Tachyonic thermal excitations and causality

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We consider an ideal Fermi gas of tachyonic thermal excitations as a continuous medium and establish when it satisfies the causality condition. At high temperature the sound speed is always subluminal cₛ<1, but there is no stable form of tachyon matter below the critical temperature Tc<0.23m that depends on the tachyon mass m. The pressure P and energy density E cannot be arbitrary small, but P can exceed E, and P=2.36E when T→ Tc.