# Is curvature instantaneous?

1. Mar 15, 2015

### TimeRip496

Assuming a mass warp spacetime such that the curvature of spacetime extend one light year away from that object. If I am standing at 1 light year away from the object and the object start losing mass by emitting light, will you feel the change in the curvature first before the radiation reach you after 1 year or feel the change in curvature instantly the emitted radiation reach you?

My apologies for such qns as I am not smart at all.

2. Mar 16, 2015

### phinds

Changes in gravity propagate at c, just as does light, BUT your question is more complex than that because to a body far away from an object there IS no change in the center of mass of the object, as far as the distant body is concerned, as photons or mass leave an object.

SO ... I assume your question is really just "do changes in gravity propagate at c?" and they answer to that is yes.

3. Mar 16, 2015

### wabbit

To the question "do you start to feel the change at the same time the first emitted radiation hits you", I would think the answer is "Yes" :

Until then the star+emitted light lies in a spherical region outside of which you are, so as phinds said nothing changes. After that you are inside the expanding sphere and the gravity you feel (i.e. the curvature) changes.

This effect might be quite tiny - after all losing mass through radiation is what stars do for a living : ) but if we're talking say a supernova explosion, then I expect it might be noticeable, provided the gravity from the star is itself noticeable 1ly away.

Last edited: Mar 16, 2015
4. Mar 16, 2015

### bahamagreen

When you say "more complex" and refer to the center of mass... are you hedging a little bit, perhaps obliquely, about how gravity does not show aberration due to relative movement the way that light does?

A lot of questions about the speed of gravity come from wondering about presumed propagation delay effects on orbit stability.

5. Mar 16, 2015

### A.T.

Light is a propagating disturbance of the EM-field, comparable to gravitational waves. Gravity itself is comparable to the Coulomb force, which doesn't show the aberration that light shows.

6. Mar 16, 2015

### TimeRip496

Why? Is the speed of light by gravity shown by general relativity or einstein field equation? If so, can you tell me?

7. Mar 16, 2015

### phinds

I don't know what the specific math is that shows that gravity changes propagate at c, but there are lots of posts on this forum discussing it so a forum search will turn something up.

By the way, I slightly misspoke when I said that there is no change in the center of mass of a radiating object as far as a distant observer is concerned. I should have added "until the radiation begins to pass the remote observer" (at which point the COM actually STILL hasn't changed, assuming equal radiation in all directions, but the effects on that observer will start to change).

EDIT: I see that wabbit's post already covered this correction.

Last edited: Mar 16, 2015
8. Mar 16, 2015

### phinds

9. Mar 16, 2015

### phinds

Right, which is why I said "changes in gravity propagate ... " (AT, I'm pointing this out as additional info for TimeRip, more than as a response to your correct statement).