• Question: How come a black hole can (insert better word than 'suck in' here) light? And why?

    Asked by infintaneousdeath to Joel, Kristian, Tim, Venus, Zachary on 14 Nov 2013.
    • Photo: Kristian Harder

      Kristian Harder answered on 14 Nov 2013:

      Many people think that gravity attracts mass, but that’s only partially true.
      Gravity works on all forms of energy, and mass is one form of energy. Light doesn’t have mass, but it has energy, and therefore it can be sucked into a black hole. (Sorry, can’t think of a better word than “suck” either. 🙂 )
      Where it gets really interesting is when light does not actually fall into a black hole, but is only bent around, somewhat like a comet flying past a massive star bending its path a bit towards it. That’s much more impressive, because then you can *see* the result. This is called gravitational lensing, see for example http://www.cfhtlens.org/public/what-gravitational-lensing for a nice photo and an explanation. On that photo, you can see several distorted images of one and the same galaxy in the background, because the light emitted from that galaxy is bent around a massive galaxy cluster in the foreground that acts like an oddly shaped optical lens, making the light of the far away galaxy appear to reach us from several different directions.

    • Photo: Tim Hollowood

      Tim Hollowood answered on 15 Nov 2013:

      black holes can feed on any kind of matter and that includes light. Once the matter has crossed the event horizon then it can never come out. If you fell in you would fall towards the singularity where gravity becomes infinitely strong and you would be mashed to a pulp.

    • Photo: Zachary Williamson

      Zachary Williamson answered on 15 Nov 2013:

      Black holes don’t suck in light, they do something much more awe-inspiring and sinister. They curve and distort the space they exist in. From the light’s perspective, its just traveling in a straight line. But that straight line has actually been curved enough that it goes directly into the black hole. They can do this because of their enormous mass combined with their tiny size.

      The event horizon of a black hole is the region of space where space is so badly distorted that no matter which direction you travel, no matter how fast you go, all paths lead directly into the maw of the black hole.

    • Photo: Joel Goldstein

      Joel Goldstein answered on 19 Nov 2013:

      Even a much smaller massive object can deflect light – that is predicted by both Newton’s theory of gravity and Einstein’s relativity, and this was measured experimentally 100 years ago with light travelling past the sun. The more massive an object gets, the stronger its gravitational field and the more it deflects light. If an object’s gravitational field is strong enough, it will pull everything including light in.

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