4 ms·

It's also worth skimming the wikipedia page on quantum decoherence [1]. Quantum mechanics forms a consistent theory without any concept of randomness or wave f

by dguest 6y ago

It's also worth skimming the wikipedia page on quantum decoherence [1]. Quantum mechanics forms a consistent theory without any concept of randomness or wave function collapse, and decoherence explains how this evolves into a classical observation when things get "big". There is nothing fundamentally random about this evolution. We still use random numbers (wave function collapse) to approximate observations, because in many cases quantum calculations for macroscopic objects would be intractable, and because they give the same answer to any precision we can hope to measure. Every experiment that demonstrates quantum entanglement is essentially just showing that quantum mechanics still applies on systems that were previously thought to behave classically. And none of these experiments have demonstrated a mechanism for wave function collapse beyond what we already understand from decoherence. [1]: https://en.wikipedia.org/wiki/Quantum_decoherence https://en.wikipedia.org/wiki/Quantum_decoherence

simiones 6y ago
No, that is a misunderstanding of what decoherence does or does not buy you. Decoherence does explain why macroscopic systems can't (normally) exhibit self-interference and some other quantum effects. However, decoherence doesn't in any way solve the other aspect of the measurement problem: why the object is only ever found entirely at one location instead of appearing with different amplitudes at multiple locations, as the Schrodinger equation predicts. That is the source of the "fundamental randomness" and it is not explained by decoherence. For a slightly more mathematical (but still very understandable) explanation of what decoherence solves and what it doesn't solve, I recommend this article [0] by Sabine Hossenfelder. [0] https://backreaction.blogspot.com/2020/08/understanding-quantum-mechanics-5.html https://backreaction.blogspot.com/2020/08/understanding-quan...
- dguest 6y ago
  Exactly, you end up with objects appearing at multiple locations that don't interfere with each other. This is completely consistent with observation, it's just weird: it means all the outcomes actually happened and the observer just can't see it. So decoherence either fails to solve the measurement problem or solves it by saying the universe is weird.
  - simiones 6y ago
    It is certainly not consistent with observations. Imagine you set up two detectors at two locations. Imagine further that the schrodinger equation shows that the particle has the same non-0 amplitude at both locations. Nevertheless, you will NEVER see both detectors detect the same particle. You know with 100% certainty that if one detector "saw" the particle, the other one didn't (this is true regardless of how far apart the detectors are). Furthermore, imagine an experiment like this: 1. A particle P1 is fired, and it has some amplitude at locations Det1 and Det2. 2. At location Det1, a second particle P2 is made to collide with P1; the collision will throw P2 towards location Det3. Now, the correct probability of finding P2 at location Det3 depends on whether Det1/Det2 are activated. If Det1 detects P1, the probability for P2 to reach Det3 is 0, but this is inconsistent with the Schrodinger equation for P1 and P2. To make the results consistent with observations, you must update the wave function for P1 and P2 after Det1 has detected P1. Now, the MWI tries to circumvent this by postulating that both results happen at the same time, just in different worlds; and that Det3 becomes entangled with Det1, so their results are perfectly correlated. But that runs into other problems, both philosophical (how could the other worlds be "real" if they are fundamentally undetectable) and formal (e.g. it is not yet agreed that a consistent, non-circular definition for probability in the face of many worlds can be given that is consistent with the observed probabilities computed according to the Born rule). Edit: I should also mention that without the Born rule, if Det1 and Det2 were both going to launch a satellite if they detected P1, then both satellites would have some amplitude in orbit; and you would start expecting to see the gravity of both satellites, which breaks off with experimental observations even more sharply.

by dguest 6y ago

simiones 6y ago

No, that is a misunderstanding of what decoherence does or does not buy you. Decoherence does explain why macroscopic systems can't (normally) exhibit self-interference and some other quantum effects. However, decoherence doesn't in any way solve the other aspect of the measurement problem: why the object is only ever found entirely at one location instead of appearing with different amplitudes at multiple locations, as the Schrodinger equation predicts. That is the source of the "fundamental randomness" and it is not explained by decoherence. For a slightly more mathematical (but still very understandable) explanation of what decoherence solves and what it doesn't solve, I recommend this article [0] by Sabine Hossenfelder. [0] https://backreaction.blogspot.com/2020/08/understanding-quantum-mechanics-5.html https://backreaction.blogspot.com/2020/08/understanding-quan...

dguest 6y ago
Exactly, you end up with objects appearing at multiple locations that don't interfere with each other. This is completely consistent with observation, it's just weird: it means all the outcomes actually happened and the observer just can't see it. So decoherence either fails to solve the measurement problem or solves it by saying the universe is weird.
- simiones 6y ago
  It is certainly not consistent with observations. Imagine you set up two detectors at two locations. Imagine further that the schrodinger equation shows that the particle has the same non-0 amplitude at both locations. Nevertheless, you will NEVER see both detectors detect the same particle. You know with 100% certainty that if one detector "saw" the particle, the other one didn't (this is true regardless of how far apart the detectors are). Furthermore, imagine an experiment like this: 1. A particle P1 is fired, and it has some amplitude at locations Det1 and Det2. 2. At location Det1, a second particle P2 is made to collide with P1; the collision will throw P2 towards location Det3. Now, the correct probability of finding P2 at location Det3 depends on whether Det1/Det2 are activated. If Det1 detects P1, the probability for P2 to reach Det3 is 0, but this is inconsistent with the Schrodinger equation for P1 and P2. To make the results consistent with observations, you must update the wave function for P1 and P2 after Det1 has detected P1. Now, the MWI tries to circumvent this by postulating that both results happen at the same time, just in different worlds; and that Det3 becomes entangled with Det1, so their results are perfectly correlated. But that runs into other problems, both philosophical (how could the other worlds be "real" if they are fundamentally undetectable) and formal (e.g. it is not yet agreed that a consistent, non-circular definition for probability in the face of many worlds can be given that is consistent with the observed probabilities computed according to the Born rule). Edit: I should also mention that without the Born rule, if Det1 and Det2 were both going to launch a satellite if they detected P1, then both satellites would have some amplitude in orbit; and you would start expecting to see the gravity of both satellites, which breaks off with experimental observations even more sharply.