we don't think too carefully we will just assume that one or the other of Bob/Bill is the forward-continuation of Albert. Why do we think this? I guess because we are used to believing that one person cannot become two, so "obviously" if Albert becomes two, one is real and one is a mere copy. But the whole point of the example is to deny that very intuition -- one person, by hypothesis here, *can* become two. So there is no *unique* forward-continuation-of-Albert. There are two.

Now, I must confess to being a bit lost as to how any of this is supposed to relate to QM. If it would help move the argument forward, I'd be happy to just concede, for the sake of argument, that one or the other of Bob/Bill is somehow picked out as metaphysically special -- the "real" forward-continuation of Albert (because of some continuity of "I experience" or because the other loser was born 5 minutes ago in the next room or whatever). So... if I just admit that, what happens next?

I guess you want to say the splitting of universes is just like that -- the universal wf splits into several branches at some moment, and this generates a bunch of "copies" of Albert, one of which is picked out as special in the sense of being the "real" forward-continuation of Albert. Yes?

And then I will point out: there is nothing in the unitary dynamics that can *perform* this job of picking out one of the copies as special. If you want one to be special, you need to add something to the unitary dynamics -- something that starts to smell suspiciously like the Born rule and an associated wave function collapse...

You will tell me that the "guy in the right bed" will of course have exactly the same reaction. But that's not the point. That only makes both indistinguishable from the outside. Your subjective experience, I hope, will continue to exist in ONE of the copies.

"My" subjective experience? I don't know what you mean. Bob's experience will exist in only one of the copies. Bill's experience will exist in only one of the copies. Which one is really the forward continuation of Albert? You tell me.

Anyway, whatever. Maybe conscious "I experience" really does work the way you assume. I have no idea, really. So I'm happy to grant whatever you want for the sake of bringing the discussion back to QM and MWI. I still don't see how any of this justifies talk of probabilities there...

 

[vanesch]

So I'm happy to grant whatever you want for the sake of bringing the discussion back to QM and MWI. I still don't see how any of this justifies talk of probabilities there...

You might be surprised, but it is extremely related.

Remember my "|ignorant-me>" body state, which was related to my "I experience. After talking to Alice, my body got into a superposition of states, namely |me+++>, |me-++> ...
Now, if you accept, for a moment, that there is a continuation of my "I experience" from the body state "ignorant me" into ONE of these |mexxx> states, with a probability given by the Born rule, then I have 1/8th chance to end up, in, say, the |me+--> state. AND I DON'T KNOW ANYTHING ABOUT ALL THE OTHERS. So my old "I experience" is now related to the body state |me+-->, and as I'm entangled with the Alice+-- state, when I will later talk to Alice we will both agree that we had first a + state, then a - state, and then again a - state. I am also entangled with the |1x+> state, so if I'm going to do an experiment on the first particle, everything will seem to me as if it was in a pure |1x+> state.
So for all practical matters, I can now continue with the state of the universe being |alice+-->|1x+> |2x-> |3x->.
So it is AS IF I applied the projection postulate ! I will never know anymore about all these other states.
If I do now a new quantum experiment, say, with Bob having done 4 measurements, my "old" I experience is now in |me+--> and after Bob tells me his results, my body state (starting out from this |me+--> state) will entangle with Bobs and his particles, and again, because my "I experience" cannot stand being entangled, chooses, according to the Born rule, ONE of these new body states to "live in" while the others are again "copies".

So my (I-experience-wise) new state will now be |me+--AACB> or something, which has been assigned to me, I recall, again by choosing randomly one of the possible states, with probabilities according to the Born rule.
As such, my "I experience" accumulates different experiences, results of measurements etc... and when you do statistical tests on these results, they are conform with the Born rule (that's maybe not so obvious, but it can be shown). More ! I only end up in a product state with bodystates of other people who ALSO have that experience of the correctness of the Born rule. So we end up writing theories about it, call them "quantum mechanics" and believe in the validity of the Born rule.

True Everettians come close to this view, but they refuse the "assignment by fiat" through the Born rule, and hope they can somehow find a logical argument on why it must be so. A recent argument by Deutsch goes even as far as saying that these probabilities must be given by the Born rule, because (in situations such as money/torture choices) these are the only reasonable probabilities a rational gambler would assign to his later becoming Bob or Bill based on symmetry arguments and so on ; and then goes on defining what a rational gambler is (and I think I found where in this definition he smuggled in the Born rule!). I think I know that this cannot work in general, so I just plug in the Born rule by fiat, as a postulate ; "the god of the Born rule re-assigns the souls of the observers to the new body states by playing dice according to the Born rule" ! Einstein would get sick of it !

In all of this, you might object: "yes, but what happens to the copies then ?". My answer is: I don't know, I will never know, because I have only one I-experience. A bit like if you were continuously being copied by aliens right now, flying over your house with zargon ray machines. You'll never know about those copies. The Born rule is what singles out one body state (and this is the essentially probabilistic part of the theory) to be your new body state in which your I experience will now live on. And it has as a consequence (and that is important!) that this succession of body states will accumulate experience and memory in agreement with statistics of the Born rule as if the projection postulate were true, and more, it will be in a product state with other people's body states who also accumulated such an agreement with the Born rule experience.
The scary thing in all this is that those "other people's body states" are probably not those in which *their* original I experience is now travelling, and they are probably now in contact with one of your copies. But you (being external to them) will never notice anything about that, the behaviour of their copy being identical.

cheers,
Patrick.

 

[vanesch]

I guess you want to say the splitting of universes is just like that -- the universal wf splits into several branches at some moment, and this generates a bunch of "copies" of Albert, one of which is picked out as special in the sense of being the "real" forward-continuation of Albert. Yes?

And then I will point out: there is nothing in the unitary dynamics that can *perform* this job of picking out one of the copies as special. If you want one to be special, you need to add something to the unitary dynamics -- something that starts to smell suspiciously like the Born rule and an associated wave function collapse...

This is true. But the nice thing about it is that somehow, maybe, you can assume that "I experiences" are, as you call it, metaphysical, and do not have to obey the rules of material physical objects.

As I said, I want to stick as closely as possible to the existing formalism (QM and SR). Everettians think they can get rid of that ugly Born rule, but I think that's wrong. So I hide it there where it doesn't harm physics per se: in my subjective experiences. That's not something physical, so I cannot write "the hamiltonian of my subjective experiences". I can write the Hamiltonian of my body and brain, so that's still physics. But as far as my subjective experiences themselves go, I can make the case that they do not belong to physics, and hence do not obey some unitary dynamics. Instead, they obey the Born rule.

And now off the record: if you ask me whether I really believe that that is how nature works ? No, of course not I have to say I don't know how nature "really" works, and I'm even convinced we will never know. But this is just, IMHO, a nice story that can go perfectly well with the current formalism of QM and SR. In fact, after thinking a long time about it, I have never found any OTHER story that goes with it and strictly sticks to the formalism...

cheers,
Patrick.

 

[selfAdjoint]

And now off the record: if you ask me whether I really believe that that is how nature works ? No, of course not I have to say I don't know how nature "really" works, and I'm even convinced we will never know. But this is just, IMHO, a nice story that can go perfectly well with the current formalism of QM and SR. In fact, after thinking a long time about it, I have never found any OTHER story that goes with it and strictly sticks to the formalism...

Aaah, Patrick, I can't tell you how much better that statement makes me feel! I personally would almost give up the formalism to avoid having to take MWI seriously, with all its implications. Shut up and calculate does OK for me (but then you know I am not much good at calculating! Or shutting up, for that matter. )

 

[ttn]

Remember my "|ignorant-me>" body state, which was related to my "I experience. After talking to Alice, my body got into a superposition of states, namely |me+++>, |me-++> ...
Now, if you accept, for a moment, that there is a continuation of my "I experience" from the body state "ignorant me" into ONE of these |mexxx> states, with a probability given by the Born rule, then I have 1/8th chance to end up, in, say, the |me+--> state. AND I DON'T KNOW ANYTHING ABOUT ALL THE OTHERS. So my old "I experience" is now related to the body state |me+-->, and as I'm entangled with the Alice+-- state, when I will later talk to Alice we will both agree that we had first a + state, then a - state, and then again a - state. I am also entangled with the |1x+> state, so if I'm going to do an experiment on the first particle, everything will seem to me as if it was in a pure |1x+> state.
So for all practical matters, I can now continue with the state of the universe being |alice+-->|1x+> |2x-> |3x->.
So it is AS IF I applied the projection postulate ! I will never know anymore about all these other states.
If I do now a new quantum experiment, say, with Bob having done 4 measurements, my "old" I experience is now in |me+--> and after Bob tells me his results, my body state (starting out from this |me+--> state) will entangle with Bobs and his particles, and again, because my "I experience" cannot stand being entangled, chooses, according to the Born rule, ONE of these new body states to "live in" while the others are again "copies".

OK, I understand all of this; it is more or less what I thought you thought, but it's nice to get it out here in black and white.

Remind me once again why I should take any of this seriously? Oh, right, because this story allows me to elude what would otherwise be an in-eludable violation of Bell Locality in my theory. So the calculus is: retaining belief that relativity is really fundamental (as opposed to merely emergent or somehow otherwise not the final story in spacetime) outweighs belief in basic, common-sense "realism" including, say, the idea that our best theories (including QM) are telling us something reliable about the world (remember, under MWI, we are deluded into thinking that experimental outcomes are governed by the Born rule, etc.), that our best friends are actually conscious beings on par with us, and so forth.

I admit there is a choice to be made here, but I frankly don't understand why anyone would give up so much merely to save something so little.

True Everettians come close to this view, but they refuse the "assignment by fiat" through the Born rule, and hope they can somehow find a logical argument on why it must be so. A recent argument by Deutsch goes even as far as saying that these probabilities must be given by the Born rule, because (in situations such as money/torture choices) these are the only reasonable probabilities a rational gambler would assign to his later becoming Bob or Bill based on symmetry arguments and so on ; and then goes on defining what a rational gambler is (and I think I found where in this definition he smuggled in the Born rule!).

Yes, I've heard Deutsch's argument. I'm sure you're right that it smuggles in the Born rule somehow, since you can't get statements about probability out of thin air from a complete deterministic theory. But it would be interesting to see specifically how this sneaks in. Kudos to you for having the, ahem, courage to take Deutsch's arguments seriously enough to give them careful scrutiny.

I think I know that this cannot work in general, so I just plug in the Born rule by fiat, as a postulate ; "the god of the Born rule re-assigns the souls of the observers to the new body states by playing dice according to the Born rule" ! Einstein would get sick of it !

Probably, but I doubt it would be the dice-rolling part that would mostly bother him.

In all of this, you might object: "yes, but what happens to the copies then ?". My answer is: I don't know, I will never know, because I have only one I-experience. A bit like if you were continuously being copied by aliens right now, flying over your house with zargon ray machines. You'll never know about those copies. The Born rule is what singles out one body state (and this is the essentially probabilistic part of the theory) to be your new body state in which your I experience will now live on. And it has as a consequence (and that is important!) that this succession of body states will accumulate experience and memory in agreement with statistics of the Born rule as if the projection postulate were true, and more, it will be in a product state with other people's body states who also accumulated such an agreement with the Born rule experience.
The scary thing in all this is that those "other people's body states" are probably not those in which *their* original I experience is now travelling, and they are probably now in contact with one of your copies. But you (being external to them) will never notice anything about that, the behaviour of their copy being identical.

Yes, this is precisely the point that led (I believe) David Albert to coin the term "mindless hulks" to describe all those people out there in our worlds which we think are people but are really mindless hulks.


By the way, do you believe that each individual person "lives" in some one particular branch at each moment, so (from god's perspective) there really are several billion distinct conscious people on Earth right now? Or would you say, instead: this view commits me to believing that it is overwhelmingly like that everyone I've ever seen and taken to be a conscious person was in fact merely a mindless hulk; hence I have no evidence whatsoever for believing in the existence of other conscious people; hence I assume I am the only one.

At some point you admitted that your view was solipsistic -- so I'm just trying to understand which of these versions of solipsism you subscribe to.


PS -- I'm glad that MWI fans don't watch old zombie movies where the good guys have to go around killing all the zombies. Or if they do watch them, I hope they don't get any ideas...

 

[ttn]

As I said, I want to stick as closely as possible to the existing formalism (QM and SR). Everettians think they can get rid of that ugly Born rule, but I think that's wrong. So I hide it there where it doesn't harm physics per se: in my subjective experiences. That's not something physical, so I cannot write "the hamiltonian of my subjective experiences". I can write the Hamiltonian of my body and brain, so that's still physics. But as far as my subjective experiences themselves go, I can make the case that they do not belong to physics, and hence do not obey some unitary dynamics. Instead, they obey the Born rule.

You seriously think it "doesn't harm physics" to bring all this blatantly non-physics loony business *into* physics? I think it does tremendous harm.

And now off the record: if you ask me whether I really believe that that is how nature works ? No, of course not I have to say I don't know how nature "really" works, and I'm even convinced we will never know. But this is just, IMHO, a nice story that can go perfectly well with the current formalism of QM and SR. In fact, after thinking a long time about it, I have never found any OTHER story that goes with it and strictly sticks to the formalism...

Hmmm... If you really honestly don't believe any of this (because deep down you're a sane person and no sane person could take this stuff seriously!) then why advocate it as if you did believe it, as if it was a serious physics theory? If you're convinced "we will never know" how nature really works, then by all means don't spend your time trying to figure it out. But some people are not convinced of that; we are trying to find out how nature works. Personally I don't think "trying to figure out how nature works" is something that physicists should have to *apologize* for! We should be trying to do it, and we should be doing so proudly.

This is why I think, contrary to what you implied above, it does harm physics to take MWI seriously or to pretend that you do. It makes others feel embarrassed that they *can't* take that seriously, or that they believe physics should be taken seriously, etc. In short, it sends off a vibe into the rest of the physics community that "physics isn't serious", or "physics can never really figure out how nature works, so we just make up stupid stories that we don't really take seriously anyway", or "physics is all a big inside joke, but don't tell the government or they'll stop funding us", or god knows what. Well, as someone who has a lot of respect for people like Newton and Maxwell and Boltzmann and Einstein and Bell -- people who did take physics seriously and didn't think it was doomed to become a failure or a joke -- I don't think it's good to be spreading this kind of attitude. It will cause future Newtons and Boltzmanns and Bells to go into a field that does take itself seriously like, oh god, cultural studies, or basket weaving, or who knows what.

 

[DrChinese]

1. BUT NONLOCAL HIDDEN VARIBLE THEORIES ARE NOT RULED OUT. That is why the existence of Bohmian mechanics doesn't cause the universe to disappear in a puff of logic.

2. Then you must be confused about what Bell proved. Bell's theorem shows that, if you try to "complete" QM by adding local hidden variables, the theory you get cannot both respect the Bell Locality condition and agree with experiment. So, as lots of people say, if you want a local theory, you'd better stick with QM and its completeness doctrine, and not go down the hidden variables road. But that strategy obviously presupposes that QM itself is local -- otherwise, saying "you should stick with QM and not pursue hidden variable theories, on pain of nonlocality" just makes no sense.

3. And the final piece: Bell states openly that, he thinks, nonlocality is a fact, period -- that it's *not* something which merely afflicts hv theories. As he says, you *cannot* dismiss the operations on one side as causal influences on the other. How can he believe this? What else would he need to have to believe to make this claim given the above paragraph? Obviously he would have to think that orthodox QM was *also* nonlocal. IF it wasn't, there'd be no grounds for claiming that all possible alternatives -- i.e., nature -- were nonlocal.

1. I have reviewed Goldstein's summary of BM at http://plato.stanford.edu/entries/qm-bohm/. Now I am more confused than ever! I just don't see where there are hidden variables at time t=0. He mentions that contextuality is assumed and is no big deal. Yet nowhere is the simultaneous reality of non-commuting observables asserted.

2. You see Locality as the essential assumption, and I see the "reality" of A, B and C as the essential assumption.

3. I think all of the QM interpretations somehow violate "local causality" which I define as: causes must precede effects, and there is no FTL propagation of causes.

 

[vanesch]

You seriously think it "doesn't harm physics" to bring all this blatantly non-physics loony business *into* physics? I think it does tremendous harm.

I guess what you call "loony business" are concepts which you would rather classify as philosophical or spiritual, and not as materialistic science. But it is a long standing tradition that concepts which "didn't have their place in science" came into science when science grew sophisticated enough to deal with it. The human body was something beyond scientific investigation ; organic chemistry was something beyond scientific investigation ; the fact that humans could decend from apes and in the end from bacteria was something unheard of ; the movement of the heavens was beyond scientific investigation. All these domains have now been "invaded" by science. So why not the concepts of consciousness, "I experience" etc... ?
After all, if something like MWI *is* an approximate description of reality, that would then be the first time that such "unheard of" concepts are integrated into the scientific machinery, all be it very coarsely.
So I don't accept the argument that this "loony business" has nothing to do with science, because I put it on the same level as saying that human blood has nothing to do with chemistry.

Hmmm... If you really honestly don't believe any of this (because deep down you're a sane person and no sane person could take this stuff seriously!) then why advocate it as if you did believe it, as if it was a serious physics theory?

You understood my statement "on the wrong side of its meaning" (like I knew a professor who told somebody that he "almost got zero". When the student asked how much he had, the professor said: "minus five" .)

When I say that I "don't take this seriously" I only mean that this stuff is still too speculative and naive. After all, we don't even have all of *material physics* into a theory. So there are still many changes to be accepted, and I think that something like MWI is only the beginning ! Probably nature is way way more weird than this simplistic view in MWI ; I didn't mean that we would get "back to good old positivism". In fact, if science is to progress, I really think that one day it will have to tackle issues which, for me, seem currently out of its scope, such as consciousness (and I'm not talking about the brain processes that are at its basis). So maybe, just maybe, quantum theory is our first indication of it.

If you're convinced "we will never know" how nature really works, then by all means don't spend your time trying to figure it out. But some people are not convinced of that; we are trying to find out how nature works. Personally I don't think "trying to figure out how nature works" is something that physicists should have to *apologize* for! We should be trying to do it, and we should be doing so proudly.

This is why I think, contrary to what you implied above, it does harm physics to take MWI seriously or to pretend that you do. It makes others feel embarrassed that they *can't* take that seriously, or that they believe physics should be taken seriously, etc. In short, it sends off a vibe into the rest of the physics community that "physics isn't serious", or "physics can never really figure out how nature works, so we just make up stupid stories that we don't really take seriously anyway", or "physics is all a big inside joke, but don't tell the government or they'll stop funding us", or god knows what.

Well, you misunderstood me, clearly. When I say that we will never know exactly what reality is about, I really believe that, because we had to change our views already a few times very profoundly, so we will have to do that again in the future, implying that at any moment, we are only in an approximative scheme (having given up, or not, to continue).
As such, I think that Newtonian physics is farther from reality than say, Maxwell's classical field view. And Maxwell's classical field view is farther from reality, than, say, general relativity. Or farther from reality than QM, with its MWI viewpoint. And MWI is farther from reality than *fill in our next paradigm*. I think however, that MWI is much closer to reality than Newton's viewpoint, for instance. But I'm aware that it will still change many times, and it is probably still too early to get it out of the speculative realm. In fact, as long as we don't have a serious theory of quantum gravity, I think it is too early to know. However, if the theory of quantum gravity DOES still adhere to the superposition principle, I think you cannot get by without MWI. So all string theorists and (I think) all loop quantum gravitists must be somehow "MWI" inspired. Superstrings makes no sense without MWI. But I'm very attentive to positions such as Penrose's, who thinks that gravity will play a crucial role in the measurement process in QM.

Well, as someone who has a lot of respect for people like Newton and Maxwell and Boltzmann and Einstein and Bell -- people who did take physics seriously and didn't think it was doomed to become a failure or a joke -- I don't think it's good to be spreading this kind of attitude. It will cause future Newtons and Boltzmanns and Bells to go into a field that does take itself seriously like, oh god, cultural studies, or basket weaving, or who knows what.

I'm affraid you could say the same about people trying to apply chemistry to organic materials, which were supposed to be "outside of the scope of science" at a certain point, and try to tell them to stick to minerals, if serious people were supposed to take chemistry serious in the future.
I really don't see why bringing in ideas such as subjective experience are blasphemy to physics. After all, one day, we will have to find out ! Before being able to make zargon ray machines of the second generation !
However, it might be that we are still far from that achievement, and that you 're right, that quantum theory as it stands, today, doesn't touch upon it yet, and we need a few more millennia of "positivist materialist science" before we get there. Nevertheless, I think it is funny that if you take QM very literally, you almost naturally arrive at MWI like situations.

However, this discussion showed me something. I think I really should learn more about Bohm's theory. All I know about it is in fact the 1-particle description.

So if you're willing, you can teach me some Bohmian mechanics ! Then I'm the guy making the nasty remarks, and you're the guy who has to explain and justify :-)

cheers,
Patrick.

 

[DrChinese]

So if you're willing, you can teach me some Bohmian mechanics !

I am interested in knowing more about how BM applies to EPR too.

As to the "paradigm" each of us uses... Who is to say what is best, really? I have seen this point argued ad infinitum in plenty of areas of science, and the answer is always inconclusive.

The fact is, any model (map) can serve a purpose. And a good idea can come from anywhere, even the most unexpected place. So there is not a lot of point to argue that one model is "better" or "more promising" because that can't really be objectively agreed upon.

For example, in the early sixties, who would have thought that the next big thing would emerge from a little city in England called Liverpool? Or that a Swiss clerk would emerge as the most powerful force in theoretical physics?

 

[vanesch]

I am interested in knowing more about how BM applies to EPR too.

As to the "paradigm" each of us uses... Who is to say what is best, really? I have seen this point argued ad infinitum in plenty of areas of science, and the answer is always inconclusive.

The fact is, any model (map) can serve a purpose. And a good idea can come from anywhere, even the most unexpected place. So there is not a lot of point to argue that one model is "better" or "more promising" because that can't really be objectively agreed upon.

This is true. I think the only thing that is not acceptable are logically inconsistent views: you know, things that give a different answer according to the way you arrive at it.

That's why I think that Copenhagen QM has a problem if we take it that the wave function is something "real". And the reason for that is that we arbitrarily have to decide which physical processes are measurements (which induce a collapse), and which physical processes are interactions (which evolve unitarily), and that we can in fact, choose between both for a great many of them, leading to different behaviours of our "real" wavefunction.

However, Copenhagen QM is perfectly all right if we see it as an abstract mathematical machinery out of which come predictions of probability (the "shut-up-and-calculate" approach). In this case, we take physical theories just to be epistemological: they just allow us to know answers, but they don't describe what is "really there". However, there is a danger that we apply the Born rule too early: we have to apply it only at the end, when we are really extracting probabilities that will be compared to experimental relative frequencies.

Nevertheless, a more ontological approach to a physical theory seems to be desirable: you would like to map things of the formalism to "things out there".
The question then is for how much deviation you allow from the existing formalism, and what principles you use in order to guide you to a formalism that IS ontologically mappable.

I would then think that an MWI like approach is what comes closest to the existing formalism: you take 2 basic principles which are at the basis of the theory (namely locality (in its minimalistic version: information locality), and the superposition principle) for "real" in that the machinery has to apply it too.
However, the result sounds "crazy" and you arrive at the use of "loony business" such as talk about subjective experience and so on.

If you give yourself more liberty with the formalism, I guess you can arrive at Bohmian mechanics. I have to say that this discussion gave me some incentive to learn more about it ; for instance, I was of the opinion that Bohmian mechanics had a serious problem with QFT ; but this may not be the case, I don't know. This is then a question if Bohmian mechanics is formally equivalent with any Hilbert state description or not. If it is not, then we're actually talking about 2 different theories, which will probably make different predictions for certain cases, and not about 2 different ways of viewing the same theory (namely quantum mechanics).

Whatever way you take, I suppose, as you say, that it is a matter of preference, as long as it is compatible with established results and is internally consistent.

cheers,
Patrick.

EDIT: Although predicting things is difficult, especially if it concerns the future, and - as you say - that inspiration can come from the most unexpected view, I would nevertheless allow myself to poder whether, say, if Bohm's view had been predominant, and not von Neumann's, whether one would have arrived at anything like quantum field theory...

 

[ttn]

I guess what you call "loony business" are concepts which you would rather classify as philosophical or spiritual, and not as materialistic science. But it is a long standing tradition that concepts which "didn't have their place in science" came into science when science grew sophisticated enough to deal with it. The human body was something beyond scientific investigation ; organic chemistry was something beyond scientific investigation ; the fact that humans could decend from apes and in the end from bacteria was something unheard of ; the movement of the heavens was beyond scientific investigation. All these domains have now been "invaded" by science. So why not the concepts of consciousness, "I experience" etc... ?

Oh, I have nothing against bringing new concepts into science. I guess I got the impression from your earlier comment that the point of bringing consciousness in the way you want to in MWI, however, wasn't really to clarify or build any new science, but, rather, merely because it was a convenient and safe place to "hide" the Born rule where it couldn't be refuted because it didn't make any contact with actual physics (i.e., physical objects). I think that is a pretty bad motive for bringing in new concepts into physics; so it's not that I'm against doing so per se, but only against doing it for what seems like really fishy, unscientific reasons.

So if you're willing, you can teach me some Bohmian mechanics ! Then I'm the guy making the nasty remarks, and you're the guy who has to explain and justify :-)

Well, maybe, but it really is easier to wear the critic's hat...

 

[ttn]

This is true. I think the only thing that is not acceptable are logically inconsistent views: you know, things that give a different answer according to the way you arrive at it.

I don't really want to get into a big debate over pure philosophy here, but I'm not sure I agree with this. In particular, I think you are using a too-narrow definition of "logical consistency" to mean merely narrowly-conceived internal consistency. I believe in scientific realism, part of which is the idea that the physical world we perceive is more or less real -- there really is a world that "looks" that way. Or putting it negatively, we aren't massively deluded (e.g., brains in vats) about everything we rationally believe. But MWI requires us to believe that we are massively deluded about pretty much everything we ever thought we believed. So why shouldn't I count *that* as a violation of "logical consistency"? To say that the only bad theory is one that makes two different predictions for the same one event, is, I think, to have a way-too-open mind about theories. How exactly to sniff out quality theories is a notoriously difficult quesiton; but there must be some sort of standards that go beyond mere blatant internal contradictions. If there aren't, then it's going to be impossible, e.g., to argue against the persistent "local realism" crowd who wants to suppose weird conspiracy theory type explanations for why 17 different experiments don't *really* prove that Bell's inequalities are violated... (not to mention a lot of other *way crazier* or even way more insidious junk.)

I would then think that an MWI like approach is what comes closest to the existing formalism:

The quantum formalism has two parts: unitary evolution and measurement/collapse. When you say MWI stays close to the existing formalism, you are already making a judgment call about which parts of that existing formalism are important and which aren't. But remember the reason for the collapse postulate -- without it, QM cannot predict the experimental outcomes which are actually *observed* and on which basis we believe in QM in the first place! So it wouldn't be all that crazy to think that *that* was the more important of the two aspects of QM. Of course, that doesn't leave you with much of a theory of anything (there being nothing to apply the collapse postulate *to* if you dump wave functions evolving unitarily...). But my point is broader: looked at from a "forest" perspective, wave functions and superpositions are the *least familiar*, most strange and new and suspicious looking parts of QM. The part that says "the probability of this needle pointing left is 50%" is by contrast very grounded, very familiar, very easy to understand. So I think one perspective on what MWI is doing is to dump the most familiar/intuitive/grounded aspects of QM and raise up in importance just those aspects that are least well understood and most unfamiliar. Again, maybe there's no blatant internal contradiction involved in doing this, but it's the kind of move which wouldn't be taken very seriously in other branches of science. That is, it would only be taken seriously if/when some other more obvious, better grounded moves had first been tried and found to fail.

If you give yourself more liberty with the formalism, I guess you can arrive at Bohmian mechanics.

I'm not sure what liberty you need. Basically, if you say, "QM is a theory about particles like electrons" -- and then you take that *seriously* and start to think about how the theory might be attached to a particle ontology -- you get Bohmian mechanics. I think it's far less "liberal" to believe that QM is about particles (if one knows the historical origins of QM especially!) than to think "hey maybe all those experimental results we thought argued for QM actually didn't have outcomes at all!"

I have to say that this discussion gave me some incentive to learn more about it ; for instance, I was of the opinion that Bohmian mechanics had a serious problem with QFT ; but this may not be the case, I don't know.

There are some problems, if you want to call them that. First, there have only been a handful of people working with Bohm's theory for 50 years. So obviously it hasn't been developed *nearly* as much as the standard approach. But several versions of Bohm-type QFT's exist, and I don't think there is any serious obstacle here. The second point is that people will often say that Bohm-type QFT's and such are "obviously wrong" since they are nonlocal and hence seem to violate relativity. But this objection is very wrong-headed. Really, it's just the same old debate about nonlocality from nonrelativistic QM all over again. Yes, Bohmian theories are rather obviously nonlocal (in the Bell locality sense...) and this remains true in the context of relativistic theories (Dirac equation, or QFT). But (drumroll please) so are all the *orthodox* relativistic theories, if you take them seriously, collapse postulate and all. And if all you mean is that regular QFT is *information local*, well, so are the "obviously nonlocal" Bohmian versions. So anyway, one has to be very careful here and keep an open mind about things like extra space-time structure like preferred frames, etc.

 

[vanesch]

I believe in scientific realism, part of which is the idea that the physical world we perceive is more or less real -- there really is a world that "looks" that way. Or putting it negatively, we aren't massively deluded (e.g., brains in vats) about everything we rationally believe. But MWI requires us to believe that we are massively deluded about pretty much everything we ever thought we believed. So why shouldn't I count *that* as a violation of "logical consistency"? To say that the only bad theory is one that makes two different predictions for the same one event, is, I think, to have a way-too-open mind about theories.

I didn't say that every logically consistent theory is a good theory ! I said that a good theory needs at LEAST this, and that the rest is open to debate, taste etc... I think it is a good idea to be very open minded a priori. And about being deluded not a right thing to do: hey, I believed in Santa Claus until I was 8 years old ! So I have the habit of being deluded. Nothing special about it.

How exactly to sniff out quality theories is a notoriously difficult quesiton; but there must be some sort of standards that go beyond mere blatant internal contradictions.

But all other standards are open to debate. You'll see that I have different standards.

If there aren't, then it's going to be impossible, e.g., to argue against the persistent "local realism" crowd who wants to suppose weird conspiracy theory type explanations for why 17 different experiments don't *really* prove that Bell's inequalities are violated... (not to mention a lot of other *way crazier* or even way more insidious junk.)

I agree with you that you need other criteria ; only, it is open to debate which criteria. If you allow to impose FIRST some criteria, then the LR crowd can put forward local realism as an unviolable criterium. You're closer to the LR crowd with your viewpoint than you think! Your viewpoint is that we cannot be "deluded" ; I guess this means: A theory that EXPLAINS correctly all results, but which has an underlying ontology which seems to differ with the intuitive picture you have of it, is a bad theory ? The problem is that this is so much dependent on what you can consider as your intuition that it is a dangerous viewpoint. The LR crowd cannot accept the violation of Bell locality ; it is against their intuition too. So if those 17 experiments have to go, and QM has to go, so be it, in order to stick to their intuition. In your case, if SR has to go, then so be it, in order to stick to your intuition.

My criteria is that one should look for fundamental principles which are at the base of the formalism and then stick to it at all costs. In (relativistic) quantum theory, these are the superposition principle and lorentz invariance (from which follows the need for information locality).
All my reserves come from the fact that another great principle, namely general covariance (of general relativity) has a serious clash with the superposition principle. So I expect a serious review of the fundamental principles when we will have a quantum theory of gravity. Such a revision will then imply a fundamental revision of all one can deduce, including MWI and everything. But usually, in such a paradigm shift, things only get weirder, not "more intuitive".

The quantum formalism has two parts: unitary evolution and measurement/collapse. When you say MWI stays close to the existing formalism, you are already making a judgment call about which parts of that existing formalism are important and which aren't. But remember the reason for the collapse postulate -- without it, QM cannot predict the experimental outcomes which are actually *observed* and on which basis we believe in QM in the first place! So it wouldn't be all that crazy to think that *that* was the more important of the two aspects of QM. Of course, that doesn't leave you with much of a theory of anything (there being nothing to apply the collapse postulate *to* if you dump wave functions evolving unitarily...). But my point is broader: looked at from a "forest" perspective, wave functions and superpositions are the *least familiar*, most strange and new and suspicious looking parts of QM. The part that says "the probability of this needle pointing left is 50%" is by contrast very grounded, very familiar, very easy to understand. So I think one perspective on what MWI is doing is to dump the most familiar/intuitive/grounded aspects of QM and raise up in importance just those aspects that are least well understood and most unfamiliar. Again, maybe there's no blatant internal contradiction involved in doing this, but it's the kind of move which wouldn't be taken very seriously in other branches of science. That is, it would only be taken seriously if/when some other more obvious, better grounded moves had first been tried and found to fail.

Well, as far as I know, they have failed as long as we stick to something that looks like special relativity !

I'm not sure what liberty you need. Basically, if you say, "QM is a theory about particles like electrons" -- and then you take that *seriously* and start to think about how the theory might be attached to a particle ontology -- you get Bohmian mechanics. I think it's far less "liberal" to believe that QM is about particles (if one knows the historical origins of QM especially!) than to think "hey maybe all those experimental results we thought argued for QM actually didn't have outcomes at all!"

I'm not denying this. But to me, basically QM is NOT a theory about particles like electrons. To me, QM is the formalism that goes with the superposition principle, and together with special relativity, it gives rise to things like quantum fields, which "look sometimes like particles".

But ok, if Bohmian theory also works for QFT, I'm interested to look at it...
Let's continue in the other thread then !

cheers,
Patrick.

 

[straycat]

Born rule and MWI

Hey Patrick,

I read the very interesting discussion that you had with Travis (hey Travis! ) back in Feb, and I have a question for you regarding how the Born rule fits into the MWI.

In your journal, you wrote:

The problem is that MOST of these "yous" will accumulate experiences which are NOT in agreement with the statistics of the Born rule. It is the problem MWI proponents never could solve (although they give a lot of "plausibility arguments"), and that's because they left out explicitly the Born rule.

This, I think, is a very interesting topic. Here's one way I might flesh it out with an example. Suppose we have N identically prepared spin 1/2 particles, prepared so that a spin measurement along the x-axis will yield spin up with, say, probability = p. (So we get spin down with probability 1-p.) If we measure the spins of each of these N particles, then we end up with 2^N worlds. At the end of these N measurements, the observer plans to calculate p by simply counting the number of particles that were observed to be spin up and dividing by N. The difficulty, as you point out, is that if p is not equal to 0.5, a significant number of these worlds will contain observers who conclude that the Born rule is incorrect! In fact, as N gets larger, the percentage of worlds such that the observed p deviates from the Born-rule prediction by some arbitrary cutoff (at least I think this is true -- I haven't actually shown this rigorously) just gets larger.

iiuc, Everett addresses this situation by defining a "probability measure" to each world which is, simply enough, prescribed by the Born rule. The conceptual difficulty here is that, in keeping with the "spirit" of the MWI, each individual world should be thought of as being on an "equal footing," so to speak. So what we have here are, in a sense, two different ways of counting worlds: by one method, we simply count the number of worlds via treating each distinct experimental outcome as one distinct world, so that, in the above example, we get 2^N worlds; by the other method, we give each of these worlds a "weighting" which is proportional to its Born-prescribed probability. The former method is in keeping with the "spirit" of the MWI by putting each physically distinct world on an equal footing, but suffers from the difficulty that most observers will think the Born rule to be false. In latter method, the set of worlds in which the Born rule is violated is of measure zero (as Everett, Hartle, and others have shown); however, we are in a way breaking with the "spirit" of the MWI.

If we want to rescue Everett's notion of attaching a probability measure to each experimental outcome, one thing we could do would be to say that -- to take a single spin measurement as an example -- instead of splitting into TWO worlds, we instead split into P_tot = P_up + P_down worlds, where p = P_up/ P_tot is the Born-rule given probability of observing spin up. P_tot, P_up, and P_down would, according to this scenario, be integers. And since in general, p can take on any value between 0 and 1, p_up and p_down would typically be expected to be able to be extremely large integers! In fact we can already see a problem here: if we assume P_up and P_tot to be integers, then we are effectively saying p must be an element of the rationals (ie, it cannot be any real-valued number in [0,1].)

A second problem is that, in the simplest scenario at least, there is no PHYSICAL distinction between one observer who ended up in one of the P_up worlds, and another observer who ended up in another of the P_up worlds. Now there is no strong reason to object to this scheme -- it's just that it seems, well, ill-motivated. Why should one observer split into P different directions, when all of them that split into the P_up direction are physically identical? It seems so unfrugal. (Is that a word? )

This is about as far as I have gotten in thinking about this particular topic. So let me turn to my question for you. Earlier you wrote:

Well, I'm not a real MWI-er. What you write (get rid of the Born rule) is indeed part of the program of MWI, and I think I have found a mathematical proof that this cannot work, which I hope to publish soon.
I think that there is some real need for the Born rule. On the other hand, I want to get rid of the projection postulate, and up to there, I follow MWI.

I've been doing a little background reading on the MWI -- for instance, I managed to get a hold of Everett's original dissertation from amazon -- but I have not yet run across anyone trying to "get rid of the Born rule." What does this mean, exactly? And what is your argument that "this cannot work?" I am intrigued.

straycat

PS The book that I got from amazon has a very interesting (not published elsewhere, afaict) decades-old article by Neill Graham, one of Bryce DeWitt's former students, addressing this very topic. He lays out the basic issue very nicely, I think, although I'll admit that I cannot follow his "solution."

 

[vanesch]

iiuc, Everett addresses this situation by defining a "probability measure" to each world which is, simply enough, prescribed by the Born rule.

But there is a problem in doing so. Indeed, in order to be able to apply a Born rule, you have to choose a preferred basis (the measurement basis). It is different if you apply the Born rule for "position" or for "momentum". It is different if you apply the Born rule in the z-spin basis, or in the x-spin basis. The only thing that distinguishes the measurement basis from any other basis, from an observer's point of view, is that in the measurement basis, he appears in a Schmidt decomposition (sum of |observer state> |system "measurement" state> components) while this is not the case in other basis states. But that's of course observer-related! So you can only define that measure AFTER having choosen already a basis. Honestly, you're then just applying the projection postulate.

The conceptual difficulty here is that, in keeping with the "spirit" of the MWI, each individual world should be thought of as being on an "equal footing," so to speak. So what we have here are, in a sense, two different ways of counting worlds: by one method, we simply count the number of worlds via treating each distinct experimental outcome as one distinct world, so that, in the above example, we get 2^N worlds; by the other method, we give each of these worlds a "weighting" which is proportional to its Born-prescribed probability.

The problem with this statement (which I endorse !) is that MWI says:
i d/dt psi = H psi AND THAT'S ALL. The original program (as I understood it) was that we could then derive all physical consequences from it, also the "observed" probabilities by observers. This was in a reaction to the Projection Postulate, which has a lot of problems, namely distinguishing "measurements" from "physical processes".

So when do we assign this "measure" to different terms in the wave function ? This is bluntly introducing the Born rule again!

Let us look at an example:
Imagine that I've been measuring the z-component of an electron spin ; so now the wavefunction is:

|psi> = Sqrt[0.9] |me+> |z+> + Sqrt[0.1] |me-> |z->

In an MWI setting, the idea somehow is that "a real me" only observes ONE of these branches. So there must be some kind of splitting of "me", so that there is now a "me" that will "have measured" the plus component, and that will now work in the first branch and a "me" that will have measured the minus component and will now work in the second branch.
There are NO PROBABILITIES involved in this game. BOTH happen.
What *I* am claiming is that in order for probabilities to arise (namely that I have 90% chance to be me+ and 10% chance to be me-), you HAVE TO MAKE AN EXTRA ASSUMPTION, and that extra assumption amounts to the Born rule: I explicitly consider this a "measurement process" and assign probabilities to the outcomes according to the Born rule. The way I prefer to do it (as I tried to explain in my journal), is by saying that although there are now 2 different body states (nothing surprising, there are also 2 different electron spin states), my "consciousness" has TO CHOOSE, ACCORDING TO THE BORN MEASURE, which body state it will live in. So I put the "measurement process" into the "inhabiting of a body state by my consciousness". But then we get easily into metaphysical discussions, and I don't want to here again. Say that it is some shortcut to make the picture work.

Remember, in a true MWI view, ALL "branches" or "worlds" (simply, Schmidt decomposition terms in the wave function and a choice of a system which is the "observer") "happen" (because simply all terms are present in the wave function), and there is no a priori probability of anything. It is very difficult to assign probabilities to terms in the wavefunction without introducing (again) a distinction between a "physical process" (you don't assign probabilities) and a "measurement" (you assign probabilities). And this was the difference (measurement/process) Everett wanted to get rid off in the first place.
The only way to introduce probabilities in a natural way is by "inhabiting" these terms with observers, according to a kind of probability measure, and the whole point of MWI, as I understood it, was that this probability measure would emerge "naturally" (by counting of something, say). Clearly, simply counting the terms doesn't work, so you can think of more complicated schemes: but then you have to POSTULATE these schemes somehow, and this postulate usually comes down to the Born rule.

cheers,
patrick.

 

[ZapperZ]

Hey Patrick,

I read the very interesting discussion that you had with Travis (hey Travis! ) back in Feb, and I have a question for you regarding how the Born rule fits into the MWI.

Well it's about TIME you join in the fun, straycat! Took you long enough! :)

Zz.