What counts as a measurement in QM?

[Careful]

But let's face it, no one knows where the next big breakthrough will come from. So maybe careful's direction will make sense in the end. Ultimately, the proof is in the pudding.

As 't Hooft says it: let's everyone mud on, the solution will come from an unexpected direction The fact that the LR program has a clear guideline (quantum chemistry) is in my opinion a strength and not a weakness (it is very unlikely that in the next few decades we shall get any further indication for QG) - no other program has such clear goals.

For the record: I am not advocating necessarily pure LR, I think however it is the most sensible STARTING strategy for QG. Concerning this visibility, I have commented on that to you already in my email. I think it is not so unlikely as you seem to think it is, since the predictions of QM on scales below 1/10'th of a meter are **indisputable** also for me. It is entanglement above this scale which is very, very strange and cannot be explained by local realism (and would indeed need a nonlocal ``perturbation´´). Let's see what the future brings !

Cheers,

Careful

 

[Careful]

I don't agree that Rowe is not meaningful. It certainly places severe restrictions on LR models. The idea for the LR program vis a vis Rowe must go something like this:
a. Rowe is efficient, but does not rule out local effects accounting for the correlations.
b. QM provides a good mechanism for the correlations regardless of local effects being ruled out or not.
c. The local realist must therefore add in a new, previously unknown mechanism to account for observed results.
.

Agreed that Rowe adds restrictions (which could be seen as a good thing) - I did not say it was not meaningful (just that it is not an exclusive Bell test)- except for (c): local realism ALWAYS needs to take into account the detailed setup (that is its philosophy), it is not said that ``new physics´´ is needed.

**So now we have traded Bell's Inequality for something just as bad. If there is such a new and previously unknown mechanism, what is it? Where is it? In other words, we are now requiring that the setting of one measurement device affects the results at the other measurement device. This must be explained by a candidate LR theory **

Aha, that is were you have to dig deeper into the well known theories, and that is not obvious by any means. As I mentioned once, local realist explanations are going to be more difficult and that is why QM, when appropriatly apllied, is going to stay around a very long time as an effective theory and perhaps even as a fundamental one (in my worst nightmares :-) ). But the only thing we need to know for QG is that there exists a plausible, albeit complicated LR explanation (at least when you adapt the LR point of view).

Cheers,

careful

 

[DrChinese]

But the only thing we need to know for QG is that there exists a plausible, albeit complicated LR explanation (at least when you adapt the LR point of view).
Cheers,
careful

I want to make sure I am following this comment. To get Quantum Gravity to make sense and ultimately produce a good theory, one should adopt a local realist perspective as a starting point. Do I have the basics correct?

 

[Careful]

I want to make sure I am following this comment. To get Quantum Gravity to make sense and ultimately produce a good theory, one should adopt a local realist perspective as a starting point. Do I have the basics correct?

I did not say that one HAS to adopt a LR point of view to make QG make sensible. However, it is for sure an approach which avoids all philosophical as well as many (unsolvable) technical problems QM oriented approaches have. It is not a popular line of thought because it actually has to reproduce all QM successes on the scales I mentioned and is without the introduction of a non locality scale not capable of explaining quantum correlations at larger distances IF these exist in nature (but these correlations are also troublesome for QM itself - see my Einstein comment). However, you can compare the spirit of what I say *in some sense* with the study of interacting quantum field theories as a perturbation of the free field theory (with that difference that we do not know yet if nature is not ``free´´). Free field theories are the easiest ones to solve (and the only ones in which we have some reasonable particle notion), in the same way, realist locality is by far the most simplifying principle in nature and leads to the cleanest theories. So, it is ALWAYS useful to study these, even though they might not have the final word.

Cheers,

Careful

 

[Sherlock]

(a) Obviously, the problem concerns isolated substems of a closed system. The only closed system is the entire universe. Jim Hartle has written good stuff on that.
(b) Let me clarify some entanglement issues to you:
(I) CLASSICAL : all matter fields and gauge fiels are REAL. Entanglement between two particles would mean that results of separate, but identical measurements (***) on both particles are correlated due to interactions between the particles in their mutual *causal* past through gauge fields.
(II) Quantum mechanically: nothing is real except the results of measurements. Here entanglement means the same except that it results from processes which are NOT restricted to the mutual causal past (such as the instantaneous collapse of the wavefunction).
(***) A measurement does NOT need to be a binar number due to interaction with a measurement apparatus, but could be the value of a function on phase space (of one particle) at a particular instant in time determined by the trajectory of the particle.
Since (II) is more liberal than (I), QM predicts correlations which cannot be reached by locally causal realist theories in THEORY (the practice is an entirely different matter).
Now, in scenario (I), suppose we want to check a property of a particle by making some measurement which takes about a nano second. Then, we know that if we control the environment in one meter around the experimental setup, we are safe since no events outside this region can influence the experimental outcome. Now, in scenario (II) this would be more troublesome since if our setup were entangled with an object outside this room and a measurement on this object were done between the preparation and registration of the relevant property of our particle, then the outcome of our experiment would have been influenced superluminally. If such superluminal influences were to occur randomly then it would be impossible to find any regularity in the outcome. This was an objection made by Einstein Podolsky and Rosen.
You can find good discussions about the difference between classical and quantum entanglement in the books of Bell - speakable and unspeakable in QM - and Selleri, the EPR paradox.

Of course, quantum physicists could argue that the influence of the environment on the subsystem would miraculously average out and therefore be of no importance. But that is exactly one aspect of the cat problem which they should SHOW and not assume. As you noticed, this issue gets different faces in different approaches (therefore I am not going to expand on this anymore). So, if you are interested in more measurement stuff, then specify your interpretation of QM.

Thanks for the reply Careful. Sorry I intruded on your European Sunday morning. :-)

So far, my interpretation of qm would be the probabilistic one.

I gather from what you wrote above that quantum entanglement doesn't mean the same thing as classical entanglement in that classical entanglement requires some sort of prior physical connection between two particles, A and B, (not necessarily a direct connection between the two such as an interaction, but at least some common influence); and quantum entanglement doesn't require this. This doesn't seem correct to me. Wave function collapse by itself isn't what entangles two particles in qm, afaik. They have to be related in some way prior to the measurement which collapses the wave function in order for their wave functions to be linearly combined prior to the measurement, don't they?

I'm still not sure what you mean by saying that the assumption that isolated systems exist in nature isn't possible in qm.

In a Bell test setup for example, once a pair of disturbances have been produced and are on their way toward the polarizing filters, and before they interact with the polarizing filters, each disturbance is a closed, isolated system. (It's during this interval that their wave functions are combined and interfere via the application of the superposition principle, but this isn't a physical interaction.)

Every particle in the universe isn't quantum entangled with every other particle. They might be classically 'entangled' in some sense (eg. wrt the motion of the universe as a whole -- expansion, rotation, etc.; or wrt gravitational behavior of the macroscopic objects that they're part of), but these aren't quantum correlations.

I don't understand what you're saying about scenario (II) being more troublesome wrt the 1ns measurement example. It seems like you're assuming that quantum entanglement of spacelike separated events implies superluminal influences -- and, afaik, it doesn't. That is, for all anyone knows quantum entangled particles *are* affecting each other superluminally, but that conclusion isn't *required* from observations and qm (or Bell's theorem for that matter).

As for the S-cat problem, or why don't we see macroscopic interference? Well, we do see macroscopic interference, don't we? That's what ponderable objects are -- regions of interference/interaction wrt a hierarchy of various media, aren't they? After all, the idea of interfering *quantum* phenomena came from observing interfering macroscopic phenomena, didn't it?