Thread: Is there a speed to gravity?

Hypothetical... If the sun disappeared in a split second, we would still get light for about 8.5 minutes because the light that left the sun would still be traveling through space to the earth.

Would we be under the gravitational pull for those 8.5 minutes. If light is the fastest "thing" that travels in the universe then we should be under a gravitational pull for at lease 8.5 minutes after the sun disappeared. Right?

Just thinking about it, you would think that we would loose gravitational pull immediately. Making... the speed of gravity faster than light... right? If so how fast is gravity?

I guess what I'm asking is how can one object affect another object over vast distances seemingly in an instant?

Originally Posted by mike365

Hypothetical... If the sun disappeared in a split second, we would still get light for about 8.5 minutes because the light that left the sun would still be traveling through space to the earth.

Would we be under the gravitational pull for those 8.5 minutes. If light is the fastest "thing" that travels in the universe then we should be under a gravitational pull for at lease 8.5 minutes after the sun disappeared. Right?

Just thinking about it, you would think that we would loose gravitational pull immediately. Making... the speed of gravity faster than light... right? If so how fast is gravity?

I guess what I'm asking is how can one object affect another object over vast distances seemingly in an instant?

The speed of light in a vacuum is the limit. That limit applies to gravity, but it requires the general theory of relativity to explain why.

If so, could a body form a "dual star" orbiting system with itself?

I mean - imagine a massive body following a circular trajectory (how to start this movement is another question, let's say we used some unimaginably powerful machinery pulling unimaginably strong ropes from a distance). At every point of this trajectory, it is subject to the gravitational field it created when it was on the opposite side, just as if it had a twin there.

Oops. Now I have written this I realize the body would have to travel at to get to the other side at the same time as its own gravitational field, so this particular setup is impossible. I will still post it because I feel there are other configurations of moving bodies where the finite speed of gravity can have curious effects. Any ideas?

that's a very good question *trundles off to find answer*

I'd assume it depends entirely on what gravity is, if it's some undiscovered FTL particle (like tachyons?) then it could feasibly stop near instantly, faster than light in fact.
if gravities waves of energy much like light, then it'l probably have a top speed of light



(i'm sure someone will plane me but that's my tuppence worth)

Originally Posted by Leszek Luchowski

If so, could a body form a "dual star" orbiting system with itself?

I mean - imagine a massive body following a circular trajectory (how to start this movement is another question, let's say we used some unimaginably powerful machinery pulling unimaginably strong ropes from a distance). At every point of this trajectory, it is subject to the gravitational field it created when it was on the opposite side, just as if it had a twin there.

Oops. Now I have written this I realize the body would have to travel at to get to the other side at the same time as its own gravitational field, so this particular setup is impossible. I will still post it because I feel there are other configurations of moving bodies where the finite speed of gravity can have curious effects. Any ideas?

You seem to be thinking that there might be some type of "aberration of gravity", where the felt force of gravity comes from where the source was, not where it is. This will not happen. The short reason for this is that effects from General Relativity cancel out any tendency for this to occur.

Originally Posted by Janus

You seem to be thinking that there might be some type of "aberration of gravity", where the felt force of gravity comes from where the source was, not where it is. This will not happen. The short reason for this is that effects from General Relativity cancel out any tendency for this to occur.

I find this strange, because it would mean - in my opinion - that gravity transmits information instantenously, making a body at point A "feel" the presence (or absence) of another body at point B at the very moment when that other body is (or is not) there.

I thought such instantenous transmission of information was impossible, precisely because of the Theory of Relativity.

As I don't understand TR I am probably wrong, but I am curious to know where I err.

Originally Posted by Leszek Luchowski

Originally Posted by Janus

You seem to be thinking that there might be some type of "aberration of gravity", where the felt force of gravity comes from where the source was, not where it is. This will not happen. The short reason for this is that effects from General Relativity cancel out any tendency for this to occur.

I find this strange, because it would mean - in my opinion - that gravity transmits information instantenously, making a body at point A "feel" the presence (or absence) of another body at point B at the very moment when that other body is (or is not) there.

I thought such instantenous transmission of information was impossible, precisely because of the Theory of Relativity.

As I don't understand TR I am probably wrong, but I am curious to know where I err.

What happens is that certain GR effects due to velocity "cancel out" the effect of aberration.

This causes the effect of gravity to come from the where the source "is". The thing is this only works as long as the velocities are uniform. It's kind of like leading a target. You can hit a moving target, not because you aim at where it is, but where you know it will be when the bullet gets there, so I don't need my bullet to travel at an infinite speed to hit a moving target (this is a really loose analogy).

Of course, if your target makes a sudden change of velocity after you pull the trigger, you will miss.

The same holds for cancellation of gravity aberration. It works if the velocity between the objects is uniform. If the relative velocity of the source suddenly changes, there will be a propagation delay before you would notice.

With objects in orbit, the relative velocity, on average, remains constant and you get no net aberration.

If this were not the case, no orbits would be stable. Consider the Earth. If the above mentioned effects did not cancel out aberration, the Earth would hit the Sun's gravity at an angle. There would be a cumulative forward component that would accelerate the Earth forward in its orbit, causing it move out from the Sun at a fairly good rate.

Is this more or less the same reason why galaxies still "look right" when viewed at an angle, even though the furthest part is thousands of light years more distant?

If so how fast is gravity?

The Gravitational force as well as the electromagnetic force both travel at the speed of light. Gravitational influence has never been shown to exceed or go below "c" However, this is not set .

Originally Posted by KALSTER

Is this more or less the same reason why galaxies still "look right" when viewed at an angle, even though the furthest part is thousands of light years more distant?

No. Consider the Earth in it's orbit around the center of the galaxy. it takes 225,000,000 years to complete an orbit and is 25,000 ly from the the center. It would take light 50,000 yrs to cross the diameter of this orbit, in which time, the Earth would move less than 1/10 of a degree in its orbit. One tenth of a degree is the distance a minute hand moves in one tenth of a sec, or not very much at all.

So the reason the galaxies look "normal" is that they rotate too slowly for any real visible difference to occur in the time it takes light to cross their width.

In answer to the original question, my guess is that light would stop hitting the Earth after 8.5 minutes, but gravity would stop immediately, as in, the gravitational field would recede or collapse, travelling at C, back towards where the Sun was, the instant the Sun vanished. Do I have a theory to back this up, " No " , but I am working on one.

I am still working dudes.

Originally Posted by Janus

Originally Posted by Leszek Luchowski

Originally Posted by Janus

You seem to be thinking that there might be some type of "aberration of gravity", where the felt force of gravity comes from where the source was, not where it is. This will not happen. The short reason for this is that effects from General Relativity cancel out any tendency for this to occur.

I find this strange, because it would mean - in my opinion - that gravity transmits information instantenously, making a body at point A "feel" the presence (or absence) of another body at point B at the very moment when that other body is (or is not) there.

I thought such instantenous transmission of information was impossible, precisely because of the Theory of Relativity.

As I don't understand TR I am probably wrong, but I am curious to know where I err.

What happens is that certain GR effects due to velocity "cancel out" the effect of aberration.

This causes the effect of gravity to come from the where the source "is". The thing is this only works as long as the velocities are uniform. It's kind of like leading a target. You can hit a moving target, not because you aim at where it is, but where you know it will be when the bullet gets there, so I don't need my bullet to travel at an infinite speed to hit a moving target (this is a really loose analogy).

Of course, if your target makes a sudden change of velocity after you pull the trigger, you will miss.

The same holds for cancellation of gravity aberration. It works if the velocity between the objects is uniform. If the relative velocity of the source suddenly changes, there will be a propagation delay before you would notice.

With objects in orbit, the relative velocity, on average, remains constant and you get no net aberration.

If this were not the case, no orbits would be stable. Consider the Earth. If the above mentioned effects did not cancel out aberration, the Earth would hit the Sun's gravity at an angle. There would be a cumulative forward component that would accelerate the Earth forward in its orbit, causing it move out from the Sun at a fairly good rate.

From what I understand, if you're moving a fraction of C, and you aim a laser beam at an angle perpendicular to your own motion toward another object moving at the same speed and direction (seems perpendicular to you), it will actually travel at an angle to intercept the other object, but the other object will still perceive it to have arrived from a perpendicular angle.

Feel free to correct me if I'm wrong about that, but it seems to be basically like what you're saying, only if we were discussing light instead of gravity. The gravitational effect travels from the point where it was "emitted" (not sure if gravity can be described as being emitted...) at the angle necessary to reach you, but exerts a force that pulls you in the direction where the emitter is now.

Originally Posted by Leszek Luchowski

If so, could a body form a "dual star" orbiting system with itself?

I mean - imagine a massive body following a circular trajectory (how to start this movement is another question, let's say we used some unimaginably powerful machinery pulling unimaginably strong ropes from a distance). At every point of this trajectory, it is subject to the gravitational field it created when it was on the opposite side, just as if it had a twin there.

Oops. Now I have written this I realize the body would have to travel at to get to the other side at the same time as its own gravitational field, so this particular setup is impossible. I will still post it because I feel there are other configurations of moving bodies where the finite speed of gravity can have curious effects. Any ideas?

This is an interesting point. I wonder if it could be applied to objects being accelerated when they're already traveling near the speed of light? Maybe it takes additional force for the object to escape its own gravitational field?

As Janus pointed out above, acceleration is different from continual motion. If you accelerate an object, say the Sun, I'm thinking that maybe you would be moving it into a space where it is being pulled backward, for a moment, because it's being pulled back toward where it was. But, if you apply enough force to overcome that effect for a moment, then it can freely continue.

Originally Posted by DrRocket

The speed of light in a vacuum is the limit.

Correct.

That limit also applies to gravity

Correct.

But it requires the general theory of relativity to explain why.

Wrong! Why would it? It is an obvious fact that our universe has a speed limit, so nothing can surpass that limit, not even gravity. Besides, theres really no such thing as "the speed of gravity", gravity is just a force, and all forces have a cause, so the cause of forces is what has a speed limit of c.

First, Einstein proved that the speed of gravity is the same as the speed of light.

Secondly, in Part One of the PBS show The Elegant Universe, Brian Greene shows what would actually happen if the sun were to disappear. What would happen is that the curvy dent in the fabric of space-time cause by the sun would flatten out. As the fabric flattens, the earth would then roll out straight into space. That's how it was visualized for us on the show.

I believe that most cosmologists have ascribed to the flat universe theory, so that scenario makes the most sense.

HTH

Originally Posted by Waveman28

Wrong! Why would it? It is an obvious fact that our universe has a speed limit, so nothing can surpass that limit, not even gravity. Besides, theres really no such thing as "the speed of gravity", gravity is just a force, and all forces have a cause, so the cause of forces is what has a speed limit of c.

Nope.

It is you who are wrong. The "speed limit" as you call it is a functin of relatifity. It is the general theory of relativity that deals with gravitation.

It is best not to attempt to contradict other people when you don't know what you are talking aboutl.