A little rusty on this, but a black hole is a supergiant that collapses on itself, right? And yet, where does the extreme gravity come from to form a black hole? If gravity is related to mass, there’d have to be increased mass. But, where does the extra mass come from? Or, have I oversimplified things?
It isn’t that there is any more mass, it’s just that it occupies so little space that the gravitational field intensity gets especially high. From the point of view of a planet orbiting the star or black hole, the gravitational field would remain equally strong.
The big deal is how concentrated the mass is, not because there’s more of it, but because it’s so tightly packed together.
You might be asking why it collapses at one point in time, if it had the necessary mass all along.
The answer to this is, if I’m not mistaken, that the processes that keep the star burning before it collapses excite the particles in it enough to keep it from collapsing.
Yeah, basically. The fusion produces heat, the heat produces pressure, and the pressure holds up the layers above it.
To the original question, the gravity of something gets stronger the closer you get to it. For instance, we’re 150 million kilometers from the Sun. If we went to 100 million kilometers, or 50 million kilometers, the gravity would get bigger. If we went to within about 10 km of a mass the size of the Sun, the gravity would be so strong that we’d start noticing effects from General Relativity. But we can’t get to within 10 km of the Sun, since the Sun itself is a lot bigger than that: We’d have run into the surface long before. However, a black hole the mass of the Sun would have a radius of only about 3 km, so we could get close enough to a black hole for the relativistic weirdness to take hold.
The difference between a black hole and any other massive object, ultimately, is just that you can get closer to a black hole.
So if a star of sufficient size suddenly collapsed into a black hole, any planets that were orbiting it would continue to orbit the black hole at the same distance? (with respect to gravitational pull only, notwithstanding any other forces related to collapse)
Yes.
Which is why it’s so weird to hear people freak out about black holes, and mini-black holes in particular. Ever been afraid a passing proton was going to destroy you and all you love?
Thought not.
" Don’t kid yourself Jimmy. If a proton ever got the chance,
he’d eat you and everyone you care about!"
To extend this question a little, several times I’ve heard “nothing, not even light, can escape a black hole’s gravity.” But apparently information does, right? The information regarding the black hole’s gravity escapes, for instance. Is this right?
You can infer the black hole’s mass and characteristics from the behavior of things orbiting it, or how it influences other bodies.
That’s not true, by the way, there’s a thing called Hawking radiation that does escape black holes.
I understand that. That information about the mass and characteristics has to travel from the black hole to the orbiting bodies though, right? That does escape at a much larger scale than Hawking radiation, I would think.
Only if you construe gravity as information.
The information about a black hole is very limited and doesn’t have to come out, it is there before and after the gravitational collapse. According to the “no hairs” theorem (black holes have no hair), the hole is completely characterized by a small number of parameters. Chronos could give us the details, but this includes the mass, angular momentum, electric charge, position, and velocity.
P.S. Despite the name, the “no hair” theorem is a hypothesis.
Actually, it is a theorem, though of course like any theorem it’s subject to a number of axioms. One, of course, is that general relativity is correct. Another is that the background in which the hole is found is a vacuum. And strictly speaking, “hair” is allowed; what the theorem actually predicts is that any “hair” beyond the few allowed parameters decays away exponentially on timescales of order the size of the black hole.
And the allowed parameters are mass, momentum, position, angular momentum, electric charge, and magnetic charge.
Actually, Interconnected Series of Tubes was afraid of that a few days ago :p.
I would say the no hairs theorum is actually a conjecture.
It is based on several genuine theorums (i.e. mathematically proven statements) in general relatvity whose scope has been extended by making a number of assumptions and several reasonable arguments.
So obviously the mass of a black hole is broadcast from the black hole, and I guess this is done by force-carrying particles that gravity does not affect (gravitons), correct?
It’s not really the best way to think of it, perhaps try the rubber sheet analogy instead.
Think of the black hole as a weight sitting on a rubber sheet causing it to distort and effect the movement of other objects on the rubber sheet. As long as the configuration of matter doesn’t change, then the configuration of the rubber sheet doesn’t change. The weight (black hole) is not continously broadcasting it’s presence to other objects on the rubber sheet so much, instead the objects are aware of it and it’s properties in the way it affects the rubber sheet.
I hasten to add though that though this is only an analogy, the point being that information needn’t pass from the interior of a black holein order for the black hole to graviationally affect other objects. The infomration about it’s mass, etc is already contained in the spacetime that describes the region outside of it’s event horizon
Pre-quote, paraphrased: So, anything orbiting that star (now black hole) would continue orbiting it?
That’s hard to accept when, perhaps from sci-fi, we picture them as the vacuum cleaners of space (within some range). I understand a black hole has a horizon (akin to a sphere of influence, you might say). Can one calculate the distance to where the sun’s event horizon would be (should it become a black hole)? Would Mercury be sucked in or continue to orbit*???
*This question ignores the fact that, as I understand it, the sun would first nova or supernova [thereby swallowing up Mercury if not all inner planets] and then collapse upon itself.
Fiction is fiction. A solar mass black hole would have a Schwarzschild radius of 3 km. So no, Mercury is not going to be ripped apart just because the Sun becomes a black hole.
The Schwarzschild radius is the horizon you’re thinking of, but it is simply the distance from the singularity where the escape velocity equals c.