Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Does wave-function collapse cause the entropy of the atom (ie. the sub-atomic particle system that makes up the atom) to increase?

share|improve this question
    
It does. W.f. collapse means that you've made a measurement so the environment (e.g an emitted photon) has info about the internal state. Tracing out the environment leaves the atom in an entropyful mixed state –  Slaviks May 10 '13 at 4:33
    
And if you do postselection instead of a trace (ie detect and measure the photon) you can prepare a pure state with $ S=0 $ –  Slaviks May 10 '13 at 4:39

1 Answer 1

Disclaimer. I'm not sure it even makes sense to talk about changes in entropy of systems that undergo wavefunction collapse unless one also includes the measuring apparatus as part of the system. Having said this, here are my two cents that I hope are informative:

The (von-Neumann) entropy of a quantum system prepared in a state (density operator) is defined as $$ S(\rho) = -k_B\mathrm {tr}(\rho\ln\rho) $$ Where $k_B$ is Boltzmann's constant. In particular, If a quantum system is described by a pure state (the notion of state as an element of a Hilbert space that you learn when you start out in QM), then its entropy is zero. As a result, if you prepare an atom such that its quantum state is pure, then its entropy will not depend on which pure state it is prepared in. For example, whether it's prepared in an energy eigenstate, or a linear combination of energy eigenstates, its entropy after being prepared in any pure state will be zero.

In this sense, perhaps it can be said that the entropy of an atom that collapses from one pure state to another via projective measuement does not change.

In order for the entropy of a quantum system consisting of an atom to be greater than zero, you would have to prepare the atom in a statistical mixture of pure states. For example, you could envision coupling a large sample of atoms to a heat bath.

share|improve this answer
1  
Given that I included approximately a million disclaimers in this response specifically so as to avoid confusion but nonetheless attempt to offer something informative, I'm curious to know: what warranted a downvote? –  joshphysics Mar 10 '13 at 23:10
    
Unexplained downvotes ought to be forbidden. The person who answers sticks their necks out but the people who downvote get to be anonymous. Doesn't seem fair. –  Paul J. Gans May 10 '13 at 2:15
    
@PaulJ.Gans I agree. I also think that downvoting without a comment robs the general community of a potentially enlightening dialog. –  joshphysics May 10 '13 at 2:39

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.