Proof of expectation value for a dynamic observable

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Homework Help Overview

The discussion revolves around proving a relationship involving the expectation value of a dynamic observable in the context of classical mechanics, specifically using the concept of Poisson brackets.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to relate the time derivative of the expectation value to the Poisson bracket but questions the validity of a substitution involving the trace operation. Other participants suggest using the cyclic property of the trace and expanding the Poisson bracket for further insights.

Discussion Status

The discussion is ongoing, with participants exploring different properties of the trace and the implications of the Poisson bracket. There is no explicit consensus yet, but various lines of reasoning are being examined.

Contextual Notes

Participants are navigating the complexities of the mathematical properties involved, particularly concerning the trace and Poisson brackets, without resolving the underlying assumptions or constraints of the problem.

digogalvao
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Homework Statement


Show that:
d<A(q,p)>/dt=<{A,H}>, where {A,H} is a Poisson Bracket

Homework Equations


Liouville theorem

The Attempt at a Solution


<A>=Tr(Aρ)⇒d<A>/dt=Tr(Adρ/dt)=Tr(A{H,ρ})
So, in order to get the correct result, Tr(A{H,ρ}) must be equal to Tr({A,H}ρ), but I don't think I can do that substitution. Is it valid? How can I prove that?
 
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Use the cyclic property of the trace: Tr(ABC) = Tr(BCA) = Tr(CAB).
 
vela said:
Use the cyclic property of the trace: Tr(ABC) = Tr(BCA) = Tr(CAB).
But there is a Poisson Bracket in it...
 
Good point.
 
So? lol
 
Try expanding out the Poisson bracket and see if there's something you can do with the derivatives.
 
No luck :(
 
Bump...
 

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