• Question: What is the strength of gravity on the verge of a black hole?

    Asked by anonymous to Steve, Emma, Christian, Bose, Alice on 9 Mar 2016. This question was also asked by Pele Pitbury.
    • Photo: Emma Dean

      Emma Dean answered on 9 Mar 2016:

      It depends on the size of the black hole.

      A black hole made from star, a bit bigger than our sun, would be quite small. The change in gravity from one point on the edge to the next e.g. across 1cm, would be really big. So if you where to fall into the black hole, you would be stretched like spaghetti until you are ripped apart. This is because the gravitational force and your feet would be a lot stronger than at your head. If you were at the edge of a supermassive black hole, like the one at the centre of our galaxy. The change in gravity from one point to the next would be a lot smaller. You could fall into the black hole and not even realise!

    • Photo: Alice Harpole

      Alice Harpole answered on 9 Mar 2016:

      To work this out, we only need two formulae: the gravitational potential V at a radius r of an object of mass M is

      V = – GM / r

      where G is the gravitational constant. We then need to know how big the black hole is – for a slowly rotating, spherical black hole, this is approximately given by the Schwarzschild radius:

      r_s = 2GM / c^2,

      where c is the speed of light.

      By substituting the Schwarzschild radius into the expression for the gravitational potential, we get

      V = -c^2 / 2 = – 45,000 TJ/kg (45,000 terajoules per kilogram)

      To put this into perspective, this is almost 1,000,000,000 times larger than the gravitational potential on the surface of the Earth!

    • Photo: Steve Marsden

      Steve Marsden answered on 9 Mar 2016:

      It’s also worth noting, that as you get closer and closer to a black hole, the strength of gravity increases at an alarming rate.

      If you were to approach feet first, the pull of gravity would be far stronger on your feet than on your head. The difference would be sufficiently large to start stretching you out like a string of spaghetti (the process is actually called spaghettification). Eventually, the pull would be so great it would start to rip you apart, atom from atom, long before you reach the surface of the black hole.

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