If gravity travels at the speed of light and nothing escapes a black hole other than travelling faster than the speed of light then surely gravity cannot escape a black hole.
That means that you can’t detect a black hole by checking for it’s gravitational force.
Unless I’ve missed the latest developments, it is in fact gravity that makes it impossible to escape a black hole. You’re arguing that gravity should somehow cause itself to stop working… or something like that.
Are you claiming that gravitons or gravity waves can’t escape? Even if that were true (I have no idea whether that’s true), would that stop the black hole from sucking things in? A large stationary lump of iron doesn’t necessarily emit gravitons or gravity waves, but will still attract other masses, right?
You could always measure its gravitational force by looking at the effect it has on other masses that are still outside of the event horizon, right?
Preface: I don’t know nothin’ about physics, other than a brief history of time and what little I pick up here and there (mostly here, SDMB).
Whatever happened to the “gravity is the curvature of space” line of thought? IIRC, some theories (specifically having to do with the expansion of the universe shortly after the big bang) postulate that space itself can move, for lack of a better word, faster than the speed of light.
If that were true, wouldn’t gravity of black hole = curvature of space caused by black hole = ‘faster than light’ = gravity of BH felt outside of BH?
I agree with the idea that gravity itself does not exist as a particle. The idea that seems more plausable is that gravity is the curvature of space and that the things we see as gravitational effects are really changes in the fabric of space itself.
If there are indeed gravitons, then we have the possiblity of blocking the action of those particles. If gravitons exert an effect on a body, then somehow they must lose some of their energy in effecting the body. Therefore, we would be able to see a difference in the gravity exerted on the moon by the sun as different say during a lunar eclipse versus during a solar eclipse.
Granted this is just speculation since we don’t have the instruments to measure it, but I’m not sure I can figure out an experiment which would demonstrate the difference between stretched/curved space and gravitons. anyone wanna give it a try?
Gravity most certainly does escape from a black hole, and the gravity of a black hole can be detected outside of the hole. But here’s where it gets interesting: A black hole can have an electric charge, and if so, the electric field is also detectable outside the hole. But the electric field is carried by photons. So does this mean that photons can escape from a black hole, after all?
Well, yes and no. Real photons (or real gravitons, for that matter) can’t escape a black hole. But when your field is static (that is, not changing in time), you don’t need real particles to carry the field, you just need virtual particles. And virtual particles can escape from a black hole. The only catch is that they can’t carry information. If you were inside a black hole and falling to your doom, you would not be able to send your last will and testament to the guys outside. Not even if you had a gravitational-wave generator.
I never paid enough attention to my Physics to fully understand fields. Can you expand some on the idea of virtual particles escaping from outside the event horizon because of a static field? Is the implication that the particles are coming from being close to the event horizon.
Also, is there an implication that the gravity carries no information (besides the mass of the hole, presumably)?
Can you postulate a condition that would show the difference between “gravitons” and bent space? For that matter, what is the thinking about what constitutes space-time itself?
“The sum of the intelligence on the planet is a constant and the population is increasing.” --Unknown
No, these particles are actually coming from inside. You’re probably thinking of Hawking radiation, where particles come from just barely outside, but those are real particles, and can be of any sort (photons, neutrinoes, electrons, etc.)
For a static field, correct. If you have a non-static field, then you can (in principle) use it to send the same sort of information you can send via radio (although the apparatus is more difficult to construct).
No, I can’t, and if I could, I’d probably be heading to Sweeden in the near future for a little ceremony. But if I do find out, I’ll let you all know :).
Just curious about the brane effect. Take the ten dimensional M theories in which the other six spatial dimensions are curled up in tiny circles. Some people have conjectured that all the other forces except are trapped in our three spatial dimensions, which acts like a membrane to keep them in, hence the name. But the weakness of gravity compared to electromagnetism comes about because gravity can “penetrate” all nine dimensions and so it weakens its force in our three.
Now the big jump. Does this play any part in the propagation of gravity in the dimensions we can perceive? And would this help explain gravitation and black holes?
It would play a significant part in our perception of gravity, but only at scales which are small compared to the size of the curled-up dimensions. If, for instance, the largest of the extra dimensions is about a millimeter, then at distances of less than a millimeter, gravity would fall off as 1/r[sup]3[/sup] , rather than 1/r[sup]2[/sup] . Since a size of about a milimeter is a theoretical possibility (for reasons I don’t yet understand, some sizes are favored over others), this was the subject of experiments. It’s been a while since those expeiments were started, though, and since I haven’t heard any more about them, I presume that they came up negative (I’ll ask around about that). If that’s the case, then the next largest “favored size” is well below the range we can experimentally measure.
This would have little or no effect on the black holes we know of in this Universe, though. All of the holes in our Universe are of stellar mass or larger, with radius measured in kilometers, so effects at the millimeter scale wouldn’t matter to them. It might make a difference for the final stages of black hole evaporation, though, and I’ve seen quite a few papers on Hawking radiation in 4+1 or more dimensions.