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If I am not mistaken, this video says that X-rays travel faster in glass than in a vacuum. Special Relativity says that information can not travel faster than the speed of light in a vacuum, so what is keeping us from making Tachyon Phones with glass and x-rays?

J. Murray
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ACertainArchangel
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  • Possible duplicates: https://physics.stackexchange.com/q/436329/2451 , https://physics.stackexchange.com/q/76663/2451 and links therein. – Qmechanic Jun 27 '23 at 18:43

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What travels faster than light is the phase (phase velocity), not the envelope of the x-ray beam. The waveform travels with the group velocity which is less than $c$.

Information is encoded into the waveform therefore it's transfered with the group velocity.

Phase velocity do not transfer information hence no tachyon phones

LolloBoldo
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    RF waveguides (hollow tubes that conduct RF power, usually in the GHz region) have the same property -- the phase velocity is higher than $c$, but the group velocity is lower. – TimWescott Jun 27 '23 at 22:54