Exactly how is causality violated by superluminal travel?

[LaserMind]

No, it assumes the observers are inertial and therefore can't travel faster than light, but you can measure any events at all. My post that I linked to begins with three events, without any mention of motion.

OK. Right. I have this simpler scenario. Its very simple- I hope.

Bob (the cause) kills Alice (the effect) with a laser beam that takes 10 minutes to arrive at her location.

Alice's friend (Martha) sees the dead Alice and jumps into an FTL rocket to try to reverse the death. She travels back to Bob at 5 times the speed of light. So at 5c time goes backwards and she arrives at Bob a minute before he fires his laser and implores him not to shoot.

Bob is prevented from shooting. But Alice is already dead.

Now t1 = t2/SQRT(1 - vSQUARED/cSQUARED)
= t2SQRT(1 - 25)
= t2SQRT(-25)













If something cannot be explained simply then you do not understand it. (Einstein)

 

[LaserMind]

Assume light travels a planks length in one clock tick - and also assume that this is ontologically how light (or information) travels in space-time.

Then Lorentz corrections can be applied and all is dandy as we know already.

Now, assume that in one click of the clock an entity can travel multiple planks lengths instead of just one - steps. The previous Lorentz correction is no longer applicable.

That 'entity' could be a wave-function that travels in single steps and leaves no trace of its path. Why no trace? Because it jumped in one step. A wave-function is not information per se - it is 'knowledge of quantum states'. So it is allowed to go ftl.

 

[Fredrik]

OK. Right. I have this simpler scenario. Its very simple- I hope.

Bob (the cause) kills Alice (the effect) with a laser beam that takes 10 minutes to arrive at her location.

Alice's friend (Martha) sees the dead Alice and jumps into an FTL rocket to try to reverse the death. She travels back to Bob at 5 times the speed of light. So at 5c time goes backwards and she arrives at Bob a minute before he fires his laser and implores him not to shoot.

An object emitted at speed 5c at the event of Alice's death, going at 5c in the direction towards Bob, reaches Bob 12 minutes after he fired the laser.

Also, the whole idea of Alice jumping into an FTL rocket is inconsistent with SR. Massive particles (like the ones in her body) can't be accelerated to, or past, the speed of light. The energy required to accelerate her to speed v goes to infinity as v→c.

If something cannot be explained simply then you do not understand it. (Einstein)

Only if you take this as a definition of what it means to understand something. I don't think that would make sense though. Physics is no longer about things that can be explained in simple terms. That doesn't mean that physics can't be understood.

SR is however one of those things that can be explained in simple terms. Spacetime diagrams are by far the simplest.

Assume light travels a planks length in one clock tick - and also assume that this is ontologically how light (or information) travels in space-time.

The Planck length has no special significance in special or general relativity. It only becomes relevant in quantum theories of gravity. Do you want to analyze this scenario using relativity or a quantum theory of gravity? What quantum theories of gravity do you know?