Posted on 7/12/2002, 10:38:57 PM by gcruse
| 10:17 12 July 02 |
| Exclusive from New Scientist Print Edition |
|
Black holes really do imprison matter and light, and sap energy from light that narrowly escapes their grip. Until now, these were only predictions of Einstein's theory of gravity, but astronomers peering at suspected black holes have at last found compelling evidence that this does actually happen.
Black hole theory says that if a very large star explodes at the end of its life and leaves behind a core weighing more than about three times the mass of the Sun, the core will collapse to a point under its own gravitational pull.
So strong would be the gravity of the resulting "singularity" that it would prevent matter and even light escaping from a region around it bounded by the so-called event horizon.
By definition, it's impossible to see black holes directly. But astronomers have located around a dozen black hole candidates in our Galaxy, because orbiting telescopes can see the X-rays emitted by a black hole's accretion disc - the disc of hot matter swirling towards it.
However, there has been no concrete proof that any of these objects really has an event horizon - until now. Jeremy Heyl of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts and his colleague Ramesh Narayan think they have found conclusive evidence.
It comes from studies of neutron stars, dense remnants of supernova explosions that are not heavy enough to collapse into black holes. Instead, they collapse into neutron-rich balls about 12 kilometres across with solid surfaces made of iron nuclei.
Most of them occasionally eject flares of X-rays called type 1 bursts that last up to 15 minutes. The bursts are thought to occur because matter trickling onto a neutron star's surface gradually piles up and then burns in a nuclear fusion explosion.
Narayan and Heyl have calculated that if very heavy objects do not collapse to a point with an event horizon but instead have a surface, they would eject as many type 1 bursts as neutron stars. But to date, we have not seen a single burst from an object thought to be a black hole.
"Since they don't burst, we can argue beyond reasonable doubt that they don't have a surface - it's pretty compelling," says Heyl.
In a separate study, scientists led by Jane Turner of the University of Maryland, Baltimore County, have confirmed that light narrowly escaping from a black hole loses energy as it emerges - the second of Einstein's predictions.
They were following up earlier work on the X-ray spectrum of a black hole accretion disc, which revealed a broad "fingerprint" generated by iron. It had a smeared-out pattern of frequencies, suggesting that X-rays near the event horizon were losing energy as they escaped from the black hole's gravitational pull.
However, critics argued that the pattern could be due to jostling electrons in the hot gases colliding with the X-rays. But now the Maryland team has proven the critics wrong.
The astronomers studied a supermassive black hole with a mass 23 million times that of the Sun. They looked at fine detail in the broad spectral fingerprint of iron using NASA's Chandra X-ray satellite and the European Space Agency's XMM-Newton satellite.
The pattern of frequencies was precisely what Einstein's theory predicts for light climbing out of an accretion disc, rather than the result of the chaotic jostling of electrons.
The researchers believe that the X-ray features they looked at are coming from two very bright "hot spots" within the black hole's accretion disc. If so, tracking the hot spots could allow astronomers to measure how fast the black hole inside is spinning.
"This research shows the possibility of watching an individual hot spot as it spirals toward the event horizon," says Fred Baganoff, an astronomer at MIT. "That would be a tremendous advance." |
I thought the black hole proper was literally a mathematical point. How can a point spin?
(Another dumb question from a guy who quit physics just before they got to the bizarre stuff.)
Gravitational collapse imparts spin, I would
think, from the conservation of angular velocity.
Much as pulling you arms in as you twirl on
your toes is supposed to speed you up,
the collapse into singularity would take
its spin with it. Being a point doesn't
mean never saying "I'm dizzy." But
the real experts should be along shortly. :)
Ah yes. Leaping Leprochauns. Aren't they predicted by string "theory"?
Something like the surprising amount of iron on an extremely redshifted quasar on another thread yesterday. That's a black hole, too.
Just trying to imagine matter being sucked toward the centerpoint after it has crossed the event horizon. I imagine it zips along pretty quickly. Whether or not it's still matter after crossing that boundary, I have NO idea. :-)
Aside from size, the are both black holes. The iron in this one is expected since it was a star, probably 2nd generation or later, of relatively recent creation and so it would be expected to have a lot of iron. The quasar would have been an entire galaxy created near the time of the Big Bang and shouldn't have had time to evolve so much iron.
Hey! It's weird out there!
Right, the event horizon definitely has a radius. But I've read that's not really the beginning of the black hole proper. It's just the solution to an arbitrary mathematical problem; namely, find the equation for the boundary beyond which you're not getting out. The black hole proper is at the very center, and is no larger than a mathematical point.
I particularly liked the frame dragging explanation, but there are a lot of other interesting comments here about it, too.
Thanks, everyone . . . I love these threads! :-) A lot of smart people hang out at FR, that's for sure.
A black hole ends in a singularity, i.e. a point where the known laws of nature break down. We don't really know much about them, it's really off the wall stuff, but perhaps the high gravity has some effect not on the black hole but on the area of space-time directly surrounding the black hole, giving it the effect of spinning.
However, it seems possible and within the laws of physics so far, to use a black hole to do some limited time travel.
Actually, it has a circumfrence and surface area. There's no way to measure the radius.
Hawking's theorem that 'a black hole has no hair' is interesting: it states that the only measurable quantities associated with one are its mass, angular momentum, and electric charge. In particular, there is no observable difference between a hole that was originally matter, one that was originally anti-matter, and one that was orginally just light or graviatational waves (assuming that's possible)
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