Speeding into a supersize black hole

1. May 18, 2005

Anders Lundberg

The galaxy NGC4261 is believed to contain a black hole that weighs about one hundred thousand times as much as our sun, with a diameter the size of our solar system. (http://www.damtp.cam.ac.uk/user/gr/public/bh_obsv.html)

Now, imagine that we clear the space around the black hole and use some kind of spaceship to first build up a near light speed velocity. Then, from a distance of a few "light days" (we suppose that this black hole has a diameter of almost a "light day") we aim straight at the black hole, turn off the engines and just plainly falls towards it.

My question is; If the spaceship already is at very near light speed, why doesn't the massive gravitational field from the black hole manage to accelerate the spaceship to above light speed before it reaches the black hole? Any relativistic mass increase of the spaceship would not make any difference since it is accelerated by a gravitational field (and not by its engines), so what is stopping it?

2. May 18, 2005

Crosson

What happens when things fall in a gravitational field? They gain kinetic energy.

There is no limit on kinetic energy. You can fall towards a black hole and gain nearly infinite amounts of kinetic energy.

But nearly infinite amounts of kinetic energy are not enough to travel faster then the speed of light. Better luck next time.

3. May 18, 2005

pervect

Staff Emeritus
Some thought needs to be given as to how the velocity is being measured. For instance, the coordiante velocity dr/dt for an object falling into a black hole will be zero! The most convenient number is not a velocity, but a rapidity - dr/dtau, which is the rate of change of the Schwarzschild coordinate 'r' with the proper time 'tau'.

For more on rapidities in SR see for instance

http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html