Quantum state: Reality or mere probability?

[stevendaryl]

Its obvious it doesn't.

All he is doing is interpreting the state in a frequentest way as a CONCEPTUALLY large number of similarly prepared systems from which the observation selects an outcome.

Note the key word - CONCEPTUAL - meaning it resides in the head of the theorist.

I'm confused: Who is "he"? Leifer or Ballentine?

If the ensemble is something in the head of the theorist, then that would seem epistemic, to me.

 

[bhobba]

I'm confused: Who is "he"? Leifer or Ballentine?

Sorry - I was being slack - its Matthew Leifer - it's colloquialism out our way that Matthew is shortened to Mati, Matty etc.

If the ensemble is something in the head of the theorist, then that would seem epistemic, to me.

My goof - you are correct - its the other way around - its not ontic - I get confused with these philosophy terms.

But to forestall other questions here is a link to Ballentine's 1970 paper:
http://www.kevinaylward.co.uk/qm/ballentine_ensemble_interpretation_1970.pdf

Now have a look at the bottom of page 361:
'In contrast the Statistical Interpretation considers a particle to always be at some position in space'

That is only possible in some Bohm like theory and is responsible for the remarks of myself and Frederik.

To avoid it you say the act of observation selects a particular position from the ensemble - its not there before observation. He is much more careful about this in his book.

Thanks
Bill

 

[vanhees71]

An "ensemble" is a very real thing. It's just the repetition of an experiment/measurement for many equally an independently prepared setups. This you do from the very beginning of your scientific career in the labs at university and that's what's done in any lab on the world without further thinking of it. Quite often my experimental colleagues say "that's a statistics hungry observable; we definitely need more statistics", which just means they have to collect more data running their accelerator further, trying to increase the luminosity at a given beam energy and what not.

I don't know of any way to verify theoretical predictions (the more those of quantum theory which is probabilistic in the first place) than to interpret probabilities (wherever their estimate comes from) in the frequentist way and use large enough ensembles to check them. I've not yet met any (Q) Baysenist who could tell me what he means with the idea that a probability applies to single events.

Of course, there are examples for very interesting things that meet great difficulties preparing large enough ensembles. One example is the measurement of cosmic neutrinos. E.g., Icecube has collected just some "cosmic neutrinos", and one would like to draw conclusions already on this, but that's a big challenge, indeed. There's no way to gain more insight beyond the standard statistical conclusions (let alone the estimate of systematic errors which must also be performed by the experimentalists).

 

[naima]

I don't think there isn't any sign of that in http://arxiv.org/abs/1301.3274.

I think that is what he writes:

In this paper, we hope to have given a new and use-
ful way to think of quantum interference: Particularly,
we have shown how to view it as an empirical addition
to Dutch-book coherence, operative when one calculates
probabilities for the outcomes of a factualizable quantum
experiment in terms of one explicitly assumed counter-
factual. We did this and not once did we use the idea of a
probability amplitude

I was very enthusiastic the first time i read this paper. Born's rule was described only with positive real numbers!
Fuchs avoided the complex amplitudes for interferences. he avoided Jones matrices, he also avoided Kraus operators. All those tools which act on amplitude functions.
But alas, he also avoided to calculate interferences. How could he calculate a fringe length?
I googled "qbism interferometry". But i found no caculation.
He starts with a Hilbert space then he says let us ignore its inner product
A part of quantum mecanics remains. Qbists are interested in this subset of QM.
You said that as a Bohmian you agree with Fuchs. Do you also avoid all that "linear stuff"

 

[bhobba]

An "ensemble" is a very real thing. It's just the repetition of an experiment/measurement for many equally an independently prepared setups. This you do from the very beginning of your scientific career in the labs at university and that's what's done in any lab on the world without further thinking of it. Quite often my experimental colleagues say "that's a statistics hungry observable; we definitely need more statistics", which just means they have to collect more data running their accelerator further, trying to increase the luminosity at a given beam energy and what not.

Its the standard way its taught in applied math such as statistical modelling. You think of it as a very large number of repetitions of the same thing.

But in some areas like statistical inference the Bayesian view is more prevalent - however that is NOT the situation in QM.

Sill Copenhagen that views it that way is a popular interpretation.

Thanks
Bill

 

[atyy]

No. I previously had similar sentiments as I considered some Bohmian models (the "minimalist" ones) to be ψ-epistemic but Leifer cleared this up in this 2014 paper linked above and points out why all Bohmian models are ψ-ontic:

I think Demystifier had tried to explain it to me but I didn't get it even though I thought I did. But I don't feel bad because Spekkens also argued similarly as us.

Yes, you are right using the definition of ψ-epistemic in Harrigan and Spekkens and PBR. But I was wondering whether using a different definition one could think that the Bohmian ψ is also epistemic? That's why I asked in post #2 "Can it be said that the Bohmian answer to this question is reality and mere probability?". Or can one have one's cake and eat it? Do these interesting comments from stevendaryl and Demystifier from another thread https://www.physicsforums.com/showthread.php?t=767672 suggest it is possible?

Maybe the answer is that in Bohm, while the wave function is objectively real, its interpretation (when squared) as a probability involves a subjective notion of probability?

Yes.

 

[stevendaryl]

I've not yet met any (Q) Baysenist who could tell me what he means with the idea that a probability applies to single events.

Well, Bayesians are equally mystified by frequentism. I mean, you never have an infinite run of anything, you have a finite run. So what does probably imply for a finite number of trials? Nothing. So ultimately, your decision as to whether a probabilistic theory has been falsified by experiment is subjective.

Everybody makes subjective decisions as to what to believe. Whenever someone does something that has never been done before, whether it's trying a new drug, or performing a new accelerator experiment, or riding in a new space vehicle, people have to make subjective judgments about how safe it is. Maybe you can say: "We've proved using QED [or GR, or whatever] that it's perfectly safe", but you don't know that those theories are correct. So everybody deals with subjective notions of what to have confidence in. It's just that non-Bayesians wouldn't associate such uncertainties with a number, a probability. But if you don't attach probabilities to those uncertainties, then there is no way to consistently reason about them, if you have several sources of uncertainty. Bayesianism is just a way of being systematic and consistent about the uncertainties that everyone deals with in one-of-a-kind events.

 

[atyy]

Well, Bayesians are equally mystified by frequentism. I mean, you never have an infinite run of anything, you have a finite run.

Isn't there a theorem that allows Bayesians to be effectively frequentist? http://en.wikipedia.org/wiki/De_Finetti's_theorem

There's a quantum version: http://arxiv.org/abs/quant-ph/0104088.

 

[Jabbu]

There is also a practical side to things being real or not. So if quantum state of an electron is real, it really means electron itself is not quite real, but rather smeared nowhere and everywhere in the same time, and with no particular position or velocity vector, right? Isn't that actually the opposite of "real"?

How could electrons follow this exact trajectory if their location and momentum vector is not precisely defined at every point in time along that path?

 

[Dale]

Closed pending moderation.