In his reply on what would happen if one fell into a black hole (http://www.straightdope.com/classics/a1_200.html) Cecil says “If our entire galaxy collapsed into an ebony aperture (I am getting tired of typing black hole), said BH would be about ten billion light years across.” That’s way off. The galaxy is only about 30,000 ly across now–why would it get 333,000 times larger if it became a black hole? Did Cecil mean to type, “If our entire universe collapsed…”?:smack:
I don’t know what Cecil meant or didn’t mean to type, but as far as I know the galaxy is still 90.000 light years across. Not that that explains anything, mind.
A slight high-jack, but…
I thought that once matter was caught in an ebony aperture’s cravity, that tidal forces would pull the matter apart, ripping the very atoms apart as it gets closer to the black hole, destroying the matter in the process. And that it is from this process that we see x-rays coming from known accretion disks of black holes.
Am I on track or not? Because if this is true, once a black hole forms, nothing ever falls into it again.
You’re right, privard. It should be universe. However, there is definitely some confusion, because he says it will have the density of a thin gas.
A galaxy-mass black hole would have a density of about 1.8 micrograms per cubic centimeter. That’s about 1/700th the density of air, a reasonable value for a thin gas, I’d imagine.
A universe-mass black hole would have a density of, well, the same as the universe, something more like one electron mass per cubic centimeter. This isn’t a thin gas. This is orders of magnitude less than the density of the interstellar vacuum, which is orders of magnitude less than laboratory vacuums.
Ficer, almost.
What pulls the matter apart is the tidal forces just outside the event horizon. Basically, the gravity gradient is so high that the difference in pull from one side of an atom’s nucleus to the other is enough to overcome the nuclear forces.
The other thing that this gravity gradient does is cause a huge difference in orbital velocity over the distance of an atom’s width. This causes incredible friction between nearby atoms, which is what generates the x-rays.
These high gravity gradients are true of small, stellar-sized black holes. The gradient is based on the square of the distance from the center-of-gravity of the black hole. For larger black holes, just outside the event horizon, the difference in gravity falls off much more slowly. I believe that for a black hole of galaxy size that normal matter could survive the fall down to the event horizon. For really big ones, it might not even rip a person’s body apart.
All this is happening just outside the event horizon.
As Cecil alluded to, one seeming-paradox is that as the matter approaches the event horizon it’s getting closer and closer to the speed of light, and thus building up more and more time dialation. So, I suppose from one point of view the matter appears to hover just outside of the event horizon for a long time. But, given the size of some black holes, apparently the matter does eventually fall in.
Using the standard approximations (primarily that any change in mass of the hole is negligible compared to the total mass), from the perspective of someone safely outside the black hole, nothing ever quite makes it all the way in. But from the perspective of someone falling in, it takes a very short time. And either way, the mass of an object falling in does still contribute to the total mass, regardless of whether it’s inside or jjust outside.
Anyone here remember Robinette Broadhead? 
-Rod-
As I understand it, if Joe falls into a black hole, then in our reference frame he does so fairly quickly, but we know that it takes forever from his PoV. I used to understand this…
Obviously from Cecil’s earlier days
I’m not sure what he means here. The density of a point singularity? Well, pretty undefined AFAIK. The density within the event horizon? My newtonian-approximation back-of-the-envelope calculation easily shows this increases. The density of a non-point body making a black hole? I wouldn’t think so, but… hell maybe he doesn’t understand it
Well, try again. It should be proportional to M[sup]-2[/sup].
rho = 1.8×10[sup]16[/sup] gm/cm[sup]3[/sup] × (M/M[sub]SUN[/sub])[sup]-2[/sup]
You’ve got it backwards. When I use reference frames, I usually mean inertial reference frames. So, I’ll use the pharse Point Of View. From our POV, Joe takes forever to cross the horizon. From poor Joe’s POV, he reaches the singularity quickly and time ends for him in a finite amount of time.
Cecil has another column on the subject http://www.straightdope.com/mailbag/mhawkingradiation.html
Achernar: I meant ‘decreases.’ :smack:
DrMatrix: ok, that makes sense. :smack:
You’d think that by now I’d have gotten used to people mistaking me for Cecil. But I haven’t, and it’s still flattering :). Thank you, Shade.