• Question: How come Black hole can have a gravity so missive to trap Light

    Asked by Bartosz to Daniel, Hannah, Maggie, Ry, Scott on 8 Nov 2017.
    • Photo: Hannah Middleton

      Hannah Middleton answered on 8 Nov 2017:

      The reason comes from something called “escape velocity”. When we are launching rockets from the surface of the Earth, we have to fire them faster a certain velocity called the escape velocity in order to escape the gravitational pull of the planet. For the Earth that’s about 11,000 km per second. But to escape from a denser planet (perhaps a planet that has the same diameter but has more mass), you need to reach a higher velocity to escape.

      So if you imagine having a objects which are denser and denser (with more and more mass), then the escape velocity needed gets higher and higher until it would need to be faster than the speed of light. Then even light could not escape from such an object and that’s what we call a black hole. And because nothing can travel faster than light, nothing can escape if it gets too close to a black hole.

    • Photo: Ry Cutter

      Ry Cutter answered on 8 Nov 2017:

      Brilliant question! Hannah answered it really well (I’m actually a little jealous ’cause black holes are my favourite thing to explain!)

      Another way you can look at it, a black hole is like putting a bowling ball on a trampoline. If you roll a tennis ball on it, the tennis ball will roll towards bowling ball. If you throw the tennis ball faster it can roll to the other side of the trampoline without getting trapped by the bowling ball.
      However, if you make the bowling ball heavy enough there will be no speed that you can throw the tennis ball that will stop it from getting trapped by the dip caused by the bowling ball. Black holes are like the heavy bowling and light is like the fastest tennis ball you can ever throw. No matter how fast anything goes they cant get out of the dip, not even light! So it looks black, that’s why we call it a black hole.
      Great question,
      Ryan

    • Photo: Maggie Lieu

      Maggie Lieu answered on 8 Nov 2017:

      It used to be just a theory, you can calculate the mass of an object needed to stop something even if it’s travelling at the speed of light. Hannah already explained the escape velocity pretty well.

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