Clarification of the postulates of QM

[vanhees71]

Sure, I can make an assumption about the state of my local environment or parts of it (like a piece of metal), and that's well testable by observation. What's also testable in cosmology are the observations in my "space-time neighborhood", including the redshift of far-distant objects to determine the Hubble diagram with better and better accuracy (assuming of course certain laws on the luminosity of the objects to determine the distance). But here I just probe a very coarse grained classical picture of the universe, and that's sufficient to describe the observables. But this is far from having a description of the "state of the entire universe" and in fact not involving any quantum theory at all (it's just GR and a very crude model for the "matter" described as an ideal fluid). The same is true for the piece of metal, I can describe by some very coarse grained macroscopic (thermodynamic) observables like temperature.

 

[A. Neumaier]

But this is far from having a description of the "state of the entire universe" and in fact not involving any quantum theory at all (it's just GR and a very crude model for the "matter" described as an ideal fluid). The same is true for the piece of metal, I can describe it by some very coarse grained macroscopic (thermodynamic) observables like temperature.

Yes, the whole universe and a piece of metal are completely analogous in this respect.

All descriptions in physics are either very coarse-grained or of very small objects. The detailed state can be found with a good approximation only for fairly stationary sources of very small objects. But this doesn't mean that the detailed state (of the metal or the whole universe) doesn't exist or that talking about it is an empty phrase. Even in classical mechanics, it is impossible to know a highly accurate state of a many-particle system (not even of the solar system with sun, planets, planetoids, and comets treated as rigid bodies) but its existence is never questioned.

Thus there is no physical reason to question the existence of the state of the whole universe, even though all its details may be unknown for ever.

 

[martinbn]

@A. Neumaier : Isn't there a difference though? For the universe it is unknowable in principle for the metal only in practice.

 

[A. Neumaier]

The detailed state of a piece of metal is unknowable in principle. To disprove this you'd have to propose a Gedankenexperiment how to find it. This cannot even be done for a classical model of the metal. I haven't seen any idea in the literature that would indicate how to reliably detect a single classical particle position anywhere in the deep interior of a piece of metal. Exact classical positions of multiparticle systems are therefore metaphysical assumptions.

 

[vanhees71]

Yes, the whole universe and a piece of metal are completely analogous in this respect.

All descriptions in physics are either very coarse-grained or of very small objects. The detailed state can be found with a good approximation only for fairly stationary sources of very small objects. But this doesn't mean that the detailed state (of the metal or the whole universe) doesn't exist or that talking about it is an empty phrase. Even in classical mechanics, it is impossible to know a highly accurate state of a many-particle system (not even of the solar system with sun, planets, planetoids, and comets treated as rigid bodies) but its existence is never questioned.

Thus there is no physical reason to question the existence of the state of the whole universe, even though all its details may be unknown for ever.

The only fundamental question is, how you then define the meaning of a probabilistic statement for something that is one single event and cannot be reproduced in terms of an ensemble. Of course, also the single bar of metal is well-described by macroscopic quantities, and there the averaging/coarse-graining is over many microscopic details/sufficiently large space-time "fluid cells", making up an ensemble in some sense.

 

[A. Neumaier]

The only fundamental question is, how you then define the meaning of a probabilistic statement for something that is one single event and cannot be reproduced in terms of an ensemble. Of course, also the single bar of metal is well-described by macroscopic quantities, and there the averaging/coarse-graining is over many microscopic details/sufficiently large space-time "fluid cells", making up an ensemble in some sense.

I had already answered this in post #115. You can easily check that in the quantum field theory of macroscopic objects, the averaging is always done inside the definition of the macroscopic operator to be measured; this is sufficient to guarantee very small uncertainties $\sigma_A$ of macroscopic observables $A$. Thus one does not need an additional averaging in terms of multiple experiments on similarly prepared copies of the system. Since all quantities of interest in a study of the universe as a whole are macroscopic, they are well-determined by the state.

 

[vanhees71]

I had already answered this in http://https//www.physicsforums.com/posts/5496600/ . You can easily check that in the quantum field theory of macroscopic objects, the averaging is always done inside the definition of the macroscopic operator to be measured; this is sufficient to guarantee very small uncertainties $\sigma_A$ of macroscopic observables $A$. Thus one does not need an additional averaging in terms of multiple experiments on similarly prepared copies of the system. Since all quantities of interest in a study of the universe as a whole are macroscopic, they are well-determined by the state.

I agree with all of that, except that I don't know what you mean by "study of the universe as a whole". What we study are pretty local tiny parts of the universe in our neighborhood. That we call this "study of the universe as a whole" is entirely based on the assumption of the Cosmological Principle, which never can be checked by experiment. Although cosmology is nowadays a very successful branch of physics, one should not forget this problem in connection with what we call the "scientific method" in all other branches of physics!

 

[A. Neumaier]

I don't know what you mean by "study of the universe as a whole" [...] which never can be checked by experiment

In 1978, Penzias and Wislon got a Nobel prize for experimentally checking an earlier theoretical prediction about the universe as a whole.

The Nobel Prize in Physics 1978 was divided, one half awarded to Pyotr Leonidovich Kapitsa "for his basic inventions and discoveries in the area of low-temperature physics", the other half jointly to Arno Allan Penzias and Robert Woodrow Wilson "for their discovery of cosmic microwave background radiation".

From the press release:

The discovery of Penzias and Wilson was a fundamental one: it has made it possible to obtain information about cosmic processes that took place a very long time ago, at the time of the creation of the universe.

Studying what took place at the time of the creation of the universe is surely a study of the universe as a whole. And no doubt it is a quantum phenomenon, since everything in the universe is one, though much of it can be described in a classical approximation. It is therefore reassuring to know that there are no fundamental obstacles in quantum field theory that prevent it to be applied even to the largest possible quantum system.

 

[Ilja]

The question is if there is such a thing as "studying the creation of the universe".

The only "evidence" about the creation of the universe is a singularity of the Einstein equations of GR. But such a singularity is not reasonably evidence about a "creation of the universe", but, instead, evidence for a failure of GR, which is plausible given that in the very early universe quantum gravity effects would become relevant, so that it is anyway well-known that the best (according to the mainstream) existing theory of gravity, classical GR, is no longer applicable anyway.

On the other hand, given the homogeneity of the CMBR, it is reasonably plausible that studying this radiation is studying the whole observable part of the universe.

What we know from observation is that the straightforward GR model, without "inflation", fails. That means, we know that there was some time in the very "early" universe where $a''(\tau)>0$. This is something very different from a naive meaning of "inflation" (which would be $a'(\tau)$ very large, see http://ilja-schmelzer.de/gravity/inflation.php ), but this is not the point I want to make. With $a''(\tau)>0$ in the very early universe, it is not even clear if there is a singularity. There can be, as well, some minimal value, with some big crunch before, a big bounce. The only observational evidence is about $a''(\tau)>0$, nothing more.

 

[A. Neumaier]

The question is if there is such a thing as "studying the creation of the universe".

I corrected the phrase I had used to studying ''what took place at the time of the creation of the universe'', which is the exact wording used by the Nobel prize committee. You'll have to argue with them whether their formulation was adequate, not with me.

 

[Ilja]

Ok, if any of the members of the Nobel committee reads this - I think you made an error here.