Do Black Holes Expand at the Speed of Light?

[glowing lights]

hi guys...i hav got this doubt abt black holes...

black holes are said to be expanding at the speed of light. so if this is true, then why doesn't a black hole gobble up the planets in its way?

 

[malawi_glenn]

Who says that they expanding at the speed of light? And for how long?

 

[tarbag]

hi guys...i hav got this doubt abt black holes...

black holes are said to be expanding at the speed of light. so if this is true, then why doesn't a black hole gobble up the planets in its way?

The black holes are traveling like normal stars in the universe under the effect of the gravitation. there is no physical reason to push them to move like photons. in the colapsing matter can drop or be accreted in the last moment with speed of light.

thanks

 

[malawi_glenn]

Yes, tarbag, the matter collapsing to a black hole is ultrarelativistic, but the black holes in space just moves with the velocites as an ordinary star.

 

[yenchin]

I think the OP is referring to statement like the one found http://cosmology.berkeley.edu/Education/BHfaq.html". The 7th paragraph under "what is a black hole" states that:

The horizon has some very strange geometrical properties. To an observer who is sitting still somewhere far away from the black hole, the horizon seems to be a nice, static, unmoving spherical surface. But once you get close to the horizon, you realize that it has a very large velocity. In fact, it is moving outward at the speed of light! That explains why it is easy to cross the horizon in the inward direction, but impossible to get back out. Since the horizon is moving out at the speed of light, in order to escape back across it, you would have to travel faster than light. You can't go faster than light, and so you can't escape from the black hole.

I wonder whether this is a strange way of trying to say that the event horizon is a null surface?

 

[malawi_glenn]

But how could you "see" the horizion moving towards you when the escape velocity is speed of light, no light can escape from the black hole.

 

[tarbag]

But how could you "see" the horizion moving towards you when the escape velocity is speed of light, no light can escape from the black hole.

Why the light can't escape from the black hole , its mass is nul and it doesn't suffer from gravitationel potentiel?

 

[malawi_glenn]

That is wrong, light is affected by gravity. here is a reference http://en.wikipedia.org/wiki/Gravitational_redshift
There is much more to be explored for you altough.

Still waiting for an answer for my question how one could see the event horizon going towards you at the speed of light.

 

[pervect]

I think yenchin has the right idea, that Tedd Bunn is simply saying that the event horizon is a null surface. Relative to any physical observer, the relative velocity between the physical observer and the event horizon when the physical observer crosses the event horizon is 'c'. Rather than saying that the physical observer is moving at the speed of light, Ted Bunn is pointing out that it is the event horizon which is lightlike. The physical observer's path is timelike.

Another way of putting this: you can think of the event horizon as a bunch of trapped photons (point-like light beams). When you reach the event horizon, you can see these photons, and they'll be moving at 'c', just like other photons.

Ted Bunn points out that from your perspective, the event horizon (which contains these photons) is "moving towards you at the speed of light", while you, of course, are stationary with respect to yourself.

What you'll see if you look at them is that the trapped photons contain (faint) images of other objects that emitted photons at just the right time while they were also falling through the event horizon.

 

[glowing lights]

Who says that they expanding at the speed of light? And for how long?

then is it only true that they move with the speed of light?

 

[Garth]

then is it only true that they move with the speed of light?

NO!

glowing lights where did you get this idea from?

Did you read something like the quote in yenchin's post #5?

Black holes are like any other massive body in the universe, they will have their own trajectory velocity relative to other bodies, but nowhere near the speed of light, which would be impossible in any case.

BHs do not expand either, unless they swallow up another mass.

BTW, welcome to these Forums!

Garth

 

[glowing lights]

thanks a lot dude...

 

[Garth]

thanks a lot dude...

Sorry, I didn't mean to sound rude, others had answered your original question and I wondered where the idea that BH's traveled at light speed like photons had come from.

Keeping asking the questions!

Garth