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> The equations of physics are algebraic and symmetrical, whereas causal relationships are directional. I don't agree with this. Quantum measurements are proje

> The equations of physics are algebraic and symmetrical, whereas causal relationships are directional. I don't agree with this. Quantum measurements are projective and they are very much one way. People seem to want to dismiss this as being just "epistemic" like the way entropy (thermodynamics) is one-way, but not fundamentally one way. Entropy increases only because we can't see all the details. Quantum measurements are not like that.

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antidesitter 8y ago
Many physicists believe quantum “measurements” are just an approximation to underlying unitary processes. See https://news.ycombinator.com/item?id=17894779 https://news.ycombinator.com/item?id=17894779 for more discussion. See also https://en.wikipedia.org/wiki/Quantum_decoherence https://en.wikipedia.org/wiki/Quantum_decoherence.
nabla9 8y ago
As far as I know the theory does not completely exclude time-reversal formulations of QM. Non-symmetry of quantum measurements has not been experimentally confirmed. The only confirmed T-symmetry violation is that of K and B mesons, I think. But that does not follow from quantum theory I think. It's experimental result.
skybrian 8y ago
I remember reading a post that entropy increases because we classify higher entropy states as essentially the same and there are more of them. Is there a good explanation of this?
a1369209993 8y ago
Actually, quantum 'measurements' are like that; decoherence happens when a entangled photon (or whatever paticle) escapes far enough away that you can't retrieve it and force it's horizonal and vertical states to feed amplitude into the same outcome. This generally happens because you lost track of that photon (and countless others), the same way you lose track of the blackbody radiation photons your heat engine emitted to cool down. Edit: And actually, causal relationships aren't directional, they just look that way because we usually care about "What can we infer about the more complicated [thermodynamically later] system state given knowledge about the less complicated [thermodynamically earlier] one?" rather than vice versa. (Well, also because most physical processes exchange information about about particle velocities (heat differentials) for information about the state and layout of macroscopic-ish objects (knowledge), and the laws of thermodynamics result in those exchanges all going in the same direction.)

deleted 8y ago

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antidesitter 8y ago

Many physicists believe quantum “measurements” are just an approximation to underlying unitary processes. See https://news.ycombinator.com/item?id=17894779 https://news.ycombinator.com/item?id=17894779 for more discussion. See also https://en.wikipedia.org/wiki/Quantum_decoherence https://en.wikipedia.org/wiki/Quantum_decoherence.

nabla9 8y ago

As far as I know the theory does not completely exclude time-reversal formulations of QM. Non-symmetry of quantum measurements has not been experimentally confirmed. The only confirmed T-symmetry violation is that of K and B mesons, I think. But that does not follow from quantum theory I think. It's experimental result.

skybrian 8y ago

I remember reading a post that entropy increases because we classify higher entropy states as essentially the same and there are more of them. Is there a good explanation of this?

a1369209993 8y ago

Actually, quantum 'measurements' are like that; decoherence happens when a entangled photon (or whatever paticle) escapes far enough away that you can't retrieve it and force it's horizonal and vertical states to feed amplitude into the same outcome. This generally happens because you lost track of that photon (and countless others), the same way you lose track of the blackbody radiation photons your heat engine emitted to cool down. Edit: And actually, causal relationships aren't directional, they just look that way because we usually care about "What can we infer about the more complicated [thermodynamically later] system state given knowledge about the less complicated [thermodynamically earlier] one?" rather than vice versa. (Well, also because most physical processes exchange information about about particle velocities (heat differentials) for information about the state and layout of macroscopic-ish objects (knowledge), and the laws of thermodynamics result in those exchanges all going in the same direction.)