# Science Questions

## How fast does gravity propagate?

Tue, 25th Jan 2011

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### Question

Does Gravity propagate instantaneously? In other words, to use a fancy analogy: I move the sun 10,000,000 km in 1 sec in 1 direction, will the earth move instantaneously or will the effect only propagate at light speed to reach the earth 8 mins later?

My feeling is that the effect will be "felt" by the earth instantaneously, due to the fact that Gravity propagates in a field around any body. Apparently the jury is still out on whether Gravity consists of both field and particle (Gravitons) components (sort of like light) but I may have my facts muddled up on this one.

Therefore, accepting that Gravity propagates instantaneously, even at stellar distances, any change, however minute, is immediately discernable. It is then logical that if it is possible to interact with the gravity field of Earth, the Moon or a very large space station, it should be possible to regulate the gravity fields generated by this body. This regulation can then be molded into communication signals that travels the universe around us instantaneously, to be picked up by our own (or alien) receivers without the wait for signals that only travel at the paltry 300,000km/s speed of light, so to speak.

We can start small by trying to regulate (or oscillate) the gravity of a small craft the size of Sputnik, building a working receiver that reads the signal from this source and then working up from there.

Obviously the ability to manipulate the gravity field of any body may be a few (or a great many) years in the future but the theory seems plausible.

Am I totally off the mark here or is there merit in this hypothesis?

Regards

Riaan from Menlyn

#### Make a comment

Riaan Engelbrecht asked the Naked Scientists: Dear Chris, I love your show with Redi every Friday on Radio 702. I have a question in multiple parts so please bear with me. Does Gravity propagate instantaneously? In other words, to use a fancy analogy: I move the sun 10,000,000 km in 1 sec in 1 direction, will the earth move instantaneously or will the effect only propagate at light speed to reach the earth 8 mins later? My feeling is that the effect will be "felt" by the earth instantaneously, due to the fact that Gravity propagates in a field around any body. Apparently the jury is still out on whether Gravity consists of both field and particle (Gravitons) components (sort of like light) but I may have my facts muddled up on this one. Therefore, accepting that Gravity propagates instantaneously, even at stellar distances, any change, however minute, is immediately discernable. It is then logical that if it is possible to interact with the gravity field of Earth, the Moon or a very large space station, it should be possible to regulate the gravity fields generated by this body. This regulation can then be molded into communication signals that travels the universe around us instantaneously, to be picked up by our own (or alien) receivers without the wait for signals that only travel at the paltry 300,000km/s speed of light, so to speak. We can start small by trying to regulate (or oscillate) the gravity of a small craft the size of Sputnik, building a working receiver that reads the signal from this source and then working up from there. Obviously the ability to manipulate the gravity field of any body may be a few (or a great many) years in the future but the theory seems plausible. Am I totally off the mark here or is there merit in this hypothesis? Regards Riaan from Menlyn What do you think? Riaan Engelbrecht , Thu, 20th Jan 2011

Even if gravity waves were found to propergate at superluminal velocity it would be quite impossible to use them for communication.
The most sensitive devices built or theoretically possible to detect gravity waves can only hope to receive them from merging black holes there is no way that we could generate waves that strong! syhprum, Thu, 20th Jan 2011

Oh yes, because the info spreads very fast into very large area, it is very close to impossible to detect any info.

jartza, Thu, 20th Jan 2011

Gravity according to our current model travels at lightspeed. The deAlembertian, a four dimensional wave equation will be represented as ▼. Not usual notation but box notation cannot be used here.

▼hμν=0

This is the same as saying

∂μ∂μhμν=0

This means it will follow the speed of light since ▼∂t2 - c2∂2x.
QuantumClue, Thu, 20th Jan 2011

Practical for applications or not, superluminal gravity would still be a very important find in physics. I have a feeling that we would have to rework a lot of physics theories if such were the case. Supercryptid, Fri, 21st Jan 2011

Notwithstanding Q C's equations, which unfortunately mean nothing to me, is there any respectable theory that suggests that gravity might propagate faster than light? Bill S, Fri, 21st Jan 2011

Well, if you want to be pedantic. Inertia do act everywhere in the universe as far as we know, ah, as good as 'instantly' locally. yor_on, Fri, 21st Jan 2011

I can take you through some quantum field theory which would help you understand. But I won't bother if I believe no one is listening. QuantumClue, Mon, 24th Jan 2011

Yes, because the referential clock of particles are in the same frame as their mass... Gravity and time... (are the same thing) CPT ArkAngel, Mon, 24th Jan 2011