Supersonic speed doesn't violate laws of nature or violates casuality. Superluminal speed in matter just produces Cherenkov's radiation.
At superluminal speed, we will be able to hit photons in any order, including reverse order, or emit photons and catch them later. Why this is a problem for casuality?
I know next to nothing about physics, but isn't the "It just so happens" part of what you said potentially misleading? Isn't it that light is massless, so that directly implies that it would travel at the maximum speed?
It's a distinction with no difference. Only massless particles can move at the "maximum speed", in our Universe these are photons and gluons. And maybe gravitons, if they exist.
Similarly, massless particles can't move below the "maximum speed", as they'll have no energy or momentum and won't be able to interact with anything.
Why would the absence of mass force something to travel at the maximum speed? I could understand it being able to travel at that speed; why would it always do so though?
Theoretically, a massless particle can move slower than light. But then it'll have zero energy and momentum, so it won't be able to interact with anything.
Thanks. The equations give the answer in the end, I found this explanation the best [1].
The only way to get a zero mass is if the energy is zero, or the velocity is c.
[1] https://profoundphysics.com/why-do-photons-have-no-mass-simp...
Supra-luminal signals will almost necessarily imply that it's possible to violate causality by arranging a series of supra-luminal communications. Not every supra-luminal communication violates causality, but it's hard to think of a way to build a consistent theory that would both preclude causality violations and permit at least some supra-luminal signaling.