• Question: Where does light go when it enters a black hole?

    Asked by to Mark, Matthew, Mike, Paul, Sabina on 17 Mar 2014. This question was also asked by .
    • Photo: Paul Coxon

      Paul Coxon answered on 17 Mar 2014:

      Black holes are fascinating things, but one of the frustrating things is how little we know about them. We know so much about the universe around us by collecting the light and radiation given off from stars, gas clouds, comets, etc but in a black hole we can’t get information this way.

      The mass and density of a black hole is so great that it distorts the region of space around it by an enormous amount. So much so that even light – the fastest thing in the universe – can’t escape. This is why is appears ‘black’ because we can’t collect anything directly from them. We know they exist, because of their influence on the region around them.

      However Stephen Hawking as discovered a special form of radiation that actually drains energy away from a black hole (called Hawking radiation), and this will gradually cause itto evaporate, and eventually disappear altogether. As to what happens to light, we know it can’t escape, but nobody really knows what happens inside.

    • Photo: Mike Lee

      Mike Lee answered on 17 Mar 2014:

      A black hole is something waves cannot escape. Since light is a wave, it cannot escape and we cannot tell where it goes. Nobody knows.

    • Photo: Matthew Malek

      Matthew Malek answered on 17 Mar 2014:

      This is an excellent question! Alas, I cannot give you an equally excellent answer… as we just don’t know!

      Astronomy is a science of observation, and physics is an experimental science. Because we don’t get light out of a black hole, we cannot make direct observations. And, up to now, we have not been able to make our own “mini-black holes” in the lab to experiment on them. So, for now, we are stuck in the land of speculation.

      Einstein’s theory of general relativity tells us that matter shapes spacetime. When you get lots of matter in one place, you distort spacetime a lot. When you get beyond a critical amount, like in a blackhole, you may actually punch a hole in the fabric of spacetime. In which case, the light going through that hole is lost to our universe.

      Of course, if spacetime is curved, it is also possible that the hole on one side opens up somewhere else in the universe, as a sort of “white hole” where everything that fell into the “black hole” comes out. Linking the two together gives you a “wormhole”, which is a popular idea in science fiction, as it allows travel across long distances as speeds that seem to be faster than light! (In actuality, you wouldn’t be going faster than light, you would just be taking a cosmic “shortcut”!)

      It’s a great idea, but do wormholes actually exist for the light (and other stuff) that enters a black hole to come out of? I personally doubt it. For one thing, studies have shown that the universe appears to be “flat” at large scales, so you wouldn’t have the big curvature needed to bridge two parts via a wormhole. Also, we have indirectly observed lots of black holes (via their graviational pull), but we have never seen any evidence for matter appearing out of a “white hole”. So I wouldn’t bet on it… but, that said, I’ve been wrong before!

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