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Exactly how is causality violated by superluminal travel? 
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#19
May2610, 10:31 AM

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See also this post (and the comment about the typo in #138).



#20
Feb1012, 12:37 PM

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I know this is an old post, but isn't the reason why there would be causality violations simply a function of the speed of light a constant  thereby making relativity mathematics useless in ANY case involving formulas involving v>c?



#21
Feb1012, 01:22 PM

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#22
Feb1012, 02:47 PM

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I agree with you that if we assume nothing can go faster than light then strong causality would fail if anything did somehow manage to go faster than light. That follows from simple logic not any physics laws. So it leaves the more interesting question of why that speed is as it is rather than lets assume its the fastest speed. Takers anyone? (without obfuscation of course).... 


#23
Feb1012, 03:04 PM

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Im a simpleton, it should be mentioned this is observed causality being discussed, in which case...big deal.
Getting information faster then expected hardly means a "break/failure" in causality right? It would mean observations of cause/effect are not invariant. 


#24
Feb1012, 04:59 PM

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See Tachyonic antitelephone for details. 


#25
Feb1112, 01:57 AM

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Secondly There is breaking 'weak causality' and breaking 'strong causality'. Breaking weak causality is just fine  and probably happening in quantum step changes. So, if the speed of 'information travel' was faster than the maximum speed (achieved by some unspecified means) it could break weak causality  sure. There is nothing wrong with that (mathematically or conceptually). But, it could not alter a history on its journey because that would break strong causality and then an 'event' could happen before the 'cause' which (I believe) is an absurdity. What do you think? 


#26
Feb1112, 05:41 AM

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*) I used infinitespeed particles in that post only to make it easier to visualize it in a spacetime diagram. 


#27
Feb1112, 09:40 AM

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Here is a third where I give a specific numerical example: Speeds greater than the speed of light, post #42 There is also an illustrative spacetime diagram (although for different values) in post #32 of that same thread. 


#28
Feb1212, 12:53 PM

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Then when we say, well, in this situation something IS travelling faster than light. But then Lorentz would be based on a false assumption and the equations are incorrect? The time turns out negative in the mathematics, but the equations used are not correct because we are assuming something is FTL (that we earlier stated was not possible). Is there a really easy example that is unambiguous and easy to follow? All the examples quoted seem convoluted and hard to follow. And surely we needn't use a spacetime diagram? Any takers? 


#29
Feb1212, 01:58 PM

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Spacetime diagrams are orders of magnitude easier to understand than almost everything else in physics. They are certainly easier to understand than Lorentz transformations, which require algebra, while spacetime diagrams require no math skills at all. 


#30
Feb1212, 04:16 PM

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#31
Feb1312, 01:46 AM

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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) 


#32
Feb1312, 01:59 AM

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Assume light travels a planks length in one clock tick  and also assume that this is ontologically how light (or information) travels in spacetime.
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 wavefunction that travels in single steps and leaves no trace of its path. Why no trace? Because it jumped in one step. A wavefunction is not information per se  it is 'knowledge of quantum states'. So it is allowed to go ftl. 


#33
Feb1312, 04:02 AM

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


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