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A black body absorbs all light/radiation in its reach. According to basic laws of physics, the more energy a body absorbs the more it can emit. Therefore, a black body absorbs all energy directed at it and also emits all energy that's been absorbed.

A black hole is known to absorb all sorts of every (light, radiation..). If it absorbs -all- energy around it, it should in addition emit all energy it has absorbed. But due to its vast gravity noting can "escape", and therefore nothing is actually emitted. Is a black hole still considered a perfect black body?

Disclaimer: I am no physicists, I am a senior year high school student with an interest for physics. I don't know the preferred terminology of things, especially not in English (2nd language), so if anything is unclear I will explain in comments below.

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up vote 8 down vote accepted

Yes, black holes are supposedly near-perfect black bodies. They emit thermal radiation called Hawking radiation, which, however, does not originate from beyond the event horizon, but is a consequence of the interaction of the strong gravitational field outside the horizon with the vacuum.

The process is sometimes described as the production of 'virtual' particle pairs due to quantum fluctuations, where one of the particles falls into the black hole, forcing the other one to become 'real'.

A graphic explanation why black holes are only near-perfect black bodies is that they cannot absorb photons with wavelength exceeding the black hole's size.

As a side note, due to the equivalence of gravitational and intertial effects due to acceleration, there's the related Unruh effect for accelerating observers.

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Simply No. It isn't a blackbody. Due to quantum vacuum fluctuations, it still emits radiation at a rate inversely proportional to its mass, which was first predicted by Hawking. So, it's not perfect as a blackbody. The only problem is that these emissions are so small and it takes up a lot of time - probably some million or billion years based on its mass.

So, it's the best blackbody ever in nature. Still, not close to our ideal one...

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protected by Qmechanic Apr 23 '14 at 16:52

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