The tachyon has a spin of 0, does any other particle have a spin of 0?
Any scalar field particle, like the Higgs, has spin zero. Of course, we've not observed one (yet) so these are hypothetical particles.
What is a "scalar field particle"?
I have trouble with this. First, nobody has seen a tachyon. Second, a spin-1/2 or spin-1 tachyon is no more and no less likely than a spin-0 tachyon.
This I think refers to the tachyon "problem" in string theories, which is the lowest angular momentum particle along the trajectory (so is probably a scalar ).
This may be a stupid question, but would a spin 1/2 tachyon break the Lorentz symmetry?
Ack. Yes. Stupid questions. Tachyons break Lorentz invariance anyway.
Maybe *I* am the one not thinking but I don't see that a tachyon breaks Lorentz invariance. They may give problems with causality and other stuff but I don't think they break Lorentz invariance (if they do, in what way?) For example, if they travel faster than c in one frame, that will be the case in all frames. Maybe I am missing something obvious. I have never worked much with theories with tachyons but I thougth they were always written in a Lorentz inavraint form.
You can have as many tachyons in your theory as you like. It's only when you allow them to couple to ordinary matter that all hell breaks loose in your theory. But sterile tachyons are fine.
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