# Thread: Gravity detection in real time?

1. ## Gravity detection in real time?

Someone posted the following statement on another forum (that is dealing with some complicated world-building related to a science-fiction comic):

Gravity can always be detected in real time, gravity (sic) waves(ripples) in space itself travel at the speed of light but its a forgotten inconvenience that gravity experiments show the suns real position and not its 8min light lag position is effecting Earth in real time. I say inconvenience because its in direct conflict with currently accepted models yet is proven repeatable and verifiable and yet ignored.

...

Quantum Gravity sensors detect actual changes in gravity and not the ripples unlike non quantum based gravity sensors that detect ripples caused by gravity not the actual "pull" of gravity.
This doesn't seem right to me, and I was under the impression that we know the opposite - gravitational waves or not.

Is there a succinct answer to this and maybe a reference or two that anyone can provide?

Thanks

CJSF

2. Gravity travels at the speed of light. I think this person is talking out of their... hat.

3. Changes in gravity (gravitational waves) propagate at the speed of light. So if the sun were to disappear into another dimension, we'd continue to orbit for eight minutes around the point where it "used to be". (Simultaneity is slippery in relativity.)

But the apparent "force" of gravity, which generates the orbital motion, has an interesting mathematical property - it doesn't point towards the retarded position of the gravitating mass (the position it occupied when light-speed delay is taken into account), but towards the point it would occupy if it continued with constant acceleration during the light-transit time. At low (ie, non-relativistic) values of relative velocity and acceleration, this means that GR looks like Newtonian gravity, with its instantaneous force transmission. But at high velocities and rapidly varying acceleration (for instance in a binary pulsar), there's a deviation from Newtonian which we're able to detect.
This way in which gravity seems to propagate with infinite velocity under GR isn't something that has been put in by hand - it actually falls out of the equations.

So your correspondent is wrong about light-travel delay being an ignored problem - it's a very well understood feature of GR.
Take a look at Steve Carlip's short treatise on the topic here.

As for the second section - quantum gravity sensors, like other gravity sensors, measure the acceleration of a falling mass in order to estimate the local force of gravity. There's nothing special about the "quantum" - it refers to the measurement technique, not the gravity, so your correspondent is wrong in saying that "non quantum based gravity sensors" can't measure the force of gravity. We've been using pendulums to do that for centuries, after all. See this news report for a summary.
The thing that detects gravitational waves is a gravitational wave detector, which is a completely different animal. I think your correspondent may be confusing "non quantum gravity sensor" with "gravitational wave detector".

Grant Hutchison

4. So, how do we know that the vectors only point there and that the force really isn't? Oh - so by dint of the extreme cases (high velocity/varying acceleration), we can see the discrepancy?

CJSF

5. [double-post]

CJSF
Last edited by CJSF; 2019-Nov-05 at 07:09 PM.

6. Originally Posted by CJSF
So, how do we know that the vectors only point there and that the force really isn't?
Well, there is no force in GR. The geodesic trajectory followed by an object in free fall follows an orbit that behaves as if it is moving under the influence of a central instantaneous force, like Newtonian gravity. This is despite the fact that changes in the gravitational field propagate at lightspeed. It all falls out of the equations of General Relativity.

Originally Posted by CJSF
Oh - so by dint of the extreme cases (high velocity/varying acceleration), we can see the discrepancy?
Yes - we see the deviations from Newtonian behaviour starting to show up in the extreme cases, but it all looks Newtonian in the less extreme cases. The mathematics of GR conspire, in a way, to produce movement that looks very like the movement that would be produced by an instantaneously propagating force.

Grant Hutchison

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•