• Question: Could you explain everything about black holes to me? I would really like to know, I already know a bit but I am confused as to why a black hole has infinite density and not a specific amount.

    Asked by curryman96 to Paul, Lily, Laura, Mark, Sarah on 23 Jun 2010 in Categories: . This question was also asked by eleanorbeedles, ck12.
    • Photo: Lily Asquith

      Lily Asquith answered on 22 Jun 2010:

      I think this is Paul’s specialism but I’ll tell you all that I know.

      When a very large star dies (it has to be huge, the sun is too small) all of its outer layers collapse into the core. When everything hits the core there is an almighty explosion called a supernova. What is left in the core can either become a very dense star called a neutron star, OR a black hole. Black holes happen when the star that dies was absolutely vast, so that when it implodes it causes a *singularity* in spacetime. Normals physics doesn’t work any more at a singularity. As you get close to a black hole stuff happens that you can’t even imagine. One of the spatial dimensions actually switches with time…

      The reason for infinite density is the fact that the black hole has *no size*: it is a singularity, just a point in space. Density is mass/volume, so if volume=zero then density becomes =infinite. This would be true even if the mass was really small, because dividing any number by zero gives you infinity.

    • Photo: Mark Roberts

      Mark Roberts answered on 22 Jun 2010:

      Alas not really my area of scientist as I’m a biochemist not an astronomer – hopefully Lily or Paul can help

    • Photo: Paul Roche

      Paul Roche answered on 23 Jun 2010:

      Black holes are actually quite simple things really…

      All you need to do to make a black hole is to crush something down until it is so dense that it’s gravitational attraction is too strong for even light to escape from. If you want to leave the Earth, you need a rocket that can travel faster than the Earth’s gravity oulls you back – we call that the escape velocity, and is is relating to the mass of an object and it’s radius (so similar to density, which is mass divided by volume – but volume is related to radius). For Earth, that means a velocity of about 14 km per second.

      If you crushed Earth down until it was about the size of a 5p coin, the escape velocity would become greater than about 300,000 km per sec – the speed of light. Einstein says nothing can go that fast, so not even light can escape from this super-dense object. The object then appears black.

      The infinite density bit only refers to the very centre of the black hole – the Singularity. This has zero volume – so any mass, divided by zero (to give you the density) will mean you get “infinity” as the answer (any number divided by zero equals infinity).

      I weigh black holes that formed when massive stars (bigger than say 20-30 times the mass of the Sun) died – usually we find them by seeing how they affect other stars in what we call binary systems. You can’t really “see” a black hole, but you can easily see the effct that it’s extreme gravity has on other stuff, like stars or gas clouds.

      I can’t tell you everything about black holes, but I hope that helps!

    • Photo: Sarah Bardsley

      Sarah Bardsley answered on 23 Jun 2010:

      Eeek – I think I will be no hope here. I’m sure one of the others can answer better than any attempt by me!

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