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There are many classical systems that have properties we consider real that are nevertheless not practically measurable. For example, grains of sand have defini

There are many classical systems that have properties we consider real that are nevertheless not practically measurable. For example, grains of sand have definite position and momentum, yet you can't in reality measure them for each grain of sand when you want to describe the flow of a sand in an hourglass. So the practical ability to measure something is not required for us to consider something "real". Now, Bell's inequalities show that any theory of local hidden variables independent from the measurement apparatus is inconsistent with QM and with experimental observations. However, that still leaves the possibility of non-local hidden variables possible, and it also leaves open the possibility that the experiments have been flawed - that the measurement decision was somehow correlated with the result. The Copenhagen interpretation is BOTH non-real and non-local, so I don't find a real non-local theory a priori problematic. Whether pilot wave theory can be made consistent with at least special relativity is still open as far as I know, but perhaps there is some equivalent of the no-communication theorem that will be found. Superdeterminism is also seeing a resurgence - I can't claim to understand how it could be made into a real scientific theory, but perhaps there is something that is escaping me and we will see a successful theory come out of it. Now, the matter of whether something can be considered "real" but fudamentally un-measurable (as opposed to just being practically un-measurable) is in the end a problem of definitions. I would also note that there is no fundamental quantum randomness - quantum phenomena behave according to a linear equation, so they are fundamentally deterministic. It is only measurement (the interaction between quantum systems and classical systems) that introduces randomness - the superposition of all the solutions to the Schrodinger equation is replaced with a single position&momentum with some probability, and with error bars that are subject to the uncertainty principle.

lisper 6y ago
> grains of sand have definite position and momentum No, they don't. The uncertainty principle applies just as much to grains of sand as it does to electrons. The only reason they appear to have definite position and momentum is that their masses are large, so a very small velocity produces a very large momentum which gives them a very small wavelength, much smaller than their physical extent. But the uncertainty principle still applies. With the right technology you could do a two-slit experiment on grains of sand. See: https://www.wired.com/story/even-huge-molecules-follow-the-quantum-worlds-bizarre-rules/ https://www.wired.com/story/even-huge-molecules-follow-the-q...
- Koshkin 6y ago
  To be fair, the parent was talking about "classical systems," not about the applicability of quantum-mechanical principles to macroscopic objects.
  - lisper 6y ago
    But there is in reality no such thing as a classical system. The world we live in is quantum. Its behavior approaches that of a classical system as the number of entangled degrees of freedom grows, to the point where classical physics becomes a damn good approximation. But reality never actually becomes classical. The wave function does not actually collapse. And grains of sand do not have actual positions and momemta.
    - simiones 6y ago
      Whether this is actually true or not remains an open question, but it is indeed the expected consequence of QM. However, there is as yet no experiment showing any kind of quantum behavior for huge objects (closest is a few hundred million atoms as far as I know). Still, the point was about a classical model, not about physical reality being classical or quantum. To rephrase, my point was that even in a classical model there are quantities that can't practically be measured (even though classical mechanics as a model assumes all objects have definite positions and momentums), and that doesn't stop us from accepting in such a model that they are 'real'. So, the idea of property that is 'real' but not really measurable already exists in classical physics, it is not an invention of exotic interpretations of QM.
- deleted 6y ago
  [deleted]
jostylr 6y ago
> Whether pilot wave theory can be made consistent with at least special relativity is still open as far as I know It can be made consistent, namely, adding a foliation (a notion of a canonical present moment) makes it work out fine. The foliation is not detectable, it gives the right predictions, it is mathematically sound, etc. But it does feel philosophically sketchy and not fundamentally relativistic. However, there is a paper https://arxiv.org/abs/1307.1714 https://arxiv.org/abs/1307.1714 which explores how to derive foliations from the wave function itself and thus would be as relativistic as in theory involving the wave function. The article also points to other articles with a variety of ideas how to deal with nonlocality in Bohmian mechanics.

lisper 6y ago

> grains of sand have definite position and momentum No, they don't. The uncertainty principle applies just as much to grains of sand as it does to electrons. The only reason they appear to have definite position and momentum is that their masses are large, so a very small velocity produces a very large momentum which gives them a very small wavelength, much smaller than their physical extent. But the uncertainty principle still applies. With the right technology you could do a two-slit experiment on grains of sand. See: https://www.wired.com/story/even-huge-molecules-follow-the-quantum-worlds-bizarre-rules/ https://www.wired.com/story/even-huge-molecules-follow-the-q...

Koshkin 6y ago
To be fair, the parent was talking about "classical systems," not about the applicability of quantum-mechanical principles to macroscopic objects.
- lisper 6y ago
  But there is in reality no such thing as a classical system. The world we live in is quantum. Its behavior approaches that of a classical system as the number of entangled degrees of freedom grows, to the point where classical physics becomes a damn good approximation. But reality never actually becomes classical. The wave function does not actually collapse. And grains of sand do not have actual positions and momemta.
  - simiones 6y ago
    Whether this is actually true or not remains an open question, but it is indeed the expected consequence of QM. However, there is as yet no experiment showing any kind of quantum behavior for huge objects (closest is a few hundred million atoms as far as I know). Still, the point was about a classical model, not about physical reality being classical or quantum. To rephrase, my point was that even in a classical model there are quantities that can't practically be measured (even though classical mechanics as a model assumes all objects have definite positions and momentums), and that doesn't stop us from accepting in such a model that they are 'real'. So, the idea of property that is 'real' but not really measurable already exists in classical physics, it is not an invention of exotic interpretations of QM.
deleted 6y ago
[deleted]

jostylr 6y ago

> Whether pilot wave theory can be made consistent with at least special relativity is still open as far as I know It can be made consistent, namely, adding a foliation (a notion of a canonical present moment) makes it work out fine. The foliation is not detectable, it gives the right predictions, it is mathematically sound, etc. But it does feel philosophically sketchy and not fundamentally relativistic. However, there is a paper https://arxiv.org/abs/1307.1714 https://arxiv.org/abs/1307.1714 which explores how to derive foliations from the wave function itself and thus would be as relativistic as in theory involving the wave function. The article also points to other articles with a variety of ideas how to deal with nonlocality in Bohmian mechanics.