# New Many Worlds objections, right/wrong ?

1. Nov 13, 2014

### Quantumental

I came across these objections in a paper about the status of Interpretations:

Why Current Interpretations of Quantum Mechanics are Deficient
arxiv.org/abs/1408.2093

In it the author raises 2 objections that he calls: The Observer energy problem and Initial entropy problem. This is way past my understanding so I am reaching out to the experts here, hoping for some interesting debate

2. Nov 14, 2014

### kith

Similar problems are also present in classical mechanics. The inital entropy problem is the question why was the initial entropy of the universe so low and the observer energy problem comes into play if we want measurements to be truly irreversible processes. For this, we need dissipation which contradicts the closed system assumption.

If someone raises these problems in the context of QM, I expect him to explain how these problems are solved in classical mechanics and where exactly the application of the solution to QM goes wrong.

Also I don't think that these problems are somehow specific to the MWI because decoherence as a measureable process is present in all interpretations.

3. Nov 14, 2014

### Staff: Mentor

Kith is correct.

There is also the so called factoring problem. Decoherence has only been demonstrated by dividing a system into what is being observed, what is doing the observation, and the environment. Theorems do not as yet exist showing its independent of that factorisation. Critiques believe the only reason it works is due to that factorisation - but as yet we don't have theorems one way or the other. Also it must be said such factorisations are used throughout physics, the assumption being it makes no difference. Simply think back to the basic physics you probably did at High School. When analysing a ball rolling down an incline, for example, no one really thinks you would get a different answer if you analysed the system differently than the ball and the incline - its simply a choice that makes things a lot easier.

Thanks
Bill

4. Nov 14, 2014

### Staff: Mentor

The following is incorrect:
'Recall that Gleason’s theorem demonstrates that any interpretation capable of reproducing the results of quantum mechanics under the Copenhagen interpretation must be contextual. That is, the result of a measurement must depend on the measuring device used to measure it. If the subsystems have properties defined without reference to a measuring device, then one has introduced non-contextuality and it is guaranteed, via Gleason’s theorem, that the interpretation disagrees with quantum mechanics in an experimentally falsifiable manner.'

Gleason's theorem is a deep interest of mine and what is said above is a garbled caricature of it. I think he knows what it really says, but should have been more careful in stating it. If you would like to see a correct explanation and proof see post 137:

What can be said 100% for sure is neither Copenhagen or the Ensemble interpretation has any issue with Gleason - nor does any interpretation I am aware of for that matter.

The real issue with most interpretations is the so called problem of outcomes - ie why do we get any outcomes at all. That is the one most interpretations, including my personal one, called ignorance ensemble, stand powerless before. With our modern understanding of decoherence it basically replaces collapse. Yet strangely for some like Bohmian Mechanics and Many Worlds its trivial. This leads to my personal view on the whole interpretation thing. It doesn't matter what issue worries you you can find an interpretation that fixes - but not all at once.

Thanks
Bill

5. Nov 18, 2014

### Ilja

I think theorem 3 of http://arxiv.org/pdf/0901.3262.pdf does the job of showing a dependence on the factorization. It proves the existence of different factorizations, with the same Hamilton operator having in all of them having the same general form $H = p_1^2 + p_2^2 + V(q_1,q_2)$ but with physically different potentials V. So, different factorizations lead to different physics, even if the Hamilton operator itself is the same (unitarily equivalent).

6. Nov 18, 2014

### Staff: Mentor

If true, that's a big issue - a VERY big issue. It means you cant trust any calculation because what you get depends on the factorisation.

The fact it hasn't caught on however doesn't inspire confidence its that big a problem. Such claims if true would be very big news. I have also seen analysis showing for some simple models factorisation doesn't matter.

I will await further developments before getting too worried - extraordinary claims require extraordinary evidence. It needs more investigation.

Still one never knows.

Thanks
Bill

Last edited: Nov 18, 2014
7. Nov 18, 2014

### Demystifier

Factorization should be a big problem only for those who take MWI very seriously (even if they do not realize it). But for all the others factorization is not really a problem, because in other interpretations of QM one can always identify a "natural" factorization.

Last edited: Nov 18, 2014
8. Nov 18, 2014

### Staff: Mentor

Cant follow that one. If it's shown any factorisation gives different results that's seems a BIG problem.

It may turn out the type of factorisations that cause issues only occur in MWI or similar - but cant follow how any factorisation is unique to MW.

If its a problem, as far as I can see, its a problem for decoherence in general.

Still its an interesting issue I think needs more work. Obviously it works for some factorisations since we have results that stand up to experimental confirmation - but exactly when does it break down - if it does.

Thanks
Bill

9. Nov 18, 2014

### Demystifier

It's a problem if you think you can use any factorization. But in other interpretations you don't use any factorization. You use the "natural" factorization, which is essentially unique.

It is not a problem for decoherence in general if, in general, you use the "natural" factorization. My point is that in all other interpretations one knows what "natural" means.

10. Nov 18, 2014

### Staff: Mentor

Ahhhh. Got it.

Yes indeed there is an unwritten natural factorisation in other interpretations eg consistent histories.

Thanks
Bill

11. Nov 18, 2014

### Ilja

Instead of "unwritten natural factorization" I would talk about a factorization defined by the actual configuration in the real world.

In the Copenhagen interpretation this would be a factorization defined by existing classical measurement devices, in Bohm-like approaches it would be one defined by the environment of the actual configuration.

The question of factorization which would be necessary for using it in the foundations is something completely different. Here the subdivision into systems should be something fundamentally predefined, no unstable particular configuration like the Earth and some measurement systems on it. Something more close to, say, a fundamental subdivision into fermionic and bosonic degrees of freedom or so.

12. Nov 18, 2014

### Quantumental

13. Nov 18, 2014

### Staff: Mentor

Closed pending moderation