Slashdot Mirror
Plans To Peer At A Black Hole's Event Horizon
mattorb writes: "From the press release: "Scientists have designed and succesfully tested a new type of X-ray telescope that, when fully developed and placed in orbit, may capture the first images of a black hole and resolve images of nearby stars as clearly as we can see our own Sun today. The report is published in the Sept. 14 issue of Nature."
Go here for more information on the project, which is known as the Micro Arcsecond X-ray Imaging Mission. Note that the proposed MAXIM mission would launch after 2010."
Black holes can be detected (in theory of course) by looking for the emissions they give off. The theory goes (extremely roughly) that as individual particles reach the "edge" (event horizon?) of the black hole (crossing this line means you never come back), some of them are torn apart, half of the particle going in, half going out, and some energy is released during this fission. It is these fissions at the edge that make a black hole appear to give off energy, and make it detectable.
That type of radiation is called Hawking Radiation (after Stephen Hawking, naturally). However, this isn't what lets us detect black holes, as Hawking Radiation is ridiculously faint. Black holes can be detected by the X-Rays that they "inadvertantly" produce. When matter is falling into a black hole it is accelerated, heated, and compressed to such a degree that it gives off large amounts of X-Rays. I believe the first black hole we detected (again, assuming black holes exist), was Cygnus X-1 (or cygnus something), and we detected it by the x-rays it gave off.
Another method of detecting black holes is to look for graviational lensing effects. Because black holes are so massive, they bend the fabric of space time. (Imagine a sheet suspended in the air. Place marbles on the sheet. The marbles make depressions on the sheet, like stars make "depressions" in space-time. A black hole is so heavy, it's like dropping something that is the size of a marble but with the weight of a bowling ball onto the sheet. The sheet bends A LOT, and it actually will have a hole where the singularity is.) Light travels in a straight line, so if space-time curves, light also curves with space-time. Gravitational lensing was proved during a solar eclipse. Astronomers observing the eclipse noted that they were able to see stars that should have been blocked by the eclipsed sun. The sun's gravitational field caused enough "lensing" so that stars directly behind the star could be seen to either side of the star. So, if we find something out in space that is causing a LARGE amount of gravitational lensing, but we can't see anything, there's a chance it's a black hole. At that point we normally observe it more to determine if it is or isn't a black hole.
Hawking radiation comes from localized fluctuations in the electromagnetic field intensity of the void. That is to say, even though on average a certain area of empty space beyond the event horizon is, say, E_0, small localized fluctuations may result in, say, differences of +/-1eV.
Now, given that photons are the carriers of the electromagnetic force, you can consider the -1 eV and +1 eV as two virtual photons. These are not a photon and an anti-photon: they're a photon of positive energy, and a virtual (i.e. whose longevity is less than Plank's time) photon of negative energy. Or, if you prefer, if you set E_0 to 10 eV, then the first photon has 11 eV, and the second 9 eV.
Now, the positive photon may have enough energy to escape the black hole's pull; the negative photon, OTOH, automatically does not. It falls into the black hole, where it anihilates with a photon caught inside.
End result: the black hole is 1 eV poorer, and a 1 eV photon has been emited by the space around the black hole. This, in effect, means energy somehow 'escaped' the black hole, and can be measured as radiation.
It's a nifty concept, but unfortunately, its intensity pales in comparison to the radiation emitted by matter falling into the black hole as it is accelerated.
They use gold in all of the X-ray mirrors I've heard of - it is a heavy enough atom that the innermost electrons are tightly bound and the energy from X-rays only causes a transition to a higher energy level rather than ionizing it.
This page describes the manufacture of an X-ray mirror for an ESA mission. I assume they're all fairly similar.
... black holes also show up in superstring theory (well technically M-theory). You can use a D-brane to model the black hole, and this technique has been used to acheive a first principles calculation of the microscopic entropy of a black hole, whereas traditional techniques used fairly general arguments and a bit of hand-waving.
On a side note, string theory may suggest ways that information can escape from a black hole due to violations of locality. This is still very much open to debate though.
For more information, see here at the Cambridge University's Relativity pages.
I don't know how relevant this is, but we just had an article (too lazy to link, sorry) about how the speed of light barrier was broken (by light, ironically). If this is true, doesn't it shoot many physics theories down the drain? And if so, how would if affect this one?
You mean this article? It wasn't really light travelling faster than light at all, merely an effect due to the fact that the pulse of light has a leading edge which travels ahead of it. When this leading edge hit the target, the entire pulse was recreated and transmitted from the other side of the caesium target.
Whilst it looked like the speed of light barrier was broken, it wasn't really, it was just a cunning effect. Whether this effect could be used to transmit information faster than light is unknown - it depends on whether this leading edge can hold information or not.
As for tachyons, well they always travel faster than light and indeed speed up as they lose energy - a tachyon with zero energy would travel at infinite speed!
When a star collapses the matter begins to implode upon a point, eventually crossing the point where the escape velocity becomes greater than the speed of light and a black hole is formed. The edge of this black hole is what we call the event horizon - anything passing within the event horizon cannot ever escape. The simple solution is described by the Schwartzchild metric.
The matter however is still collapsing to a point at the centre of the black hole. According to general relativity there is nothing to stop this collapse and we end up with a point of infinite density and zero volume - a singularity.
However when you come to rotating black holes (described by the Kerr metric) there are differences. The angular momentum of a collapsing star is conserved, and this causes the black hole's event horizon to bulge out along the equitorial plane, much like the Earth has a slight bulge around its equator. Indeed, the central singularity itself forms a torus rather than a point when the black hole is rotating.
As angular momentum is increased this bulge gets bigger and the polar size of the event horizon shrinks, until eventually you are left without an event horizon at all, but just a torus-shaped singularity, which is said to be "naked".
Of course, whether a naked singularity can ever exist is an open question. There is something called the "Cosmic Censorship Principle" which states that the laws of physics will never allow a naked singularity to form, but the final answer is "we don't know".
Also of interest is that since the naked singularity would be in the shape of a torus you could theoretically pass through the centre of the torus and find yourself somewhere completely different, possibly even in another universe!
For a fairly technical intro to black holes and singularities, see this article at suite101.