Comments by "Lepi Doptera" (@lepidoptera9337) on "Why Going Faster-Than-Light Leads to Time Paradoxes" video.

Because nothing in Cherenkov radiation is going faster than the speed of light. The charged quanta are merely going faster than the group velocity in the Cherenkov medium, which is always slower than c.
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The Fermi paradox is a social phenomenon, not a physical one. That somebody doesn't want to talk to you because you are not attractive to them has nothing to do with the speed limit on your local highway. You are just not their type. ;-)
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The problem with these explanations is that they are focusing on the wrong scenario. There is no non-causal behavior in the rest system of the two endpoints A and B or the ship S that traverses the distance between them at v>c. The problem occurs when an external observer tries to establish causality from a rest system that moves at a non-zero velocity relative to A and B. FTL brakes the causal consistency conditions, i.e. an outside observer could see you being alive before your grandfather was and they could see the timeline between the two of you running backwards.
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The problem does not occur in Earth's reference frame. It occurs in SOME reference frame moving relative to Earth. As long as all causality is restricted to speed <c, all observers will agree that event A happened before event B if A happens before B in the reference frame of A and B. If the causality is connecting these events at >c, however, there is a reference frame in which B will come before A, i.e. in case of the grandfather paradox some observer will see you being born before your grandfather. I didn't think this through entirely, but intuitively one might expect time to flow in the opposite direction on the FTL ship itself (apart from the fact that it acquires an imaginary component which makes the system unstable). I Might be wrong about the last part, but the general lore about tachyonic systems is that they are always unstable. Now, all of this only holds if nature is actually relative. If it is not, then one can construct systems which have different energy bands, just like phonons and charge carriers in semiconductors. We would be in one of these bands and tachyons in another. There would be a gap between the two. By supplying enough energy one can "jump" the gap and then the new state would be in the higher band. We have accelerated protons to roughly 7TeV so far without observing this energy gap and "jump". We have been able to observe much, much higher energy cosmic rays (I believe by a factor of a million) and they still didn't become tachyonic, so if such a band exists, then it's at energies above the highest energy cosmic ray that has been observed. It would be completely useless for either communication or travel.
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@bryansjodin What about ALL observers vs. SOME observers did you not understand just now? I the distinction too subtle? :-)
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It is not in the past from the viewpoint of the ship. It is in the past from the viewpoint of Earth. In relativity the finite speed of light is simply a consistency criterion that assures that causality is guaranteed for ALL observers and not just for some. Of course you can always pick observers even in the FTL scenario that won't see a causality violation, but that's not the case for ALL observers. There will always be some who will see unphysical effects that simply do not happen in nature.
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It's not black magic. It's just a necessary consistency condition that has been violated. The universe is keeping its laws for all observers at all time, not just for some observers some of the time.
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That is correct. One can not derive from either these diagrams or the math of special relativity what would happen in situations that simply can't happen. ;-)
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The finite speed of light is a necessary consistency condition for causality. There is no way around it. If we break this consistency condition even once, then we break causality in an infinite number of ways. Nobody can put that Humpty Dumpty back together, again.
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Of course. None of this requires any consciousness. You can play the entire game with high energy scattering processes involving tachyons. If you do, then there will be a flood of tachyons coming at you before you even turn on the accelerator. If you just think about building a really high energy accelerator there will be a flood of tachyons coming at you. And boy, if you are thinking about building a really, really high energy accelerator, then the Neanderthals will see a bright light in the sky that will completely wipe out all life on Earth. So don't you dare thinking about accelerators for tachyon research! You don't want to hurt the Neanderthals. ;-)
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@bryansjodin This has nothing to do with ONE observer. Relativity makes a statement about ALL observers. If relativity holds AND causality holds, then nothing causal can happen beyond the speed of light. You can let go of relativity, causality or both and then these algebraic restrictions fall away. So either your grandfather is not responsible for bringing one of your parents in this world (and you just randomly appeared with your grandfather being correlated to some observers) or physics is not relative. We do not have any evidence for either of these conditions being violated. That makes FTL fiction at this moment.
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So how comes you asked but you didn't receive? ;-)
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Time is not a dimension. Time is that which the clocks show. The diagram basically relates clocks on Earth to clocks on Vega to clocks on the STL ship. The problem is that clocks order causal events by assigning time stamps to them. The only way you can have FTL is by allowing these time stamps to change almost randomly. So what is a causal event for one observer is not one for the other. If you make a movie out of this then you would actually see the frames in the opposite order, i.e. FTL could make it look like somebody on Earth could predict a supernova that hasn't happened, yet. Or predict next week's lottery numbers... which would make all kinds of games of luck completely irrelevant in an FTL universe.
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Yes, that's what tachyons do. They step out... and as a result they are immediately gone. :-)
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It does when you are on an FTL ship, but that's because the diagram is actually wrong for that case. The light cone gets narrower as we approach the speed of light and it contracts to the diagonal as we reach it. That's the famous "light does not experience time" case. However, if we evaluate it past that speed, then the cone doesn't start opening up, again, instead if flips 90 degrees out of the diagram. Time becomes imaginary. That's tachyons. Tachyons are not experiencing time because they are constantly decaying. There simply is no FTL coordinate system in which stable matter can exist.
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The speed of light is always the same for all observers.
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That sounds cool until you realize that you will have to branch the universe an infinite number of times because the causality violations happen differently for an infinite number of different observers. Just where is the energy coming from to create all those branch universes? :-)
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The universe does not have an FTL transmitter. It does everything at or slower than light speed. But since the universe has actual consequences, you can't avoid paradoxes. You need to pay more attention in school, kid. ;-)
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@ Yes, you are correct about that. My attention span was just about enough to earn a PhD in physics, but I never even came close to mastering your job as a parking attendant. ;-)
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Quantum entanglement is the equivalent of Newton's third law. It's simply momentum/angular momentum conservation. Conservation laws aren't effects at a distance. They are the absence of conservation law breaking mechanisms.
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That's because there is nothing fundamental about the speed of sound.
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It doesn't. Distances are to the right and the left. Time is towards the top.
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Why are you telling us that you failed in high school? :-)
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Time is that which the clocks show. Clocks are not illusions. They are local energy reservoirs that are distributing their energy in equal amounts towards infinity.
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