[url]http://news.bbc.co.uk/1/hi/sci/tech/2639043.stm[/url]

Gravity appears to be limited to the same speed as the propagation of light.

i was just thinking about that the other day .
i am glad Einstein was right.

You make my head hurt.

That's a nice bit of information.  The big question is what causes gravity.  Physicist like to dance around that little detail.

Little off topic..

How can a black hole produce enough gravity to bend light and not let light escape? is there something at the end? Something I'm missing?Just a question I've pondered in the back of my head for a while. Any astrophysicist here to explain?

edited to add: Also, What is a black hole? How is it formed?

G!

How would that work if the speed of light is not necessarily a constant?  Planerench out.

I imagine the rate of gravity propagation would change in the same way.  We know that the speed of light changes depending on what medium it is in--that is what causes refraction and allows lenses to work--so it would be interesting to know if gravity is effected in the same way.  Would the effects of gravity propagate more slowly in water than they would in air?  More slowly in air than in vacuum?  According to these findings, that is the implication.

Edit: [red]Disclaimer:  The above is probably wrong.[/red]

Edit:  Also, there was evidence found a few months ago that the speed of light used to be higher than it is now--which would imply the effects of gravity used to propagate more quickly as well.

If I remember correctly gravity acted instantaneously but I took Gen Phys I  and II many years ago (1988 and 1989).

Well, I am getting a minor in astrophysics [:E]

The principle of equivalence states that a gravitational field is indistinguishable from an accelerating reference frame.  In other words, all things behave the same here on the surface of the earth, where the acceleration due to gravity is about 9.8 meters per second per second, as they would in, say, an elevator accelerating at 9.8 m/s^2 at some isolated point in space.  If you drop a ball on earth it would behave the same as if you dropped it inside that elevator.

Similarly, if you had a flashlight shining through a hole in the side of the elevator (perpendicular to its acceleration) the light beam would appear to bend "downward" to someone inside that accelerating elevator.  The light is moving in a straight line but the elevator is moving, and accelerating, perpendicular to it.  This light beam would appear curved, if the acceleration is great enough.  By the principle of equivalence, this light beam would behave the same in a gravitational field.

That is how gravitational lensing works.  Light from some distant object passes by a very massive, nearer object and is bent:

[img]http://imagine.gsfc.nasa.gov/Images/news/lens_fig4.gif[/img]
[img]http://imagine.gsfc.nasa.gov/Images/news/lens_fig1.gif[/img]

Anyway, if you look at more massive objects, the light is bent more and more.  A black hole is just an object so massive that light originating from its matter is bent so far in on itself that it doesn't escape beyond a certain radius (i.e. event horizon, or Schwarzschild radius).

So in answer to your other question, no, there is nothing at the end...because there isn't an end.  It is just a massive object, in its most generic sense.  The intense gravitational field produced by that object would cause significant time dilation, according to Einstein's theory of general relativity.  This theory also (mathematically) allows for objects called "white holes" which could be used in conjuction with specific black holes to form "wormholes."  Here's one set of FAQs I found with a google search which talks about that stuff more:

[url]http://physics7.berkeley.edu/Education/BHfaq.html[/url]

That is what was classically believed.  Einstein (apparently, correctly) assumed it was limited to the speed of light to develop his theory of general relativity.

The problem is that light interacts very strongly with materials, since it is just a traveling version of the electromagnetic force that holds atoms and molecules together. So it shakes the particles around in a material that it passes through. Gravity is so much weaker, it would just pass through unnoticed. This is what makes it a real bitch to detect and study gravity waves.

What you are suggesting may happen on a larger scale. Like gravity waves generated by a star exploding might be "lensed" by hitting the broad side of a spiral galaxy.

Of course, I don't claim to really understand this stuff, but the smartest people who have ever lived haven't figured it it out either, so I don't feel so bad.

This is a good point, but I don't think it is caused because gravity is weaker (which it is, in most situations).  But rethinking my earlier post, the propagation of em-waves is only slower in certain media because the photons are repeatedly absorbed and reemitted.  The speed of light in a medium should be the same as it is in vacuum between the particles of that medium.  Therefore, I would imagine that the propagation of gravitational effects would not be slowed by the presence of any media, because from my understanding, those effects would not be absorbed and reemitted by particles....  Or would they?  Depending on what theory of gravitation is correct....  Is it the "standard model" which predicts the existence of gravitons?  Would gravitons be "absorbed and reemitted" and thus slowed?  Although the standard model has been challenged recently, if I understand correctly, with physicists being unable to find any trace of the Higgs boson.  This is all over my head, I am just an undergrad.  Maybe in a few years I'll understand particle physics and general relativity...we'll see.


Interesting, but probably not observable for the time being, even if it did occur.


[:D]  I can't make that claim either.

Let's see how much EMAG I can remember..

Light travels through various media at different speeds, because the photons themselves are absorbed and re-emitted by the atoms of the media.  If a medium is transparent, then the light will go straight through, with a little loss of speed, and perhaps a little diffraction.  If the medium is opaque, then the light is absorbed, but not re-emitted, or it may be re-emitted back out the way it came (reflected).  

I don't think gravity is absorbed and re-emitted by atoms, so the speed of gravity wouldn't be affected by medium.

(I am not a physicist, but I played one in class for a while)

Yep, I agree.  I corrected that in a later post, after [b]MadMatt[/b] jump-started my brain (hey, I'm still on break, that's my excuse).

Like so many little orbiting planets ;)

I read a different article somewhere that said something different.  Well sorta.

The article I read said they determined that gravity was not 'instanteous', and that it definitly was not slower than light, and that it probably wasnt more than 10 times faster than light.

Dunno if that really means anything in theories or not.

Was the other article older or did it refer to this same discovery?

If the former, it fits with this new data.  It gave a range from c to 10c and this new data says it is c.  If it is the latter then one of the sources is dropping the ball, which wouldn't be the first time the media misreported a scientific discovery....

In simple terms.

A black hole is simply super compressed matter. The more dense the matter the stronger the gravity. For instance, if the Earth were compressed to half it's size it would have twice as much gravitational pull (Note to physicists: I'm using gross generalities here.)

A black hole is a LARGE sun that expanded and collapsed on itself so dramatically that one four times the size of our sun now occupies the space of a soccer ball. it exerts massize gravitational forces that destroy anything that comes near it.

The limit of it's influence is called the "event horizon." A black hole has matter and takes up space but you still cannot view it. This is because even light is destroyed by it's gravitational field. As a result the black hole itself becomes invisible.

Also it is not a tunnel or anything like that. Just a inconceiveably dense object in space destroying anything that comes near it.

"If gravity travelled at the speed of light it would mean that if the Sun suddenly vanished from the Solar System, the Earth would remain in orbit for about eight minutes - the time taken for light to travel from the star to our planet. Then, in the absence of gravity, Earth would move off in a straight line."

Something tells me that if this were to happen, gravity wouldn't really ( matter )anymore. The question here is. How long would life as we know it survive under these circumstances?

If you removed the sun, the atmposhere would instantaniously freeze and fall to Earth as snow. All life as you know it would end instantly.

Sparsky,

While you're question was partway answered by another poster, let me take a shot at it in slightly more accessable terms, starting with the second question first, that is, What is a Black Hole and how is it formed.

A black hole is formed when a very large star, (much larger than our own sun) exhausts its nuclear fuel that keeps the fires of fussion burning. The mass of material in the star wants to collapse on itself due to gravity while the energy of the fussion process in the star's core wants to blow it appart. Up to this point the forces are balanced.

Anyway, at the end of the star's life, several processes occur (too much to detail here), but the end result is the star blows up in a massive explosion called a supernova. This is very important to you and me because it is only in a supernova explosion that elements heavier than iron can be formed. (All that gold, uranium, etc we have came from supernovas!) The dead star's core will now begin to collapse again on itself, but now without fussion forces to balance the collapse. The only forces that are left are the nuclear forces between the atoms of the material in the star. IF the star has enough mass left after blowing itself apart, the forces of gravity will overcome even these forces and crush the material of the star into A SINGLE POINT called a singularity.

Now, the strength of a gravitational field is determined by (among other things) the density of matter. Since the density of the singularity has approached infinity at this point, nothing, not even light (which is affected by gravity) can escape the singularity if it gets close enough. This parameter of no return for light is called the singularity's 'event horizon'. The size of the event horizon is determined by how much matter is in the singularity. As more stuff is sucked in, the "black hole" grows in size.
---------------------

This is a very simplified explanation of what happens and there are many things to know about black holes, gravitational theory, and quantum theory. Strangely enough, Stephen Hawking has theorized that black holes can evaporate! (too long to explain here).

Anyway, I hope this is somewhat clearer to you.

That would really suck......

[img]http://home.earthlink.net/~thegardenweasel/bearslippin.jpg[/img]

Then that means, a spaceship can travel at the speed of lght.

Just got to figure out how to catch a ride on a gravity wave.

waterdog

Damn... some of the explanations on this thread  are deep enough to form a black hole all to themselves.
They are good though.

In 1978 I checked out and read the book "Red Giants and White Dwarfs" by Robert Jastrow. It is one of those books that I will always remember. It was fasinating to find out stuff about the universe that I had not known until then. The book covered the spectrum of stars and the life phases they go through to form, red giants, white dwarfs. pulsars, quasars, twin stars, novas, supernovas, black holes etc... etc.  If I remember correctly all the way back then, it basically stated that black holes were theorized to form when a bright star had the right combination of mass, density and gavitational pull  that it would eventually collapse upon itself after it petered out as a bright star. A star's mass density makeup is the main determining factor on the ultimate fate of a star.  As big as our sun is, it is nowhere even remotely close in total mass density to ever become a black hole... hell just read the book :) ... seriously, it is a good book, easy read.

Post from DigDug -

The Earth sucks?

Eric The(Huh?)Hun[>]:)]

This is not correct.  Density has nothing to do with an objects gravity.  The force of gravity is described by:

F = GMm/r^2

Where G is the "gravitational constant" (about 6.67 Newton*meter^2/kilogram^2), M is the mass of one object, m is the mass of the orbiting object, and r is the distance between the two.

What this means:
In SteyrAUG's example, if the earth were compressed such that its mass were held constant but the diameter was cut in half, then--
1. If we were on the new surface of the earth (half as far from the center as it is now) then we would feel a gravitational force 4 times stronger than we experience now, because the radius is cut in half, and notice the gravity changes with the inverse [b]square[/b] of the distance.
2. If we remained the distance we are now from the center, meaning we would be in orbit around the new, compressed earth, the force on us would be exactly the same as it is now.  Notice the above equation-- in the example, neither mass is changed, nor is the distance (r), so the force does not change.  The equation is not concerned where the surface is.  Compressing the earth only moves the surface, so to speak.

Replacing the sun with a black hole of the same mass would not alter the orbit of the earth.


I hate to nitpick, but that is what I'm being trained to do. [:D]

The event horizon isn't really the limit of the black hole's influence--because its gravitational field will have effects far beyond it.  The event horizon is just the point at which nothing will escape (nothing except, theoretically, Hawking radiation, as someone else mentioned...but I haven't studied that at all).  It also can't be said that light is destroyed...it just does not escape.

Well, it would take about 8 minutes from the sun's removal for us to notice because of the speed of light [:D]

Also, the atmosphere would not instantly freeze.  It would appear to be night and everything would continue to grow colder until that did eventually happen.  I am not sure how long that would take, but I think we would survive for hours or days, not seconds.

This is the same reason someone can be exposed to the vacuum of space and not instantly die, as some sci-fi movies portray.  I have heard from the folks at NASA that someone could be exposed to space for somewhere around 10-20 seconds before losing consciousness and dying (as long as he didn't try to hold his breath).  He won't freeze right away because in the near-vacuum of space the density of matter is negligible.  Negligible density means there are no particles to absorb your thermal energy (no convection or conduction) which leaves only radiating heat as a means of thermal dissipation (that is, thermal energy, your body's heat, will only leave via photons).  That is tremendously slow.  It would take quite some time for the earth to radiate away its heat.

No, see a couple posts up.  A gravitational field is in no way determined by an object's density.  The only thing that high density means is that the object will be smaller than an object of equal mass but lower density, and therefore, another object can approach more closely to the center of mass without colliding with any surface; meaning it is possible to experience a gravitational field of higher magnitude.

Thank you Zonan, SteyrAUG, legrue, and levi for all the answers. Highest I ever learned in the science ladder was Physics and Chemistry. We talked about gravity in Physics, but not really in depth. Thanks again for explaining.

Oh yeah... also a thanks to the knowledgeable Hun...lol

Oh hell, I dont remember.  And I cant find the article.  It was one about the jupiter thing, I know that much, and they talked about the 1c to 10c, and I remember them talking about some huge margin or error in their findings too.  Cant remember the entire context of the whole thing, I just sorta skimmed it.

[img]http://bmrc.berkeley.edu/~trey/images/pics/homer.head.gif[/img]


[img]http://utenti.lycos.it/TribeGenerations/Cartoni_jpg/homer3d.jpg[/img]

So would a 'tractor beam' be possible in theory?

Nah...  

Remember, matter is equivalent to energy.  (E=Mc^2)  Whenever an object gains energy, it also accumulates mass.  The amount of mass gained in ordinary earth stuff heating up and moving around is immeasurably small.  But, when you're talking about relativistic speed (measurable in terms of the speed of light), then the energy gain, and accompanying mass gain, would be immense.  

As you gain mass, it takes more energy to accelerate you, then you gain more mass, and take more energy, etc. etc...  The energy requirements to accelerate are asyptotic approaching the speed of light.  This means that the amount of energy curves sharply upward, becoming infinite when you reach the speed of light.  So, it would take infinite energy to accelerate any normal matter to the speed of light, and once it got there, it would have infinite mass.

This is, fortunately, impossible, otherwise some yahoo from Nebulon V would have done it already, and everything would be sucked into the Gravity Vortex of Doom by now.

If you want to go faster than light, there has been some new research using paired quarks to transmit information.  It's way beyond what I got in school, but might eventually lead to transluminary teleportation.  I doubt I'll live to see it, though.


... Unless my physics prof was pulling my leg, of course.