relativity & black holes

[rasbedo]

If Einsteinian relativity is true -- mass approaches infinity as any mass approaches light speed , how can black holes exist & accrete matter as any mass approaching the Shwartzchild radius would have infinite mass & no length . Infinity is infinity .

[timo]

They seemingly can, so perhaps you should elaborate where you see a problem. "Infinity is infinity" is a nice tautology but not exactly a well laid-out scientific argument. Acceptable grammar and punctuation would help understanding your issue, too.

[MigL]

What is your reason for thinking infalling mass approaches light speed at the event horizon ?

The fact that a mass needs light speed to escape ( escape velocity = C ) doesn't mean infalling mass reaches that speed. If it did it would posses escape velocity and would pass right through the black hole.

[IM Egdall]

What is your reason for thinking infalling mass approaches light speed at the event horizon ?

The fact that a mass needs light speed to escape ( escape velocity = C ) doesn't mean infalling mass reaches that speed. If it did it would posses escape velocity and would pass right through the black hole.

 

Speed is relative. From the infalling mass's reference frame, it is at rest. From this point-of-view, the event horizon is going by it in the opposite direction at the speed of light. That is why the infalling mass cannot escape the black hole.

[MigL]

Don't quite understand what you mean.

A black hole will act just like any other massive gravitational source. It will have an escape velocity just like any other gravitational source, but, its escape velocity happens to be faster than C, such that light cannot escape its 'pull'.

If an object falling towards the earth reaches its escape velocity ( about 18000 mi/hr ) and is not on a collision course, it will escape into interplanetary space.

Similarily if an object reaches a velocity above C at the event horizon of a black hole ( fairly small with an escape velocity just above C ) then it would escape. This is clearly not the case. Objects with mass will not reach C at the event horizon.

[IM Egdall]

Don't quite understand what you mean.

A black hole will act just like any other massive gravitational source. It will have an escape velocity just like any other gravitational source, but, its escape velocity happens to be faster than C, such that light cannot escape its 'pull'.

If an object falling towards the earth reaches its escape velocity ( about 18000 mi/hr ) and is not on a collision course, it will escape into interplanetary space.

Similarily if an object reaches a velocity above C at the event horizon of a black hole ( fairly small with an escape velocity just above C ) then it would escape. This is clearly not the case. Objects with mass will not reach C at the event horizon.

 

I think we agree here. All I was trying to point out is that from the point-of-view of an astronaut falling into a black hole. he is at rest and he sees the black hole approaching him faster and faster. At the event horizon, it is passing by him at the speed of light. So from his point-of-view, he would have to go faster than the speed of light to escape beyond the event horizon.

 

(Edited for clarity.)

Edited July 7, 2011 by I ME

[Cap'n Refsmmat]

he is at rest and he sees the black hole approaching him faster and faster. At the event horizon, it is passing by him at the speed of light. So from his point-of-view, he would have to go faster than the speed of light to escape beyond the event horizon.

I don't think that's necessarily the case; presumably, I could build an incredibly fantastic rocket engine and sit just outside the event horizon, stationary. You don't have to approach light speed.

 

The other problem with the OP's idea is that it conflates relativistic mass and rest mass; the black hole won't gain infinite rest mass if a particle hits it at near the speed of light.