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View Full Version : Supermassive Black Holes Spin at the Limits of Relativity



Fraser
2007-May-29, 09:24 PM
You know the saying: nothing, not even light can escape a black hole. That makes them invisible. Amazingly, researchers from the University of Maryland have determined how fast a supermassive black hole is spinning. ...

Read the full blog entry (http://www.universetoday.com/2007/05/29/supermassive-black-holes-spin-at-the-limits-of-relativity/)

mknorr
2007-May-30, 08:44 AM
So, what's the bloody spin rate? :P

dasguptachandan
2007-May-30, 09:50 AM
Are there any difference of speed between the photons comming from the centre of the black hole and that comming from it's circumference ?

John Mendenhall
2007-May-30, 01:01 PM
Are there any difference of speed between the photons comming from the centre of the black hole and that comming from it's circumference ?

Photons in free space travel at the speed of light, wherever they are. See Wikipedia article on special relativity. By definition, photons cannot escape from the center of a black hole - or from anywhere inside the event horizon. I assume you mean photons from the accretion disk? Which, by the by, probably doesn't reach to the center (spin axis?) of the black hole, at least externally.

Additionally, this article and the three following (this is for May 30, 2007), are absolutely mind boggling. The more we know, the more we know we don't know.

ronjust1
2007-May-30, 03:09 PM
Fraser,

I don't understand Mr. Hawking's Black Hole Radiation theory, simply named - Hawking Radiation. If nothing can escape a black hole - not even light,...then how can it radiate anything? Thanks :)

R o n G e r l a c h .

isidoor rabi
2007-May-30, 03:47 PM
They have measured the rotation speed of the accreation disc if I am not mistaken!
Should not those discs have a way of movement which is dictated by the mass of the black hole (and the distance of the disc from it) much as in celestial mechanics?

John Mendenhall
2007-May-30, 05:04 PM
Fraser,

I don't understand Mr. Hawking's Black Hole Radiation theory, simply named - Hawking Radiation. If nothing can escape a black hole - not even light,...then how can it radiate anything? Thanks :)

R o n G e r l a c h .

A pair of virtual particles - one normal, one antimatter - which occur spontaneously and usually self destruct almost immediately, may occasionally appear near the boundary of the black hole. In this situation, one particle may fall into the black hole, leaving the other particle in our universe. This particle appears to us as Hawking radiation. I forget the details of the mass loss for the black hole, but it's in the Wikipedia article.

Cecil
2007-May-30, 09:21 PM
The article states "...least 98.7% of the maximum spin rate allowable by Einstein’s Theory of General Relativity"

So what *is* this spin rate? 98.7% the speed of light?

Also, how large is this supermassive black hole? If you dropped it into our solar system, to what planets would it cover? Mercury? Venus? ... Just to get an idea of the size...

abraham
2007-May-31, 04:23 AM
i think this report is not an example of "bad" astronomy but excellent astronomy coupled with horrible reporting....

the accretion disk is spinning at some rate. maybe 98.7 of the allowable spin rate of by Einsteinís Theory of General Relativity maybe not the article didn't specify.


Not the black hole proper.

we're not really absolutely sure about precisely what happens in a black hole (were not even that sure about what causes gravity) so unless the scientists assumed that the accretion disk is spinning at the same rate as the black hole or they assumed that the black hole is spinning at a faster rate than the accretion disk it is impossible to conclude at what speed the actual black hole is spinning. isn't possible and maybe even plausible that the black hole is spinning at a completely different rate (or axis) than the accretion disk? perhaps its not spinning at all?

it makes sense to wonder if a black hole spins ... it just doesn't make any sense to conclude that because the accretion disk spins the black hole must spin at a faster rate.

A possible explanation they could have used is that the mass of the observable particles (read accretion disk) exert some gravitational or other force on the black hole proper. and cause it to spin. if that were the case then it would follow that the black hole itself would be spinning no faster than the innermost particles that haven't yet crossed the event horizon and become part of the black hole itself.

another is the former explanation plus the amount of kinetic energy in the particles which collide with the black hole after crossing the event horizon. this raises two more questions. the particles that cross the event horizon accelerate but at what rate? do the particles ever reach the speed of light? if so then couldn't radiation pressure account for how much the particles push on the sides of the black hole to make it spin? if so then if mass of the supermassive blackhole doubles by absorbing particles traveling at the speed of light (for ease of calculation the mass of these particles could be substituted with the relative mass of photons) hey... perhaps everything that travels at c has no rest mass (Einstein's idea) that doesn't exclude relativistic mass.

I'm sure that much speculation has been proposed on these questions and I honestly don't have any answers. But these are the questions that need to be answered before any conclusive evidence can be presented.

If there is new evidence that proves me wrong I want to hear it. Being proven wrong only means that I've now learned something.

Abraham Adams.

Widespread intellectual and moral docility may be convenient for leaders in the short term, but it is suicidal for nations in the long term. One of the criteria for national leadership should therefore be a talent for understanding, encouraging, and making constructive use of vigorous criticism.
Carl Sagan

John Mendenhall
2007-May-31, 12:17 PM
I'm sure that much speculation has been proposed on these questions and I honestly don't have any answers. But these are the questions that need to be answered before any conclusive evidence can be presented.

There is a wealth of information and speculation available on spinning discs where the rims are traveling at relativistic velocities. See Wiki et al. There is also a thread on ATM from about six months ago about the problem. As I recall, even Einstein considered the problem. Essentially, even though we normally consider a solid disc as continuous, it is actually composed of atoms and molecules, and various expanding and contracting forces operate on the material from the quantum mechanical level on up at relativistic velocities. There is still not agreement on exactly what happens (check the sources, there have been some fine brawls over this), but close study of these objects may resolve some of the questions. It's certainly convenient of mother nature to provide us with such a nice testbed.

Added: And just for fun, beside the usual three spatial and one time vectors of general relativity, in this case one of the vectors rotates continuously. Makes my head spin.

Nick4
2007-Jun-06, 06:00 AM
How do you mesure the speed of a black hole?

John Mendenhall
2007-Jun-08, 09:11 PM
How do you mesure the speed of a black hole?

Verrry carefully. Don't get too close.

Seriously, the spin rate or the proper motion?