A star starts at rest at infinity, how do you calculate the kinetic energy of the star when it crosses the event horizon of a black hole?

Asked Mar 15 '16 at 08:40

Active Mar 17 '16 at 01:10

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A star that begins very, very far away from a black hole (infinity) is brought in by the black hole's gravitational pull.

How do you calculate the kinetic energy of the star as it passes the point of the event horizon (/Schwarzschild radius)?

edit - I'm looking specifically on how to calculate the kinetic energy, not the velocity of an object (in this case, the star).

edited Mar 17 '16 at 01:10

Qmechanic

asked Mar 15 '16 at 08:40

user111395

1

Energy is a complicated concept in GR. For example since in our coordinates the star comes to a halt at the event horizon its kinetic energy must be zero there. For objects falling into a black hole there is a conserved quantity that is equal to total energy, but splitting it into kinetic and potential bits is not an especially meaningful thing to do. – John Rennie Mar 17 '16 at 07:16

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