 98346 H.J. Borchers, J. Yngvason
 Modular groups of Quantum Fields in Thermal States
(52K, Tex, figures in Postscript)
May 15, 98

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Abstract. For a quantum field in a thermal equilibrium state we discuss the
group generated by time translations and the modular action associated
with an algebra invariant under halfsided translations. The modular
flows associated with the algebras of the forward light cone and a
spacelike wedge admit a simple geometric description in two
dimensional models that factorize in lightcone coordinates. At large
distances from the domain boundary compared to the inverse temperature
the flow pattern is essentially the same as time translations, whereas
the zero temperature results are approximately reproduced close to the
edge of the wedge and the apex of the cone. Associated with each
domain there is also a one parameter group with a positive generator,
for which the thermal state is a ground state. Formally, this may be
regarded as a certain converse of the Unruheffect.
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