Interpretations of Quantum Mechanics

[Grey]

Over here, Spherical suggested it might be interesting to discuss the various interpretations of quantum mechanics. This is a pretty good introduction to the most prevalent, with some explanation of the similarities and differences. So, what's your opinion of the various choices? Is there one that's "obviously" the most reasonable? Should we hold out on making a judgment until someone comes up with a way to distinguish these cases observationally, or is it worthwhile to talk about it even we can't distinguish them?

And what about things like Feynman's sum over histories method? This is really a mathematical technique, but it treats particles as taking all possible paths from a given starting point to a given destination, including paths that take a side trip to another galaxy on the way. Should we take that as a serious representation of what's really going on? If reality doesn't actually behave anything like that, why does the math work out so darn well?

Or is this all just philosophical nonsense, and we should just shut up and calculate? What do you think?

[Daffy]

Wish I understood the math. John Von Neumman's speculations (particularly the "chain") are especially fascinating to me. Does consciousness exist because of matter? Or vice versa? Or both?

[Tensor]

Over here, Spherical suggested it might be interesting to discuss the various interpretations of quantum mechanics.

snip...


What do you think?

I'm of the "shut up and calculate" school. Trying to figure out the different interpretations is, to me, philosophy, not science. But, I'm sure there are others with different interpretations.

[JohnD]

All,
I'm not capable of these heights.
But I would use the example of the discipline of pharmakokinetics, the modelling of drug movement into and out of the body. There, it may be necessary to assume that the drug dissolves in a volume that is several times the size of the human (in this case) body. Nonsense? No, it works, it allows you to predict the behaviour of drugs, to decide rationally on dosage and to control infusions.
That it demands an impossible condition doesn't matter.
In other words, shut up and calculate!

John

[hhEb09'1]

I'm of the "shut up and calculate" school. Trying to figure out the different interpretations is, to me, philosophy, not science. But, I'm sure there are others with different interpretations.

At this point, I agree. It appears that they all come up with the same conclusions--but they also seem to be strikingly different.

I think the hope is, as far as each of their proponents is concerned, that by pushing the philosophical arguments far enough, we'll eventually attain a quantifiable difference that would allow us to test each version.

But another goal has already been served. There were all sorts of philosophical conclusions drawn from the randomness and uncertainty principles of quantum mechanics. The other interpretations show that those sort of conclusions are not necessarily forced by the physics. In other words, ethics and teleology are still not on as firm of a foundation as molecular biology or pharmacology. But we probably guessed that.

[gzhpcu]

What is the goal here? To understand how the universe works or to find out what corresponds to reality? IMHO, as long as we do know (if ever) the fundamental answers to what is matter, energy, space and time, we will have myriads of mathematical models and myriads of interpretations. They can not all be right. There is only one answer, but will we ever know what it is?

[Grey]

But another goal has already been served. There were all sorts of philosophical conclusions drawn from the randomness and uncertainty principles of quantum mechanics. The other interpretations show that those sort of conclusions are not necessarily forced by the physics.

That's a good point. So if you really think the universe is deterministic rather than random, you can find an interpretation that allows that, though you'll end with some weirdness somewhere else...

[Tensor]

Ok, here's my interpretation:

The universe is weird. How's that?

[Disinfo Agent]

Ok, here's my interpretation:

The universe is weird. How's that?

Weird.

I'm of the "shut up and calculate" school.

I knew I had mentioned that quote a while ago in another interesting discussion.

[Nereid]

If philosophising can help produce some new theories which, upon testing, turn out to be better than anything we've got today - specifically, an extension to QM - then let the philosophising begin!

History, that ever unreliable guide, counsels against putting too much hope into such an endeavour however.

[Van Rijn]

Well, there may be ways to distinguish between interpretations. From here, a column by physicist John Cramer:

http://www.analogsf.com/0410/altview2.shtml

Many (including me) have declared, with almost the certainty of a mathematical theorem, that it is impossible to distinguish between quantum interpretations with experimental tests. Reason: all interpretations describe the same mathematical formalism, and it is the formalism that makes the experimentally testable predictions. As it turns out, while this "theorem" is not wrong, it does contain a significant loophole. If an interpretation is not completely consistent with the mathematical formalism, it can be tested and indeed falsified. As we will see, that appears to be the situation with the Copenhagen and Many-Worlds Interpretations, among many others, while my own Transactional Interpretation easily survives the experimental test.

I'm not sure about more recent work on this, though.

[Grey]

What is the goal here? To understand how the universe works or to find out what corresponds to reality? IMHO, as long as we do know (if ever) the fundamental answers to what is matter, energy, space and time, we will have myriads of mathematical models and myriads of interpretations. They can not all be right. There is only one answer, but will we ever know what it is?

Actually, that can be the subject of debate, too. If you have two models of reality with completely different underlying frameworks, but that can be shown to make identical predictions in all cases, do you really have two models of reality after all? I'd probably say yes, but the point is arguable, I think.

I'm not sure about more recent work on this, though.

There haven't been any published journal articles about this yet, but it's created a lot of buzz. Most physicists think Afshar is mistaken, and doesn't understand a few important points. For example, from here:

In the complementarity principle, we first determine which set of photons we consider, and then we calculate both V as well as K from this set of photons. The contrast is computed from the pictures created by all these photons, and we also want to determine the "which way" information of all of them if we want to claim that K=1.

If you read the section 3 of his PDF file, Afshar seems to compute the contrast of his interference picture from a very small subset of his photons that he uses for the calculation of K - only from the photons that interact with the wire grid.

If true, that's of course silly. We can always arrange an experiment with 2 million photons - the first million will be used to create a perfectly sharp interference picture, and for the second million we will be exactly able to determine the pinhole. But this does not mean that we have K=1, V=1. We must consider the same set of photons if we want to determine K, V.

Still, your general point is valid, that we might be able to come up with some other experiment which would let us distinguish between the various interpretations.

[Peter Wilson]

What is the goal here? We have myriads of mathematical models and myriads of interpretations. They can not all be right. There is only one answer, but will we ever know what it is?

Only one answer? What about the wave-particle duality of QM? Neither interpretation is "right;" neither is "wrong." If getting the "interpretation of QM" right means holding mutually contradictory ideas, then I guess we have to accept mutually contradictory ideas (i.e. more than one).

Am I not both right and wrong?

[Disinfo Agent]

Am I not both right and wrong?

There's a 50% chance you are.

[gzhpcu]

Only one answer? What about the wave-particle duality of QM? Neither interpretation is "right;" neither is "wrong." If getting the "interpretation of QM" right means holding mutually contradictory ideas, then I guess we have to accept mutually contradictory ideas (i.e. more than one).

Am I not both right and wrong?

Not so sure. It is wave-like when not observed, and particle-like when observed. Since it is not both at the same time, I see no contradiction.

[snarkophilus]

And what about things like Feynman's sum over histories method? This is really a mathematical technique, but it treats particles as taking all possible paths from a given starting point to a given destination, including paths that take a side trip to another galaxy on the way. Should we take that as a serious representation of what's really going on? If reality doesn't actually behave anything like that, why does the math work out so darn well?

It does work well. Really well. Really, really, really well. It shocked me when I first learned how well.

Or is this all just philosophical nonsense, and we should just shut up and calculate?

Yes and no. How's that for a quantum answer?

There are good reasons for finding a deeper physical basis for this. First of all, curiosity. Second, because a good physical hypothesis will suggest new experiments that can be done. Third, I'm as sick of the EPR paradox as I am of Riemann's hypothesis. Somebody solve the stupid things already.

That was a joke, by the way. But it's one of the reasons to shut up and calculate. Maybe we're nowhere near knowing enough to solve those problems yet. Maybe our time would be better spent learning about different facets of the world, and if we go back to those problems fifty years from now, we'll have the tools to investigate more efficiently. A lot of it really is philosophical nonsense, but some of it may have value if we can figure out how to use it. The problem may be that we don't yet know enough to intelligently decide what is useful -- maybe we're really just in the alchemy stage of physics.

For most people, I really lean toward "no." If it was, for instance, Neils Bohr telling me that he was looking into a deeper meaning of it, then who am I to argue? (As an aside, I went to see that play the other day, but it was sold out ) But if it's some guy who took an English class in university and decides that because of that he can lecture to me about how the universe really has eleven dimensions and we are not responsible for our actions because of Heisenberg's principle, I feel an obligation to tell him to shut up (and calculate, I suppose, though really I just wanted him to shut up ).

[Nereid]

This is the wikipedia entry; in part:

Papers by Afshar on the experiment have been published in the American Institute of Physics and SPIE conference proceedings; however, as of May 4, 2006, neither a description of the experiment, nor any discussion of its theoretical interpretation, has been published in a refereed physics journal.

Does anyone know what's happened since early May?

In particular:
a) has anyone replicated the experiment?
b) done a similar one, with only one photon in the apparatus at a time?

The Afshar Q&A blog (ref 15 in the wikipedia article) seems to be unmoderated, so is (completely?) full of spam (and so useless).

[gzhpcu]

Only one answer? What about the wave-particle duality of QM? Neither interpretation is "right;" neither is "wrong." If getting the "interpretation of QM" right means holding mutually contradictory ideas, then I guess we have to accept mutually contradictory ideas (i.e. more than one).

Am I not both right and wrong?

What I meant was the different interpretations of QM: Everett's many worlds, Copenhagen interpretation, etc.

[Ken G]

If you have two models of reality with completely different underlying frameworks, but that can be shown to make identical predictions in all cases, do you really have two models of reality after all? I'd probably say yes, but the point is arguable, I think.

I'll take the position that the connection of theories to reality is defined entirely by their testable predictions. Thus two such theories you describe would be different theories, but not in any physically significant way-- the difference is purely, yes, pedagogical. Now I've used that word quite a bit around here, because I find a persistent confusion between what is a theory (just shut up and calculate) and what is a pedagogy (how can I picture what I just calculated, to help me do the calculation right, and get a sense of meaning behind the calculation?). Once one understands the crucial difference between these, and how one is the objective testable component of science and the other is the subjective human component, a lot of these seemingly tricky questions just go away. So my answer is, certainly do not shut up and calculate, rather, talk a lot and calculate. But never confuse the talk for the calculation. Ultimately, the test of the calculation is its accuracy and reliability, and the test of the talk is its track record for leading to proper calculations and the all-important sense of comprehension that it conveys. Note the latter may be different from individual to individual, but the former should not.

[gzhpcu]

I'll take the position that the connection of theories to reality is defined entirely by their testable predictions. Thus two such theories you describe would be different theories, but not in any physically significant way-- the difference is purely, yes, pedagogical. Now I've used that word quite a bit around here, because I find a persistent confusion between what is a theory (just shut up and calculate) and what is a pedagogy (how can I picture what I just calculated, to help me do the calculation right, and get a sense of meaning behind the calculation?). Once one understands the crucial difference between these, and how one is the objective testable component of science and the other is the subjective human component, a lot of these seemingly tricky questions just go away. So my answer is, certainly do not shut up and calculate, rather, talk a lot and calculate. But never confuse the talk for the calculation. Ultimately, the test of the calculation is its accuracy and reliability, and the test of the talk is its track record for leading to proper calculations and the all-important sense of comprehension that it conveys. Note the latter may be different from individual to individual, but the former should not.

I would think that if you have two different theories delivering the same testable predictions, then IMHO, we are not covering all aspects and making all possible measurements. Not sure if we can ever get a pedalogical view of physics (much too complex). Increase the scope of the testable environment and one of the two will hit its limits.

We are talking about 0 dimensional points, about vibrating 1-dimensional strings, about 10 spatial dimensions, but this is just math. We can't really picture it. Sure we can talk about "curled" up dimensions, but it is really not imagineable from our experience.

[Ken G]

I would think that if you have two different theories delivering the same testable predictions, then IMHO, we are not covering all aspects and making all possible measurements.

I think another key distinction between mathematics and pedagogy is that mathematics is so self-contained that if two different mathematical processes lead to the same answer in all situations, they may be said to be equivalent and for all intents and purposes they are identical. For example, 2+2=4 (the "length" approach) can also be solved by 2=1+1 and 1+1+1+1=4 (the "counting" approach). No one would say these are two different ways of doing math, but they really are pedagogically different. The equivalence is more obvious on the mathematics side than the pedagogy side, as the first can be seen in a more formal way.

An example that is more relevant to this thread is the equations of wave mechanics. One way to find what a wave will do is to solve a second-order differential equation, the "wave equation", but one can also use a mechanistic approach-- Huygens principle (each point on the wave crest is a new wave-crest source, that idea). The latter is really just a technique to solve the former, and the former is a way to describe the latter in terms of equations, but pedagogically they sound quite different. So it's not always obvious when two pedagogies are equivalent.

But the point is well taken that two different pedagogies might diverge in a realm that has so far been untested (example, Newton's laws and relativistic mechanics make all the same predictions at very low speeds, so seem like equivalent pedagogies but only over a restricted input domain.) When this is true, the mathematics are not equivalent either, but we don't know that until we consider the new domain. Thus we have another reason to favor multiple pedagogies-- not only do they allow us, as individuals, to choose the one we "prefer", it also gives us an opportunity to discover new physics in domains where the diverging pedagogies lead to different mathematical results.

[gzhpcu]

Thus we have another reason to favor multiple pedagogies-- not only do they allow us, as individuals, to choose the one we "prefer", it also gives us an opportunity to discover new physics in domains where the diverging pedagogies lead to different mathematical results.

I like the second part of the statement, but the first part, less. I don't like choosing what I "prefer", I'd like to know what corresponds to the truth.

I suspect that no current theory is correct for all possible cases, and that consequently, it is treading on thin ice when one attempts to correlate the underlying math in a theory with a physical picture/image.

[Ken G]

I like the second part of the statement, but the first part, less. I don't like choosing what I "prefer", I'd like to know what corresponds to the truth.

I suspect that no current theory is correct for all possible cases, and that consequently, it is treading on thin ice when one attempts to correlate the underlying math in a theory with a physical picture/image.

Yes, but we are just intelligent apes, when will our pedagogies ever be "the truth"? And how would we know? We never get beyond the inside of our head, yet we can interface with nature, and its great consistencies, and so we progress in practical terms. But as a human exercise, science does inevitably involve personal preference ("Occam's razor" being the classic example). Of course, that message must be made very carefully to avoid being heard wrong-- the testable aspects are not personal preference, and they are the anchor of the usefulness of science as something more than an intellectual pastime. Still, you cannot test a pedagogy, only its predictions.

[hhEb09'1]

But as a human exercise, science does inevitably involve personal preference ("Occam's razor" being the classic example).

I've said this before, and I'm only saying it again because I think it is gzhpcu's point as well, that Occam's Razor is not a scientific principle. It's more of a personal preference, as you say.

[Ken G]

And I will briefly reiterate that scientific principles are defined by the practice of science. Part of that practice is to value understanding, which involves simplification, which is where Occam's razor comes in, right in the middle of the foundations of the practice of science. (And note, the fact that you and I don't seem to agree on much of anything lately, even though we use all the same mathematics, is proof positive of the important role of pedagogy in scientific understanding.)

[gzhpcu]

Yes, but we are just intelligent apes, when will our pedagogies ever be "the truth"? And how would we know? We never get beyond the inside of our head, yet we can interface with nature, and its great consistencies, and so we progress in practical terms. But as a human exercise, science does inevitably involve personal preference ("Occam's razor" being the classic example). Of course, that message must be made very carefully to avoid being heard wrong-- the testable aspects are not personal preference, and they are the anchor of the usefulness of science as something more than an intellectual pastime. Still, you cannot test a pedagogy, only its predictions.

Correct me if I am misunderstanding you here. Are saying essentially "we are limited intellectually and incapable of understanding the truth because it is so complex and foreign to our way of thinking?".

[Ken G]

Yes, that seems quite likely.

[gzhpcu]

Then would it be better to have theories based on abstract math, like Matrix mechanics of QM, which do not lend themselves to imagery?

[Ken G]

I'm not sure I'd agree that abstract math doesn't lend itself to imagery. Indeed, the more abstract, the greater the need for imagery to guide one's thinking. When's the last time, when someone said 2+2=4, you actually imagined two things and two more things coming together?

[gzhpcu]

Yet, there is a reality out there... the search may very well futile, but fascinating regardless.... and it would be interesting to be aware of as much of it (as accurately as possible) as our limitations permit us to be...