What the hell is a blackhole?
I have seen them mentioned on Scifi films before but I don’t understand anything other than that they pull you in.
They are objects so massive that their escape velocity exceeds the speed of light.
Escape velocity is the speed something has to travel to escape the object’s gravitational field. The earth’s escape velocity is about 25,000 mph – that’s how fast the Apollo astronauts had to go to get to the moon.
Since, according to Einstein, nothing can go faster than the speed of light nothing can escape from a black hole. Not even light. That’s why they’re black. And, since nothing can escape, everything that falls into them never comes back out. That’s why they’re a hole. In actuality they are neither black (they radiate profusely) or holes (they are massive, dense bodies).
Does that mean that black holes inexorably suck everything in? That’s what is usually portrayed in science fiction – somehow we’ve gotten too close and now we can’t get out! But there’s a reason it’s called fiction. Yes, if you get too close you’ll be sucked in. But the same thing is true of the sun. If you get too close you’ll be sucked into it, too. But we have managed pretty well to avoid that catastrophe for billions of years
Very simply, a blackhole is a collapsed star. We all know that massive objects have gravity. Blackholes are so dense that their gravitational pull is extremely strong. So strong that nothing can escape, not even light(except for some stray particles, but that’s another story)
-Diver
Enormous mass and therefore enormous gravitational pull, in an iddy-biddy-li’l space.
The term “hole” is commonly misunderstood by popular science fiction, as if there were an actual “hole” in the fabric of space-time (which does get wrinkled and needs ironing, but that’s another issue.) There’s only a “hole” in the sense of the enormous gravity attracting lots of stuff that doesn’t escape, hence the “going down a drain” interpretation (as in the Disney movie, THE BLACK HOLE.)
Correct me if I’m wrong, but there’s one other interesting characteristic of a black hole–they have a measurable diameter but an infinite radius. This is due to the distortions they cause to space.
Although the escape speed of a black hole does exceed the speed of light, the escape speed is not necessarily the speed required to ‘leave’ the orbit of a massive body, as suggested by one of the repliers (apollo astronauts).
Escape speed, by definition, is the speed required to escape the gravitational pull of a planet or other large mass immediately starting from the mass surface.
However, since the effects of gravity are reduced greatly as you leave the surface, you can actually ‘escape’ the gravitational forces in a more gradual way at much lower speeds.
I now have a much greater appreciation of blackholes…the only problem I have now is that I don’t understand how light can’t escape a blackhole due to gravity!!!
My brain is beginning to bubble!!!
This thread brings back the memory of an astronomy class I took at Georgetown where we spent a week or so on the physics of black holes. The two things I remember were:
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If the Sun were to go black hole on us, the Earth would still remain in orbit since the mass of the Sun would be the same, just not occupying as much space as before. Not that it would matter to us since we’d all be fried, but what the hell.
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Calculating how big a black hole made entirely out of peanut butter would have to be. Had something to do with the density of the given substance over the amount of space it occupied, IIRC.
How stinky would that be? Not only are you trapped on the damn thing, you just can’t get it off the roof of your mouth.
OK, let’s get back to gesh’s question. Light, as defined by quantum physics, exhibits the properties of both wave (energy) and particle (matter). Given that it possesses material properties, it’s subject to the same physical laws as all the rest of the matter in the Universe. So, when it comes near a massive source exerting a strong gravitational pull, its path is diverted. (This was proven by looking at the position of some stars near the location of a solar eclipse. The light emanating from them was bent by passing so near the sun, and it looked like they’d shifted position.)
Given a large enough gravitational pull (i.e. a black hole), the path is bent so much that light is pulled towards the object in question.
So much for the particle side of the story. Light is also energy, and the energy isn’t affected by the pull of gravity, so black holes radiate like gangbusters, which is how we can see 'em with radio telescopes and all.
And it all becomes clear!!!
Thankyou one and all for enlightening a silly little man!!
Er, I don’t agree … E = mc^2 is an identity, not a mere equality. Energy is mass is energy.
The material falling into a black hole but not yet inside the event horizon radiates like gangbusters for various reasons, and it appears that black holes themselves emit Hawking radiation (virtual particle pairs appearing near or at the event horizon may not immediately annihilate each other when one of them happens to enter the black hole and the other doesn’t).
I think the reason we get so much radiation from the vicinity of black holes is that matter tends to emit radiation, x-rays I think, as it is accelerated. And it certainly gets accelerated a lot falling into a black hole, so the neighborhood of a black hole has a lot of irregular bursts of x-rays. The irregularity of these bursts, IIRC, is one of the ways they were first distinguished from pulsars, which radiate very regularly from their surfaces.
The surface of a black hole is still dark - energy can’t leave a black hole except by the theoretical Hawking process.
Isn’t there energy transfer through gravitational transfer of momentum to the infalling matter? I seem to rcall that black holes can lose angular momentum through that process.
Yep. That’s right. Thanks for the clarification.
Somebody mentioned something about the radius and diameter of a blackhole. The way I understand it is actually, given the huge graviational properties, the black hole collapes to the point of infinite density…thus having no radius or diameter at all. This is the “singularity” you might have heard of.
A black hole would appear “black” because photons of light are pulled in by the gravitational forces like pretty much else. Interestingly enough, the graviational forces themselves are able to escape (I think I heard them referred to as gravitrons somewhere, but perhaps I am making that up). It might appear as if there were a hole in the universe due to the “event horizon” the point at which some light particles can neither escape nor are drawn in but sort of “hover” in space.
Again, none of this is actually PROVEN and is pure speculation for now. They might not really exist at all. But they make for great Disney movies.
That’s a non-sequitur. If gravitational forces could be “pulled into” a mass, the mass wouldn’t demonstrate any gravity and wouldn’t form a black hole.
Graviton have never been shown to exists, outside of Star Trek.
Escape velocity is calculated not from an object’s surface, but from any point in the gravity well. It is derived from equating kinetic energy to the amount of work necessary to move the object to infinity (i.e. the potential energy of the object at some point in the gravity well).
Percentage-wise, the value doesn’t change much whether you’re talking about calculating from the Earth’s surface or calculating from low orbit (only 100-200 miles away–trivial considering the Earth’s diameter of ~7500 miles).
JonF:
Oh, heck. I hadn’t thought of that. Okay, let me amend my statement to, “It’s kind of hard for energy to escape a black hole.” “Kind of hard”, is, of course, highly technical terminology.
avalongod:
I think black holes have positive diameters. A point of infinite density would still have no mass, right? I thought the estimated size of a black hole was a fuel miles across. The whole “infinite radius” thang is way to intense for my finite brain to comprehend right now.
The diameter of a black hole is the diameter of its event horizon. The diameter of the event horizon is defined by the surface at which the escape velocity is exactly equal to the speed of light.
What’s going on inside the event horizon is unknown … although there’s speculation.
Three clarifications:
First off, Hawking radiation does occur, or at least it’d better, or theoretical physics as we know it is screwed, but for a normal-sized black hole (about the mass of a star or larger), it’s completely insignificant. A stellar-mass black hole has a Hawking temperature on the order of a millionth of a degree above absolute zero. By contrast, the universal microwave background has a temperature of 2.7 degrees above absolute zero.
Second, the energy loss that JonF descibes only occurs for rotating black holes, and it only occurs for as long as the hole is rotating. There’s an upper limit on the amount of energy you can get out that way.
Finally, while the graviton has never yet been directly detected, we have a pretty good theoretical grasp of what it should be like, and if it is indeed as we think, then we wouldn’t have been able to detect it yet with any experiment to date. The search goes on.
Other than that, you’uns seem to have the topic pretty well in hand… Why can’t people warn me when these relativity threads come up, so I can weasle in while there’s still something left to say? 
Could probably use Chronos here to verify this or debunk…
Olentzero:
I think energy is affected by gravity. The wave/particle duality of light does not mean that the particle form of light has mass. It can’t have mass and go light speed.
When they say nothing escapes a black hole they mean nothing. I believe the effects described above happen because a massive object warps space. Kinda like steel ball warps a paper towel if dropped in the middle. I believe this warping happens in four dimensions. As light passes a star it dips into its gravity well and seems to change course. In fact, it’s following a straight line in four dimensions that seems to us mere 3-D creatures as if it’s bending.
Nothing can escape a black hole because space is folded back on itself. I.e. There is no path in 4-D that can lead a light wave/particle out.
As for black holes radiating I thought Stephen Hawking described it something like this (forgive my butchering here…read his books for the real stuff):
There is an event horizon to every black hole. Basically the point of no return for light. It is known that particles can spontaneously ‘pop’ into existence. A particle and an anti-particle. Usually they annihilate each other instants after creation. However, if this happens at an event horizon then one particle might have a path that moves it past the event horizon while the other particle stays outside. The one that stays outside can get away since there is no anti-particle to annihilate it and it’s not in the black hole. Voila…black holes seem to radiate energy.