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In Crown Glass, the speed of light is 1.52 times the speed of light in vacuum. Consider the following hypothetical situation:

Hypothetical Situation

If a black hole is surrounded by a giant crown glass media (or any denser medium), will it's event horizon's surface area be less than that of an identical BH in vacuum, with the same mass, electric charge and angular momentum? This may be a weird question, but I just want to know the behavior of a singularity in different media.

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    $\begingroup$ Light is 1.52 times slower in crown glass than vacuum. $\endgroup$
    – lemon
    Mar 15 '15 at 11:25
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The event horizon is not an optical effect, so changing the refractive index of the space around it will make no difference. Adding extra mass will make a difference, but it's the mass that makes the difference not the refractive index.

You couldn't make your spherical shell of glass go right up to the horizon because at the horizon no material is strong enough to resist falling into the black hole. The best you could do is have the horizon, then a gap, then your shell of glass.

There are modifications of the black hole metric that describe the black hole in the presence of matter. One such is the Vaidya metric, though this is in the presence of infalling dust (specifically null dust) rather than a spherical shell of glass.

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