> This doesn’t mean that the atoms themselves are smeared out like waves; rather, what spreads is the probability distribution of them being found subsequently in a given location
No, it really does mean they're smeared out like waves. Prior to the measurement they are in superposition, relative to you. When your experiment images the atoms, that's a measurement that entangles you with the atoms.
If the observable value takes values in (A,B) then when you get entangled you end up in a state (A, measured A) + (B, measured B), each of which perceives a definite value of the measurement. The whole system would still be (to an outside, non-entangled observer, if you could pull such a thing off) in a superposition which could continue to interfere with itself, but the observer who's inside the superposition will never be able to tell.
Afaik this is the standard interpretation nowadays. Particles are waves (well, in the sense that what we call a particle is usually a momentum eigenstate that evolves in space like a wave), but the measurement process that entangles us to the causes them to come in quantized packets which we call particles.
Maybe I'm missing the point of the article somehow though...
I feel like there isn't really, not anymore, and most of the semblance of a debate is people reporting on historical debates. Although hard to be sure without, like, a survey.
At least in what I read it seems like the remaining debate is over how literally to interpret the words "many worlds", rather than whether the basic idea is correct. Ideas like entangling with the experiment and the decoherence of quantum states are very well-established.
> Ideas like entangling with the experiment and the decoherence of quantum states are very well-established.
Yes, but neither of those explains why we observe measurements to have single results. The MWI "explains" that by saying that actually, measurements have all possible results (which is what a straightforward application of the Schrodinger equation says), but we don't experience that, we experience a single result in each branch of the overall entangled wave function.
This "explanation" is treated with skepticism by many because it is completely different from how we treat individual branches of an entangled superposition in any other context besides "measurement". Normally, when two quantum systems are in an entangled superposition, we say that neither of them individually has any well-defined state at all; only the total joint system containing both of them does. We don't say that each branch of the entangled superposition is a different "world" in which each individual system has a well-defined state. The MWI has to claim that somehow when a "measurement" involved we can say that.
> Yes, but neither of those explains why we observe measurements to have single results.
IMO Everett is in fact the only interpretation that explains this quite clearly.
"Why do we observe measurements to have single results?"
"Because when you entangle with (an observable in a superposition), you enter a superposition. I.e., a mapping from states of you to states of (observable)"
People who don't believe it's possible for themselves to enter a superposition should ask themselves whether they believe atoms may, and whether they (themselves) are made of atoms.
> "Why do we observe measurements to have single results?"
"Because when you entangle with (an observable in a superposition), you enter a superposition. I.e., a mapping from states of you to states of (observable)"
This doesn't explain why we observe single results. A single result is not an entangled state. Mathematically, it's just one term of the entangled state: so observing a single result would mean the entangled state would be replaced by just one of its terms. This is what "objective collapse" interpretations say happens. But it's not what the MWI says happens.
> People who don't believe it's possible for themselves to enter a superposition
"Superposition" is not the correct term here; "entanglement" is. "Superposition" is basis dependent. "Entanglement" is not.
The relevant question is not whether people can become entangled. A state in which a measurement has a single result is not entangled. At least, not if you interpret the math the way standard QM normally interprets the math.
Edit: I should add here that this whole discussion is assuming a "realist" interpretation, where the quantum state describes the actual physical state of an individual quantum system (which might consist of subsystems, such as a measured object and the measuring device and the brain of the person reading the result off the measuring device). Not all interpretations are like that; for example, ensemble interpretations or statistical interpretations. In those interpretations the issue we are discussing doesn't even arise and "many worlds" seems like a straightforward confusion of concepts, like thinking that (to use an example from Beyond the Fringe) Venezuela must be all blue because it's colored blue on a map.
There is no real resistance I think, it's just that the various interpretations are not very relevant in practice. All of engineering that depends on quantum mechanics make do with the standard treatment that has been taught since 100 years now.
The MWI doesn't offer any other way to calculate anything (as it's an interpretation, not a theory). It's a good mental image of what happens though.
I guess at some point there will be some experiment that can be performed that requires a more advanced theoretical method than what current QM provides and then maybe the MWI (or other interpretations) get a chance.
> There's still a surprising amount of resistance for some reason.
It's not surprising at all; it's a perfectly rational unwillingness to treat our current quantum mechanics (and not even relativistic quantum field theory, but non-relativistic QM, which is already known to be just an approximation) as though it were an exactly complete fundamental theory.
Personally, it leads me to "shut up and calculate". You can use QM to make physical predictions without adopting any interpretation at all. Interpretations are not tools of physics. They're ways for people to tell themselves comforting stories about "what really happens" that have nothing to do with actually using physics to make predictions.
It's because the Copenhagen interpretation comes with the standard method of actually calculating stuff. You might call it a cheap shortcut, but it works (so far).
Please explain. I've only ever seen handwaving in pop-sci books like Sean Carrolls suggesting this (something about that the Born-rule was the "only possible equation" which seemed somewhat tautological to me), and IIRC this wasn't very encouraging either because it didn't come with any experiments that could be done to show a difference to the standard methodology and then you tend to pick the simplest method which is the one everybody else uses every day.
Would be cool if there were of course. I'm not saying it isn't interesting, I'm just saying I completely understand why most physicists don't bother as it doesn't bring anything new to their table - there are plenty of other practical quantum effects that require research as well.
> what we call a particle is usually a momentum eigenstate
Not really. In non-relativistic QM, it's a wave packet. True momentum eigenstates are not physically realizable.
In quantum field theory, "particle" is just a shorthand name for certain quantum field states. Quantum fields aren't waves or particles, they're quantum fields.
No, the superposition is a real physical thing and not just a probability distribution, the particle is in all parts of the superposition and not just randomly at one of the spots, we can see in experiments that it is in every spot. If you measure it then the superposition squared is the probability distribution of where you will find something, but it isn't just a probability.
This is what makes quantum mechanics hard, if it was just a probability distribution nobody would find that vexing its just a normal probability.
To me 'probability distribution' could be interpreted as a bunch of classical particles whose emergent behaviour resembles a wave in some way. Or a series of observations of a classical system whose errors add up to some wave-like phenomenon.
But others in this thread have mentioned that you can interfere a single electron with itself (which to my mind rules out classical-phenonema-which-appear-quantum.). And 'superposition' seems like a better word for that.
> This doesn’t mean that the atoms themselves are smeared out like waves; rather, what spreads is the probability distribution of them being found subsequently in a given location
No, it really does mean they're smeared out like waves. Prior to the measurement they are in superposition, relative to you. When your experiment images the atoms, that's a measurement that entangles you with the atoms.
If the observable value takes values in (A,B) then when you get entangled you end up in a state (A, measured A) + (B, measured B), each of which perceives a definite value of the measurement. The whole system would still be (to an outside, non-entangled observer, if you could pull such a thing off) in a superposition which could continue to interfere with itself, but the observer who's inside the superposition will never be able to tell.
Afaik this is the standard interpretation nowadays. Particles are waves (well, in the sense that what we call a particle is usually a momentum eigenstate that evolves in space like a wave), but the measurement process that entangles us to the causes them to come in quantized packets which we call particles.
Maybe I'm missing the point of the article somehow though...