Quantum Entanglement and time travel

[Demystifier]

Vanesch, I was thinking about the MWI and I concluded that a version of MWI that seems reasonable to me is the version which I will call
"MANY-WORLD SINGLE-MIND" interpretation!

What is this? According to this, the wave function never collapses and there are many coexisting branches of the wave function (MANY "WORLDS"). However, not all these branches enjoy the same rights. One (and only one) of them is picked up randomly as the one which corresponds to the reality percieved. There are no "paralel universes" in which other realities are realized. Only one reality, one branch, is the "right" one. In particular, this branch is subjectively perceived by our minds, but this branch has a preferred role even without our minds, so the theory is self-consistent even without a theory of mind or consciousness. It is not clear what the "mind" is, but at least it is clear that there is only one mind for each person (SINGLE MIND), not many copies with different hystories in different parallel universes.

It can also be compared with classical physics described by a deterministic equation of motion. The equation of motion containes many possible "worlds" corresponding to many solutions. They can be viewed as different "branches" of the equation of motion. But only one solution is picked up as the right one. There is no theory that predicts which solution (or which initial condition) is the right one, so, effectively, the right solution is picked up randomly.

Now in quantum mechanics we have two levels of the unpredictable. One is the choice of the right solution of the Schrodinger equation. The other is the choice of the right branch of that solution.

What do you think about such a version of MWI?
(You will probably note that this version combines some desirable features of MWI and BM and eliminates some undesirable ones.)

 

[vanesch]

What do you think about such a version of MWI?
(You will probably note that this version combines some desirable features of MWI and BM and eliminates some undesirable ones.)

It is non-local, unfortunately. The "synchronisation" of the different minds associated with physical structures (bodies) at space-like intervals, would require us to do something non-local (such as projection in Copenhagen, or the particle guide equation in BM).

 

[Demystifier]

It is non-local, unfortunately. The "synchronisation" of the different minds associated with physical structures (bodies) at space-like intervals, would require us to do something non-local (such as projection in Copenhagen, or the particle guide equation in BM).

I seems to me that it is not more nonlocal than the usual original (but not yours) form of MWI. This is probably the main motivation for a relational version of MWI introduced by Rovelli and advocated by you. Am I right?

But note that I was inspired by your note on the analogy between the block-time interpretation of time and MWI. In particular, the block-time interpretation combined with the self-consistency principle (which, in fact, is nothing but a tautology) automatically resolves the time-travel paradoxes, but it also requires a sort of nonlocality in order to prevent initial conditions that do not lead to globally self-consistent solutions. What do you think about that? (Recall also that this is actually a topic on time travel.)

 

[vanesch]

I seems to me that it is not more nonlocal than the usual original (but not yours) form of MWI. This is probably the main motivation for a relational version of MWI introduced by Rovelli and advocated by you. Am I right?

I don't know what other versions you are thinking about. I'm not aware of any MWI version where there is a "single world" which is shared by remote observers. A "world" is an observer-dependent concept, and the "slicing up of the wavefunction in different worlds" is also an observer-dependent concept, in all MWI versions I know of.

But note that I was inspired by your note on the analogy between the block-time interpretation of time and MWI. In particular, the block-time interpretation combined with the self-consistency principle (which, in fact, is nothing but a tautology) automatically resolves the time-travel paradoxes, but it also requires a sort of nonlocality in order to prevent initial conditions that do not lead to globally self-consistent solutions. What do you think about that? (Recall also that this is actually a topic on time travel.)

I'm affraid I don't see what you're aiming at...
The analogy between block-time and MWI I had in mind was the following:
in both cases, we seem to be subjectively experience something else but the "whole". In MWI, we don't seem to experience subjectively the entire wavefunction (with all its alternatives, in the form of superpositions of classical situations, which we call "worlds"), and in block-time, we seem subjectively to experience only "now" and not the entire "time dimension". But in both cases, the relationships between observations/events (in other words, the physics) are all right. In MWI, in each branch, you have a consistent set of observations (the Alice that saw "up" will be entangled with the "up" state of the particle etc...), and in "block time" you have the correct temporal relationships (between event A and event B, the two twins have observed a certain number of rotations of the hands of their clocks, and that number is exactly what is expected etc...)

So, true, in both cases there seems to be a fundamental difficulty in explaining our subjective experience (which makes some people reject the ideas of MWI or of block time), but in both cases all relational aspects between observations are ok. In both cases, the proposed "ontology" fits close to the formalism based upon the fundamental ideas of the theory (superposition and unitary evolution for MWI, 4-dim spacetime manifold for block time).

 

[Demystifier]

I don't know what other versions you are thinking about. I'm not aware of any MWI version where there is a "single world" which is shared by remote observers. A "world" is an observer-dependent concept, and the "slicing up of the wavefunction in different worlds" is also an observer-dependent concept, in all MWI versions I know of.

Then I probably misunderstood some versions of MWI. I thought that, in some versions, observers are irrelevant, while it is the whole universe itself that splits in many branches. Since the universe is a nonlocal object, such spliting would necessarily be nonlocal.

 

[vanesch]

Then I probably misunderstood some versions of MWI. I thought that, in some versions, observers are irrelevant, while it is the whole universe itself that splits in many branches. Since the universe is a nonlocal object, such spliting would necessarily be nonlocal.

This is indeed an often encountered misunderstanding of MWI. If it were the case, as you point out, MWI would have no particular advantage over Copenhagen "projection" because the magical "split" would be just as unexplained and mysterious as the "projection by observation" - and would imply just as non-local a happening. But it is an often-encountered misunderstanding, leading also to an often-encountered objection to MWI, which is that each time there is an entanglement of two electrons on Andromeda, my worlds would "split".

Branching only makes sense wrt an observer.

 

[Demystifier]

This is indeed an often encountered misunderstanding of MWI. If it were the case, as you point out, MWI would have no particular advantage over Copenhagen "projection" because the magical "split" would be just as unexplained and mysterious as the "projection by observation" - and would imply just as non-local a happening. But it is an often-encountered misunderstanding, leading also to an often-encountered objection to MWI, which is that each time there is an entanglement of two electrons on Andromeda, my worlds would "split".

Branching only makes sense wrt an observer.

I think that MWI people are responsible for that misunderstanding. They should call their interpretation MOI (Many Observer Interpretion) or something like that.

 

[vanesch]

I think that MWI people are responsible for that misunderstanding. They should call their interpretation MOI (Many Observer Interpretion) or something like that.

The original name (by Everett) was much better: relative state interpretation.

 

[tehno]

I wonder guys how you never get tired of such topics (like "time travel","exceeeding the speed of light" etc)?

-tehno

 

[Demystifier]

I wonder guys how you never get tired of such topics (like "time travel","exceeeding the speed of light" etc)?

Everyone has something that never gets him/her tired. We have "time travel", "exceeeding the speed of light", and stuff like that. And you?

 

[tehno]

I enjoy watching series of "Star trek" .

 

[vanesch]

I wonder guys how you never get tired of such topics (like "time travel","exceeeding the speed of light" etc)?

If this question really needs to be answered, I'll give you my PoV. We have accumulated a certain amount of knowledge, by past experiences, by our own personal experience and so on, and from this knowledge results some model of reality. In fact, according to different categories of experiences, we extract different models of reality: we have our "common sense everyday" model of reality (I know my home, my car, my family, some animals, some weather, ... which is a certain model of reality I have set up, since I was a child, of "the world") ; scientific experiments have lead to some scientific theories which also give us some (mathematical / formal) model of reality. Now some of these models are highly counter-intuitive (in other words, don't seem to conform to our common sense model of reality we have set up based upon personal experience). They are, nevertheless, just as good (or even better) models of reality as are our "naive" views from everyday life. If you take these models completely seriously, then you arrive at sometimes very strange predictions/concepts. One of it is time travel. Of course, it is an extrapolation, far beyond the data that were gathered in order to set up the mathematical model. Nevertheless, it is an intriguing possibility. So it is fun to explore it, on the purely theoretical side.

 

[drphysica]

This is reply 4 vincentm:
I would have to agree with most physicist who say that time travel is impossible. But you have to be careful when you say anything like that because time is actually a relative concept. However there are some mathematical prediction of time travel but it bring a lot of questions with paradoxes that we cannot comprehend. The thing is if "time travel" as you imagine is not the same as what I have in mind. The time travel as what I understand is a possible action however there is no such thing as paradoxes of going back in my past and killing my grandfather because what most of us call time travel is actually a travel to other space-time continuum all together. By warping time we actually travel to different universe and the funny thing is we do it all the time just cannot tell the difference. If anyone familiar with atomic clock experiment when one set of clocks put on a plane and taken around the world for a trip, when plane returns and the clocks compared it is apparent that time was warped by few nanoseconds during the plane trip and hence it an evident that time travel is indeed possible. Just now we arrived to the universe which is almost identical copy of the previous one. So you see it is possible with no paradoxes but it is not a time travel as you might think. :)

 

[vincentm]

Backwards time travel might be problematic for other reasons, but it's definitely allowed in GR (though a theory of quantum gravity may change this), and your arguments for why it's nonsensical don't work, for the reasons I tried to explain above.

So one can travel backwards in time via relativity. Sorry to ressurect this thread but i didn't understand most of the discussion here, perhaps a more laymen's type terminology is best for the purposes of my understanding.

 

[rewebster]

Quote Brian Greene :
"Despite years of debate, scientists still haven't completely ruled out the possibility of going back in time. "Many physicists have a gut feeling that time travel to the past is not possible," said Columbia University theoretical physicist Brian Greene. "But many of us, including me, are impressed that nobody's been able to prove that."


To me, this sounds like political double-speak as not to offend anyone


He could as well have been talking about the existence of unicorns.

 

[MattE]

I'm not too sure the question really was ever answered. To put it fairly simple, we don't really know. The idea of why it is believed to violate special relativity is that entanglement means that objects that are interacting will react at the same time when something happens to one of the objects no matter how far away the two objects are. But this would mean that some sort of signal would have to be interacting with both objects and would be moving faster than the speed of light. This is where the paradox comes from, if the reaction happens instantly than the signal is moving faster than the speed of light and violated special relativity, unless the signal moves backwards through time.

 

[Demystifier]

Motions/signals faster than light do not violate special relativity.
Motions/signals backwards in time are not forbidden.
To understand why, see:
http://xxx.lanl.gov/abs/1002.3226
http://xxx.lanl.gov/abs/1006.1986