Among professional physicists, is there any rejection of quantum mechanics, including the uncertainty principle, as a violation of causality?
How do you get that the uncertainty principle violates causality? Would you also claim that it violates causality in the classical, macroscopic systems where it’s observed?
Now, Bell’s inequality, that can be argued to violate causality. It’s pretty definitively established by experiment and by Bell’s work that something that seems like it ought to be intuitively true must be violated, and causality is one of the options. But if you hold causality to be a sacred cow, then there are other options to sacrifice at that altar, while still keeping quantum mechanics as we know it and as experiment shows it.
I can’t see how Bell can be interpreted to imply causality violation?
Obviously, the experimental results per se don’t involve causality violation!
If we try to form an intuitive classical account of entanglement, we end up with Einstein’s “spooky action at a distance”. But to be consistent with the experimental results, this spooky instantaneous action must lack the usual cause-effect property of “action”, since no superluminal communication is possible. Since no classical “action” is ever like that, ultimately don’t you just have to accept that there is no classical intuitive picture that is consistent with all aspects of QM reality?
Einstein, Albert; Letter to Max Born (4 December 1926); The Born-Einstein Letters (translated by Irene Born) (Walker and Company, New York, 1971) ISBN 0-8027-0326-7.
dem Geheimnis des Alten … words to live by
Well, if you accept Special Relativity (which I think pretty much everyone does), then causality is nearly synonymous with locality, and nonlocality is indeed one possible resolution to Bell’s inequality. Though it’s only nearly synonymous, and nonlocality of the sort required to resolve Bell’s inequality could still preserve classical causality perfectly (though not on the quantum level, and of course nothing is ever truly classical).
EDIT: That Einstein quote gets trotted out a lot, but keep in mind that it was from before Bell’s work and the corresponding experiments had been done. At the time, there was a lot more room to disagree with QM than there is now, or even at the time of Einstein’s death.
Yup, thanks.
Can you explain what the other option would entail exactly - i.e. giving up Realism. I’ve never fully grasped what a non-Realistic theory would look like, and how that would “save” locality given the experimental results.
Yup. And his coining of “spooky action at a distance” was almost like Hubble coining “Big Bang”. Saying - this is the crazy place that this theory takes us, so our current picture can’t be quite right, future experiments will show something more.
Einstein might have told them not to do the experiments on his lawn, but I think he would have accepted the results of the Bell Inequality expts had he been alive to see them, and realized that he was probably wrong. He does not seem to have been an immodest fellow given his status.
I was watching a PBS documentary a few years ago (NOVA I think). They made the claim that every single experiment conducted to test QM resulted in affirming it, even the crazy loony-tunes stuff. I was certainly impressed …
Now where’s my hover-car?
It will be delivered last Tuesday.
Hoyle of course.
No.
The OP asked “is”. Certainly there was a lot of opposition/concern 100 years ago when the theory was new.
There are two general ways that people oppose quantum physics.
First, the crazy-ass denialists, who, like the Nazis, suspect the origin of the theory. Some complain that it, like Relativity, implies moral relativism, and thus it can’t be true. Like evolution.
But there is a small and relatively legitimate group of people who argue for “hidden variables” – and I have to be careful, because this is a term of art and can mean different things in different contexts. For instance, there are some who suggest that a Uranium nucleus has a “mechanism” for “counting down” to the point where it shall fission, and that the half-life effect is not random at all, but totally determined.
(Again, this is tricky, because “determinism” also has more than one meaning. SDMB threads have derailed completely over this.)
There are those who suggest there are inner mechanisms, of which we know nothing as yet, that inform an electron which slit of the double-slit experiment to pass. In this view, there is no randomness; everything is mechanically determined, even pre-determined, reviving Newtonian (and theological?) determinism.
This latter view is not insane. It’s just completely unfounded and has zero experimental support. It’s “not even wrong.” There’s no way, at this point, to falsify it, as it is purely speculative. But it’s not bugfuck crazy, which is nice, after listening to those in the first moiety.
Local hidden variables are ruled out by the Bell’s Theorem experiments.
So, are you talking about Superdeterminism? I though that was the only hidden variable theory left - and that really is pretty much batshit insane. It basically says that you can’t sample randomly because the entire universe is a conspiracy.
There are perfectly valid nonlocal hidden variable formulations of quantum mechanics. Are they right? Well, they’re formulations of quantum mechanics, so as far as science is concerned, they’re as right as any other formulation of quantum mechanics. All of them get you to the same place; they just describe the route differently.
Out of my depth; I can’t swim here, so I’ll sink…
(My objection to the “mechanism that tells a Uranium atom when to decay” is that it would have to contain a huge amount of information. It would have to have an amazingly complex “timer” circuit, able to count down to zero. Also, that timer seems to be initialized to random times, so, really, what’s the difference? But there are people who insist that atoms do not decay “randomly” but according to some physical timing mechanism. I don’t know enough to say whether that’s batshit crazy, or just really far-out speculation supported by no evidence whatever.)
Trivia note (for anyone who didn’t already know): Max Born’s grand-daughter is Olivia Newton-John.
When a photon travels from one mirror to another, the two interactions are just two events in space-time. The photon doesn’t “know” which was in the past and which in the future — just that the two events are correlated. Thus, information travels “faster than light” by simply considering the two events reversed. If time’s arrow just emerges from macroscopic correlations, E-P-R is no longer paradoxical.
No one rejects quantum mechanics itself. That would make no more sense than rejecting logarithms as a means of multiplying numbers. More than any other fundamental physics theory that can before it, it nearly explicitly rejects the idea that it should describe how things “actually are,” and instead restricts itself severely to calculating the results of well-defined and strictly realizable experiments. Meaning you can’t ask QM to answer the question 'What happens if I observe the electron with some as-yet-undiscovered apparatus using undiscovered physics that does not perturb it the way ordinary light or particles would? Could I then catch it in the very act of tunneling?" In response to such a question, QM says “mu.” That may not seem at first important, but bear in mind it is not how any previous theory worked. Newtonian mechanics, for example, casually assumed that absolutely anything about a physical system was, in principle, observable without perturbation. Sure, maybe it required some instrumentation so delicate as to not yet have been invented, but in principle you could measure any property whatsoever at any time – that, therefore, these things should be assumed to actually exist and have certain nature, obtained by observation, assumption, and common sense. And then we deduce experimental observation from there.
QM works entirely the other way around. We assume nothing at all about what things are “really” like, and only seek to predict the results of experiments we actually know how to do. We may not ask the results of experiments we cannot precisely define. QM has nothing to say about them. Where is the electron “while” it is tunneling? Until someone comes up with a precise, experimentally realizable definition of what “while” means in that system, QM refuses to say. Classical mechanics would say, and what it would say is it can’t be where it seems it must be for the observation of tunneling. QM gets around the fact that we nevertheless observe tunneling by simply refusing to speculate on how the electron gets from one place it can be to another without going through a place it can’t be. At least, again, until someone dreams up a precise definition of the measurement that we want to make to resolve the puzzle.
So strictly speaking QM is just a method, like logarithms, for calculating stuff, like products. You can use other methods – there are many other ways to formulate QM – but of course you have to get the same results, because those results are experimental facts. Would a mathematician argue that logarithms aren’t “real” because the idea of 10^2.445 makes no sense? How can you multiply 10 by itself 2.445 times? Well, they might, but most mathematicians would shrug and say, it’s just a method, and it works, and we can interpret 10^2.445 in a way that’s a logical extension of 10^2 = 10*10, so…meh. Who cares? That’s more or less how it is as far as QM itself goes.
What you might be getting at, however, is the question of the interpretation of QM in terms of what “ultimate reality” is like. What is the electron actually really doing, to the extent “actually” and “really” have any meaning outside of experiment? Now we get into the real of quantum metaphysics, which is often called “the measurement problem.” It centers around the question of what we want to say is beyond what we measure. Do you continue to exist when you’re unconscious? If a tree falls in the forest and no one hears it, does it still make a noise? That kind of stuff. We’re asking: does an electron still behave like what we think a particle should be like even when we can’t measure it?
There are various answers, all philosophical, none with any (so far) experimental support whatsoever. There are some who draw a hard line, and say “if you can’t measure it, I have no opinion on whether it exists or not, so bugger off.” Where is the electron “while” it’s tunneling? Nowhere. Anywhere. Everywhere. Doesn’t matter, because existence that can’t be measured has no meaning. Then there are some who say the problem is our lack of imagination. QM and the funky behaviour is pointing us towards some “intuitive” picture of what is going on, but we’re just to dumb and unimaginative to see it. We’re blind men fumbling with the elephant: we feel a trunk, we feel the legs and the tail, but we just can’t imagine the whole creature, so we’re baffled – we make up some story about a trunk that morphs into a tail sometimes and a legs others, all very logical, but no clear image. I think people who like pilot-wave theories or many-worlds theories are kind of in this camp. Finally, there are people who think QM is just incomplete in some as yet unknown way. There is a way we could use a fundamental physics theory to figure out what an electron “really” is, and what it’s “really” doing all the time – but QM isn’t it. It’s somewhat how Newtonian mechanics is a very precise approximation to relativistic mechanics for very low energies. It’s as if we were now doing some experiments at high energies, but lacked the imagination to develop relativity, and so were using weird ad hoc extensions of Newtonian mechanics that gave us the correct answers, but lacked the key insights of relativity about “what is” (e.g. the relativity of interval, the constancy of c). People in this camp are discouraged by the fact that there are no experiments at all that point to some limitation or inadequacy of QM. Or rather, if there are such experiments, they are probably at energies (Big Bang energies) that lie permanently out of our reach.
Several things could happen:
(1) Someone may imagine a new physics that predicts everything QM does (it must) and some other stuff, stuff we somehow haven’t thought of measuring yet but have the capacity to measure, so that we can know it’s better – and it provides some description of reality that suits our intuitive understanding of what it means to say what “is.” Unfortunately, in the entire history of empirical science, this has never happened. All major advances in theoretical understanding have been driven by puzzling experiments. We do not think of the revolutionary theories first, then discover the experiments that provoke them.
(2) Someone may stumble across the right kind of experiment, which provides a result QM cannot calculate right – and then we’re off to the races. No one can imagine such an experiment right now (at achievable energies), but that doesn’t mean no one ever will. The future goes on for a very long time.
(3) We may just be stuck here. We may have gone as far in understanding how the universe works, fundamentally, as our intelligence can take us. The species of cats can be said to understand Newtonian mechanics in some sense, because they land on their feet, but I think we can reliably say that no cat will ever ever understand relativity. It’s simply beyond their intelligence. There is no guarantee that the true nature of the universe is simple enough for our species to understand it, but, weirdly, too complicated for every other known species to understand it. When you think about it, that does seem a little strange, that we should be so perfectly positioned – that we should be the one and only species with no limits to what it can understand. Probably the only way (3) could be confirmed would be if we met some alien species that was as much smarter than us as we are smarter than cats, and they told us that they understood the universe, and moreover, that it was forever out of our capacity to understand. Assuming, that is, we have the ability to understand that communication itself.
Hidden variable theories are not as disreputable as you think - for example J S Bell himself favoured hidden variable theories.
In vanilla QM alpha decay of uranium-238 can be modelled semi-classically the quantum tunelling of the alpha particle out of the potential well of the uranium atom. In de Broglie-Bohm theory (a hidden variables theory) the similarly-simplified explanation for the escape of the alpha particle would be the trajectory of the particle of the combined potential of the potential well and the quantum potential. The apparent randomness of alpha decay is then interpreted as our inability to precisely determine initial conditions and the highly non-trivial nature of the quantum potential.