Why isn't Manyworlds obviously correct?

[bhobba]

Why would decoherence suddenly "end" just because a system gets large enough?

It doesn't - it simply happens so quick you never notice it.

Thanks
Bill

 

[bhobba]

I don't understand why you equate decoherence and manyworlds!

Indeed.

Many interpretations such as Consistent Histories and Ignorance ensemble use it.

Thanks
Bill

 

[GofG]

It does *not* tell you that there is a 50-50 chance of a given photon going left or right. That requires an extra postulate.

Yes: that postulate is the Born probabilities, which state that the probability of experiencing a given brain is equal to the squared modulus of the thickness/density of that brain's amplitude blob. I have already admitted to this postulate.

But what is built-in to Schrödinger's equation is the fact that each brain is just as real as each other brain; that is, from within Schrödinger's equation, there is no mechanism for collapsing down into one brain.

Schrödinger's equation gives me the density/thickness of a particular blob, and the Born probabilities tell me how to turn that density/thickness into a subjective anticipation/probability.

I tend to be a Bayesian too (and I agree with your description of how a Bayesian would use QM to derive anticipations about future observations), but that doesn't mean Bayesians get to dictate all uses of the word "probability". As that word is used in the phrase "Born probabilities", it refers to predicted relative frequencies, not epistemic uncertainty on the part of observers. That's because comparing predicted relative frequences with actually observed relative frequencies is how we experimentally test QM.

But my epistemic uncertainty reflects exactly the predicted relative frequencies; it's a 1:1 mapping, they're isomorphic. I am translating the predicted frequencies, as given by Schrödinger and Born, into subjective anticipations, as given by Bayes. I don't think there's anything wrong with this?

 

[DrChinese]

What do you mean by this? Can you give me an example in terms of, say, polarized photons?

Sure, check this article:

http://arxiv.org/abs/quant-ph/0201134

Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping
Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, Anton Zeilinger
(Submitted on 29 Jan 2002)

"Quantum teleportation strikingly underlines the peculiar features of the quantum world. We present an experimental proof of its quantum nature, teleporting an entangled photon with such high quality that the nonlocal quantum correlations with its original partner photon are preserved. This procedure is also known as entanglement swapping. The nonlocality is confirmed by observing a violation of Bell's inequality by 4.5 standard deviations. Thus, by demonstrating quantum nonlocality for photons that never interacted our results directly confirm the quantum nature of teleportation. "

And note on page 5: "Therefore, this result indicate that the time ordering of the detection
events has no influence..." Alice (in this case acting after Bob has seen his results) is in a world and decides to entangle the 2 particles via entanglement swapping using her Bell State Analyzer (BSA). How does that world know to connect the outcomes of these 2 independent (at this point) photons so as to make sense of what Bob already saw? Keep in mind that the angle setting of her BSA need have no relationship to the angles Bob is measuring at, and could be at a 45 degree angle relative to Bob's. So presumably collapse for the 2 photons has already occurred when they arrive at Alice's BSA. Why should anything related to entanglement of Bob's photons even occur?

 

[GofG]

It doesn't - it simply happens so quick you never notice it.

Thanks
Bill

But if the decoherence happens, if it doesn't end, then there are two brains, both experiencing different things. One experiences the photon going left, one experiences the photon going right. Both of those brains are real, both of those brains are actually experiencing the events. That's manyworlds.

 

[GofG]

Sure, check this article:

http://arxiv.org/abs/quant-ph/0201134

Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping
Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, Anton Zeilinger
(Submitted on 29 Jan 2002)

"Quantum teleportation strikingly underlines the peculiar features of the quantum world. We present an experimental proof of its quantum nature, teleporting an entangled photon with such high quality that the nonlocal quantum correlations with its original partner photon are preserved. This procedure is also known as entanglement swapping. The nonlocality is confirmed by observing a violation of Bell's inequality by 4.5 standard deviations. Thus, by demonstrating quantum nonlocality for photons that never interacted our results directly confirm the quantum nature of teleportation. "

And note on page 5: "Therefore, this result indicate that the time ordering of the detection
events has no influence..." Alice (in this case acting after Bob has seen his results) is in a world and decides to entangle the 2 particles via entanglement swapping using her Bell State Analyzer (BSA). How does that world know to connect the outcomes of these 2 independent (at this point) photons so as to make sense of what Bob already saw? Keep in mind that the angle setting of her BSA need have no relationship to the angles Bob is measuring at, and could be at a 45 degree angle relative to Bob's. So presumably collapse for the 2 photons has already occurred when they arrive at Alice's BSA. Why should anything related to entanglement of Bob's photons even occur?

I'd like to see replication, and in particular I'd like to see a replication where states 0 and 4 aren't thrown away. I suspect that when Alice communicates to Victor her bell-state measurement (and, therefore, entangles herself with Victor's self in the same causal slice of the wavefunction), that produces the supposed violation of Bell's theorem. If you imagine her sending various results, depending on what blob of amplitude she is, and then entangling that particular blob with Victor's blob... I'm not sure, I need to do some diagrams to think it through.

I'll see about replicating this in my uni's lab...

 

[bhobba]

However, none of the other interpretations imply the Born probabilities either.

Hmmmm.

You might like to rethink that:
https://www.physicsforums.com/showthread.php?t=758125

What implies the Born rule is non-contextuality so you can apply Gleason.

The many world guys think they can derive it from a betting argument. I have been through it in Wallace's book on Many Worlds and, IMHO, there is an assumption of non contextuality involved.

Thanks
Bill

 

[bhobba]

But if the decoherence happens, if it doesn't end, then there are two brains, both experiencing different things. One experiences the photon going left, one experiences the photon going right. Both of those brains are real, both of those brains are actually experiencing the events. That's manyworlds.

No.

What happens is decoherence converts a superposition into an improper mixed state.

There is no way to tell the difference between a proper and an improper one, so there is no way to tell if collapse actually occurred or not.

MW, while very elegant is not without issues eg the wavefunction is exponentially diluted at an alarming rate.

Thanks
Bill

 

[PeterDonis]

I have already admitted to this postulate.

Then you've admitted that QM, as we actually apply it, is not "just the MWI".

from within Schrödinger's equation, there is no mechanism for collapsing down into one brain.

Agreed.

my epistemic uncertainty reflects exactly the predicted relative frequencies

No, it doesn't. It reflects the predicted relative frequencies *in certain particular worlds*. For example, if we run the photon experiment ten times, the relative frequency of the photon going left is only 50% in some worlds. There are others where it's quite different. There is one, as I pointed out, where it's 100%--all the photons go left. There is another where it's 0%--all the photons go right. Your epistemic uncertainty only reflects the predicted relative frequency if you end up in the right world.

 

[PeterDonis]

There is no way to tell the difference between a proper and an improper one, so there is no way to tell if collapse actually occurred or not.

Is this true in principle, or only true FAPP ("for all practical purposes", as Bell put it)? AFAIK it's the latter; in principle, one could concoct an experiment which would distinguish the two.

 

[bhobba]

Is this true in principle, or only true FAPP ("for all practical purposes", as Bell put it)? AFAIK it's the latter; in principle, one could concoct an experiment which would distinguish the two.

There is no way in principle, or practice, to tell the difference between a proper and an improper mixed state. If its a proper one then collapse would have occurred, but since you can't tell the difference you can assume it has. That's why its called apparent collapse - which in this context means FAPP.

The OP is correct about one thing - collapse has not gone away - its still there hidden. But that doesn't mean all the possible outcomes must still exist as in MW. You simply need to assume - somehow - it's a proper one. That's a problem.

But as I pointed out MW has its own problems.

Like all interpretations its merely what problems you like best.

Thanks
Bill

 

[GofG]

No, it doesn't. It reflects the predicted relative frequencies *in certain particular worlds*. For example, if we run the photon experiment ten times, the relative frequency of the photon going left is only 50% in some worlds. There are others where it's quite different. There is one, as I pointed out, where it's 100%--all the photons go left. There is another where it's 0%--all the photons go right. Your epistemic uncertainty only reflects the predicted relative frequency if you end up in the right world.

I'm totaling up all of the worlds and counting those frequencies.

 

[DrChinese]

I'd like to see replication, and in particular I'd like to see a replication where states 0 and 4 aren't thrown away. I suspect that when Alice communicates to Victor her bell-state measurement (and, therefore, entangles herself with Victor's self in the same causal slice of the wavefunction), that produces the supposed violation of Bell's theorem. If you imagine her sending various results, depending on what blob of amplitude she is, and then entangling that particular blob with Victor's blob... I'm not sure, I need to do some diagrams to think it through.

I'll see about replicating this in my uni's lab...

Since it's a top team, I would expect results to be replicated. There are other teams who have done much the same, though not exactly the "after" variation shown in the cited article. For example (which also stretches the MWI scenario):

http://arxiv.org/abs/1209.4191

Entanglement Between Photons that have Never Coexisted
E. Megidish, A. Halevy, T. Shacham, T. Dvir, L. Dovrat, H. S. Eisenberg
(Submitted on 19 Sep 2012)

"The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving, can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the non-locality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spatial separation, but also with temporal separation. Here we demonstrate these principles by generating and fully characterizing an entangled pair of photons that never coexisted. Using entanglement swapping between two temporally separated photon pairs we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime."

 

[atyy]

@GofG, which version of many-worlds would you like to discuss? There are several versions of many-worlds, eg. the Vaidman approach is different from the Deutsch-Wallace approach in many details. From your posts, I think you are implicitly using the Deutsch-Wallace version, which is an approach I do like a lot, even if I don't understand it well enough to know whether it really solves all the problems. Some experts do, eg. Matt Leifer http://arxiv.org/abs/1311.0857 writes "Crucially, this sort of argument can only really be made to work if there is an objectively existing external reality. There needs to be some sort of "quantum stuff" such that events that are always assigned the same probability correspond to physically equivalent states of this stuff. In the context of the many-worlds interpretation, the Deutsch-Wallace [18, 19, 20, 21] and Zurek [31, 32] derivations of the Born rule are arguments of this type, where the quantum stuff is simply the wavefunction.".

You are entitled to say that the Deustch-Wallace version is obvious, since I guess the solution to the Poincare conjecture might also be considered obvious to some people. Anyway, why don't you tell us which version of many-worlds you are using so the discussion can be more focused? If you can give a reference, that would be helpful.

 

[PeterDonis]

I'm totaling up all of the worlds and counting those frequencies.

You can't count those frequencies, because you can't observe how many worlds there are or what their relative weights are. You can only observe one world.

 

[Quantumental]

A lot of good responses here.
I would like to see you explain how Many Worlds "obviously explains" the factorization/basis problem: http://arxiv.org/abs/1210.8447

 

[Demystifier]

First, Schrödinger's Cat:

Photon emitter, pointed at a half-silvered mirror, such that 50% of the wave hits a sensor that activates a gaseous poison emitter inside a box that has a cat in it.

Standardly, we think about it this way: the photon hits the halfsilvered mirror and decoheres into a superposition of "straight" and "reflected", then the sensor decoheres into a superposition of "hit" or "not hit", then the poison emitter decoheres into a superposition of "activated" or "not activated", then the cat decoheres into a superposition of "dead" or "alive"... so the cat is both dead and alive, etc.

Isn't it obvious that upon opening the box, the human decoheres into a superposition of "sees an alive cat" and "sees a dead cat"?

Isn't that blindingly obvious? If the cat can decohere, why can't the human?

And that's all Manyworlds is, is assuming that decoherence doesn't suddenly "stop" at macroscopic levels.

Can someone explain to me why this isn't selfevident?

If Schrödinger equation is correct, then there is no doubt than many branches of the wave function simultaneously exist. However, it does not yet mean that many WORLDS exist. The difference is that many-world interpretation takes one additional assumption: It assumes that wave function is not merely a computational tool, but a real material object. This assumption certainly has some merits, but is not obvious.

 

[Demystifier]

A lot of good responses here.
I would like to see you explain how Many Worlds "obviously explains" the factorization/basis problem: http://arxiv.org/abs/1210.8447

It does not.