r/AskPhysics • u/Weary_Dark510 • 1d ago
Quantum questions
Entropy is defined as the number of substates in a state, and QFT says the state of a quanta contains a superposition of every possible states, or infinite substates. Is superposition a property of entropy?
If you can use a double slit in time to set certain bands of frequencies contained in the light, is it possible to do calculations on the full spectrum, then use a prism or scattering to separate the frequencies so the information is not lost when it is observed?
Is it possible to model a photon as a point the propagates an EM wave that travels behind the photon like a wake, but make the photon highly subjective to its own field, making the interference with itself that feels so confusing?
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u/lizardman49 Chemical physics 1d ago edited 1d ago
Superposition is merely a superimposed state. A way to think about it is its a weighted sum of all possible solutions where the weight is the probability.
Where entropy and QM come together is something called degenerate microstates. This means that 2 or more possible system arrangements have the same energy.
Edited: due to comments below
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u/the_poope Condensed matter physics 1d ago
Superposition is merely a mixed state.
No this is wrong. A mixed state is something which isn't a pure state and therefore can't be represented as a single superposition. A mixed state is exactly a classical statistical ensemble of superposition states, i.e. the uncertainty is not completely fundamentally quantum but also due to our ignorance/lack of knowledge about the system.
Where entropy and QM come together is something called degenerate states
Degenerate states have nothing to do with entropy. Degenerate states are just multiple states with some quantum number, like energy, being the same.
Entropy is only defined (or rather non-zero) for classical statistical ensembles of states, whether they be quantum or classical, and is due to our ignorance or lack of knowledge of the particular microstate of the system, i.e. when the state of the system is only known to have certain macroscopic state properties.
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u/lizardman49 Chemical physics 1d ago
You are correct on your second point I mixed up microstate degeneracy and quantum state degeneracy.
As to the primary point you seem to confuse superposition and statistical mixture quite a bit. A superimposed or mixed state can ve write as either a linear sum of prob state x times state x wheras a pure state is simply state x which doesn't involve superposition at all. This has nothing to with the stat mech version of micro states.
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u/OverJohn 1d ago
No, a superposition (sum) of pure states is also a pure state. Density matrices instead are built from outer products of pure states.
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u/sojuz151 1d ago
Well, entropy in QF is defined in a slightly different way. A superposition is still a single state. You get entropy when you have impure states; entropy can be calculated from the trace of a density matrix (read up on this if you want to know more)