I think the thread title asks the question. More specifically, I’m thinking of the supermassive black hole in the center of our galaxy (does it even have a name? Beyond an astronomical number identifier). Could you get close enough to it to see - well, not IT, of course, but rather the accretion disk and such around it - without the aid of a magnifier?
If you could, or if you looked at a black hole with an optical telescope (that is, no X-ray imaging or anything), what would you see? Obviously, the hole itself will be, y’know, black… but would the accretion disk be visible? Would there be enough “background” stuff in the surrounding starfield to clearly make out the lack-of-light spot that the black hole would create?
Ultimately, I’m asking: If you set up, say, an observation post near our supermassive black hole (like in the movie Black Hole… god, that movie was cool), what would you see and how would you see it?
Oh yeah, it could be seen from a long way away. The accretion disk would be very bright long before it passed the event horizon. There is a certain balance to reach, where you can see it with the naked eye and not die from radiation, but you would definitely have a show.
Gravity tends to bend light around, so you would definitely see some distortion along with the black, assuming that you got a good view with the accretion disk going behind the hole. The background stars would be very visible as well because of the increased density around the center of out galaxy.
I’d have to say no. The visible effects described are the effects of the hole, not the hole itself (It’s hard to see Prarie Dogs, for example, but you know where they are because the dirt they burrow shoots pretty high up into the air; but the dirt ain’t the Prarie Dog).
The actual Black Hole is about the size of a golf ball. You can get within a couple hundred thousand miles of it without getting sucked past its event horizon (which, presumably, you don’t want to do) and even at that distance, you’re looking for a black object against a black background. And even if you had a white object (like a moon or a planet) on the other side of it from you, light would still warp around it.
You’d have to suspend the laws of physics five or six different ways to get a good look at a Black Hole, so the short answer is “no.” There’s a ball peen hammer floating around Earth’s satellite belt that you could see from your front lawn before you could see that black hole under the circumstances you describe.
A black hole can theoretically be any size but the ones you are most likely to encounter will be far bigger than a golf ball. A stellar mass (i.e. our sun) black hole would have a radius of about 3 kilometers. For comparison the earth would make a black hole with a radius of 1 centimeter.
Further, the only place you absolutley cannot esacpe from a black hole is within the event horizon. Anywhere outside and you have a shot at escaping although the closer you get the more energy you will need to get away. As such the event horizon (the point at which escape impossible) for an astronaut is probably somewhat greater than the actual event horizon at which light cannot escape. For a supermassive black hole it is conceivable an astronaut could pass the event horizon and not even know it. For much smaller black holes the astronaut would be torn apart long before getting to the event horizon.
A black hole does not necessarily have to have an accretion disk. One can conceive of black holes that long ago gobbled up everything in the vicinity and are quietly sitting by themselves slowly evaporating.
Technically, in order to see something, you need to receive energy reflected from that object. So in this sense you cannot ‘see’ a black hole. What you may see is its effect. If no accretion disk then you may notice some dramatic warping of light coming from behind the balck hole. Some light will skirt the edge without falling in but its path will be severly bent (this effect has been observed with our sun). If there is an accretion disk it is hard to miss. As for what CURIOUS_GEORGIE7 mentioned I believe he is thinking of Hawking radiation. While it exists I think the energy release is exceptionally low from this. Still, if you could detect that then it may be another way to notice a black hole is nearby.
Yes, the black hole would bend light so that you would be able to see things that you would not normally be able to were the object not so massive. The sun does this to some extent as well, but it would be easier to see when the glowing of the object doing the bending is not in the way.
The actual size of the matter in the black hole cannot be proven, and has no bearing on how it will look when viewed. The event horizon is what you will be looking for, and while light will be warped around it it will not be in a manner as to make it invisible. I am sure that there are sims of this effect somewhere on the net…
Krokodil, with all due respect, you’re wrong. A black hole the size of a golf ball would be only a little more massive than the Earth, not the two million solar mass behemoth at the center of our Galaxy. The size of the black hole is the size of the event horizon… There’s simply no other sensible way to describe the size of a black hole.
The most immediately apparent things you’d see would be the effects of the black hole, but the hole itself is “visible” in the same sense that any black object is visible: You won’t see any light coming from it, but you’ll see the silouette against things behind it. And for a large black hole like the one at the Galactic core, you could even cross the event horizon before it ripped you apart, whereas you could clearly see it from a distance many times greater than the event horizon.
And yes, CURIOUS_GEORGIE7, you could see around the hole, and more than that, even. If light comes in at just the right angle, it can even wind completely around the black hole many times before swinging out, so you could even see things behind you looking just past the edge of the hole. However, most of the scenery is going to be stars, and unless you know where the stars are “supposed” to be, you probably wouldn’t notice that their positions were “wrong”.
The particles you refer to emitted from just above the event horizon are called Hawking radiation, and they really do exist, but you wouldn’t be able to see them. The larger you make a black hole, the dimmer and colder the Hawking radiation gets, and even for a stellar-mass hole (the smallest we think it’s possible to form in the current Universe) the effective temperature is about a millionth of a degree above absolute zero. For a galactic black hole, it would be that much weaker. If you could somehow get a black hole with the mass of, say, an asteroid (some of these might have been formed in the Big Bang), you might be able to see it then.
The way you describe it, it sounds like the apparent (from the viewer’s perspective) density of stars should increase right around the edge of the black hole.
So let’s get things straight: You can’t see the hole itself (obviously), but you can see the silhouette. You CAN see the accretion disk (if there is one), and this will be all swirly 'round the hole. And, finally, you CAN get within visual distance of this sucker 'fore the gravity gets dangerous.
Leftover questions: Light will bend around the sucker… will this make it appear as if the starfield doesn’t have a “break” where the hole blocks light? How close can you get to the supermassive black hole in our galaxy’s center, anyhow? How dangerous is the radiation that the beast spews out?
As for light bending, it’s going to be complicated work out exactly what it looks like and is very much dependent on where you are. For example if you’re at 1.5 * the Schwarzchild radius (where the Scwarzchild radius is the radius of the evnt horizon for a Schwarzchild black hole) then you are in the photosphere, were light orbits around the BH.
You can get as close as you like to a galatic centre black hole (as long as it wasn’t a very active galactic centre such as which produces quasars, blazars and radio galaxies, in which case the accretion disc may cause some problems), but I wouldn’t recommend it, as you could cross the event horizon black hole of sufficent mass without even realizing it.
Hawking radiation for any black hole that has occured via the process of stellar evolution is extremely puny and virtually undetectable (though for a very weeny black hole it would make it glow white hot), that said the radiation realeased (in the form of a jet) by a supermassive black hole’s accretion disc is very powerful, which means they can be seen many,many light years away.
Yes. On a smaller scale this is called microlensing, the weak form of gravitational lensing. If a black hole (or other heavy mass) which is too small to resolve passes in front of a luminous object, the object will actually appear brighter while the black hole is in front of it. This is another indirect way to detect black holes.
No, there is reasonably strong observational evidence that they do exist, for a start there are several objects observed that are so dense they could only be black holes. On a large scale observations simply do not deviate from those predicted by genral relativity so even without observational evidence a very strong case for there existance can be made.
Initially black holes were only suggested by mathematics. Subramanyan Chandrasekhar (I think) was the first to suggest the upper mass limit for a star before it must collapse in on itself. This was a result of doing some relativity calculations. At the time lots of scientists didn’t like it and attacked the results. Nonetheless the math couldn’t be denied. Relativity was showing itself to be more and more accurate all the time. To ignore this particular result would throw the Theory of Relativity itself into doubt. Unless there was some mechanism that kept stars below this critical mass a black hole was almost a foregone conclusion. Asronomical observations indicated that such stars (of sufficient mass) did indeed exist.
So, eventually scientists started looking for black holes and sure enough they’ve found them. frixxxx provided a link to one earlier in this thread. Technically no one can be 100% sure these are black holes but the evidence we do have fits a black hole and there is no alternate theory that fits nearly as well for what we do see.
I think black holes are pretty much accepted by the scientific community as about as close to fact as we are likely to get till we can voyage out and study one up close.
That’s exactly the scenario I’m thinking of, RickJay (though I imagine, in the far-flung future, they’ll set up a tourist trap near the galactic black hole instead of a research station). I’m just trying to get a better understanding of precisely how it’ll look to the naked eye, so’s I can describe it better in a story.
There may well be a large gap around the black hole in the centre of our galaxy, which has been swept clean of stars; occasionally stars and stuff does fall in, and I believe that recently such an event has been detected; but the accretion disc is not as active now as it may have been earlier in the history of the galaxy, when the central black hole might have been very busy.
Our galactic black hole is a quiet feeder…
This is a possible reason why distant galaxies seem to have more active in the past, when their light set off to reach us; thay hadn’t cleared out the space around them yet.
Interesting visual and dynamic effects occur near a black hole.
At 1.5 Schwarzschild radii a light ray would form a perfect circle around the hole. To a person in a large tube enclosing this circle the tube would appear to be perfectly straight, and he would see infinite images of his back. If he started moving in the tube, no matter how fast, he would feel no centrifugal force (no correction please) even though he is circling the hole.
If the tube were inside this radius the person would see it curving away from the hole, even though the actual curvature is exactly the opposite. This also means the centrifugal force would tend to throw him inwards toward the singularity.
Not for a black hole formed by stellar collapse. It would be absorbing more from the background radiation that it emits. It would be a very very long time before the universe cools enough for a stellar black hole to radiate more than it absorbs. Primordial black holes perhaps (if they exist), but not black holes formed by stellar collapse.