# Is gravity instant?

1. May 25, 2014

### 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?

2. May 25, 2014

### 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.

3. May 25, 2014

### 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.

4. May 26, 2014

### Vanadium 50

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

5. May 26, 2014

### adjacent

6. May 26, 2014

### ModusPwnd

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.

Last edited: May 26, 2014
7. May 26, 2014

### Buckleymanor

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.

8. May 26, 2014

### 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

9. May 26, 2014

### phinds

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

10. May 26, 2014

### stevendaryl

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.

11. May 26, 2014

### phinds

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.

12. May 26, 2014

### Staff: Mentor

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.

13. May 26, 2014

### dauto

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.

14. May 26, 2014

### Buckleymanor

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

15. May 26, 2014

### dauto

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.

16. May 26, 2014

### Staff: Mentor

Why "therefore"?

17. May 26, 2014

### Matterwave

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.

18. May 26, 2014

### MikeGomez

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).

19. May 26, 2014

### dauto

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.

20. May 27, 2014

### A.T.

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

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

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".

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

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.

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