So I got into a mini-debate in science class today because I proposed that black holes aren't really black, they only look black because light can't reflect off them. But if you were to take the material that makes up that black hole and decrease it's density so that gravity isn't so strong that light can't reflect, then what color will this new object be? Think of it like this: if you put a red apple in a room that is completely pitch black, the apple will appear black but it's actually red (we just can't observe this because there's no light).

  • $\begingroup$ If you decreased the gravity to the point light can reflect, it would cease to be a black hole $\endgroup$ – Jim Mar 1 '19 at 14:30

The material part of a black hole is (classically) compressed into a zero volume area, and almost all of the information of the matter that eventually became the black hole was dissipated away, so the original notion of your question is unanswerable.

There IS another sense in which we can think of your question, though. Black holes are known to shine light through a process known as Hawking radiation, which has a blackbody spectrum. To first approximation, stars are known to also have a blackbody spectrum.

The key point there is that the blackbody spectrum's color is determined by the temperature of the distribution. In the case of stars, this means that the hotter the star, the bluer the color of the star. For black holes, the more massive the star, the redder the blackbody distribution. For black holes that have masses anywhere near that of the sun, the "color" of the star will be very, very far into the radio edge of the light spectrum, and therefore, the black hole will not be visible to the naked eye.

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  • $\begingroup$ Thanks for the explanation. It makes much more sense now. $\endgroup$ – Badr B Apr 1 '16 at 23:18
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    $\begingroup$ At some point during their life they will heat up enough to become visible, though. If we believe xaonon.dyndns.org/hawking, then a 5800K black hole has a radius of $3.141749\times 10^{-8}m$ (i.e. the size of a small virus), a mass of $2.115868\times 10^{19} kg$ (the mass of a large mountain and a luminosity of $7.959627\times 10^{-7} W$ (barely enough to be visible with the naked eye from a few meters, I think). One couldn't get close enough to see it with bare eyes, though, the gravity would be a killer at that distance. I might be wrong about that bit. $\endgroup$ – CuriousOne Apr 2 '16 at 2:56
  • $\begingroup$ @CuriousOne: yes, I'm talking about stellar mass black holes at a time within the natural lifetime of the sun from now. At arbitrarily late times, the black hole mass goes to zero, so the temperature will eventually diverge. But that's not really relevant to the question, at least as I see it. $\endgroup$ – Jerry Schirmer Apr 2 '16 at 6:31
  • $\begingroup$ If someone is asking for the color of a black hole, the actual answer can only be "When?", since it changes. There is no valid physical reason to apply an anthropomorphic time scale. We are not the ones setting the pace in this universe. $\endgroup$ – CuriousOne Apr 2 '16 at 13:40
  • $\begingroup$ @CuriousOne We are doing all of the observing, though, and talking about anything that requires a timescale larger than 13.6 billion years is kind of silly and close to being outside the realm of science. $\endgroup$ – Jerry Schirmer Apr 2 '16 at 13:42

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