4 ms·

Measurement effects are captured by projection operators (at least in standard QM). This is an equation and it is certainly not time-symmetric.

danbruc 10y ago
But who seriously believes the collapse postulate is a fundamentally true law of physics? Unitary time evolution and the collapse postulate are incompatible, so at most one is fundamental and true. And over almost a century we have collected a gigantic amount of experimental data confirming that unitary time evolution is correct. You might argue that the same is true for the collapse postulate and the Born rule but I have to disagree. Yes, it gives the correct answer in as many experiments, but those are parts of the experiments that we don't really understand. We have classical system interact with quantum systems and our measurement devices are just such gigantic quantum systems that we can't treat them quantum mechanically. The collapse postulate is obviously true in a kind of classical limit, but it is really just an emergent law and nothing fundamental. If you could show that a wave function collapse can occurs in quantum system before we measure it, that would change the game. But that is not the case, the wave function collapse only occurs when we throw a gigantic classical system at the quantum system which we can not treat properly due to its complexity. So I argue people are rightly ignoring wave function collapse when they talk about time symmetry because it is just an emergent phenomenon. And there are several ideas that can explain why it looks like a wave function collapse occurs, so we are not simply throwing a corner stone of our theory out of the window.
- mathgenius 10y ago
  > who seriously believes the collapse postulate is a fundamentally true law of physics? I would love to find out the answer to this! There needs to be some more surveys of physicists... > And there are several ideas that can explain why it looks like a wave function collapse occurs I don't think it is at all as simple as you are implying. This is still very much an open question, even if most physicists don't care to think about it.
- marcosdumay 10y ago
  How does a non-linear irreversible phenomenon emerges from repeating unitary linear interactions? Given that a reversible linear mechanics is so much more useful than one that isn't, I think it's way more likely that the universe isn't in fact linear, but the best way people got to mapping it was to shove all the non-linearity at the border of the mechanics, right before they get something from the experiment.
marcosdumay 10y ago
Projection operators do not collapse the wave function. Put some mixed state in, and you'll probably get some mixed state out (for some states it may be pure, but not all).

danbruc 10y ago

But who seriously believes the collapse postulate is a fundamentally true law of physics? Unitary time evolution and the collapse postulate are incompatible, so at most one is fundamental and true. And over almost a century we have collected a gigantic amount of experimental data confirming that unitary time evolution is correct. You might argue that the same is true for the collapse postulate and the Born rule but I have to disagree. Yes, it gives the correct answer in as many experiments, but those are parts of the experiments that we don't really understand. We have classical system interact with quantum systems and our measurement devices are just such gigantic quantum systems that we can't treat them quantum mechanically. The collapse postulate is obviously true in a kind of classical limit, but it is really just an emergent law and nothing fundamental. If you could show that a wave function collapse can occurs in quantum system before we measure it, that would change the game. But that is not the case, the wave function collapse only occurs when we throw a gigantic classical system at the quantum system which we can not treat properly due to its complexity. So I argue people are rightly ignoring wave function collapse when they talk about time symmetry because it is just an emergent phenomenon. And there are several ideas that can explain why it looks like a wave function collapse occurs, so we are not simply throwing a corner stone of our theory out of the window.

mathgenius 10y ago
> who seriously believes the collapse postulate is a fundamentally true law of physics? I would love to find out the answer to this! There needs to be some more surveys of physicists... > And there are several ideas that can explain why it looks like a wave function collapse occurs I don't think it is at all as simple as you are implying. This is still very much an open question, even if most physicists don't care to think about it.
marcosdumay 10y ago
How does a non-linear irreversible phenomenon emerges from repeating unitary linear interactions? Given that a reversible linear mechanics is so much more useful than one that isn't, I think it's way more likely that the universe isn't in fact linear, but the best way people got to mapping it was to shove all the non-linearity at the border of the mechanics, right before they get something from the experiment.

marcosdumay 10y ago

Projection operators do not collapse the wave function. Put some mixed state in, and you'll probably get some mixed state out (for some states it may be pure, but not all).