# The Naked Scientists Forum

### Author Topic: How fast does gravity propagate?  (Read 4931 times)

#### Riaan Engelbrecht

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##### How fast does gravity propagate?
« on: 20/01/2011 11:30:03 »
Riaan Engelbrecht  asked the Naked Scientists:
Dear Chris,

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?
« Last Edit: 20/01/2011 11:30:03 by _system »

#### syhprum

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##### How fast does gravity propagate?
« Reply #1 on: 20/01/2011 11:51:19 »
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!

#### jartza

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##### How fast does gravity propagate?
« Reply #2 on: 20/01/2011 11:59:27 »
Oh yes, because the info spreads very fast into very large area, it is very close to impossible to detect any info.

#### QuantumClue

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##### How fast does gravity propagate?
« Reply #3 on: 20/01/2011 21:42:53 »
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

μμhμν=0

This means it will follow the speed of light since ▼∂t2 - c22x.

#### Supercryptid

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##### How fast does gravity propagate?
« Reply #4 on: 21/01/2011 03:05:13 »
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.

#### Bill S

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##### How fast does gravity propagate?
« Reply #5 on: 21/01/2011 14:07:47 »
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?

#### yor_on

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##### How fast does gravity propagate?
« Reply #6 on: 21/01/2011 15:32:18 »
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.

#### QuantumClue

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##### How fast does gravity propagate?
« Reply #7 on: 24/01/2011 07:40:58 »
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?

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.

#### CPT ArkAngel

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##### How fast does gravity propagate?
« Reply #8 on: 24/01/2011 07:56:19 »
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.

Yes, because the referential clock of particles are in the same frame as their mass... Gravity and time... (are the same thing)
« Last Edit: 24/01/2011 07:58:17 by CPT ArkAngel »

#### JP

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##### How fast does gravity propagate?
« Reply #9 on: 24/01/2011 10:10:26 »
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?

Longer answer: general relativity seems to indicate that gravity moves at the speed of light.  It seems to be the best theory we have for gravity.  No doubt there are theories that propose gravity moves superluminally.  Some of them might even be compatible with what we've observed (which so far seems to support GR).  But if it does move superluminally this would mean major problems for our understanding of the universe, since we believe that information can't travel faster than the speed of light.  It's probably healthy to be very skeptical about FTL gravity, since it would cause us to re-examine our understanding of the universe if it were true.

Fortunately, there are currently experiments under way to measure gravitational waves which should be able to determine if FTL gravity exists.

#### yor_on

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##### How fast does gravity propagate?
« Reply #10 on: 30/01/2011 16:56:23 »
The problem with gravity isn't how fast it may 'propagate'. In a universe controlled by one 'speeding' constant, namely lights invariant speed in a vacuum as measured in, and from, any frame possible it seems very plausible that gravity will behave the same.

The problem is 'inertia'.

#### Fururism

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##### None
« Reply #11 on: 06/09/2013 21:36:21 »
Gravity is instantaneous! how can an object disappear and its once great gravitational force now gone and travel at the speed of light with no substance !
All space travel calculations are done in the Newtonian way ,we on Earths orbit around the Sun project 8.5 minutes ahead of the apparent position of the Sun !

#### The Naked Scientists Forum

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« Reply #11 on: 06/09/2013 21:36:21 »