# Q & A: Speed of gravity and light?

Q:
It is said that gravity propagates at the speed of light, does this mean that if our sun were to vanish in an instant the earth would still faithfully follow its orbit round the now missing sun for 8 or 9 minutes.?To me this seems very unlikely.
- Terry (age 71)
UK
A:
As usual, in problems of special relativity, it depends on where the observer is located relative to the phenomenon.   If you were on earth and you see the sun vanish, then the gravity would vanish at the same time and you would notice the change in orbit changing synchronously with the change in light.   If you were at the sun's position you would see the earth's orbit be the same for twice the light distance, about 16 minutes.   Then it would change.

LeeH

There's another complication. Just as the field effects can't transmit instantly, the mass-energy which is the source of the fields can't move faster than the speed of light. So the starting premise- the sudden appearance or disappearance of the field source- itself violates the rules.  Mike W.

(published on 09/14/2010)

## Follow-Up #1: what's the speed of gravity

Q:
If the sun were to suddenly disappear (explode) would the gravitational effects of that be experienced here on earth instantaneously or eight minutes. I guess I am asking if information can travel faster than the speed of light?
- Tom Wells (age 62)
Cypress. TX., USA
A:
A nice straightforward question, very clear.

Any change in the Sun will have no influence at all here for 8 minutes- gravitational effects propagate at the speed of light.

On a related point, the mass-energy of the Sun is conserved, so it won't disappear in the explosion. If the explosion is spherically symmetric the gravitational field here (in the good approximation counting only the lowest-order deviation from flat spacetime ) won't change until some of the actual stuff reaches here, which of course would take longer than 8 minutes. If the sun exploded into two blobs heading opposite directions, the gravity here would start to change in 8 minutes, but the initial change would only start to grow as the square of the time after the event was seen because the center of mass of the blobs would stay put.

Mike W.

(published on 04/02/2011)

## Follow-Up #2: The speed of gravity.

Q:
Assume the sun were to be sucked away instantly by some force. At the speed of light, the last light would reach earth in several minutes. The force of gravity would be gone the moment the sun was removed. The question is, how can the force of gravity move faster than the speed of light?
- Ken Jones (age 52)
A:
It doesn't. It propagates at the speed of light. I've marked your question as a follow-up to some earlier ones that have a more extensive discussion.

Mike W.

(published on 09/04/2011)

## Follow-Up #3: photon gravity

Q:
"There's another complication. Just as the field effects can't transmit instantly, the mass-energy which is the source of the fields can't move faster than the speed of light. So the starting premise- the sudden appearance or disappearance of the field source- itself violates the rules. Mike W." ->What if you convert all the solar mass to photons at once (maybe by annihilation?)? What will happen to the gravitational field? Sea of photons can still exert gravitational force on earth?
- Anonymous
A:
Yes, they can.

Mike W.

(published on 07/08/2011)

## Follow-Up #4: Gravity after the sun vanishes?

Q:
Let us consider our solar system only and our Earth and other planets are in their orbits as usual, and it takes around 8 minutes for light to reach the Earth from the sun. Now, let us assume, sun has vanished from this moment, and naturally this event will reach our earth after 8 minutes at the velocity of light. So, during that 8 minutes, our earth will be in its orbit undisturbed, is that true?( assuming nothing moves faster than the light). In that situation, having nothing at the center of the solar system, and earth remains in its orbit perfectly during that 8 minutes, how the basic Physics principles will hold during that 8 minutes. Any explanation?
- Md. Noor Alam (age 58+)
A:
This is a common question, so I've marked it as a follow-up.

Mike W.

(published on 06/03/2012)

## Follow-Up #5: gravity waves from planets?

Q:
If gravity travels at the speed of light, does it work in the same manner? Is gravity a wave in the same manner that light is a wave? I'm not talking about the gravitational waves from binary black holes, just regular gravity from a star or planet. This makes more sense than gravity warping space into a funnel shape that we often see on documentaries. Instead of warping space into a funnel, it warps it into waves. The more gravity, the larger the amplitude and frequency which would cause more space/time to be around massive objects effectively pushing other massive objects into orbits like a needle on a record. I hope my question came across clearly and looking forward to the answer.
- bryan brekhus (age 48)
78413
A:

Although those pictures you've seen are somewhat misleading, they are correct that the basic effect is a warping of spacetime. Waves only show up when something is accelerating. Ordinary planetary orbits involve some acceleration, of course, but the resulting gravitational waves are extremely weak. A static picture of distorted spacetime captures the gravitational effect well.

Mike W.

(published on 11/14/2017)