Speed of Gravity

[VladTheImpaler]

Elegant Universe

I'm just wondering, if nothing can exceed the speed of light and if gravity [or gravity waves] travel at the speed of light then how can black holes exist? The gravitational forces of a black hole is so great that it exceeds the speed of light, doesn’t that mean the matter in the depths of the black hole, beyond the event horizon, somehow communicates (not sure if this is the proper word to use) with the light waves outside at a faster rate than the speed of light?

Can someone shed some light on my confusion?

[VladTheImpaler]

What happens if the Sun ceases to exist?

This video seems to indicate that Earth will just instantly drift away. I don't understand why, though.

[Choobus]

VladTheImpaler wrote

Can someone shed some light on my confusion?

geodesics

A couple excerpts from the Wiki article on black holes that may (or may not) answer your first question:

1)'...the light is running on a spacetime "treadmill;" the light is moving away from the black hole at the rate of c, but the spacetime is being sucked into the black hole at the same rate, so the two cancel each other out, much like a treadmill.' Or, I think a better metaphor might be, it's like trying to to run up the down escalator that is moving as fast as you can possibly run.

2)'Within the ergosphere, space-time is dragged around faster than light—general relativity forbids material objects to travel faster than light (so does special relativity), but allows regions of space-time to move faster than light relative to other regions of space-time.'

So I think -- and I'm no physicist, so feel free to suspect I'm talking out my ass, for I surely think so -- that nothing is really pulled faster than light. It is merely that the mass of the thing means its escape velocity prevents light (and mass and information) from moving out beyond its event horizon. Photons don't move into the black hole at superlight speeds, and nothing is pulling them in at superlight speeds.

To answer your second question, earth would continue to move with whatever momentum and direction it had, according to Newton's laws. It isn't drifting away, really. It was in motion and would stay in motion, along that same vector and with the same speed. Next, it would start drifting toward whatever object was massive/close enough to make its own little well or dip in the grid sheet of these animations. Newton said that every object in the universe attracts every other, proportionally to its mass and inversely proportionally to the distance between the two objects. The earth and some other massive object would find each other (this is too cutesy, really, for I make it sound like lovers running slow motion toward one another in some perfume advert) and meet in a lovely marriage (I may as well go for broke with that metaphor) of orbits around their common center of gravity.

But as I say, I'm just a student of these things and may be wrong. Or I might be right but totally unclear in my explanation, and if that is so, wait for a real scientist to come and answer with more clarity and accuracy.

BTW, I like Brian Greene a lot. While reading his bookThe Fabric of the Cosmos, I had the only moment I ever felt I got what the hell a Higgs field was--of course, a week later, I no longer really understood it, but for a moment there, it seemed to be within my grasp. What more can I ask of a popular science book explaining very technical matters?

[ILOVEJESUS]

well put hippy i concur. space is like a thin rubber sheet. when something so massive rips though ( black hole) then nothing that falls through that hole will escape. thats as simple as i can make it. the earth only stays where it is because its "dip" in the fabric of space is inside that big dip of the sun. if the sun wasnt there it would then traverse till it found another massive dip to orbit in... or maybe fall into. think of space like a thin rubber sheet and all the objects as balls of different weight and questions about gravity etc become clearer.

ty ILJ, I'm glad you found it coherent. You know, sleeping on this, I had another black hole question.

As you all know, we see only a tiny range of electromagnetic radiation, which we call light. Our light really is much more than that tiny slice, of the full range of light, as wavelengths from what we call radio through gamma rays also exist, invisible to our eyes. The reason we see exactly the bit we do is because the sun burns at 5800K, and the black body radiation curve of such an object peaks at just -- how 'bout that? -- the center of our visual range. (Nocturnal animals evolve with a reshifted range for vision, because the evolution of vision is driven by the advantage of seeing both prey and predators.) If multicelled life had evolved around an O type star (impossible, though), it would have significantly blueshifted vision compared to ours. If life exists around M-type stars (maybe it does, they say, but I am skeptical), the diurnal creatures there will see a longer bunch of wavelengths (a redder range to us, though they'd see it as normal and see us as the oddball blue-shifted creatures.)

So I was musing, what if life evolved around a double star system where the two "stars" were a star and a small black hole? (Putting aside for the moment that in a double star system, planetary orbits would probably be so eccentric that life wouldn't evolve there at all. And assuming no dangerous-to-their-life radiation from the black hole's event horizon reaches the planet's surface.)

Would it pay, evolutionarily, for these creatures to develop a sense organ that detected gravitons?

[ILOVEJESUS]

well it would pay to. or maybe in some way sense other larger reactions caused by gravitons ( if they can be found to exist ) that would be beneficial to their survival. once they are sufficiently eveolved like ourselves then they could use technology to sense the gravitons with more accuracy and detail. since we dont think life would prevail round those environments does that not hint that evolving life there would either be tooo complicates with better options elsewhere... or just not possible to evolve life where those kinds of senses would need to be in place pretty sharpish to ensure survival

[Choobus]

oldhippychick wrote

Newton said that every object in the universe attracts every other, proportionally to its mass and inversely proportionally to the distance between the two objects. The earth and some other massive object would find each other

Actually it's inversely proportional to the square of the distance between the two.

oldhippychick wrote

So I was musing, what if life evolved around a double star system where the two "stars" were a star and a small black hole? (Putting aside for the moment that in a double star system, planetary orbits would probably be so eccentric that life wouldn't evolve there at all. And assuming no dangerous-to-their-life radiation from the black hole's event horizon reaches the planet's surface.)

Would it pay, evolutionarily, for these creatures to develop a sense organ that detected gravitons?

I realized, with some thought, that as fun an idea as graviton sense organs was for a moment there, there is no reason to evolve with them. They wouldn't help you get food, avoid being food, or attract a mate.

Just because there is a surfeit of some particular particle on planet X does not mean the creatures on planet X would evolve to detect them. There are, after all, plenty of neutrons decaying into protons all around me, but I didn't evolve with an anti-neutrino detector. In fact, being able to detect microgravities near me would be counterproductive to survival, for I wouldn't know if that 200kg mass thing just ahead was a bear or a motorcycle or a tasty cow or a rock. I'd forever be running from Toyotas and trying to eat rocks.

Which of course I do, but that's a different story.

[ILOVEJESUS]

well alright then. tell me do you like belinda carlisle by any chance? hippy?