(Disclaimer: I’m not an astronomer, so please forgive naive misspellings or references, thanks.)
Last evening, on the Science Discovery Channel, there was a program about black holes. It seemed relatively new, unlike the dated shows they put on so often. For one thing, it was formatted in HDTV proportions, and for another, they made many references to discoveries and conclusions from the nineties.
They were discussing how to “see” super massive black holes by looking for a black disc in the center of the bright super heated matter that was spiraling into it. They were unable to see such a disc even with the best Hubble (and other) images, looking at objects they were convinced are black holes and surrounding matter.
It occurs to me that, space being three-dimensional and all, they will never see a disc in a circle of light, since the black hole is not a disc in a disc, but a sphere in a sphere. The super heated matter will surround the black hole, not just on one plane, but on every plane.
Am I missing something, or has the stretched rubber analogy of gravity so diverted their focus?
One thing is that black holes revolve (rotate? Anyway, they spin). This spinning affects the movement of matter around the hole. It’s kind of like how Saturn’s rings are more or less aligned with the direction of the planet’s spin.
Hopefully a more knowledgeable Doper will show up with a better explanation, but I think that’s the main reason.
here’s my wag:
think of a traslucent hollow ball (or even a light bulb) looking at the center (minus the filliment) you can see right through it almost like it’s air (with some refraction) but the edges are much easier to see and appear solid.
The Saturn rings had occurred to me, but there are documented cases of matter (in the form of moons) around gravity masses (in the form of planets) whose orbits are “irregular”, i.e., not coplanar in an equatorial plane: at least two around Uranus, eight around Jupiter, and one around Saturn and Neptune.
If the spinning of a core object causes the gathering of sorrounding matter into (basically) a single plane, is the distribution of the matter, and therefore the election of one plane over the others, predictable, based, perhaps, on the momentum of the average mass or something? If the rings of Saturn are a smashed moon, it stands to reason that the pieces would continue in orbit according to the momentum of their ancestral mass.
The lightbulb analogy isn’t clear to me [smile]. My understanding is that the super heated matter is not transluscent, but opaque, i.e., white.
sorry didn’t go into that much detail. the lightbulb glass is like the matter in orbit of the BH. you will ot be able to see through the matter itself but since the matter is sepperated by empty space it is somewhat transparrent like the glass. if you look directly at (right angle to) the matter (or glass) you can pretty much look right through it. If you look at it along it’s plane (the edge of the ‘bulb’ or BH sphere) you are looking through much more matter.
Forgive my density, k2dave, but given your position for the sake of argument that the super heated matter is not opaque, that still doesn’t explain how you might see something black through it.
Experiment: Open your favorite graphical software. Create a new image that is solid black (like space). Now, use your software’s paint brush to paint a black circle in the center (like a black hole). This is your black hole in black space. Now use your software’s paint-bucket tool to spill white on the image at, oh, say 50% transparency. Do you now see the black hole as distinguished from the black space? No.
You are correct that black holes (the event horizons, that is) are spherical. Large black holes are at the centers of large galaxies. Large galaxies tend to be disk-shaped (spiral galaxies). So, the in-falling material is already preferentially within one plane. But not 100% of course as the centers of galaxies are less disk-like than the outer regions (i.e., the central bulge). Most in-falling material does not shoot straight in but instead, due to its initial velocity (speed + direction) it follows a orbiting spiral around and eventually into the black hole. This massive amount of in-falling material still has gravity and attracts other in-falling materials. Plus there is a bit of “friction” from colliding bits of of this in-falling material. Plus there is conservation of angular momentum (like how pizza dough flattens out when it is spun). All this tend to pull stuff into one main plane called the accretion disk.
So, material gets pulled toward the black hole, falls into a decaying orbit, gathers with other material into a disk, gets superheated as it gets closer to the black hole, gives off lots of energy, and then enters the event horizon and disappears forever. So an incredibly bright and fast moving disk going around what appears to be nothing would be an indicator of a black hole.
FWIW, gravity/friction/angular momentum are also what flattened out the cloud of material that our solar system formed from into a flat plane (i.e., all the planets more or less orbit the sun in the same plane even though the solar system formed from a non-disk cloud of material).
If the original material that formed the planet & moon were from the same source (i.e., one cloud), then they would tend to rotate/revolve in the same direction and would have parallel axes of rotation. If the moon was smashed into smithereens, then the resulting fragments would stay in the same orbital plane as the moon was (and form a ring). If the moon was a captured asteroid, then it could be in any plane around the planet…however, it would be more susceptible to collisions because it’s orbit would cross the orbits of other moons in the “proper” plane.
I also think it is essentially opaque enough that you can not see the central black hole. But odds are that the galaxy you are viewing is not in the same plane as our galaxy, therefore, you can look down upon the accretion disk at some angle and see what’s in the middle.
In order for this experiment to work, the software would have to simulate an event horizon. The white paint wouldn’t be randomly scattered, it would collect (due to time dilation) just outside and dissappear within, the event horizon.
Phobos. The accretion disk explanation helped a lot, thanks. Searching the phrase on Google, I found this Animation of Accretion Disk Around Black Hole mpeg which purportedly is a representation of the accretion disk from the point of view of an observer in polar orbit.
At no point in the orbit is the black hole (or rather its event horizon) completely invisible. Does that mean that not only must the spiral galaxy be on a different plane from ours for us to see the central blackness (sort of like a corona?), but that the polar axis of the black hole itself must also be perpendicular to the polar axis of our plane?
I’m going to modify my previous statement a little. Even with the distant galaxy being at an angle to ours, it’s still tough to see the central black hole through the surrounding material (after all, the black hole is a small space in the middle of a large bulge in the galactic disk). Seeing the empty middle of an accretion disk is probably a bit easier for a black hole out in the galactic arms (one resulting from the end-stage of a large star).
That said, I would think that looking down the polar axis of a black hole would be a bit difficult. Large amounts of the energy released from the accretion disk is emitted perpendicular to the plane of the accretion disk. This is somewhat of an unrelated link, but it has a drawing of what I mean. http://itss.raytheon.com/cafe/qadir/q385.html
Looking-down-the-axis can be a bit of a trick for astronomers. For example, if they are looking at a bright quasar, are they seeing something that is bright in all directions or are they just in the line-of-sight of the main energy output? Obviously, if the latter is the case but they assume the former, then they will overestimate the size/power of that quasar.
Physics aside, let’s examine the geometry of the empty sphere inside the translucent glowing sphere for a moment.
Blow up a balloon. Put a round dot on it with a magic marker. Rotate it so that you can see the edge of the dot, just receding over the visible horizon of the balloon. Now, mentally note the position of the balloon, and rotate it until only the smallest portion of your round dot is visible over the horizon. The bigger your balloon, of course, the smaller the angle will be, but the area of surface will be the same.
The visibility of the translucent glowing sphere depends on the amount of light emitted, (proportional to the surface area) and the arc subtended in the field of view (proportional to the apparent width of the dot as it rotates around.) In addition, the glowing sphere is translucent, and the portion over the horizon is visible as well as the portion on this side. Double brightness. The portion opposite the center of the surface from any point does not shine through, because the black hole is in the way.
So, the edges of a circle defining any translucent sphere of glowing material will appear brighter than the surface perpendicular to the observer. (Various ring shaped nebulae will show a less pronounced exapmle of the effect without a black hole.)
There’s no need to go to other galaxies to see black holes. The nearest outer galaxy is 2 million ly away and most are much much much further. There are dozens of black holes within our own galaxies and are closer then 140 thousand ly, so the apparant sizes is much larger to see. And you probably wouldn’t be able to see the black part of a black hole swalling matter since usually the matter gets so bunched up near the event horizon that you get two streams shooting out at nearly the speed of light, one up and one down. Probably our best chance to ‘see’ a black hole would be by looking at one that is not swallowing anything. The way you would see this is by see it bend the light of galaxy behind them. I believe that there was a ESA seach for MACHOs a couple years ago and theyfound a handful of events like that. There are probably a ton of black holes orbiting above the Milky Way in the halo. All that being said there’s nothing much we could really learn by ‘seeing’ a black hole. Other then to be able to say “Yup, it’s not there.”
[nitpick]
The nearest galaxy outside our own would be one of the Magellanic clouds. To be honest, I am not sure which of the two is closer, but the Large MC is about 55,000 parsecs away, give or take 10,000.
The Milky Way is also colliding with a galaxy right now! So technically it’s not outside us, but it came from there.
