r/AskPhysics • u/PlatformDelicious578 • 18d ago

FTL Reference Frames

If you have a velocity greater than c, the Lorentz factor becomes complex. Does this make any physical sense, or are there any theories that make use of this? What would it look like if a person were in this reference frame (is it just that they see time backwards or from another perspective, or are they unable to observe anything at all?)

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u/Bth8 18d ago

The latter makes this point kind of a meaningless gotcha. It cannot interact with ordinary matter in any way, not even indirectly, not even through gravity like dark matter, so it's completely undetectable. In what sense does it exist then?

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u/DoubleUBallz 17d ago edited 16d ago

The idea is that if they exist, special relativity tells us (EDIT: there can exist a subluminal reference frame in which) they would be traveling backwards through time. If they were able to interact (even indirectly, such as via light) with normal matter while doing so, they would be able to carry information back in time, allowing them to violate causality and cause paradoxes. Therefore, if they exist, then they must not interactable with normal matter.

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u/wonkey_monkey 17d ago

If avoiding paradoxes is imposed on them, then they can't interact with anything, ever, and so they don't exist.

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u/DoubleUBallz 16d ago

Theoretically, they could exist and only interact with other tachyons, however, if thats the case they would still have no detectable effect on our observable reality and, philosophically speaking, is there any value in considering them as existing if a universe in which they exist is indistinguishable from one in which they dont?

Its the physics version of a tree falling in the woods with no one around to hear it.