why light cannot escape a black hole

Nothing, not even light can escape a black hole if it gets too close. This is why they are called black holes. It is difficult to explain precisely without introducing some complicated ideas. The simplest analogy is to consider throwing a ball up in the air: if you throw it it can escape off into space, but if not gravity will pull it back down to Earth. If we were to increase the gravity you would need to throw the ball faster and faster. Eventually there would come a point when you would have to throw the ball at the, which is the maximum speed anything can travel. At this point we have a black hole, and nothing will be able to escape. This is quite easy to understand, but has a number of flaws.

For example, in this picture light would slow down, then stop, and then fall back towards a black hole: this isn t right, as light always travels at the speed of light. A slightly more accurate but complicated picture is to think about gravity as the effect of the bending of space. In, which is our best theory of gravitation, mass bends space. This is often visualised as a rubber sheet being stretched by something heavy. Particles (whether light or matter) want to travel along, but when space is curved these become bent: they instead follow curving paths which we describe as the effect of gravity. Black holes have very high curvature, beyond a certain point (known as the
), all straight lines are curved such that they point inwards towards the black hole.

There is no direction you can go that will take you outside the event horizon! That might sound a little odd, but black holes are strange places. It s best to keep a safe distance. Photons (which are the "particles" that make up light) have zero rest mass. To understand why photons "fall" into a black hole, you need to know a bit of general relativity. What general relativity says is that any massive object warps the spacetime around it. You can think of this with a simple analogy. Imagine a stretched rubber sheet that is completely flat. This represents the spacetime when there is no mass. Now, if you put a heavy ball in the rubber sheet, it will cause a distortion in the sheet.

This is exactly what happens in space, except that it is in 3 dimensions instead of two. Further, a photon always travels by the shortest distance between two points. As spacetime is warped, the light appears to bend around a massive object. In reality, it is not that the object is attracting light, but it is just that the photons are traveling by the shortest distance in a curved spacetime. Around a black hole, the distortion of spacetime is extreme. At the event horizon of a black hole, the spacetime curves into itself and as a result, light cannot escape from a black hole. This page was last updated June 27, 2015.

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