Quantum Collapse: Unravelling the Mechanics

[ZapperZ]

The CNOT gate has been demonstrated in experiments. So, starting from:

[|0> + |1>]|0>

we get the entangled state:


|0>|0> + |1>|1>

Apply the CNOT operator again, and we get the original state back.

But is this really relevant here? I could do the same thing. Let light passes by a double slit setup with a detector at one of the slit (i.e. the coupling to some detector/environment). This "collapses" the wavefunction because I now know which slit each photon passes by. Then, after that photon passes by that slit, make it pass through another double slit, but with no detector, and voila! I've restored the superposition again!

But this is not the same thing. You still cannot restore the state of the first set of slit to be in the "pristine" superposition state. By having the detector there, you have completely changed that system.

And note, I am aware of the "weak measurement" cases that have been used recently, such as the recent direct demonstration of the Hardy's Paradox.

Zz.

 

[Count Iblis]

Well, isn't this similar to an argument one could have about time reversibility? The moment you consider macroscopic objects you effectively don't have time reversibility. Entropy always increases., etc.. But all this can be explained using reversible laws of physics. So, one does not need to assume new physics that explicitely violates time reversibility at the macro-level.

However, it is very difficult to rule out the existence of any explicit time irreversible effect that operates far enough from the microscopic realm. This difficulty is in fact a consequence of the time reversible laws themselves. So, it is then unfair to demand that time reversibility a the macro-level should be demonstrated in an experiment.

In case of quantum physics, you always have some macroscopic measurement device. So, here too we cannot, in practice, restore the initial state after measurements.

 

[ZapperZ]

Well, isn't this similar to an argument one could have about time reversibility? The moment you consider macroscopic objects you effectively don't have time reversibility. Entropy always increases., etc.. But all this can be explained using reversible laws of physics. So, one does not need to assume new physics that explicitely violates time reversibility at the macro-level.

However, it is very difficult to rule out the existence of any explicit time irreversible effect that operates far enough from the microscopic realm. This difficulty is in fact a consequence of the time reversible laws themselves. So, it is then unfair to demand that time reversibility a the macro-level should be demonstrated in an experiment.

In case of quantum physics, you always have some macroscopic measurement device. So, here too we cannot, in practice, restore the initial state after measurements.

Exactly! That's why I asked you how you were able to argue that coupling to the environment somehow allows you to get those quantum effects!

The difference that I mentioned in my references is that those measurements were made of the non-commuting, non-contextual observable. This leaves the other observables in a superposition of states, and the effect of such superposition CAN be observed! Again, the bonding-antibonding states in chemistry clearly show this, without having to collapse the position measurement of the electron involved in the state. That's the beauty of this, and why Leggett proposed the SQUID experiments to detect even more of such superposition.

Zz.

 

[Ilja]

this is a question that has been bugging me from the beginning of my learning of quantum mechanics. why does the the wavefunction collapse when we do a measurement?

Such a question makes sense only in a more fundamental theory.

The only reasonable candidate for such a theory is pilot wave theory. In pilot wave theory, where is a wave function of the universe which does never collapse, and a configuration of the universe q(t). One can derive from these data (putting the configuration of the non-interesting part - the environment - into the wave function) an effective wave function of the interesting part.

This wave function sometimes collapses, if there is an interaction between the interesting and the non-interesting part.

now bell has proved that no hidden variable theory of quantum mechanics is not possible.

That's wrong. Bell has himself proposed pilot wave theory, which is a hidden variable theory.
It is non-local, that means, among the hidden variables we have also a hidden preferred frame.

What Bell has proven is that every hidden variable theory has to be non-local. Simply, quantum theory is also non-local, if one uses an appropriate notion of locality.

 

[Ilja]

I think I've highlighted a paper in the "Noteworthy papers" sticky that showed that even when one invokes non-locality, one still cannot save realism.

Another type of nonsense which is obviously false or makes obviously meaningless assumptions? We have a simple counterexample known as pilot wave theory.

 

[ZapperZ]

Another type of nonsense which is obviously false or makes obviously meaningless assumptions? We have a simple counterexample known as pilot wave theory.

Then I look forward to you publishing your rebuttals to S. Groeblacher et al., Nature v.446, p.871 (2007), and also your counter to Leggett's non-local realistic inequality.

Zz.

 

[Ilja]

Then I look forward to you publishing your rebuttals to S. Groeblacher et al., Nature v.446, p.871 (2007), and also your counter to Leggett's non-local realistic inequality.

Reading what Groeblacher writes about Bohm (some short comment after eq. (4)), there is no reason to publish any rebuttals. These results rebut some class of nonlocal realistic theories,
which fulfill some inequality found by Legett, which differ from SQM, but not dBB theory, which agrees with SQM.

 

[ZapperZ]

Reading what Groeblacher writes about Bohm (some short comment after eq. (4)), there is no reason to publish any rebuttals. These results rebut some class of nonlocal realistic theories,
which fulfill some inequality found by Legett, which differ from SQM, but not dBB theory, which agrees with SQM.

Then which part exactly did you find to be "... of nonsense which is obviously false or makes obviously meaningless assumptions..." when I made a reference to it, or did you not remember making such a statement?

Zz.

 

[Ilja]

Then which part exactly did you find to be "... of nonsense which is obviously false or makes obviously meaningless assumptions..." when I made a reference to it, or did you not remember making such a statement?

Initially I have responded to your claim:

I think I've highlighted a paper in the "Noteworthy papers" sticky that showed that even when one invokes non-locality, one still cannot save realism.

without reading the paper, and therefore I have not made a claim about this paper, but formulated it as a question. This has been marked with the "?" sign.

Now it is clear that it is your claim about the paper which is wrong. One clearly can save realism, in a very simple, well-known, and old way - by using pilot wave theory - and nothing in this paper questions this fact.