Is Gravity Instant? A Thought-Provoking Question

[zsweyd]

This is something I have been thinking about for a while, but i want to hear some one else's reasoning.
Say a planet sized mass appears instantaneously in Earth's orbit. Would the gravitational effects from it affect us instantaneously, or does the acceleration of gravity from that mass take time to reach us?

 

[phyzguy]

Changes in gravity propagate at the speed of light. This is clear from Einstein's theory of General Relativity. Note that it is not possible for a mass to suddenly "appear". Because mass-energy is conserved, if you want a mass to appear in the Earth's orbit, you have to bring the mass from somewhere else.

 

[phinds]

If the sun were to magically vanish (a physical impossibility) we would not know about it for 8 minutes via either light OR gravity, both of which, as phyzguy pointed out, travel at c, not instantaneously.

 

[Vanadium 50]

Messages from a thread hijack have all been removed. Let's keep to the topic, everyone.

 

[adjacent]

Op, you may be interested in reading this:http://en.wikipedia.org/wiki/Gravitational_wave

 

[ModusPwnd]

Messages from a thread hijack have all been removed. Let's keep to the topic, everyone.

pffft, that was no thread hijack at all. You are ridiculous. It was on topic and relevant (if not correct (edit - but it appears to be)). It was claimed that the force was instant, as the topic asks and the implications of that claim were questioned.

 

[Buckleymanor]

This is something I have been thinking about for a while, but i want to hear some one else's reasoning.
Say a planet sized mass appears instantaneously in Earth's orbit. Would the gravitational effects from it affect us instantaneously, or does the acceleration of gravity from that mass take time to reach us?

According to Newton the gravitational effects would be instant.Einstein predictions would have gravity propergate at the speed of light.
To do that the mass-less particle, the graviton, would have to exist as only these along with other particles that don't have any rest mass travell at the speed of light.
For all that to happen you would have to have a correct theory of quantum gravity, which at present
we don't.
So at present it's all a bit speculative.

 

[stevendaryl]

There is actually a subtlety to the question of the "speed of gravity" which causes a lot of confusion among non-experts.

In the case of the image of a distant planet, if that planet is moving, we don't "see" the planet at its current location, but we see it at the location of the planet at the time the light left the planet--the so-called "retarded position" (no offense intended). When people talk about gravity having a finite speed, you might think that a similar thing would happen with the gravitational force due to a distant planet--that the force would not point toward the current location of the planet, but toward the retarded position. This is actually not the case; the force of gravity points at the current location of the planet, as if gravity were instantaneous. (A similar thing happens in electrodynamics)

This is discussed here:
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

 

[phinds]

There is actually a subtlety to the question of the "speed of gravity" which causes a lot of confusion among non-experts.

That's very interesting. Thanks for posting that, and the link

 

[stevendaryl]

If the sun were to magically vanish (a physical impossibility) we would not know about it for 8 minutes via either light OR gravity, both of which, as phyzguy pointed out, travel at c, not instantaneously.

I'm not comfortable with that way of putting it, because the same equations that say that gravity propagates at speed c also says that it is impossible for mass/energy to suddenly appear or disappear. GR cannot be used to describe a mass that suddenly disappears (without traveling somewhere else).

However, you can say it this way: Suppose that we are far away from any stars, so that the most prominent source of gravity for billions of miles in any direction is a small asteroid. Now, suppose that we attach a tremendously powerful rocket to the asteroid and move it to a new location. The gravitational field due to the asteroid will not change instantaneously, but the changes will propagate outward at the speed of light.

 

[phinds]

I'm not comfortable with that way of putting it, because the same equations that say that gravity propagates at speed c also says that it is impossible for mass/energy to suddenly appear or disappear. GR cannot be used to describe a mass that suddenly disappears (without traveling somewhere else).

I agree that it is an unrealistic oversimplification, which is why I said it's a physical impossibility. I have no argument with your caveat and your more realistic way of stating it.

 

[Drakkith]

There is actually a subtlety to the question of the "speed of gravity" which causes a lot of confusion among non-experts.

In the case of the image of a distant planet, if that planet is moving, we don't "see" the planet at its current location, but we see it at the location of the planet at the time the light left the planet--the so-called "retarded position" (no offense intended). When people talk about gravity having a finite speed, you might think that a similar thing would happen with the gravitational force due to a distant planet--that the force would not point toward the current location of the planet, but toward the retarded position. This is actually not the case; the force of gravity points at the current location of the planet, as if gravity were instantaneous. (A similar thing happens in electrodynamics)

This is discussed here:
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

Yes, this was what my earlier posts were saying. I'm not quite sure why they were considered off topic. Obviously in the OP's question we don't have a static field, so that the sudden appearance of the object would send out gravitational changes at c. But that example isn't one that can happen in reality, and as Stevendaryl has pointed out, the "speed of gravity" is a little more subtle than that.

 

[dauto]

There is actually a subtlety to the question of the "speed of gravity" which causes a lot of confusion among non-experts.

In the case of the image of a distant planet, if that planet is moving, we don't "see" the planet at its current location, but we see it at the location of the planet at the time the light left the planet--the so-called "retarded position" (no offense intended). When people talk about gravity having a finite speed, you might think that a similar thing would happen with the gravitational force due to a distant planet--that the force would not point toward the current location of the planet, but toward the retarded position. This is actually not the case; the force of gravity points at the current location of the planet, as if gravity were instantaneous. (A similar thing happens in electrodynamics)

This is discussed here:
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

That's a very important point. Also worth pointing out that that's only the case if the body (either a planet or an electric charge) does not accelerate. If it does accelerate than it's force will appear to act from the place where it would have been had it not accelerated.

 

[Buckleymanor]

I'm not comfortable with that way of putting it, because the same equations that say that gravity propagates at speed c also says that it is impossible for mass/energy to suddenly appear or disappear. GR cannot be used to describe a mass that suddenly disappears (without traveling somewhere else).

However, you can say it this way: Suppose that we are far away from any stars, so that the most prominent source of gravity for billions of miles in any direction is a small asteroid. Now, suppose that we attach a tremendously powerful rocket to the asteroid and move it to a new location. The gravitational field due to the asteroid will not change instantaneously, but the changes will propagate outward at the speed of light.

Are you not uncomfortable with gravity propargateing at the speed of light and therefore displaying electro- magnetic qualities with regards speed.

 

[dauto]

Are you not uncomfortable with gravity propargateing at the speed of light and therefore displaying electro- magnetic qualities with regards speed and the fact that the graviton has not been found yet.

No, the speed of light isn't an electromagnetic quality. It is a physical constant related to space-time geometry. Any massless particle moves at that speed no matter what other properties it might have. The fact that gravitons haven't been directly observed isn't a particularly serious problem. Off course it would be nice to be able to observe them but gravity's interaction strength is too small for it to be remotely possible to be done.

 

[jtbell]

gravity propargateing at the speed of light and therefore displaying electro- magnetic qualities with regards speed

Why "therefore"?

 

[Matterwave]

There is actually a subtlety to the question of the "speed of gravity" which causes a lot of confusion among non-experts.

In the case of the image of a distant planet, if that planet is moving, we don't "see" the planet at its current location, but we see it at the location of the planet at the time the light left the planet--the so-called "retarded position" (no offense intended). When people talk about gravity having a finite speed, you might think that a similar thing would happen with the gravitational force due to a distant planet--that the force would not point toward the current location of the planet, but toward the retarded position. This is actually not the case; the force of gravity points at the current location of the planet, as if gravity were instantaneous. (A similar thing happens in electrodynamics)

This is discussed here:
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

You have to add the caveat that the objects must be moving at constant velocity for this to hold, in fact, the force points to the "linearly extrapolated position" as is explicitly stated in the link you gave. An acceleration would not follow a linear extrapolation law for its position obviously.

Actually Drakkith already made this exact point, and I already made this exact comment about restricting the motion to constant velocities...but the posts were deleted. I wonder if this new post will be deleted also. I myself think this is very reverent to the OP's question.

 

[MikeGomez]

You have to add the caveat that the objects must be moving at constant velocity for this to hold, in fact, the force points to the "linearly extrapolated position" as is explicitly stated in the link you gave. An acceleration would not follow a linear extrapolation law for its position obviously.

I’m not understanding this “moving at a constant velocity” concept as it applies to the motion of planets. As they have elliptical orbits, are they not undergoing constant acceleration? Wouldn’t that indicate the constant velocity argument in not valid in the case of celestial bodies?

If gravitation travels at the same speed as light, it makes sense to me that the effect of gravitation would be exactly in line with our visual interpretation, meaning we should see the sun and feel it’s gravitational effect both at the same time, and at its retarded position. It seems to me that to say otherwise is to indicate a separation from the rays of light from the sun with the rays of gravitation (whatever form that may take).

 

[dauto]

I’m not understanding this “moving at a constant velocity” concept as it applies to the motion of planets. As they have elliptical orbits, are they not undergoing constant acceleration? Wouldn’t that indicate the constant velocity argument in not valid in the case of celestial bodies?

If gravitation travels at the same speed as light, it makes sense to me that the effect of gravitation would be exactly in line with our visual interpretation, meaning we should see the sun and feel it’s gravitational effect both at the same time, and at its retarded position. It seems to me that to say otherwise is to indicate a separation from the rays of light from the sun with the rays of gravitation (whatever form that may take).

Yes, Naively one would think that. But turns out both electromagnetic waves and gravitational waves seem to emanate from the retarded position but the electric field of a moving charge and the gravitational field of a moving mass point to the actual position of the object if the object is moving at constant speed. If the object accelerates than just pretend it doesn't and use the speed at the retarded location to extrapolate where the object would've been had it not been accelerating and that's where the fields point to. To understand that somewhat surprising fact one must thoroughly study the Liénard–Wiechert potentials and the fields derived from them.

 

[A.T.]

I’m not understanding this “moving at a constant velocity” concept as it applies to the motion of planets. As they have elliptical orbits, are they not undergoing constant acceleration?

The key for planet orbits is how the Sun moves. The changes in direction of the planets are relevant for their moon orbits.

Wouldn’t that indicate the constant velocity argument in not valid in the case of celestial bodies?

It's an approximation, based on the assumption that the velocity of the source doesn't change much during the propagation duration.

If gravitation travels at the same speed as light,

Changes in gravity propagate at the same speed as changes in the EM-filed. Gravity itself doesn't really "propagate", just like a the E-filed doesn't "propagate".

it makes sense to me that the effect of gravitation would be exactly in line with our visual interpretation, meaning we should see the sun and feel it’s gravitational effect both at the same time, and at its retarded position.

But that's not the case. If the Sun was charged, the E-force would be towards the current position, and so is gravity.

It seems to me that to say otherwise is to indicate a separation from the rays of light from the sun with the rays of gravitation (whatever form that may take).

Yes, for EM there is this separation: If the Sun was charged, the E-force would be towards a different direction that the visual image of the Sun. Similarly for "gravitational pull" and gravitational waves.

 

[MikeGomez]

Thanks duato and A.T.

Also, after reading the reference by Stevendaryl (and Wiki), my understanding of gravitational waves is that they are the result of the third time derivative of position of a massive body (jerk). Is that correct?

 

[WannabeNewton]

No. In the linearized approximation, the power radiated is related to the third time derivative of the mass quadrupole moment so it isn't that simple. This is akin to electromagnetic dipole radiation wherein the power radiated is related to the second time derivative of the electric dipole moment. Gravitational waves themselves just arise from linear perturbations of a background curved or flat space-time through the resulting wave equation.

 

[Buckleymanor]

Why "therefore"?

Not quite sure , if it flies like a duck etc, but as duato points out the speed of light isn't an electromagnetic quality?

 

[stevendaryl]

According to Newton the gravitational effects would be instant.Einstein predictions would have gravity propergate at the speed of light.
To do that the mass-less particle, the graviton, would have to exist as only these along with other particles that don't have any rest mass travell at the speed of light.
For all that to happen you would have to have a correct theory of quantum gravity, which at present
we don't.
So at present it's all a bit speculative.

You don't need a quantum theory of gravity to predict that it propagates at the speed of light. General Relativity predicts that.