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NEOWatcher
2006-Jan-10, 04:31 PM
It's a long ride into a black hole (http://www.cnn.com/2006/TECH/space/01/09/space.hole.reut/index.html)
What exactly are they saying here? I'm not sure what they mean by the "last leg", or how this relates to the EH. Doesn't the initial distance or the direction of travel in the "last leg" have some effect, or are they talking about some point at which there is some black hole spiral equivalence of a "terminal velocity"?
I'm sure there's a good story here, but I'm having a hard time grasping it.

Tim Thompson
2006-Jan-10, 04:50 PM
The material is falling into the black hole from an accretion disk. It's moving fast in its orbit, and is being additionally pulled by the rotation of the black hole. It can't fall in, until it sheds energy & angular momentum by virtue of the viscosity of the disk, and radiating energy away. That all takes time.

NEOWatcher
2006-Jan-10, 06:16 PM
The material is falling into the black hole from an accretion disk. It's moving fast in its orbit, and is being additionally pulled by the rotation of the black hole. It can't fall in, until it sheds energy & angular momentum by virtue of the viscosity of the disk, and radiating energy away. That all takes time.

gee; my original response is gone - did I even add it? - if anyone finds it, let me know. That's ok; it gives me a chance to re-phrase it.

I can understand it taking time, but "It would take 200,000 years for gas to travel the last leg of its one-way journey" is the statement I'm stuck on. Is there some sort of constant between accretion disks? Are they talking only galactic core black holes? Could the material start from farther away, or with a different angular momentem? Maybe I don't understand what "last leg" means, or the properties of an accretion disk.

Blob
2006-Jan-10, 07:38 PM
Hum,
i guess that the `last leg` means `from the distance of about 10 light-years away from the centre`.

NEOWatcher
2006-Jan-10, 09:09 PM
Hum,
i guess that the `last leg` means `from the distance of about 10 light-years away from the centre`.
Ok, I did a little research on my own, so I could possibly hang myself with this question.

Looking up information on an accretion disk kind of pointed me toward Roche Limit. I might surmise that the last leg as they are talking about is when the object has reached the liquid Roche Limit of the BH. Now what I'm wondering is that since the Roche Limit depends on G, would there be a direct relationship with the distance of the Roche Limit, and the diameter of the EH? And if that distance varies, would it make sense that G is fixed at the EH and fixed at any Roche Limit, therefore a fixed amount of angular momentum needs to be lost?
(Am I even anywhere in the ballpark?)

Blob
2006-Jan-10, 10:11 PM
Hum,
You have put forward some very good questions, however the answer i guess is a lot simpler.
There isn't a cut off limit to gravity.
Treated as such the `last leg` bit is bit vague if not meaningless.

The term needs a defining spatial aspect to it, which is i think simply just how close to the EH they managed to observe and resolve and measure the velocities of the infalling gases.
The resolution achieved with the Very Large Telescope is about 0.15 arcseconds, which corresponds to about 30 light-years across.

ozark1
2006-Apr-27, 12:43 PM
Actually the last leg never ends - not to an observer at a safe distance from the black hole. He can never see anything cross the event horizon as in effect time is stopped at the eh.