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by cyberax 603 days ago
Not quite. There are two speeds in relativity: the maximum speed and non-maximum speeds (i.e. everything else). It just so happens that light travels at the maximum speed.

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.
2 comments
ThrowAaaaway 603 days ago
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?

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cosignal 603 days ago
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?
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cyberax 603 days ago
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.

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MattPalmer1086 603 days ago
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?
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cyberax 603 days ago
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.
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MattPalmer1086 602 days ago
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...

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