Undergrad Quantum mechanics is not weird, unless presented as such

[ddd123]

But you wouldn't have a plethora of conflicting interpretations.

 

[Ken G]

I'm saying we would have a plethora of conflicting classical-mechanics interpretations (such as, whether or not the universe is deterministic), had classical mechanics not been superceded by quantum mechanics. It is always the most fundamental theory that we try to interpret, because we think that's where the "fundamental interpretation" lives. It's just the nature of the beast, it has always been that way in physics. Action at a distance, or not? Material particles, or fields? Aether, or no? GR or QM? At any point in the history of physics, if you want to find where the debate on interpretations was, just look at whatever was regarded as the most fundamental theory of that age.

 

[ddd123]

Fair enough. It's notable though that the pre-quantum mood was that of "we've mostly figured it out", then came the radical crisis of quantum discoveries. Although it was debatable even then, say, determinism wasn't really menaced because those you mention are only theoretical problems, whereas QM manifests in real experiments.

 

[Ken G]

I agree the loss of local realism seems to have had more in the way of aftershocks that the loss of the aether. People dropped Poincare's aether pretty much overnight, but dropping local realism seems weirder. I don't really know why though, both relativity and quantum mechanics have a very satisfactory elegant structure, and neither seems at all like what we experience day to day.

 

[ddd123]

I don't really know why though

Probably because realism was/is the main tenet of the scientific endeavor. You figure out the objective properties of things. So, relativistic space-times are removed from everyday experience but they consist in a very definite objectivity. In a sense they strengthen the realism of science because science convinces us of something that is so intangible: it means it's so powerful. The loss of realism does the opposite.

 

[Ken G]

So the weirdness simply stems from a certain brand of realism. But I don't agree this is the main tenet of science, the main tenet of science is to make sense of observations. So if a certain way of doing that makes us regard science as weird, then get rid of that way of doing it. That's what we did to the aether, and for the same reason.

 

[ddd123]

So the weirdness simply stems from a certain brand of realism. But I don't agree this is the main tenet of science, the main tenet of science is to make sense of observations. So if a certain way of doing that makes us regard science as weird, then get rid of that way of doing it. That's what we did to the aether, and for the same reason.

That's why I wrote "was/is. I can agree it is not the main tenet, but was? I think we can say objectivity isn't the main goal only because we've started doubting we can have it. So we reflected upon that and came out with the more lax requirement of "making sense of observations" (it's a little vague but I think it's not necessarily bad).

 

[Ken G]

Ah I see, I didn't notice the was/is! Yes, I think that's the main point here, what we regard as weird depends on our philosophy, but we should always expect our philosophy to need to change as science advances. So we should expect constant weirdness, and I think that is exactly what the history of science has always been. We tend to focus only on the current weirdnesses as if they were somehow special.

 

[ddd123]

So we should expect constant weirdness

You might see it differently: we should expect increasing weirdness. Since this is related with our intuition, the more we explore phenomena that are distant from us, the weirder it gets for our intuition, and with QM it even altered the very nature of our knowledge of things, something which we didn't think possible.

 

[adeborts]

When do you think your book will be published?
(You state that what is available on the net is just a draft.)

 

[adeborts]

No, the more I think about quantum mechanics, the less weird it is. I have written a whole book about it, without any weirdness; see post #2.

Quantum mechanics is weird only in the eyes of those who take the talk about it too serious and neglect the formal grounding which contains the real meaning.

You state that what is available on the net is only a draft.
When will the book reach the final format and be published?

 

[adeborts]

Sorry for the repetition!
My first inquiry was not immediately posted and I thought it got lost...

 

[A. Neumaier]

When do you think your book will be published?

A revised version is scheduled to be published in fall 2017. I'll probably add a much more polished and complete discussion of nonequilibrium thermodynamics (except for its field theoretic aspects) and take out the stuff on general manifolds. Field theory needs a second book, and I haven't yet a schedule for its publication.

 

[Feeble Wonk]

Yes. It is only surprising and looks probabilistic to us, because we do only know a very small part of its state.

Please forgive my ignorance on this matter, but is determinism a fundamental principle of QFT? I assume that there are a range of conceptual variations regarding quantum fields. While the version you subscribe to is deterministic, are there versions of QFT that are not?

 

[ddd123]

Please forgive my ignorance on this matter, but is determinism a fundamental principle of QFT?

Actually the opposite, non-determinism is fundamental to QFT. It's a theory, if a deterministic underpinning is found to QFT it will be something else. Actually, since QFT seems to prefer locality in some sense, its non-determinism is pretty essential for it not to violate Bell's inequalities: http://arxiv.org/abs/hep-th/0205105 .

 

[A. Neumaier]

While the version you subscribe to is deterministic, are there versions of QFT that are not?

Most of QFT is applied only to small systems, in which case it is probabilisitc like any (classical or quantum) model that excludes part of the full dynamics from its set of relevant observables.
Whether the full universe (the only system containing us not coupled to an environment) is or is not deterministic is unknown. I believe that it may be taken as deterministic, while those who subscribe to a statistical interpretation would say a quantum model of the universe is meaningless since one cannot replicate it often enough to make statistics about it.

 

[vanhees71]

Well, QFT is also applied to very large systems, and there it's very successful too in at least making it very plausible, why such macroscopic systems usually behave according to classical physics. Particularly QFT of the thermal-equilibrium state is very well developed and very successful in the wide area from condensed-matter physics to cosmology.

 

[rkastner]

It it available without a paywall?

Sorry, I just saw this. You can find a lot of free material on PTI on my blog:
transactionalinterpretation.org

 

[A. Neumaier]

Particularly QFT of the thermal-equilibrium state is very well developed

But it is determinsitic in the thermodynamic limit, and no trace of probabilities is left.

 

[stevendaryl]

But it is determinisitic in the thermodynamic limit, and no trace of probabilities is left.

I don't find that completely satisfying. If you treat Brownian motion using statistical mechanics, then it's deterministic. If you analyze a dust particle suspended in a liquid, your statistical mechanics will give a probability distribution for the location of the particle as a function of time, and that distribution evolves deterministically. But of course, if you're actually looking at a dust particle under a microscope, you'll see it jerk around nondeterministically.

In classical mechanics, we have a theory explaining the actual observation (the dust particle moves when a molecule of the liquid collides with it), as well as the statistical mechanics description. If you only had the statistical mechanics, I would consider the theory incomplete.

 

[ddd123]

As you know, the thermodynamic limit has for some cases shown noncomputability of the gap in quantum many-body theory though, which is even worse than nondeterminism, so it's a double-edged sword.

 

[A. Neumaier]

If you treat Brownian motion using statistical mechanics, then it's deterministic. If you analyze a dust particle suspended in a liquid, your statistical mechanics will give a probability distribution for the location of the particle as a function of time

That makes it nondeterministic. Once probabilities are the basic quantities, one has a stochastic system. Note that in any classical stochastic system, probabilities have a deterministic dynamics, but they nevertheless describe stochastic, nondeterministic processes.

To go from the probabilities to the actual events is the classical version of collapse; cf. the companion thread. But nobody working on stochastic processes uses that weird language for it.

On the other hand, for a system in equilibrium (which involves a thermodynamic limit), quantum statistical mechanics produces the deterministic equations of equilibrium thermodynamics, where no trace is left of anything probabilistic or stochastic. This is quite unlike Brownian motion, which is about the interaction of a macroscopic fluid and a microscopic 1-particle system, restricted to the microscopic system. Stochasticity characterizes the microscopic world, but is foreign to much of the macroscopic world - even when the latter is described as a quantum system.

 

[A. Neumaier]

shown noncomputability [...] which is even worse than nondeterminism

?

We already cannot compute most things about most classical systems with more than a few degrees of freedom, thus the whole discussion about theoretical limits of computability is moot.

 

[vanhees71]

In quantum theory the probabilities are also deterministic in the sense that the statistical operator and the operators representing observables follow deterministic equations of motion. That doesn't make quantum theory a deterministic theory in the usually understood sense. Determinism means that, as within classical physics, all observables at each time have a determined value and these values change via an equation of motion which let's you know any value at any time $t>t_0$, if you know these values at a time $t_0$.

 

[C Davidson]

A. Neumaier claims that quantum mechanics has no weirdness, despite demonstrations that objects as small as photons can share properties over more than a kilometer in Bell theorem tests. This sort of fuzzyheaded thinking has led to a "mass boson" called the Higgs which is so massive it cannot exist for a fraction of a second, despite the evidence the Universe has existed for 13 billion years. So the physicists "cook the books" with "virtual particles", and where the claims of "magic" cannot be refuted (as in entanglement), they simply demand it be accepted without explanation. No mechansim, nothing to see here, move along now.

Quantum mechanics isn't weird, but the explanations we have historically accepted are wrong. We will discover better ones.

 

[stevendaryl]

That makes it nondeterministic. Once probabilities are the basic quantities, one has a stochastic system. Note that in any classical stochastic system, probabilities have a deterministic dynamics, but they nevertheless describe stochastic, nondeterministic processes.

Then I misunderstand what you mean about the thermodynamic limit of QFT being deterministic.

To go from the probabilities to the actual events is the classical version of collapse; cf. the companion thread. But nobody working on stochastic processes uses that weird language for it.

That's because it's pretty clear what the relationship is between the actual events and the statistical model: The actual case is one element of an ensemble of cases with the same macroscopic description. The collapse is just a matter of updating knowledge about which case we are in.

On the other hand, for a system in equilibrium (which involves a thermodynamic limit), quantum statistical mechanics produces the deterministic equations of equilibrium thermodynamics, where no trace is left of anything probabilistic or stochastic.

I wouldn't say that. Equilibrium thermodynamics can be interpreted probabilistically: the actual system has a probability of e^{- \beta E_j}/Z of being in state j, where E_j is the energy of state j, and \beta = \frac{1}{kT}, and Z is the partition function. (Something more complicated has to be done to take into account continuum-many states in classical thermodynamics...)

You can use the equilibrium thermodynamics to compute distributions on particle velocities, and thus to analyze the stochastic behavior of a dust particle suspended in a fluid.

 

[stevendaryl]

A. Neumaier claims that quantum mechanics has no weirdness, despite demonstrations that objects as small as photons can share properties over more than a kilometer in Bell theorem tests. This sort of fuzzyheaded thinking has led to a "mass boson" called the Higgs which is so massive it cannot exist for a fraction of a second, despite the evidence the Universe has existed for 13 billion years. So the physicists "cook the books" with "virtual particles", and where the claims of "magic" cannot be refuted (as in entanglement), they simply demand it be accepted without explanation. No mechansim, nothing to see here, move along now.

Quantum mechanics isn't weird, but the explanations we have historically accepted are wrong. We will discover better ones.

I've been one of the ones arguing on the side of QM being weird (or at least, nonlocal), but the stuff that you're saying about the Higgs isn't really relevant to these foundational issues. There is a distinction between the Higgs "field" and the Higgs "particle". The particle is fluctuations in the field, and those fluctuations might be short-lived. But the field itself is stable over billions of years (if not forever---it may not be forever).

Anyway, I think it's important to distinguish between two different kinds of weirdness:

1. A topic can seem baffling and weird to a novice, because it involves unfamiliar concepts, or because familiar concepts no longer apply. This is a matter of learning the subject thoroughly. Special Relativity seems bizarre to those first exposed to it, but after you become familiar with it, and understand it, much (all?) of the weirdness disappears.
2. There can be lingering questions about the foundations of a topic, even after someone has thoroughly mastered the topic.

A. Neumaier is claiming that the only weirdness of QM is of type 1: If you understand it in the right way, then it stops being weird. I claim that there is some type 2 weirdness.

There might be unanswered foundational questions about the Higgs or the use of virtual particles in calculations, but I don't think so. I think that the weirdness there is due to lack of understanding of the (very complicated) subject. I think you're talking about type 1 weirdness.

 

[stevendaryl]

In quantum theory the probabilities are also deterministic in the sense that the statistical operator and the operators representing observables follow deterministic equations of motion. That doesn't make quantum theory a deterministic theory in the usually understood sense. Determinism means that, as within classical physics, all observables at each time have a determined value and these values change via an equation of motion which let's you know any value at any time $t>t_0$, if you know these values at a time $t_0$.

So in what sense is the thermodynamic limit of QFT deterministic?

 

[A. Neumaier]

That's because it's pretty clear what the relationship is between the actual events and the statistical model: The actual case is one element of an ensemble of cases with the same macroscopic description. The collapse is just a matter of updating knowledge about which case we are in.

Yes, and in the quantum case it is the same, if you drop the word ''macroscopic''.

I wouldn't say that. Equilibrium thermodynamics can be interpreted probabilistically: the actual system has a probability of e^{- \beta E_j}/Z of being in state j, where E_j is the energy of state j, and \beta = \frac{1}{kT}, and Z is the partition function. (Something more complicated has to be done to take into account continuum-many states in classical thermodynamics...)

Equilibrium thermodynamics doesn't have the concept of a partition function. One needs statistical mechanics to relate the former to a probabilistic view of matter.

You can use the equilibrium thermodynamics to compute distributions on particle velocities, and thus to analyze the stochastic behavior of a dust particle suspended in a fluid.

You can use statistical mechanics to do that, but not equilibrium thermodynamics, which is a 19th century classical theory that doesn't have a notion of particles. Statistical mechanics is much more versatile than thermodynamics, as one isn't limited to locally homogeneous substances.

 

[stevendaryl]

Yes, and in the quantum case it is the same, if you drop the word ''macroscopic''.

But that sounds like a hidden-variables theory of the type that is supposed to not exist.

Equilibrium thermodynamics doesn't have the concept of a partition function. One needs statistical mechanics to relate the former to a probabilistic view of matter.

Okay. I'm lumping thermodynamics and statistical mechanics together.