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The postulate of the preferred reference frame in which the signalpropagation is governed by retarded causality is a must for any theory offaster-than-light particles and signals. Such a system does exist and is thecomoving system of the relativistic cosmology. Restrictions imposed by thecausality conservation can be expressed via a causal $\Theta$ function assumedto be acting in both, the classical and quantum field theories of tachyons. ALorentz-covariant introduction of this $\Theta$ function, which ensures acausal behaviour of real tachyons (asymptotic tachyon states) preventing theappearance of causal loops constructed with the use of faster-than-lightparticles and signals, into the tachyon quantum field operators is suggested.

How to implant a causal $\Theta$ function into a tachyon field operator, or why tachyons do not violate causality