# The Naked Scientists Forum

### Author Topic: How can Black Holes Vary in Size?  (Read 9914 times)

#### DoctorBeaver

• Naked Science Forum GOD!
• Posts: 12656
• Thanked: 3 times
• A stitch in time would have confused Einstein.
##### How can Black Holes Vary in Size?
« Reply #25 on: 24/05/2008 19:23:46 »

In the case of a BH, the EH marks the point along the gravity gradient from the BH where the escape velocity from the BH is equal to c, so nothing within an EH can escape it by being given a sufficient initial velocity, because nothing can be imparted with a speed > c.

My mistake. I should have put =c rather than >c.

The light falling into the BH will be blue-shifted but as you accelerate towards the singularity the light will appear to be more and more red-shifted until you reach the point where your velocity approaches c and the apparent energy of the light reaching you approaches zero.

I did wonder about that, and that's what prompted my other thread about wave stretching. You've made me think that maybe I'm not a total dork after all

#### LeeE

• Neilep Level Member
• Posts: 3382
##### How can Black Holes Vary in Size?
« Reply #26 on: 24/05/2008 19:47:41 »
Do you mean like 2 cones base to base? That's what I was thinking.

It wouldn't necessarily result in a symmetry along its time axis, though. Imagine the case of a cone the entire base of which intersects the EH at an angle. At the EH the cone would invert with its apex at the singularity.

Sort of - because the cone would be in a gradient, the base would have to contract towards a point as it approached the singularity but I would imagine the sides of the cone to be smoothly curved and it would end up as a sort of elongated egg-shape, but with pointed ends instead of rounded ones.

A fat, pointy-ended sausage!

Quote
All I think it actually signifies is that in such a situation there's only one possible future, which we know already

Indeed. Until the entire base of the cone intersects the EH, all possible futures are still options. But when it does - SCHLOOOOOP!

heh - can we say that the diminishing fatness of the sausage depends on the schloooopiness of the BH? Imagine presenting that in a paper!

Hmm... fat pointy-ended sausages being drawn into a BH - just my base sense of humour but I can't help remembering a certain phrase concerning a fan and something that hit it.

#### DoctorBeaver

• Naked Science Forum GOD!
• Posts: 12656
• Thanked: 3 times
• A stitch in time would have confused Einstein.
##### How can Black Holes Vary in Size?
« Reply #27 on: 24/05/2008 19:57:07 »
Do you mean like 2 cones base to base? That's what I was thinking.

It wouldn't necessarily result in a symmetry along its time axis, though. Imagine the case of a cone the entire base of which intersects the EH at an angle. At the EH the cone would invert with its apex at the singularity.

Sort of - because the cone would be in a gradient, the base would have to contract towards a point as it approached the singularity but I would imagine the sides of the cone to be smoothly curved and it would end up as a sort of elongated egg-shape, but with pointed ends instead of rounded ones.

A fat, pointy-ended sausage!

Quote
All I think it actually signifies is that in such a situation there's only one possible future, which we know already

Indeed. Until the entire base of the cone intersects the EH, all possible futures are still options. But when it does - SCHLOOOOOP!

heh - can we say that the diminishing fatness of the sausage depends on the schloooopiness of the BH? Imagine presenting that in a paper!

Hmm... fat pointy-ended sausages being drawn into a BH - just my base sense of humour but I can't help remembering a certain phrase concerning a fan and something that hit it.

What are you insinuating, young man?  Or are you just being coarse? [:(!]

#### graham.d

• Neilep Level Member
• Posts: 2208
##### How can Black Holes Vary in Size?
« Reply #28 on: 24/05/2008 20:40:10 »
Hmm. The escape velocity is the velocity at the surface (in this case the EH) at which you have suficient energy to escape (just) to infinity. Consequently, if you start from infinity and just fall under the gravity of the object, you will achieve this velocity at this surface. In practice we are talking about just fairly large distances; it is just that infinity makes the calculations a bit easier, but the principle is the same. As I said before, if somehow you could use retro rockets to slow yourself down this would not apply, but you would still have a huge velocity. In an oblique entry you would probably end up in a decaying orbit into the BH. I am unsure how the universe would look in this case. Well, I am unsure in every case actually.

#### LeeE

• Neilep Level Member
• Posts: 3382
##### How can Black Holes Vary in Size?
« Reply #29 on: 24/05/2008 23:01:50 »
I did not intend to Insinuate anything - It is purely my coarse sense of humour.

However, were one to find oneself in such a situation (and assuming that one didn't want to be in it) I think that it's quite possible that some folk, obviously less cultured and refined than yourself, might actually use the phase I alluded to, but without realising how literally appropriate it might actually be, especially if the BH has any angular momentum.

#### DoctorBeaver

• Naked Science Forum GOD!
• Posts: 12656
• Thanked: 3 times
• A stitch in time would have confused Einstein.
##### How can Black Holes Vary in Size?
« Reply #30 on: 25/05/2008 09:06:42 »
I did not intend to Insinuate anything - It is purely my coarse sense of humour.

However, were one to find oneself in such a situation (and assuming that one didn't want to be in it) I think that it's quite possible that some folk, obviously less cultured and refined than yourself, might actually use the phase I alluded to, but without realising how literally appropriate it might actually be, especially if the BH has any angular momentum.

Ah yes, I can see the analogy.

#### The Naked Scientists Forum

##### How can Black Holes Vary in Size?
« Reply #30 on: 25/05/2008 09:06:42 »