h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There is actually another way to detect them. Sometimes they go in front of stars light, blocking the light out.Just thought you might like to know that.

Wait, what do you mean? Do you mean that they orbit or move? Or that the stars are moving and they move behind the black hole?

well, actually they also release radiation, like the one in the center of some Galaxy that the name escapes me. and alsl they can cause matter to be ripped from a star that is close by, such as the
X-1 system. It also causes them to rotate oddly, as if an invisible gravity source were pulling them, which the could be.

as for orbiting, tehy really don't, but *everything* in the galaxy is moving, and also could come between us ans said star as *we* orbit.

Hoped that helped

Not that I am an expert or anything, but yes, both stars and Black Holes can move. Most stars exit in clusters or two or more. (Well, 'most' being two and sometimes three. Singles like ours are more rare then two, and probaly equal to the probability of three. more then three is more rare then singles.. but I digress).
Clusters of stars will have orbits around each other. They spin around a common 'center' which can be determioned by a somewhat comple looking but not completly incomprehensible equation which I once had to memorize for my Physics degree but now forget.
It is possible that if there is a binary star system, one of the stars could collapse on itself and turn into a black hole, while the other just keeps going like any other star. They then would keep orbiting around eachother, and eventualy from our point of view in the universe, one would pass in front of the other.
Just to blow your mind and make myself look even more geeky, there is a third way.
Objects of large mass like stars and black holes and even some big planets, bend light. Just like light can get sucked into a black hole because it's gravitational pull is so incredible, some rays of light get sucked towards it but are far enough away that they dont get sucked in. they bend. The same thing happens around other stars and big planets.
Now imagin that the light from a star is passing near a black hole. it's light gets bent (say to the left, not that left or right means a whole lot), and then continues on a straight line(or vector for all the geeks in the crowd). To an obserever it can look like the star is to the right of where it really is because it's light has been moved. Now if we know the possition of a certain star, and it's light is not where we think it should be, it could be because an object of mass has altered it's light. I believer the Phenomenon is called Paralax.
(and mom said a degree in physics was a waste of time!)

There is another way to detect black holes (BHs), although this method is several orders of magnitude more difficult to use than the others mentioned already.

BHs radiate. They actually emit radiation, which could be detected (theoretically!).

It's a very strange thing, when you consider that nothing including radiation can leave the grip of a BH (after all, that's what makes it a BH in the first place!).

It's all to do with virtual particle pair production in the vicinity of the event horizon. One 'particle' falls into the hole, the other escapes. This, in effect, adds negative energy to the BH; this energy appears as the escaping particle.

To an outside observer, it looks like the BH has radiated!

Very weird indeed.

Isn't the radiation released by black holes called Hawking radiation?

That's the one!

There's supposedly a singularity effect around black holes, caused by the gravity field. The effect of time dilation will increase as you approach the center; however, you'll be too busy being torn apart by the intense gravitic forces to notice that your watch has stopped.

Although there is a singularity inside a BH, that isn't what causes the time dilation. *Any* point of lower gravitational potential than you will run slower than you. It's just particularly noticeable when you're near a BH.

And even if you could hold together, you wouldn't notice anything strange about your watch; to *you* it would look fine. It's just that it would be running slow compared to other observers further away from the BH.

You're absolutely right - I was too busy being clever to pay attention to what I was saying... most of what I know about black holes is gleaned from "Gateway" (Fred Pohl) and other old sci-fi classics.

You've probably heard of the experiment they did with two synchronized clocks - one on top of a tower, one at the bottom. The clock at the top of the tower ran a little faster - by a few thousandths of a second.

Which reminds me of a rather good mystery/sci-fi story by Isaac Asimov. The murder weapon: a null-gravity field and a billiard ball. As the billiard ball entered the field, it immediately accelerated to the speed of light, exited the field, and punched a billiard-ball sized hole in the victim.

The billiard ball wouldn't accelerate to the speed of light, even with the use of a null-gravity field.

(OK, so we have no idea how to generate a null-gravity field, but what the hell! )

You could set up an analogous experiment. Get an electromagnet under the middle of a horizontal piece of paper. Roll a small iron ball in a circle around the centre so that the ball is in 'orbit' around the electromagnet. It's being held in orbit by the balance between its kinetic energy (which is trying to make it fly away) and the electromagnetic force (which is trying to pull it toward the centre).

Now the equivalent to a null-gravity field in this case is switching off the electromagnet. The ball will just fly away because there's nothing to hold it in orbit anymore. But it won't fly off at the speed of light.

Yeah... I think the story was written in 1940-50, some time before I was born, so some of the details might not be as accurate as we'd like.