Water waves can have forward momentum. Just like everybody else, I’m curious about quantum mechanics. The OP is interesting because it’s hard to tell the difference between observe and influence.
Sure. Momentum is another question again. You don’t even need mass to have momentum.
The difficulty is trying to use wave movement in water to reason about the quantum reality of the double slit. Molecules of water are not behaving like photons. If you just want to understand the issue of measuring the wave, fine. But that doesn’t have anything to with the linked analogy.
The hydraulic analogy of QM isn’t about water waves. As the linked article notes, droplets bouncing is closer to the Bohmian interpretation. And nobody has been able to verify interference using it experimentally. It really doesn’t help in understanding.
I get that. Does a single electron pass through both slits at the same time? I still don’t get that. I am trying to learn. Please be patient with me.
It’s not known what the electron “really” does, as it depends on the interpretation of QM that you go with. The electron does produce an interference pattern as if it were a wave that passed through both slits. But what of the particle that actually hit the screen at a single point? Did it pass through both? Did it even exist before it hit the screen? We don’t actually have answers to these, and may never.
The evolving probability of finding an electron passes though both slits and interferes with itself as a wave.
But that is just what the mathematics tells us. Sadly it tells us nothing about the underlying reality. The “shut up and calculate” interpretation of QM would have us look no deeper.
There is some form of wave like excitation in a field that represents the electron. That excitation evolves accordingly to the rules of QM. When you measure the system, something gives you a result that is governed by the probability that that evolving excitation defines. But how you regard the nature of the result, whether it is a concrete value or itself an evolving entanglement of history is not well defined. A collapsing wave function yields a concrete result visible at a macroscopic level. But that isn’t the only possible interpretation.
Again, hoping for anything more tangible or deeper is asking for something that would get you a free trip to Stockholm if you could answer it.
There are schools of thought amongst physicists, but this is more philosophy than physics, and physicists are not necessarily good at philosophy.
If that’s the case, did an electron going through both slits break into some other particles? I guess if an electron or a photon can go through both slits at the same time, then the water-wave analogy doesn’t work.
Electrons (or any particles) are in a constant flux of breaking apart into other particles and recombining, but that’s not what’s going on here. Depending on your interpretation, there are a few possibilities:
- The wavefunction for the particle goes through both slits. There’s a single point particle that follows the path set by the wave, and gets deflected to create the interference pattern.
- The wavefunction goes though both slits. There’s no point particle at all until you measure it (i.e., it hits the screen)–in which case somehow a location is selected for out of the probability-weighted positions on the screen.
- There are a zillion parallel universes and the electron travels on all possible paths (including weird ones where it’s going backwards or faster than light or whatever). In each one there’s a single path. But the ones where it goes through the slits interfere with each other in a way to form the probability distribution on the screen. We are living in one of those universes, and which universe we find ourselves in is determined by the probability distribution.
- Stop asking these questions. They have no meaning and will have no answers. The calculation is all there is.
There are more interpretations; these are just some common ones.
ETA: The fourth item in the list isn’t a command to you; it’s commonly called the “shut up and calculate interpretation” of QM. I actually find the questions interesting and that people shouldn’t stop asking them.
Does QM predict a specific value? It seems to me that as soon as you kick down the door of locality, you can have a CHSH bound of 4.
Wikipedia claims that there are three principles that limit the bound to the measured 2\sqrt{2}: no-advantage for non-local computation, information causality, and macroscopic locality. Ok, great: except that all this says is that if the Tsirelson bound were exceeded, then some bad stuff would happen. Pretty much the exact complaint about superdeterminism.
Unless an actual enforcement mechanism has been identified, then non-local interpretations have the same problem as superdeterminism in my view. Giving particles a little backdoor FTL communicator, even when limited to not transmitting classical information, still gives them too much freedom to “conspire” against you.
Curiously, Wikipedia also notes that the Tsirelson bound isn’t even known to be computationally decidable. How does the universe even “figure out” the bound in complicated cases?
That’s just one way of looking at it. You can also say that the value of the observable depends on the fact of your observations.
And it’s not just about not having definite values to the observables. It’s that the universe has to reach infinitely fast across time and space to resolve the value. Losing locality really sucks, IMO. Of course, there’s MW, but that has its own philosophical problems. Or maybe locality and even cause and effect are just emergent phenomena, and we’re just some holographic projection of some other topology in a different dimension…
No, the value 2\sqrt{2} is the maximum that can be achieved using quantum states and Hermitian observables. Those other principles you mention are part of an effort to find physical principles to limit the maximum violation of Bell inequalities to the value predicted by quantum mechanics—efforts to understand why quantum mechanics predicts this value, and no other. For instance, at one point, you might have thought that every theory that does not allow nonlocal signalling also obeys the same bounds as quantum theory, but Popescu and Roehrlich showed that that’s not the case, you can in fact reach the algebraic bound if 4 in this case. So principles like information causality and so on have been proposed to limit the maximum attainable value
Every concrete formulation of quantum mechanics with hidden variables I know of (basically Bohmian mechanics, modal theories, and Nelsonian stochastics) does include such a mechanism, the guiding field in the Bohmian case, for instance, which produces a nonlocal potential influencing particle positions.
Many problems in physics are undecidable—essentially, any system that is sufficient to implement universal computation will have undecidable statements associated with its evolution.
But standard quantum mechanics is perfectly local. Only if you insist that every quantity must have a definite value at all times (i.e. if quantum mechanics is incomplete) do you find a need to rebuke locality.
I believe that’s the gist of the
Wigner's friend - Wikipedia paradox, what happens when an observer is the thing being observed.
Does the Many Worlds interpretation of quantum mechanics purport to explain why there are separate universes of measurement outcomes at all, instead of reality being one giant wave function of everything since the Big Bang?
It kind of does say, "reality is one giant wave function’. It’s just that some branches of the wave function become unable to influence each other. (I think that’s why they call it they use the word “Worlds” instead of “Universes”).
The interesting part to me is that for the “cat in a box” or a hypothetical sapient electron in the double slit experiment is that to them, they are two separate branches in different “worlds”, but for us both branches are in the same world, at least for a little while, and for the electron it is long enough to have an observable effect.
@Grrr I am fascinated by this thread. This kind of discussion is what I like best about SD. I hadn’t seen this link yet, I found from the wiki-page link from Lumpy . If I missed it before, my apologies:
As @leahcim already said, there’s a sense in which it does exactly that—indeed, that was Everett’s original idea: that everything, including the universe as a whole, should have a quantum description. Thus, his original proposal (his PhD-thesis) was titled ‘The Theory of the Universal Wavefunction’.
Everett himself also didn’t speak about ‘worlds’—that’s a much later development, mainly due to the efforts of Bryce DeWitt in the 70s (Everett’s thesis was published in 1957, to relatively little notice). Instead, his formulation is in terms of relative states: there’s a wave function of the whole universe, such that every object in it is part of some huge, highly entangled superposition of possible states. But you can condition on any given particular state in that superposition and obtain a definite state for other entities in the world, relative to that state you conditioned on. So, for instance, relative to an experimenter seeing an electron in a spin-up-state (i. e. seeing a particular detector response), the electron will be in that state—although at the same time, one could have conditioned on the experimenter observing spin down, and have the electron be in a spin-down state as a result. ‘Separate universes’ then only appear from measurement outcomes when counting all the possible ways to condition on possible experiences of experimenters—which will in every case contain definite outcomes.
There are many ways to clarify Everett’s original idea (and they do need clarification), and one of them is to reify these relative states as separate ‘worlds’, which is justified by the fact that any possible interaction between them vanishes very quickly, thanks to decoherence. Other interpretations of Everett’s interpretation instead multiply states of mind (the ‘many minds’-interpretation), take facts to be relational quantities (‘relative fact’-interpretation), or only yields approximate different worlds in general (‘emergent multiverse’).