Hidden Black Holes Discovered

mknewman wrote to mention a Space.com article discussing the discovery of a large group of hidden black holes. From the article:"Black holes cannot be seen directly, because they trap light and anything else that gets too close. But astronomers infer their presence by noting the behavior of material nearby: gas is superheated and accelerated to a significant fraction of light-speed just before it is consumed. The activity releases X-rays that escape the black hole's clutches and reveal its presence. "

Comments

chicken and egg..

[rd4tech]

what was there when it all started: galaxy or a black hole?

Geek explanation required.

[pwnage]

OK, can one of you physics geeks explain to me why x-rays are able to escape the gravitational clutches of a black hole when light cannot? I've never understood this.

Re:Geek explanation required.

[poopdeville]

If you look at Einstein's (or Newton's even) equations for gravitation, you'll see that it is an inverse square law. This means that the further away you are from an object, the weaker the gravitational force is. Indeed, if you move twice as far away from an object, the force is four times weaker.

Black holes have an associated event horizon. This is more or less the closest light can get without being sucked in. Obviously, the force of gravity is monumentally strong here. It is stronger still inside. But if you're outside of the event horizon, light can escape. If we're 2 event horizon units away from a black hole, the gravitational force is a fourth of what it is at the event horizon. Still a tremendous force, but light can escape.

What's going on here is that xrays are being produced as an object moves towards the event horizon. I'm not sure the mechanism that causes this. I'm a math guy, not a physicist. Perhaps someone else can enlighten us both.

You could also have Hawking Radiation in mind. I don't know anything about that. In fact, I'll click that linky myself.

Re:Geek explanation required.

[MillionthMonkey]

OK, can one of you physics geeks explain to me why x-rays are able to escape the gravitational clutches of a black hole when light cannot? I've never understood this.

Those x-rays you're seeing are coming from hot gas outside the event horizon. Undoubtedly much more radiation is emitted inside it than outside, but any photon inside the horizon has a world-line ending at the singularity and not your eye. And the photons you do see have lost a lot of energy in the trip up from the horizon's edge. They could have started out as gamma rays, but they get redshifted and softened as they climb up out of the field as they lose energy. Or here's a different way to look at it. Just as a clock near the event horizon will appear to a distant observer to be ticking slowly, a photon emitted with a given frequency near the horizon will have a lower frequency as measured by a distant observer.

Re:Geek explanation required.

[Quadraginta]

Well, this is silly: ...if we used heavy enough elements, we could conceivably get them "xray hot". But by that point, we would very likely have reached the ultra-unstable elements that have only been created for very brief periods of time in the lab before decaying.

The lowest electronic energy level goes like Z, and X-rays start at about 100 eV. So you could easily get a soft X-ray out of something as small as oxygen or neon, which while not common are hardly the unstable transuranics you're talking about.

But the larger point is that a plasma, which is that of which an accretion disk is made, is perfectly capable of absorbing and emitting throughout the X-ray region. There's nothing strange about a plasma at 10^6 K, and it emits quite ordinary black body radiation with a peak in the X-ray.

So I think the parent was perfectly reasonable in saying the simplest way to think of this process in a general way is the conversion of gravitational potential energy to thermal energy (i.e. the kinetic energy of the the plasma) and then to radiation in the usual way.

I suspect any radiation produced by the direct gravitational acceleration due to the hole is miniscule. The acceleration required to keep a particle in an orbit of radius 10 km or so is trivial compared to the accelerations during an inelastic collision with another particle at (say) 60% of the speed of light.

Re:Geek explanation required.

[gardyloo]

Although I don't necessarily think that your wording was very accurate, you've essentially hit one of the nails of strangeness in quantum mechanics quite squarely on the head.
    One of the reasons that it took Bohr so long to come up with the (admittedly extremely simple) orbital model of the atom is that, hey, charges should radiate extremely fast at those accelerations (based on all sorts of measurements, most notably by Rutherford), and all matter should basically collapse very quickly. He eventually just sort of waved his hands and said that one of this postulates was that electrons simply can't radiate unless they're in transition from one "allowed" state to another, and then derived various neat consequences from that.
      I could give various hand-wavy arguments based on such things as eigenstates and Wilson-Sommerfeld quantization rules (or, in more modern terms, show a bunch of stuff from quantum electrodynamics) but it's basically a lot of nomenclature and rules which have been shown to work exceedingly well and make awesome predictions... and not make a damned bit of sense from an intuitional standpoint. Seriously.
      If you can dream up some way to connect the nonradiation of a point charge which is undergoing accelerations to everyday language which makes sense, I'm willing to bet you'd get a Nobel in record time.
      (Incidentally, invoking smeared-out charge distributions only sweeps the problem under the rug, as it were.)

Re:Geek explanation required.

[MillionthMonkey]

This is the classical "white death" of the universe predicted by classical physics. An electron in a hydrogen orbital does go in circles. But it can only radiate to a lower energy state. As it becomes more localized around the proton, the uncertainty in position goes down and the uncertainty in momentum goes up. The electron eventually "floats" on this uncertainty in momentum, since radiating more photons does not get rid of it and can only increase it. This process is what forbids further radiative transitions to even lower states closer to the nucleus (which is why they do not exist, to crudely sum up a lot of math).

Sometimes the ground state electrons do fall in, but only if they are destroyed in the process. In nuclei that decay via electron capture, an electron in the innermost shell is captured by the nucleus in a p+e -> n+neutrino interaction. As it moves from the innermost shell into the nucleus, the electron emits an X-ray photon. The energy of this photon can be used to distinguish between K-capture and L-capture (from the lowest and second lowest shells), although I think they use Auger electron spectroscopy for that.

Nanoscule Macroscopes

[Doc Ruby]

Black holes bend space in every direction. Their effect on space is strongest closest to them, especially within their event horizon. But they bend all of spacetime, in every dimension, infinitely. At least to the distance in lightyears of the duration since their forming, and even before, when their spread-out mass still bent space, just not all in one place, and without the counter-intuitive effects within the event horizon.

So it seems that relying on detectors which detect only the behavior of light between the Earthly observer and the unobstructed black hole is pretty crude. How long before we have nanodetectors that detect the miniscule (nanoscule?) deflection of a laser within a small space on Earth, away from the "straight" path we'd expect from the influence of the space matter that we can see? Maybe we have to account for the "dark" matter also bending space in the Universe. But such a detector seems like a lot more reliable mapping instrument, for all these cosmic masses, than just waiting for some gas to drift across the view of our traditional scopes. How long until we can start to use really sophisticated Einsteinian relativity detectors?

Re:Nanoscule Macroscopes

[gardyloo]

Two points to be made:
          1) NASA can EXTREMELY accurately predict the motion of coasting space probes. One of my favorite diagrams is in Marion and Thornton's _Classical_Dynamics_ book (Chapter 8, pg. 316 in my 4th edition copy). The diagram shows an approximation of the International Sun-Earth Explorer 3's orbit, and eventual rendezvous with comet Giacobini-Zinner. There were (I'm copying from the text here) two close trips by Earth and five flybys of the moon (within 75 miles of the lunar surface once). The text states, "The entire path could be planned precisely because the force law [the inverse-square law for macroscale -- and, now, mesoscale -- gravitational force] is very well known." That satellite traveled for 3 years, and used almost no fuel because its orbit was so precisely calculated far in advance.
      2) You're right -- every mass bends spacetime. Even energy (equivalent to mass via E=mc^2) bends spacetime. The coolest thing ever: GRAVITY ITSELF PRODUCES GRAVITY. My gen. relativity prof. did a riff on this in class for a couple days, and was able to show that at the level of a black hole, the "extra" gravity produced by the energy of a strong gravitational field basically leads to a runaway situation, and this is what happens at (or just "inside") an event horizon.

      Much research has been done on this recently, and, if it weren't so late, I could give you the names of some of the experiments to measure this "self-gravity" effect. Erdos?

Re:Let's bring people up to date

[kale77in]

In a 'cold death' scenario, where gravity is too weak to pull the expanding universe back together (this seems to be the majority opinion, and people even talk about the expansion accelerating), I've heard the final distribution of matter estimated at: 9% black holes, 90% dead stars, and 1% dust and gas at 1030 years. I can't find a reference for that online now though; so obviously look it up if it interests you. Maybe some astrophysicist type can confirm or deny this?

Re: Let's bring people up to date

[Black Parrot]

> Can anyone explain if the curent theories still speculate that eventually all the matter in the universe will be sucked up by black holes? Also, once that happens will the black holes (as the only remaining objects in spacetime) start attracting each other?

Here is the most interesting thing I've ever read about the fate of the universe.