Smolin: Realistic and anti-realistic interpretations of QM

[A. Neumaier]

you are carefully seperating out two aspects as two idealizeable systems

But it is well-known that these are two idealizations; general (possibly dissipative) quantum mechanics is governed by other equations, those of Lindblad type!

isn't the universe as a whole is an isolated system?

Yes; strictly speaking, it is the only isolated system containing us!

QM - i.e. unitary evolution and the Born rule - as a mathematical model is as inconsistent as it gets

Not more than conservative and dissipative differential equations. But there is no inconsistency since they model different aspects of a dynamical system.

Smolin (or more accurately his book) is my source for the argument of quantum Poincaré recurrence.

No matter whose argument it is, it is meaningless for the real universe. Anything displaying macroscopic motion, be it an electric current in a wire, the Moon orbiting the Earth, light coming from a distant star, or the universe as a whole, requires a Hamiltonian with a partly continuous spectrum.

 

[bhobba]

This is anti-realist in the sense above because your interpretation does not treat the observer as a physical system subject to the same mathematical treatment/physical theory as the world he observes, eg your observer is not a factor of a tensor product Hilbert space.

That issue is gradually being resolved:
https://www.sciencenews.org/blog/context/gell-mann-hartle-spin-quantum-narrative-about-reality
You will also find people, some that post here, (I or Vanhees are not amongst them) that think QM may not even involve the common-sense concept of 'reality'. It is actually a difficult thing to pin down.

Thanks
Bill

 

[Auto-Didact]

But it is well-known that these are two idealizations; general (possibly dissipative) quantum mechanics is governed by other equations, those of Lindblad type!

That's not the point in speaking about idealization; instead the point is whether the separate solutions for these idealizations carry over into the practical unideal case where they are not treated seperately.

Not more than conservative and dissipative differential equations. But there is no inconsistency since they model different aspects of a dynamical system.

Not so much different aspects of different systems, but different 'phases' of the same system, namely the 'being measured'-phase and the 'not being measured'-phase.

 

[A. Neumaier]

into the practical unideal case where they are not treated separately.

In practical cases they are always treated separately; people in their right mind never apply both. They know when to apply which idealization and when neither works. Confounding the two cases is a sure sign of insufficient understanding.

The measurement problem is not about the conflict of the two idealizations but about how to derive the rule for handling a subsystem measured inside an isolated system. If everything is known about the isolated system, everything about the subsystem should follow. (This is one of the key points in my critique of the wave function as basic object in QM, given in my recent paper Part I.) Hence such a derivation should exist. In the thermal interpretation it does.

 

[Auto-Didact]

In practical cases they are always treated separately; people in their right mind never apply both. They know when to apply which idealization and when neither works. Confounding the two cases is a sure sign of insufficient understanding.

You misunderstand; the point is not to apply both simultaneously, but to give a single mathematical expression which can describe both sequentially, using a single mathematical concept i.e. a single branch from pure mathematics. Everything else - especially seperating and treating each using idealizations i.e. approximative schemes - is just fluff.

 

[A. Neumaier]

You misunderstand; the point is not to apply both simultaneously, but to give a single mathematical expression which can describe both sequentially,

Why should that be needed? No subsystem of the universe is isolated, hence the latter need not be described by precisely the same mathematical concept as the former.

using a single mathematical concept i.e. a single branch from pure mathematics.

The piecewise deterministic processes (PDP) of Breuer and Petruccione discussed in Subsection 5.1 of Part III of my series of papers do precisely that.

 

[Auto-Didact]

Why should that be needed?

Because all of physics and all of applied mathematics have ultimately been capable of giving such descriptions, QM being the sole exception so far; to give up on this is to give up on the original goal of mathematical physics.

As I posted before in #89, during much of the 18th and 19th century mathematicians and physicists struggled with a similar problem in fluid mechanics, which was ultimately resolved when boundary layers were discovered; dynamical collapse models are the analogue of this for QM.

The piecewise deterministic processes (PDP) of Breuer and Petruccione discussed in Subsection 5.1 of Part III of my series of papers do precisely that.

I'm looking forward to that part. But first things first: Game of Thrones!

 

[samalkhaiat]

Again, just because you don't find fundamental physics important,

How very charming of you! Fundamental physics is not important to me?? Mister, it is very important to me because I make my living doing fundamental physics. Fundamental physics (as opposed to philosophical gibberish) is a mathematical structure with testable predictions. If you can’t translate your sentences to meaning-full mathematical statements (which I’m sure you can’t), then what you say is just philosophical gibberish having nothing to do with fundamental physics (my job).

 

[samalkhaiat]

Well, I think you are right. I shouldn't waste my time anymore to discuss philosophical issues in this forum. It's kind of fighting against religious beliefs rather than having a constructive scientific discussion.

It is even worst. When there is only one philosopher, he throws at you terms such as Platonist and Realist. If you bring in another one, you have it: modern realist; post-modern realist; neo realist; “far-right” realist; etc. And with few more philosophers, the realist spectrum may become the entire real line $\mathbb{R}$. As long as they keep their “classification” for themselves, I am not bothered because what they say about “reality” is not physics.

I (as a physicist) don’t require that a theory correspond to “reality” because I don’t know what reality is. Stephen Hawking once said: “Reality is not a quality you can test with litmus paper”. A physical theory is nothing but a mathematical structure with predictive power. So, it is meaningless to ask whether it corresponds to “reality”. All that we can ask is that its predictions should be in agreement with the experimental results. QM does this extremely good.

“Roger is worried about Schrodinger’s poor cat. Such a thought experiment would not be politically correct nowadays. Roger is concerned because a density matrix that has $| \mbox{cat alive}\rangle$ and $|\mbox{cat dead} \rangle$ with equal probabilities also has $|\mbox{cat alive} \rangle + |\mbox{cat dead}\rangle$ and $|\mbox{cat alive}\rangle – |\mbox{cat dead}\rangle$ with equal probabilities. So why do we observe either $\mbox{cat alive}$ or $\mbox{cat dead}$? Why don’t we observe either $\mbox{cat alive} + \mbox{cat dead}$ or $\mbox{cat alive} - \mbox{cat dead}$? What is it that picks the alive and dead axes for our observations rather than alive + dead and alive - dead. The first point I would make is that one gets this ambiguity in the eigenstates of the density matrix only when the eigenvalues are exactly equal. If the probabilities of being alive or dead were slightly different, there would be no ambiguity in the eigenstates. One basis would be distinguished by being eigenvectors of the density matrix. So why does nature choose to make the density matrix diagonal in the alive/dead basis rather than in the alive + dead / alive – dead basis? The answer is that the $|\mbox{cat alive}\rangle$ and $|\mbox{cat dead}\rangle$ states differ on a macroscopic level by things like the position of the bullet or the wound on the cat. When you trace out over the things you don’t observe, like the disturbance in the air molecules, the matrix element of any observable between $|\mbox{cat alive}\rangle$ and $|\mbox{cat dead}\rangle$ states average out to zero. This is why one observes the cat either dead or alive and not a linear combination of the two. This is just ordinary quantum mechanics. One doesn’t need a new theory of measurement, and one certainly doesn’t need quantum gravity”
S. Hawking in “The Nature of Space and Time”.

 

[stevendaryl]

I don't like to have discussions with people who start insulting those who don't agree with them. If someone doesn't understand things in the same way, then they are speaking gibberish. I have found this all too common in Physics Forums. I was almost at the point of dropping out of the Forums because of this.

 

[PeterDonis]

Thread closed for moderation.