Some peoples avatar pics really make me wonder. If you look like an unfriendly person, why use it as a face on the web?

hi

ok, never mind. it told me a chat session was going on. There doesn't seem to be.

@AlanSE I'm here :)

hi

@davidzaslavsky You around?

Sometimes it's a bit quiet if there's nothing to complain about!

whats up?

Well, we are trained here to give short and clear answers only, without much discussion...

The level of the questions varies enormously.

@annav That's good, we get a wide audience...

Arnold did you see the blog of Lubos today? He is carrying on the measurement issue

I found that discussion interesting too. I was always a bit unclear about what constituted a measurement

As an experimentalist these discussions bemuse me.

@annav: I don't know his blog site address, could you please give the link? (He also didn't respond further to my comments in chat.)

motls.blogspot.gr/2012/11/…

thanks. I am just reading it...

I think he doesn't understand my position. his arguments are about quantum versus classical, and he equates classical with objective. Whereas I think that everything in the universe is quantum, and still there is an objective world od science to talk about. This has nothing to do with Wigner's friend, but everything with statistical mechanics.

He thinks that density matrices simply encode subjective uncertainty, but in the traditional foundations this is simply assumed without any real justification. In fact, nothing at all in the formalsim of QM (i.e., in the shut up and calculate version) commits one to this assumption,.

For me, a measurement is a number on a scale, a figure on an oscilloscope, and in this computer age, arrays of numbers in xyz., etc. Then come the convolutions to go back and peel the onion. From the bubble chamber pictures to the momenta of the particles to the fits to hypothesis, a series of convolutions (proxies) going into smaller and smaller dimensions.

At dimensions of hbar we measured behaviors inconsistent with classical mechanics and lo, quantum mechanics fits at that level , so we can state that the microcosm is quantum mechanical. The come the philosophical navel gazings :)

ack, daylight savings time :-P

In fact, statistical mechanics proves it wrong. Engineers routinely measure the thermodynamic state of homogeneous pieces of material, and statistical physics states that these determine a corresponding grand canonical ensemble - a density matrix that reproduces the experimental findings. On the level of the enginerr, this is completely objective, and any two engineers or physicists will agree. This is what i mean by objective. Our subjectivity never enters the picture.

Arnold, I cannot really enter this type of philosophical cogitations. I sort of glaze over.

I would agree that at the everyday level classical works fine.

A number on a scale, this is fully consistent with my answer to the question that gave rise to the dispute with Lubos. It means somethinh that is close to thermal equilibrium on a time scale long enough that one can look at it and read the scale reliably. This makes it objective. It doesn't matter who is reading it - no subjectivity.

But the everyday level is the quantum statistics level! There is no inconsistency, and uncertainties predicted by statistical mechnaics for a macxroscopic system are so small that they can usually be neglected.

If the everyday world behaved quantum mechanically, i.e if we were of hbar scale, we would never have discovered classical mechanics :)

It is only for tiny system that uncertaincies become big (except in eigenstates) , and then there is a subjectivity in the single-time reading (just as when people try to read a scale given a heavily oscillating pointer). In this case, it is only the statistics under frequent repetition that is objective,, and this is why physicists then reprot probabiltiies and cross sections rather than individual results.

" It means somethinh that is close to thermal equilibrium on a time scale long enough that one can look at it and read the scale reliably. This makes it objective." What do you make of decoherence?

If the world were of a scale where hbar cannot be neglected there would be no observers around who could make measurements, as the latter require a sufficiently dissipative (and hence large) structure of the universe. In fact whenever we are in a similarly uncertain classical environment (for example when it snows) we don't make detailed observations but resort to gross statements that have a statistical nature (''it snows'').

Well, once one has the concept of a density matrix and the mathematical formulation of eigen states the density matrix is a logical extension from classical to quantum and quantum to classical, imho

Decoherence is the main quantum mechanical process that ensures that the world becomes observable at mesoscopic time and space scales.

Yes, the phases (correlations) of QM disappear

Do you think it rather peculiar that Von Neumann built his density matrix description such that the decay of off diagonal components resembled thermodynamic irreversibility?

Hey everyone (@AlanSE @Manishearth etc.) , sorry I missed the scheduled time for the chat session - I forgot that the change from daylight savings time would have an effect. I just want to mention before the hour ends that I will be much less involved in the site for at least the next few weeks.

Classical mechanics and quantum mechanics are very similar on the level of the density matrix. if Schroedinger would have discovered the quantum Liouville equation before the Schroedinger equation, the interpretation of quantum mechanics would have remained much more rational than it is when QM is formulated via wave functions, which looks completely divorced from the classical world.

It took me many years of reading between the lines of the standard literature to see that there is in fact much more similarity than difference between the classical and the quantum world....

The decay of off-diagonal components of the density matrix in the right basis is thermodynamic irreversibility.

Yes, this seems right.

... so I am emphasizing in my asnwers here this closeness, as it is much too little known.

Do not worry about Lubos' interpretation of what you say. He is very dogmatic about quantum mechanics and thinks people do not understand how fundamentally nature is quantum mechanical, at the drop of a hat.

@DavidZaslavsky that is sad for physics SE, will you keep being one of our mods ?

I write for everyone, not just to refute someone's comments. I try to be informative in what I add to a discussion.

As I read it, Lubos was using the term "subjective" to mean that there's some arbitrary threshold at which you deem the off diagonal components to have effectively vanihed. This arbitrariness is what he's refering to as subjectivity. Is that what others read too ?

@Dilaton maybe, or maybe not. For now I'm just taking a partial break. I'll probably have a more definite update at the next chat session.

The threshold may be put as arbitrary as the number of decimals given to a meter reading. This doesn't make the meter reading subjective ecept when you insist on too much accuracy than available. The decoherence effect is as real and objective, no matter whether one sets the threshold to 1e-7 or 1e-10, (It just takes very little longer to reach the stricter level).

@ArnoldNeumaier yes, I'm not sure subjective is the word I'd apply in this case

I wouldn't want to make statements of other's opinions, but my interpretation is that classical objectivity, or more correctly objective reality is a convenient fiction that emerges. Fundamentally their is no underlying objective reality, or classical existence, it only emerges as a result of repeated measurement of mutual accessible observables. This is consistent with experiment.

@DavidZaslavsky I like you being here at physics SE. If an additional mod is needed to halp because you have not enought time for it sometimes, I vote for Qmechanic :-) ! Even though I have not enought rep to vote :-P ... Have to go no, the discussion was nice to read. By

@Dilaton well... it's more complicated than that, but let me not say too much for now. There will probably be a moderator election at some point soon and then we can discuss who would be good for the role at length ;-) See you later

We don't know that is the case fundamentally - it is an assumption. Objectivity means that different people agree on the result if they have had a similar training. This meaning of objectivity is given through our culture, not through an abstract definition in physics.

And we can verify that there are lots of objective measurements, though the objectivity doesnt' carry beyond a natural scale of accuracy, which can also be determined objectively. This is what ordinary measurements and statistics are about.

@I.T. in that sense we, as entities are emergent from the underlying consecutive levels of atoms, molecules, cells, organs, ...

@I.T.: Can you explain how objectivity would emerge from subjectivity just by repetition? Even this needs an objective basis!

@David hope everything is OK and you are just having a hiatus

Well decoherence is the mechanism. We can even calculate the decoherence time under certain assumptions. This depends on mass , temp, potential, which all are variables indicating environment interaction. Essentially, as a set of mutual observations takes hold, a new quantum systems will interact with another system and as interactions take place, mutual information is exchanged and the are brought into an equilibrium condition where eventually they can be understood classically.

So you are using an objective description of the environment (e.g., temperature!) to deduce that ultimately the system under observation will also get an objective property!

Not really, these are just approximations when small systems encounter larger systems.

Everything observed is just an approximation, including what we call a system, what we measure, and the Hamiltonians with which we do quantum mechanics. So it is no surprise that objectivity is also approximate, and we need to start with something approximately objective to deduce something other approximately objective.

I think the point though is that quantum uncertainty is more fundamental. Here's a fun video:

newscientist.com/blogs/nstv/2012/11/…

Thanks to all for the discussion. Bye.

good night