Theoretical Physics [OT thread] - Page 3

Suicide Junkie · #21 March 1st, 2001, 05:48 PM

It may have infinite density at the centre, but it only has finite mass.

If it had infinite mass, the entire universe would be inside the event horizon. (as soon as the gravity's effect spread out)

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PS. as for the 'weightless environments near earth' Might you be referring to the Vomit Comet, or something more dramatic that I can't remember?

[This message has been edited by suicide_junkie (edited 01 March 2001).]

dmm · #22 March 1st, 2001, 07:14 PM

quote:
Originally posted by suicide_junkie:
as for the 'weightless environments near earth' Might you be referring to the Vomit Comet, or something more dramatic that I can't remember?

He might be referring to the demos where they used REALLY strong magnets to levitate little frogs. The upward force was due to the magnetic field causing the spins of all the froggie's atoms to line up, and they could tune it to exactly counteract the downward force from gravity. So, from the frog's point of view, it was weightless. They won't be using it to train astronauts anytime soon, though, because the region of uniform field is small compared to the size of the magnets.

Suicide Junkie · #23 March 1st, 2001, 08:55 PM

Hey, thats a ready-made artificial gravity for spaceships!

Just scale it up ten times, and you've got a Grav-bed you can set to 1g for when you sleep in orbit. Build it with a superconductor, and it won't use up any electricity once its running, either.

You could also use it for when you're boosting to orbit, to cancel the 14g peak acceleration.

With a nuclear/antimatter fuelled engine, you could run continuously at high g's without crushing your crew.

Heck, maybe thats what SE4 races use when they go from a tiny moon, to a huge Rock world.

apache · #24 March 1st, 2001, 09:23 PM

Hawking radiation is a theory, and not a very developed one from what I know. Actually all this stuff is theory, but that is the title of the thread.
Yes, an escape velocity greater than c is one technical description of the area beyond a black hole, but the other part of it is a star that has collapsed to infinite density, creating a singularity, and that generates an infinite gravitational force on the 'surface' of the singularity since it is at a zero distance from the total mass of the star.
And yes, I was referring to the 'vomit comet'. Technically, the space shuttle and other spacecraft also qualify, because they are not in a real zero-g environment, but they are not that close to the ground.
I haven't heard the one about the magnets, but that is interesting.

Suicide Junkie · #25 March 1st, 2001, 09:30 PM

I guess that depends on what you consider "inside" a black hole.

quote:
(you have a spec of dust that was your space ship, try getting that out of infinite gravitational force)

I consider "inside" to be inside the event horizon, and didn't realize you meant "inside" the singularity.

That happens to be a volume of size zero, so thats why I don't see it as defining "inside"

Derek · #26 March 1st, 2001, 09:35 PM

Exactly. Inside a black hole is generally considered to be inside the event horizon.

Another thing: Although none has ever been found, it is possible to have a black hole with an event horizon on the scale of meters, centimeters, millimeters, or smaller.

None of these, nor regular sized black holes, have an infinite gravitational force anywhere, except on the surface of the singularity, which, as someone pointed out, is 'not very big'

Derek

apache · #27 March 1st, 2001, 10:16 PM

Well, my understanding is that once inside the event horizon, the object can't escape, and it gravitates towards the singularity, and then becomes part of the singularity. Everything that 'falls' in becomes part of the singularity, and pushes the event horizon outward. As far as I know, there is no way something can stay within the event horizon, but not become part of the singularity.

Suicide Junkie · #28 March 1st, 2001, 10:27 PM

That is true, but there is a finite time before the object actually gets to the center.

Is an object halfway between singularity and event horizon inside or not?

A) It dosen't really matter, but its fun to argue the point, right?

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Ah, just thought of why.

You were talking about escaping from a black hole. Escaping is just as "impossible" from 1m inside the horizon, as right in the singularity. The only difference is, if the hole is big enough, your ship can be intact 1m from the event horizon, but will be torn apart (due to gravitational shear) near the singularity and then crushed.

So, it would only be worth trying to escape if your ship wasn't crushed or smeared, but still inside the hole.
Therefore, inside the hole should be inside the horizon.

Note: by gravitational shear, consider the force on your feet vs head as you fall in. 2m from the singularity, the force on your feet is 4x the force at your head, you get smeared alll over.

[This message has been edited by suicide_junkie (edited 01 March 2001).]

dmm · #29 March 1st, 2001, 11:35 PM

Yous guys is all stoopid. I happen to know, from watching the Disney movie "The Black Hole," that just inside the event horizon of black holes is a perfect place for evil scientists to have their secret laboratory. Sheesh! Get a ejerkayshun!

jimbob55 · #30 March 2nd, 2001, 01:15 AM

The speed of light is a constant. The speed of light depends on the media through which the light is travelling... So it's a variable constant?

Magnetising frogs seems a little cruel. How much power would it take to magnetise a human? If you didn't calibrate your field properly and were relying on it to protect against 14g acceleration your spleen could remain weightless as the rest of you was spread over the acceleration couch....