- #1
fluidistic
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Homework Statement
Hello guys,
I am extremely worried because I do not understand something. My statistical mechanics course somehow follows Reichl's book for some parts.
For a system of N particles, Reichl's define ##\rho (\vec X^N, t)## as the density of probability on the phase space. ##\vec X^N## is a point in a 6N dimensional phase space. (page 288).
A few pages later he gets that ##\rho (\vec X^N, t)=e^{-i\hat L ^N t}\rho(\vec X ^N ,0)##.
Then my lecture notes go on and skipping several steps they reach that ##\rho (\vec X^N, t)=\sum _{j=1}^N e^{-i \lambda _j t} c_j f_j (\vec X ^N)## where the lambda _j's are the eigenvalues of the Liouville's operator and the f_j's are the eigenvectors of such operator. c_j's are constants (could be complex numbers I suppose).
But when I look at that expression I cannot see how rho can be real.
I took the special case of N=2 and wrote the first 2 terms of the series.
Homework Equations
Euler's equation.
The Attempt at a Solution
I believe that for rho to be real for all t, every single term of the series must be real but I'm not 100% sure.
So, by using N=2 I got the condition that ##r_1 \sin (\theta _1 -t \lambda _1)=r_2 \sin (\theta _2 -t \lambda _2)## but in my opinion that is impossible to be satisfied for all t's. Mainly because every lambda is different and so the 2 sines have a different frequency...
Anyway I also had a look a Fourier expansion of a function but in general the series starts from -j up to j in such a way that it's easy to see that all complex terms vanish. And when the series starts from j=1, everything is real. So overall there is no ambiguity that the function is real.
But in my case it's different... and I see no way how rho could be real.
Any thoughts will be appreciated. This boggles me a lot.