Quantum Physics: I need some sanity here x_x

[69dodge]

This need have nothing to do with conciousness. The measurement problem in quantum mechanics always and only happens when a system we can model with quantum mechanics interacts with a system which we cannot model with quantum mechanics (because it's too complex and/or we cannot know its initial state). The result of this looks like probabilistic collapse of the wave function, but that may be just an artifact of not being able to track the dynamics anymore. One explanation for what happens is to privilege conciousness on the part of the observer, and say that conciousness is responsible for the collapse, but there's no actual need to do so, and in my opinion no good reason to either.

Ok. Your theory says there's never a collapse. Rather, an experimenter and his experimental apparatus become entangled, and the wave function describing both continues to evolve unitarily. When then, according to your theory, does the experimenter become aware of the result of his experiment, and which result does he become aware of, out of the many that are possible? It is undeniable that experimenters do become aware of experimental results. Shouldn't any theory that purports to solve the measurement problem be able to explain how this happens?

I don't mean "how" in any deep philosophical sense. Just that the theory should at least acknowledge that it does happen, and should tell us when. A theory that involves only wave functions evolving says nothing about conscious observers, whose observations it supposedly is trying to explain. How could that possibly work?

Collapse serves two purposes in the current theory. It tells us what wave function to use for predicting future results, namely the collapsed wave function rather than what it was before the collapse. But it also tells us when the experimenter observes this result, namely when the collapse occurs.

You can get around the first point by saying that, to predict a future result, a second experimenter can use an uncollapsed wave function describing both the first experimenter and his apparatus, instead of using a collapsed wave function describing just the apparatus. But how do you explain why the first experimenter observed the result that he did, or any result at all for that matter?

A theory can't explain conscious observation if it refuses even to talk about it.

 

[stevea]

So one can know the time of an event but not the location of the event with precision governed by the Heisenberg Indeterminancy Principle and versa visa.

Well you can know the time and location of an event according to the Uncertainy principle, what is limited is the location*momentum product or any combination of variables that reduce to this.

I have a background in physics, but I cannot suggest any layman's books on the topic. I find that any abstract topic leads many popular authors into the arena of wholesale unsupported extrapolation. Books that try to relate QM to consciousness (without any serious supportive evidence) should be recycled immediately. I recall years ago scanning a book on holography that also included the authors pet unsupported theory of the human thinking operating by a similar mathematics - all hooey !

Personally I'd set aside any book that uses Schoedinger's Cat as a primary model. It's a cute joke, but as you can see on this forum it misleads one into many silly questions about intelligent observers and consciousness. One can readily observe photon diffraction with a sheet of film and an automatic analyser and this is done every day. To speculate that human observation changes these results is magical thinking (and that's exact where the 'cat problem' entices the reader).

The wave/particle cartoon was a good little snippet, but as already mentioned it becomes shakey at the point where they discuss observation at the double slit. They have of course ignored the discussion of observation it the back screen, and the necessary consideration of Heisenberg's uncertainty principle wrt observation. The problem is that 2 minute cartoon cannot follow all the topics to their ends.

I wish it wasn't true, but a mathematical conception of QM is really necessary. Everything else is a cartoonish view. Sadly most undergrad physics texts give a rather poor presentation of QM IMO (historical development approach).

It is the place where our classical conception of how large objects behave runs into the reality of how very small things behave. The wave/partcile duality is a human concept for sure, the actual objects will interact as they interact and don't have to meet our expectations. This really bothered Bohr even as he and a bunch of other people struggled to make sense of QM.

Let me suggest that the 'particle' viewpoint is the anthropomorphic view, and so there is no paradox to puzzle over here. The QM wavefunction descriptions are consistent without any difficulty and we can even measure single particle diffraction and make other 'wave' properties of things we normally consider to be 'particles'. Particles in macroscopic ensembles just have very tightly confined wavefunctions. The valence electrons wavefunction in a penny may extend over the entire volume of the coin, but it drop off at a high exponential rate beyond the edges. These same valence electons are repulsed by the electons of the molecules in your palm - thus giving the 'particle' sensibility for this ensemble. Still, it's simple enough in our moden era to observe the coin's electron wavefunction extend through a classically 'forbidden' potential and tunnel into a nearby detector where the small but non-zero wavefunction penetrates. 'Particles' are merely an unwarranted extrapolation from our macroscopic observations to a more microscopic domain. We were not expecting such short wavelengths.

I am not aware that N.Bohr was particualy puzzled by the duality as you suggest. "Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems". (Niels Bohr, Atomic Physics and the Description of Nature, 1934).

 

[Dancing David]

My comment on Bohr comes from readin a number of biographies about him, I cae away feeling that he wished that the equations would conform to expectations and was frustrated by it, but he was very open to accepting that they didn't meet his expectation as well. I should not have expressed it as being directly related to the alleged wave/particle duality but QM in general.

 

[Ziggurat]

Ok. Your theory says there's never a collapse. Rather, an experimenter and his experimental apparatus become entangled, and the wave function describing both continues to evolve unitarily. When then, according to your theory, does the experimenter become aware of the result of his experiment, and which result does he become aware of, out of the many that are possible? It is undeniable that experimenters do become aware of experimental results. Shouldn't any theory that purports to solve the measurement problem be able to explain how this happens?

How do you know multiple results are possible? You don't actually know that, except to the extent that multiple initial states are possible. But if you somehow could know the initial state of the entire apparatus and model its time evolution, how do you know that you wouldn't end up with only one deterministic result for any single initial state? Quantum mechanics suggests that's exactly what should happen: there is no equation anywhere in quantum mechanics that describes a discontinuous time evolution for a wave function. Collapse only happens when we stop using quantum mechanics, it is not what quantum mechanics itself describes.

We can't test whether or not collapse is actually happening because we don't know the initial state of our entire setup, we couldn't model its time evolution even if we did, we can't prepare the apparatus in specific quantum states even if we could know what that state was, and the number of quantum states for a macroscopic measurement system is astronomically larger than the number of "result" states. These practical problems prevent us from testing the theory fully, and they leave a hole in which collapse might indeed be some discrete, real process. But that's only a guess. It might be correct, but we most certainly do not know that it is correct.

I don't mean "how" in any deep philosophical sense. Just that the theory should at least acknowledge that it does happen, and should tell us when.

That's easy. "Collapse" happens whenever our quantum system interacts with a system with an unknown quantum state but with a large number of possible states. In other words, it happens when we find ourselves unable to use quantum mechanics to describe the dynamics, not because it's wrong but because we can't do the calculations. Find any example of collapse, and you'll find that such an interaction is occuring. So there's simply no need to posit that anything more.

A theory that involves only wave functions evolving says nothing about conscious observers, whose observations it supposedly is trying to explain. How could that possibly work?

That last question is exactly my question about "collapse": it either leaves unanswerable questions (what exactly are the criteria for collapse to occur? What counts as "counscious"?), or else it just becomes a sophisticated version of solipsism.

Collapse serves two purposes in the current theory. It tells us what wave function to use for predicting future results, namely the collapsed wave function rather than what it was before the collapse. But it also tells us when the experimenter observes this result, namely when the collapse occurs.

Except that it can't answer the question of what counts as an experimenter, which makes all the other answers pretty much just hand-waving.

A theory can't explain conscious observation if it refuses even to talk about it.

Sure. But that assumes that conscious observation is a critical component. But we cannot conclude that it is.