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Sistemi relativistici
Corso di laurea magistrale 201718
F. Becattini
Sommario lezione
Insieme grancanonico rotazionale
Formulazione covariante della Meccanica Statistica
Grandcanonical partition function with fixed angular momentum – J large
Saddlepoint expansion for J (and V) large: introduction of a rotational potential (=angular velocity)
L
S
Macroscopic rotating systems:GCE of an ideal Boltzmann gas with large J
J
Solution: if the region is symmetric with respect to the J axis:EXERCISE
Definition of the ``rotational potential”
The usual procedure canonical grandcanonical→based on saddlepoint expansion leads to the socalledGrandcanonicalrotational partition function:
For an ideal Boltzmann gas
Landau's argument on the equilibrium of macroscopic bodiesE
i, P
i
Maximize entropy with constraints
Local temperature
ifv=0
v
Rotating relativistic system in thermodynamical equilibrium:outer layers are HOTTER than inner layers
BEWARE the distinction between LOCAL temperature (in the local rest frame) and GLOBAL temperature
Similarly, the local chemical potential also depends on the distance from the centre
(Tolman’s law)
Quantum grandcanonical derivation
We can obtain the same statistical operator by maximizing entropy with the constraints of constant mean value of angular momentum, fourmomentum and charge
Maximization (remember first lecture) leads to, regardless of commutation of the single operatorsSo one can write expressions like:
Generalized entropy expression
We will see the most general expression later. For the present, let us focus on a systemdescribed by:
Which can be taken as the expression in the rest frame
Covariant formulation
In quantum field theory:
This statistical operator is seen from the frame where the system is at rest.Can we make it fully covariant?
The stationarity becomes a condition on independence of the hypersurface S
1
2
If the flux of the stressenergytensor through the spacelike part of the hypersurface vanishes,a sufficient condition is the vanishingof the divergence of the integrand
Without a spin tensor, the stressenergy tensor is symmetric and thus a sufficientcondition for the vanishing of divergence is:
Killing equation
The vector field fulfills the above equation, whose general solutionis:
with b constant and w antisymmetric constant field
Thermodynamic equilibrium conditions
The rotational case is recovered by setting
But there might be other interesting cases.