I Is the Following How Most Students Think of Einstein's View Of QM

[A. Neumaier]

Again, since QT, including Born's rule, leads to a very good agreement between theory and experiment, from a physical point of view that's what justifies the rule (together with the rest of the formalism).

This only justifies that QT works (at least on terrestrial scales and below) FAPP.

But physics always worked FAPP, even when it was from today's point of view very incomplete - only the meaning of FAPP (''for all practical purposes'') changed with time. Thus an empirical argument such as yours doesn't justify the claim that QM is complete.

Of course, a real-world experiment always needs the detailed study of the imperfections of the used measurement devices, but that's part of experimental physics not of concepts of the theory.

Well, this means that Born's rule is only valid for perfect (i.e., theoretical) measurements, not for real measurements.

But then what is a perfect measurement? It cannot have an experimental definition because of the later's imperfections, hence it must be a theoretical concept. As long as there is no clear such concept, quantum mechanics is incomplete.

 

[vanhees71]

All you need in physics is that a theory works FAPP since it's the goal to describe objective quantitative observations in nature.

You know better than me that non-ideal measurements are described by the POVM formalism.

 

[Elias1960]

In local relativistic QFTs by construction there is no problem with Einstein causality. The microcausality condition ensures that no action whatsoever can cause effects at spacelike separated events. So this problem is already solved by construction.

You confuse signal causality with Einstein causality. But, don't worry, the difference between the two is evil philosophy, so don't care about this. Signal causality is good enough for you, once you care only about observables. It is what survives even in the dBB version, which requires a preferred frame, but has no compatibility problem with signal causality.

 

[vanhees71]

Of course I only care about observables. Nonobservables are not what's investigated in the natural sciences. Maybe philosophers have nice problems to fight against each other about, but it's not belonging to the realm of the natural sciences. I guess there must be a lot of debate also about the electromagnetic potentials, because you can fight a lot of their meaning, because they are unobservable and only determined up to a gauge transformation. Sometimes you have such debates also within physics. In this context the Aharonov-Bohm effect gave some puzzles. The point of course is that the AB effect is observable and there's no problem with gauge invariance too.

 

[bhobba]

Of course I only care about observables.

So do I. But the argument is along the lines of QM is a theory about observations that appear here in the macro world. Yet the macro world is supposed to be explained by QM. Exactly how do you do this? It can be done, but is interpretation dependant. That worries some people. Not me personally - but others find an issue.

Thanks
Bill

 

[vanhees71]

What is interpretation dependent in quantum many-body theory, which successfully explains the classicality of the classical behavior of many aspects (but of course not all) of macroscopic systems?

 

[martinbn]

You confuse signal causality with Einstein causality. But, don't worry, the difference between the two is evil philosophy, so don't care about this. Signal causality is good enough for you, once you care only about observables. It is what survives even in the dBB version, which requires a preferred frame, but has no compatibility problem with signal causality.

Why do you call it Einstein causality? Is this your terminology or not? Can you give a reference?

 

[bhobba]

What is interpretation dependent in quantum many-body theory, which successfully explains the classicality of the classical behavior of many aspects (but of course not all) of macroscopic systems?

That problem is basically solved. The issue is when two systems interact you get a mixed state, and various interpretations have their own take on how the outcome that occurs is 'selected'. Personally I do not care - all theories have to do is conform to experiment - and that probabilities can be predicted is all that is needed. Others however hold different views. I am just reading Weinberg's take on it now - he thinks, to use his own words - 'even with this clarification there still seems to be something missing in our present understanding of Quantum Mechanics'. IMHO it has to do, like with Einstein, your instinct in what a scientific theory should explain. I have no issue with it, and I think you do not either, but some do and that includes people owed the utmost respect like Wienberg. Einstein's final view had more to to with the ideas detailed in EPR (yes I know he had an issue with that paper thinking his main point had been 'buried by the errududation'). I do not think Dirac worried much about it - he had a view that science constantly progresses and it will likely be sorted out one way or another which contrasts to Heisenberg who thought it already complete. BTW it's not really shut up and calculate IMHO, it's how you view the issue of you can only predict probabilities from a mixed state.

Thanks
Bill

 

[vanhees71]

The probabilities of a macroscopic system concerning macroscopic observables usually are as good as being certainties, because the fluctuations around an average value (average over microscopically large but macroscopically large space-time points) are very small compared to the accuracy necessary to observe them.

On the other hand, if you deal with small systems the fluctuations are not small at the resolution of observations of the then relevant observables, but then the probabilities predicted by QT are found to be correct, and it does not look as if there is something missing when accepting that on this level nature is indeterministic as predicted by QT.

I don't know, what precisely it is what Weinberg thinks is missing, because in his book he just says that which interpretation is "correct" is an open question, but I did not see clearly what he thinks is missing from what to be expected of a physical theory to describe beyond what QT successfully describes.

 

[RUTA]

I am just reading Weinberg's take on it now - he thinks, to use his own words - 'even with this clarification there still seems to be something missing in our present understanding of Quantum Mechanics'.

Here is another quote from Weinberg:

An electron spin that has not been measured is like a musical chord, formed from a superposition of two notes that correspond to positive or negative spins, each note with its own amplitude. Just as a chord creates a sound distinct from each of its constituent notes, the state of an electron spin that has not yet been measured is a superposition of the two possible states of definite spin, the superposition differing qualitatively from either state. In this musical analogy, the act of measuring the spin somehow shifts all the intensity of the chord to one of the notes, which we then hear on its own. ...

So if we regard the whole process of measurement as being governed by the equations of quantum mechanics, and these equations are perfectly deterministic, how do probabilities get into quantum mechanics?

QM makes exact predictions of probabilities, which means when you do an individual trial of the experiment and obtain one of the possible outcomes, you have the “collapse of the wavefunction” to that one outcome. So, Weinberg’s quote here is another way to articulate the measurement problem. That’s what he thinks is missing.

Here is another quote from that same article:

What then must be done about the shortcomings of quantum mechanics? One reasonable response is contained in the legendary advice to inquiring students: ``Shut up and calculate!'' There is no argument about how to use quantum mechanics, only how to describe what it means, so perhaps the problem is merely one of words. On the other hand, the problems of understanding measurement in the present form of quantum mechanics may be warning us that the theory needs modification.

 

[bhobba]

I don't know, what precisely it is what Weinberg thinks is missing, because in his book he just says that which interpretation is "correct" is an open question,

He believes there is no interpretation that does not have serious flaws. But says 'this view is not universally shared'. I personally do not share it, but his arguments are coherent and clearly detailed. For example his objection to Consistent Histories is its use of the Born rule seems to bring people into the laws of nature. Indeed he thinks any instrumentalist approach has this problem. To me, like John Baez says, often it's the old disagreements about the meaning of probability bought into another area, and I think Weinberg's arguments are to some extent along those lines.

Thanks
Bill

 

[bhobba]

Here is another quote from Weinberg:

He tries very hard to be carefull, to his great credit, and in understanding his views for people like me that have thought long and hard about the issues. For example he points out when in a state that when measured will give a value close to a certain value, it is tempting to think it has a value close to that value. But that is not what it says - it says if you measure it that's what happens - when not measured it says nothing. From this he draws the conclusion that such instrumentalist approaches are human dependant, and this inclusion of the 'experimenter' into the theory is not compatible with an objective reality. That I do not agree with - I believe because the Von-Neumann cut can be placed anywhere, you can consider it as having that value by placing the cut there. But the view is coherent and obviously with a lot of thought behind it. I really would have liked to see a discussion between Gell-Mann and Wienberg on this point because he was a proponent of Consistent Histories that often looks at things this way.

Thanks
Bill

 

[Elias1960]

Why do you call it Einstein causality? Is this your terminology or not? Can you give a reference?

It is what is called Einstein causality or locality in the discussions about Bell's theorem. I prefer not to use to name it "locality" because this is misleading - a completely local theory with maximal speed of information transfer 100 c would have to be named "nonlocal" if one follows this convention. Therefore I name it either Einstein-local or Einstein-causal.

The terminology, however chosen, has to distinguish two notions of causality. There is a weak one which forbids only to send signals, and therefore usually named signal causality. It is a theorem in QFT, but also holds in dBB, despite the fact that in dBB causal influences happens faster than light. These FTL influences can nonetheless not be used to send signals, and therefore signal causality is not violated. And there is the strong one which is necessary to prove Bell's theorem. With signal causality alone you cannot prove it, as QFT and dBB show, where it cannot be proven.

 

[martinbn]

It is what is called Einstein causality or locality in the discussions about Bell's theorem. I prefer not to use to name it "locality" because this is misleading - a completely local theory with maximal speed of information transfer 100 c would have to be named "nonlocal" if one follows this convention. Therefore I name it either Einstein-local or Einstein-causal.

The terminology, however chosen, has to distinguish two notions of causality. There is a weak one which forbids only to send signals, and therefore usually named signal causality. It is a theorem in QFT, but also holds in dBB, despite the fact that in dBB causal influences happens faster than light. These FTL influences can nonetheless not be used to send signals, and therefore signal causality is not violated. And there is the strong one which is necessary to prove Bell's theorem. With signal causality alone you cannot prove it, as QFT and dBB show, where it cannot be proven.

OK, but my question was if this is your own terminology or is it used by other people. Any examples you can point out?

 

[bhobba]

OK, but my question was if this is your own terminology or is it used by other people. Any examples you can point out?

It is terminology sometimes found in foundational discussions of SR. Strictly speaking SR does not forbid speeds FTL - just the ability to sync clocks by signals FTL ie signals carrying information. I don't see too much about it these days because the geometric/group theory approach to SR, while not doing away with the issue, makes it not really needed in 'deriving' the Lorentz transformations eg:
http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

Because it's not germane to the main argument, the speed c that derivation requires the fixing of experimentally, is really the maximum speed information can be sent. It does not rule out faster speeds that can't be used to send information, but a full discussion of this fine point is best left for a thread in the relativity forum where other forms of clock syncing can be discussed such as slow clock transport. Although I can't recall specific examples, I seem to recall they sometimes give different synchronizations than using, say light signals - but that takes me way back to when I was interested in the foundations of relativity, which these days is more a vague memory. Ohanian, the author of the book - Einstein's Mistakes, and that 'different' book on GR (it its not derived by geometrical methods) was careful about it:
https://pdfs.semanticscholar.org/41f5/72b841a358998a0aaab03fed3561d80c5491.pdf

Note the above does not analyse the newer methods like the group theoretic argument I gave before which gives SR without any issues. The clock sync issue applies to Einstein's original papers so the conclusions the above paper reaches is very dubious , and while a peer reviewed Journal it would not pass my review without pointing out better ways of deriving/explaining SR now exists. Added later - but even aside from this, the postulate of relativity is that the laws of physics is the same in all inertial frames. Now whatever laws of physics determine the speed of light if you reverse the coordinate system it obviously is still an inertial frame - but the light travels in the opposite direction, but at the same speed. You are invoking the POR but by reversing the coordinate system it becomes very clear.

Thanks
Bill

 

[Pony]

It is terminology sometimes found in foundational discussions of SR. Strictly speaking SR does not forbid speeds FTL - just the ability to sync clocks by signals FTL ie signals carrying information.

What does this mean? How could SR forbid the ability to sync clocks by signals FTL?

Also, AFAIK aether theories or other preferred frame theories give the same predictions as SR, and are consistent with a restricted form of FTL communication, that is, we can allow FTL communication in a preferred frame, still no causal loop or causality paradox could occur.

 

[bhobba]

What does this mean? How could SR forbid the ability to sync clocks by signals FTL?

Please, as I mentioned, take this up on the relativity forum.

But just as an overview, SR says the fastest information can be sent is the speed of light. The reason is the c that appears in the equations must the unique speed the same in all frames. Let s be the fastest speed information can be sent - it could be infinity. Now to sync clocks by signals you can't do that with signals faster than s. But by the Principle Of Relativity that must be the same in all frames. However only one speed is the same in all frames - that is the c in the SR equations ie s=c. Now if you can't send information, then you can't use it to sync clocks - or anything really - so the argument breaks down.

Thanks
Bill