Leifer’s main target in his talk was the Copenhagen interpretation of quantum mechanics, which he says most physicists subscribe to (though there were doubts expressed about that in the room). I’m wary of attempting to define what it is because a large part of Leifer’s argument is that there is no consistent definition. But it’s the interpretation attributed to a bunch of quantum theory’s founding fathers — and the one that physicists are often taught at school. It says that before you look, a quantum object is described by a wavefunction that encompasses a number of possibilities (a particle being here and there, a cat being dead and alive), and that when you look, this collapses into definiteness. Schrödinger’s equation allows you to calculate the probability of the outcome of a quantum experiment, but you can’t really know, and probably shouldn’t even worry about, what’s happening before you look.Leifer is one of the more sensible quantum theorists, and it is nice to see him acknowledge that Copenhagen is the dominant interpretation.
On top of that, Leifer argues that Copenhagen-like interpretations, rather than being the most sensible option (as is often claimed), are actually just as whacky as, for instance, the Many World’s Interpretation.
He hates Copenhagen, and defines it this way:
Observers observe. Universality - everything can be described by quantum mechanics. ... No deeper description of reality to be had. ... Quantum systems may very well have properties, but they are just ineffable. For some reason, whatever properties they have, they're fundamentally impossible to represent those properties in language, mathematics, physics, pictures, whatever you like.He objects to this philosophically, but admits that it is a reasonable position.
The real problem is his "universality" assumption. To him, this means that a time-reversible Schroedinger equation applies to everything, and it enables an observer to reverse an experiment. He goes on to describe a paradox resulting from this reversibility.
I don't remember Bohr or anyone else saying that observers can reverse experiments.
Time reversibility is a bizarre philosophical belief, as I discussed recently. The reasons for believing in it do not have much to do with quantum mechanics. Much of physics, including quantum mechanics, statistical mechanics, and thermodynamics, teaches that time is not reversible.
Leifer claims to have an argument that Copenhagen is strange, but he really has an argument that time reversibility is strange.
His real problem is that he rejects positivism. To the positivist, a system having ineffable problems is completely acceptable. I do not expect to understand subatomic physics by relating properties to ordinary human experiences of the 5 senses. I think it would be bizarre if an atom had a wave function that perfectly represented reality. Get over it. That is not even what science is all about.
On the subject of time symmetry, Quanta mag article:
“That signifies nothing. For us believing physicists, the distinction between past, present and future is only a stubbornly persistent illusion.”
Einstein’s statement was not merely an attempt at consolation. Many physicists argue that Einstein’s position is implied by the two pillars of modern physics: Einstein’s masterpiece, the general theory of relativity, and the Standard Model of particle physics. The laws that underlie these theories are time-symmetric — that is, the physics they describe is the same, regardless of whether the variable called “time” increases or decreases. Moreover, they say nothing at all about the point we call “now” — a special moment (or so it appears) for us, but seemingly undefined when we talk about the universe at large. The resulting timeless cosmos is sometimes called a “block universe” — a static block of space-time in which any flow of time, or passage through it, must presumably be a mental construct or other illusion.
Considering that ALL the precious 'space times' are calculated spaces, that is, they do not exist outside of mathematical constructions done by humans or with computers, why is reversibility even being considered?ReplyDelete
I can look at a graph representing stock trading activity over a single day, or a single year, or several years, or decades. The fact that I can REPRESENT data graphically over varying spans of time has no bearing on the fact that such representations are dependent upon change over time to even exist, much less have the data collected, collated, processed, and graphically depicted. Causality trumps bad interpretation...always.
Just because you can 'pretend' via a graph that time isn't real has no bearing whatsoever on reality that it took a certain amount of time to create and view the damn graph, and even more time to evaluate it.
You don't address the most problematic part of the modern physicists' beliefs---the one which Leifer has correctly isolated here: ``No deeper description of reality to be had.''
IMO, it is this conviction of the mainstream physicists which represents the real crux of the matter. Not this feature vs. that feature of this interpretation vs. that interpretation. The most problematic aspect of QM is this (working) position, and the set of obnoxious attitudes which it (actually) generates.
[To me, the epistemological status of this position seems to be at par with the arbitrary assertion that people having bachelor's and master's degrees in Metallurgy cannot become professors of Mechanical Engineering---even if they have PhDs in Mechanical.]
Ajit, I am a logical positivist. Reality is what we observe, and what we can determine to be true. Some questions are unanswerable, and it doesn't bother me. This search for a deeper reality is misguided by anti-positivist beliefs.ReplyDelete
No, IMO, the search for a deeper mechanism lying ``beneath'' QM does not have to be supposed as springing from a desire to challenge the actual facts of reality and the proper conceptual considerations which _have_ gone in building the QM theory.
A deeper-than-QM theory could keep those facts just the same, and still manage to be a theory. The following is a rough outline of what I have in mind.
(i) A measurement is supposed to collapse the wavefunction. (ii) The collapse is supposed to be something that is in principle ``ineffable.'' Experiments can only ever refer to the initial and final states---that is the reason put forth.
I can be OK with (i) (at least for the sake of argument), but never with (ii), because the latter is tantamount to the idea of an _acausal_ collapse.
Now, suppose, someone refused to take the ineffability seriously, and _also_ then succeeded giving a hypothetical mechanism for the collapse. Suppose that this new hypothetical mechanism wasn't just a smart conceptual rephrasing of the same previously known facts; suppose it had an actually new physical content. Which implies that it could, and did, make some new quantitative predictions that could be subject to experimental validation.
For instance, in the current QM, nothing is known (or even blindly asserted) about any aspect of the timing of the QM jumps. Not just the time to the next jump, but not even average frequencies. The aspect of time simply does not enter when it comes to quantum jumps.
Now _assume_, for the sake of argument, that the new mechanism is in some way analogous to the catastrophic buckling of a slender column loaded under compressive forces. Analogously, the quantum wavefunction too ``buckles,'' so to speak, from a superposition state to a measured eigenstate. Suppose that this is the new hypothesis put forth, with the required quantitative details (say concerning some aspects related to the timing of the quantum jumps).
What _then_ would _really_ matter in science is whether the ensuing quantitative predictions matched with reality or not. Whether past Nobel laureates had banned thinking of such a mechanism would not matter---only the correspondence to physical reality would.
Now, if sufficiently refined experiments could be conducted to the required level of precision (and of course assuming that the proposed mechanism also tied in well conceptually, in a non-contradictory manner, with the rest of knowledge), then, this hypothetical mechanism _would_ get validated.
The development _would_ thus yield a new theory that was deeper than QM as we know it as of now.
There is nothing _in_ the QM theory that can rule out such possibilities. No physics theory in fact can have anything in it which, just qua theory, can proscribe its own generalization. The calorific theory of heat couldn't assert that a kinetic description is ineffable, esp. on the grounds that it itself was powerful enough to explain the heat phenomena as they were known at a certain point in the history of physics. Further down the road, there was nothing in Classical Mechanics that could rule out QM. Similarly, the logic can be taken forward. That is my basic point.
Assuming a new theory of the kind outlined above actually becomes established. Even then, the central role of ``measurements'' and operators in QM would still remain intact---it would only get delimited and subsumed under the new more general theory, that's all.
It's just that the supposed ineffability of the collapse would have been chucked out the window.
The admonishment to stop looking for a deeper-than-QM description is the same as stopping progress in physics. It is either that, or, it is: willfully ejecting oneself out of physics. They are the two sides of the same coin. Too bad physicists chose the coin.
You're right, there is always that the possibility that a improved theory will give a deeper understanding. Let's hope one does. But Leifer is saying that because Copenhagen does not address some unobservable issues, it is just as strange as many-worlds. I do not agree with that.ReplyDelete
Turns out that once again my writing went too lengthy; thanks for reading through it.
I had not listened to Leifer's talk; I had relied just on the one phrase you reported.
But no, if that is his position, it's not tenable. The two are not equally crazy. The ``no deeper theory'' sure was a dogma first generated by none other than the CI (e.g., before the baby Bells turned dogmatic too). Yet, even then, CI is _less_ mindless as compared to the MWI.
As a matter of fact, CI is quite intellectually demanding to understand and pin down exactly, because it is pretty highly convoluted in terms of its ``logic''; the intricacy of Bohr's (and others') intellectualizations made sure of that. In contrast, MWI is very, very straight-forward a madness. So straight-forward, that to me it mostly looks plain humorous---if at all that.
If developments such as CI could be compared to a riches-to-rags story of physics brought about by a washed down Kant, then MWI can be compared to the plain hippiness of drugs-induced dreams bubbling in nothingness on a nude beach.
Then why do some reasonable people run after it---I mean the MWI? I have a hypothesis.
Just the way in CS Data Structures, you have this abstract diagram whereby you can go from a stack to a tree which makes the process easier to understand because of the _simultaneous_ visualization of all the events (say function calls), similarly, some people may find some talent in the suggestion of visualizing all the various branches of all the quantum events as a tree. (Qua visualization it works only for a discrete Hilbert space, but then, who cares for the continuous one these days?)
It's ``just'' that while in the tree visualization of the stack, each branch from a node is taken up for traversal at _different_ times, whereas in the MWI, they take _both_ (or _all_) the branches off a node at the _same_ time. And, whereas a stack's operation does not pretend to be spanning the universe, MWI's steps do. So what if they have to go 100% subjective by spinning entire universes at every node, in the process.
I wouldn't mind if they were to regard it merely as a tool to help understand the logical spread of a discrete quantum system. But then, they don't stop here. They go overboard. All the way, and in toto. _That_ makes it funny. The resulting error is so bad, so thoroughly bad, that one doesn't take it up even for criticism. One just laughs at it and let it go at that.
[OK, let me stop. Some time in future, I will move these comments to my blog.]