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Jul
19
awarded  Yearling
Jan
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awarded  Popular Question
Dec
6
awarded  Popular Question
Jul
19
awarded  Yearling
Jul
10
comment commutator to entropy in an uncertainty relationship?
@Trimok, al-Hwarizmi: even if $\rho \ln \rho$ is Hermitian (I'm not certain that it is, actually), it is not the operator for entropy. $S$ is a scalar quantity - it's the trace over $\rho \ln \rho$. As Trimok points out, not every Hermitian operator corresponds to an observable.
Jul
9
comment commutator to entropy in an uncertainty relationship?
@Qmechanic: thanks!
Jul
9
comment commutator to entropy in an uncertainty relationship?
@joshphysics - Of course! Well spotted! Unfortunately stack exchange doesn't let you edit comments after five minutes, so my mistake remains frozen as a relic for all eternity.
Jul
9
comment commutator to entropy in an uncertainty relationship?
Entropy is a macroscopic quantity, defined in terms of the density matrix: $S = -\text{Tr}(\rho \ln \rho)$. It doesn't have an associated Hermitian operator. Think of it as a kind of expectation value.
May
22
comment Chaos is predictable?
There's another great quote that I can't find the source for that goes something like "Chaos is like Justice Stewart's view of pornography - you know it when you see it"
May
22
comment Chaos is predictable?
@J.C.Leitão "there is no universally accepted mathematical definition of chaos"
Mar
19
revised Multiple measurements of the same quantity - combining uncertainties
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Mar
19
revised Multiple measurements of the same quantity - combining uncertainties
added 30 characters in body
Mar
19
asked Multiple measurements of the same quantity - combining uncertainties
Jan
17
accepted What is the nature of the correspondence between unitary operators and reversible change?
Jan
16
comment What is the nature of the correspondence between unitary operators and reversible change?
That's helpful, thank you. Please could you explain in a little more detail why it is that finite-time evolutions of this sort should be reversible?
Jan
16
revised What is the nature of the correspondence between unitary operators and reversible change?
added 89 characters in body
Jan
16
asked What is the nature of the correspondence between unitary operators and reversible change?
Nov
2
reviewed Reviewed Finding the diameter a parachute must be to land a probe on Mars at specific speed
Nov
2
reviewed Reviewed Is there a symmetry associated to the conservation of information?
Nov
2
reviewed Reviewed What are the challenges to achieving cold fusion?