# Warp 0: Why warp drive technologies might never happen

The connection between one of Whiteâ€™s collaborators and the Institute for Advanced Study at Austin is really unfortunate. Kind of casts a pall over the whole operation, at least in so far as NASA is involved. Makes me think some sort of investigation is in order. (The Austin people can spend private money to their heartsâ€™ content, but NASA is putting public money and its reputation on the line.)

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Somebody once explained to me why FTL communication would directly cause a time paradox. The explanation was the kind that totally make sense, but the next day youâ€™re scratching your head saying, â€śhow did it go again?â€ť I wish I could reproduce it.

But the bottom line is that, while in the movies paradox causes awesome, in the real world it just doesnâ€™t happen - another pretty strong argument those warp drives can never happen.

The reason has to do with the relativity of simultaneity which says that observers in different reference frames define simultaneity differently, and a side consequence of this is that if you could have an FTL signal, then if you identify the event â€śsignal being sentâ€ť and â€śsignal being receivedâ€ť, then although in some frames the signal will be received after it was sent as expected, in other frames these events will be simultaneous or happen in reversed order, meaning the signal went â€śback in timeâ€ť in this frame (the same issue doesnâ€™t happen with signals moving at the speed of light or slowerâ€“in that case all frames agree on their order). And relativity says that the laws of physics have to work exactly the same in all frames, so if itâ€™s possible in some frames for an FTL signal to be received before itâ€™s sent, this must be possible in all frames.

Then if you take into account exactly how simultaneity works in each frame, itâ€™s possible to come up with a situation where you have two observers moving apart slower than light, and A sends a signal to B that moves FTL but forwards in time in Aâ€™s frame, but backwards in time in Bâ€™s frame; then when B gets the signal B can send a reply that moves FTL but forwards in time in Bâ€™s frame, but backwards in time in Aâ€™s; and the net result is that A receives Bâ€™s reply before A sent the original signal, so thatâ€™s a violation of causality in all frames. This setup is sometimes called the tachyonic antitelephone, and for anyone whoâ€™s studied a bit of special relativity and is familiar with lines of simultaneity in spacetime diagrams, thereâ€™s a good set of diagrams explaining it here.

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Also, here is a paper which verifies that the theoretical â€śwarp driveâ€ť solution in general relativity could indeed be used for backwards time travel. This is a pretty good reason to be skeptical, I thinkâ€“although Iâ€™d like a time machine as much as the next guy, from what Iâ€™ve read physicists have a bunch of theoretical results which hint that when quantum physics is combined with general relativity to produce a theory of â€śquantum gravityâ€ť, it will probably end up ruling out all backwards time travel solutions, an idea known as the chronology protection conjecture.

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Thatâ€™s it. Thatâ€™s exactly the way I remember it, and I still find it utterly convincing while at the same time baffling.

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