Exploring the Relationship Between Schroedinger and Bohm's Quantum Mechanics

[Roberth]

You are not alone - Pat

I am apparently almost alone in this . It sounds as if most people just say "well, no information (in the usual sense) is transmitted, no energy is transmitted so everything is fine. End of story. Whereas I think that a more fundamental theory would present a more clear picture of the measurement process, of the collapse of the wavefunction, etc.

But it seems that people have got so used to the weirdness of QM that it does not elicit much desire to dig deeper.

Pat

You almost said exactly the same thing that I said at the General Physics about "Speed of Gravity".

Here is an extract:

My point is this: Quantum entanglement shows non-locality, hence there is something that is not bound by the speed of light. This 'something' causes entanglement. I believe that our known "Physics" is only a subpart of a larger structure and entanglement or the Aharanov-Bohm effect are evidence of that structure.

The fact that you cannot transmit information via entanglement is always used to 'save' locality (ie. the speed of light barrier). However, it does not matter whether you can use it to send meaningful information. The fact remains that there is an action that has a physical effect which acts faster than the speed of light.

Roberth

 

[selfAdjoint]

No I keep saying that in orthodox QM there is no signal between the particles, non3e at all, let alone an FTL one. In QM the correlation is caused by the extended entangled state, which does not exist in spacetime, and so does not have anything to do with relativity or "sending". What the state "is" truly is of course problematical, but that doesn't affect the truth of this statement.

 

[Roberth]

But this is my point

No I keep saying that in orthodox QM there is no signal between the particles, non3e at all, let alone an FTL one. In QM the correlation is caused by the extended entangled state, which does not exist in spacetime, and so does not have anything to do with relativity or "sending". What the state "is" truly is of course problematical, but that doesn't affect the truth of this statement.

I am not saying that there is a 'signal' as such in the conventional way. Your comment about spacetime is exactly what I meant in this other post.

Most likely, there is a higher dimensional (5-dimensional or higher) space above Minkowsky space where symmetry considerations are observed and they act instantly. Minkowsky space is then a sub-space of this higher dimensional 'order'.

You can think of it as something like a Hawking´s wormhole, if you like (although I do not believe that it is a wormhole). The 'information of the entanglement' does not go through our spacetime but ' cuts' through it.

Now, how could one start with a more 'flesh on the bone' theory? A candidate would be a group theoretical approach and see if there is a symmetry that must be observed in order to explain the result of the spin entanglement. The problem with that is, however, that group theory is also an epistemological and not ontological approach, similarly to the major parts of QM.

Bohm at least tried to acknowledge that there is something, ie. his quantum potential. This could be in the right direction but must probably be explained from an upper-dimensional level to forecast new physics.

Roberth

 

[nrqed]

Hi Pat,

What you write here bothered me also quite a while, until I realized that collapse of the wavefunction does make sense as long as it is an observer-bounded concept. This is a viewpoint which is somewhat intermediate in between the standard interpretation and MWI. Indeed, thanks to decoherence, you cannot distinguish between a measurement that gives rise to a true collapse, and one that just "decoheres".
So if you have two entangled particles, A and B, and you have two observers, P and Q, P which observes A and Q which observes B (and we assume these observation interactions to be spacelike separated - you write everywhere timelike but I suppose you mean spacelike),

Yes, sorry about that. For some reason I was thinking "spacelike" and typed timelike throughout the post

let us then take the point of view of P.
When P observes A, this is a true observation for P. But at that point, he doesn't know anything about B or Q, so it doesn't make sense for P to talk about a collapse of the state at Q. When Q travels to P to tell him the result of his measurement, P just considers this message from Q as another measurement (P makes a measurement on Q). All this is completely local at P, and his successive measurements (with collapse) make completely sense.
Q can do exactly the same, but then of course we have different quantum discriptions according to the observers.

Very interesting, Patrick. That's the kind of ideas that Iwould like to see people discussing more, instead of just saying "well, no energy is transferred and the setup can't be used to transmit the results of a baseball game faster than the speed of light so there is no problem. End of story". I find it hard to understand how anybody could feel satisfied with the present status of QM and SR in light of Bell type experiments. Anyway, sorry for the rant...

You idea is a very very interesting one. But it brings up tricky issues...See below.

I didn't work this out, maybe one can think of a propagating collapse wavefront going out from each observer ; or one considers that there is only one true observer in the universe, namely me :-)

cheers,
Patrick.

hehehe...

Patrick added, in a later post:

What I'm saying is that from P's point of view, Q doesn't make a measurement. It is only P who makes two measurements: first on particle A and second on "pseudoobserver Q" which remains itself in a decohered superposition until P (the only true observer in the universe) observes Q. P can only start to observe Q's results (Q's entanglement with B) when Q is in the past lightcone of P after the entanglement took place at Q.

So my point is that there is not necessarily a collapse at Q when P makes his first measurement. It is only when Q is in the past lightcone of P after the entanglement with B that potentially P can observe Q's results and hence that there must be a projection.

You can ask: and what about Q ? Well, Q is a different observer, and hence lives in a different quantum observer world. So he can observe completely different things, P can never find out. P can only make measurements on Q, and then P's measurements will be coherent with other measurements P made in his history record. This is the same issue as how different people perceive the color blue. When presented with something blue, both say that it is blue because told so since they were a child. But you'll never find out if what you perceive as "blue" isn't perceived as "orange" by the other person.

As I said before, this is very strange to me too! But it is the only way I found to have peace of mind with SR and QM, the way they are formulated.

Ok, but here's a question: what is "observer Q" is a piece a paper on which the results of a polarization measurement are printed out. No human consciousness is involved there. So when P (who is human, let's say!) receives paper Q, what happens to the results printed on the paper? Would you say that the numbers on the paper are not well-defined before P reads it? It almost starts to sound like the question: if there is nobody in the forest, does a falling tree make any noise...

In any case, at least I appreciate the fact that you are struggling with these issues and trying to make sense of them, which seems to be th eexception rather than rule in the physics community.


Regards

Pat

 

[vanesch]

Would you say that the numbers on the paper are not well-defined before P reads it? It almost starts to sound like the question: if there is nobody in the forest, does a falling tree make any noise...

Well, if you take unitary evolution literally, such as MWI proponents do, then your piece of paper (and everything it potentially interacted with, so the whole universe within it's past lightcone) is in a superposition in exactly the same way as the original system was ; the "measurement" at Q is nothing else but an entanglement:

Piece of paper state |empty> (there exists 2 other states |+> and |->, when we've written respectively "+" and "-" on the paper)
System: |s0> = a |spin up> + b |spin down>

"measurement hamiltonian" gives rise to:
|spin up> x |empty> ----> |spin up> x |+>
|spin down> x |empty> ---> |spin down> x |->

so: |s0> x |empty> ----> a |spin up> x |+> + b |spin down> x |->

So the state of the paper simply entangled with the spin state, and it is only P who did the measurement with collapse:
probability |a|^2 to find a + sign and probability |b|^2 to find a - sign, and at this point, you can also say that the spin state is determined: because in the first case, the state is |spin up> x |+> ; remember that this collapse only happens when the paper arrives at P, so causality for the collapse of the (past) spin state at Q is preserved.

cheers,
Patrick.

 

[Rothiemurchus]

Vanesch:
When presented with something blue, both say that it is blue because told so since they were a child. But you'll never find out if what you perceive as "blue" isn't perceived as "orange" by the other person.

Rothie M:
Unless one day it is shown that perceiving a particular colour in the brain uses quanta of energy and that everyone's brain gets stimulated to use the same number of quanta.