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And remember, the "speed of light in the medium" depends on the wavelength of the light. This is why prisms separate light by color.

by jchallis10 hours ago|

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Optical sonic boom.

by rramadass6 hours ago|

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Actually called a "Luminal Boom" or "Photonic Boom".

by rramadass10 hours ago|

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This is quite nuanced and not as most people assume.

It is the "phase velocity of light in that medium" that is exceeded.

phase velocity of light in a medium = speed of light / refractive index of the medium.

Thus the EM wave is slowed down in a medium and so a charged particle can exceed it producing Cherenkov radiation. This is similar to a sonic boom in atmosphere when speed of sound is exceeded. In both cases the object is traveling faster than the wavefront.

It is only in vacuum that "phase velocity of light" = "group velocity of light" = c (i.e. 300,000 km/sec)

What really is the speed of light in a medium/vacuum, group or phase velocity? - https://physics.stackexchange.com/questions/450377/what-real...

by ndriscoll5 hours ago|

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IIRC the group velocity in a medium can also exceed c when you send pulses through certain nonlinear media. Basically, the "lump" of the pulse can appear to "exit before it entered," but what really happens is that the tiny tip of the pulse has already gone through at <= c, and starts growing into the body at the exit before it has completely shrank at the entrance because of the nonlinearity. So it is actually the information/"signal" velocity (the tip of the signal) that cannot exceed c.

by fooker5 hours ago|

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Are there interesting relativistic effects related to this other than this Cherenkov radiation?

by IAmBroom4 hours ago|

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To be more perfectly precise, it is only in a vacuum with an infinitely repeating regular signal that "phase velocity of light" = "group velocity of light".

The deviation caused by uncertainty in the package arrival time is typically extremely short, but not zero, for finite signals.

The actual "entity" that is restricted to be <=c is "information velocity". An infinitely repeating pattern would have travel at (phase velocity)=c ... but would ironically transmit no actual information (the bit value is restricted to 1, not 0 or 1).