• Question: why are black holes black?

    Asked by hassan721 to Arttu, Ceri, James_M, Monica, Philip on 22 Jun 2011. This question was also asked by evilumbrella.
    • Photo: Ceri Brenner

      Ceri Brenner answered on 21 Jun 2011:

      Black holes are super, SUPER strong sources of gravitational fields, so strong that not even light can escape it’s gravitational pull and in that way they are described as ‘black’ since no light (classically thinking) can be emitted from within a black hole (saying that though, Hawking radiation is a quantum description of how a black hole can emit radiation, so in this description they are not actually ‘black’). However, we can ‘observe’ black holes because of the radiation (visible and invisible) and jets of particles that are given off as the particles on the outer perimeter of the hole spiral into it.

    • Photo: Arttu Rajantie

      Arttu Rajantie answered on 22 Jun 2011:

      According to general relativity, gravity is caused by curvature of spacetime. Near a massive object, gravity is so strong that time points inwards. Therefore nothing can come out, even light, because it would mean going backwards in time. Because no light can come out, the object appears black.

      However, Ceri is absolutely right that a black hole would actually emit some so called Hawking radiation because of quantum mechanical effects, and stuff falling into a black hole would also emit radiation, so they would not be completely black.

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