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I've been wondering what the current state of the art (both theoretically and practically) for sending messages using changes in a gravitational field is, and I have had a hard time finding information about it online. I've seen the Wikipedia page on gravitational waves, but it seems that gravitational waves per se are not yet practical as a communication medium, since they have yet to be detected.

However, a different (unoriginal, I know) idea is simply to cause a change in a gravitational field from some point, and measure the change at another. You could encode the magnitude and/or frequency of the change into bits and communicate wirelessly. Although this is still subject to the speed of light, it has other benefits over radio communication like easily penetrating all matter.

A particularly slow and inefficient implementation of this might be to attach a large number of powerful rockets to the moon and alter its velocity enough that it changed the timing of ocean tides in a measurable way.

How far can we extend this? Can I build a gravitational detector sensitive enough to measure a bowling ball oscillating near it? How about from the other side of the earth? How little mass can I use?

Can I do it without any mass at all? We know that massless particles are still affected by gravity, and I believe we also know that massless particles have their own gravitational field (in accordance with mass/energy equivalence). Can we exploit the gravity of massless particles to generate high frequency changes in a gravitational field without having to worry about inertia? Can we then use the high frequency of these oscillations to isolate them from the high level of noise generally present in low frequency gravitational detectors?

Danu
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Gibybo
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    No gravitational waves have yet been seen. Considering that we still cannot detect gravitational waves, even from merging black holes, I feel we will need to wait just a little bit longer before we can send a message back, or even to someone else on earth ;-) – hdhondt Nov 20 '13 at 09:03
  • why not using light (or electromagnetic waves in general) to send messages at the speed of light ?! Gravitational wave detectors such as LIGO, LISA and VIRGO are quite large and complicated... – Andre Holzner Nov 20 '13 at 09:05
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    possible duplicate of Would it be possible to transmit information through gravitational waves? – John Rennie Nov 20 '13 at 09:23
  • @AndreHolzner If I was trying to build something today, I suppose I would. But what's the potential for this 100 years in the future? Is it a viable path? It does seem to have some nice benefits over electromagnetic radiation (you can easily send it through matter, like the earth. That means low latency connections to the other side of the world, ships at sea, etc). – Gibybo Nov 20 '13 at 09:45
  • I think you're mixing up gravitational waves with changes in a gravitational field. They are quite different, and most of your question seems really to be about the latter. – N. Virgo Nov 20 '13 at 09:45
  • @Nathaniel I agree, and I was trying to differentiate them in my question, although gravitational waves (in the Wikipedia sense) are new enough to me that it's hard to clearly express that. I am interested in communication with gravity in any form. – Gibybo Nov 20 '13 at 09:50
  • I've edited it to try and make that clearer. Please feel free to roll back the edit if you don't like it. I tried to keep the edit minimal, but it might be better to remove a bit more of the discussion of gravitational waves, since they're not very relevant to what you're asking, and mentioning them too much might muddy the issue a bit. – N. Virgo Nov 20 '13 at 10:05

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At the moment, it's impossible to use gravitational waves for communication purposes. If no gravitational waves have been observed, how do yuo expect to use them?

jinawee
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  • I don't think detecting the wave is strictly necessary for communication. Two 50kg masses 1 cm apart impart a ~.002 N gravitational force upon each other. If I move one of the masses back and forth, I can encode a message in Morse code and read it on a force scale attached to the other, I just need reasonably sensitive equipment. – Gibybo Nov 20 '13 at 09:33
  • @Gibybo Try to calculate the force, it's insignificant. – jinawee Nov 20 '13 at 09:43
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    ? I did calculate it, it's ~0.002 N, we can measure that. I can do the gravitational force napkin math, but I'm not sure where the theoretical limits are (I'm sure they are much less than .002N). – Gibybo Nov 20 '13 at 09:53
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    @Gibybo Now calculate the change in the force. The absolute value of the force is not important, the change in force is. – Manishearth Nov 20 '13 at 13:18
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    @Gibybo: I don't think 1 cm is a reasonable distance for 50 kg objects. The closest you could get them without touching is probably 0.5 meter, in which case the field drops to $10^{-7}$ N. And as Manishearth said, it's not the force which is important, but its gradient. – DumpsterDoofus Nov 20 '13 at 14:07
  • Manishearth, the gradient drops several orders of magnitude in half a meter.

    @DumpsterDoofus I'm a little fuzzy in this area, but can't I take two square sheets of iron (say, 1 cm thick, a meter on each side) and put them parallel to each other? Their center of masses would be pretty close and I think the math for two equal planes is the same for two spheres (center of mass to center of mass).

     – Gibybo Nov 20 '13 at 15:25
  • @Gibybo And that's at what distance of separation? If you want to use it for communication, you need to have some appreciable distance between them. – Manishearth Nov 20 '13 at 17:49
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Not to mention that the force diminishes with distance, making small changes more useless the further you seek to communicate

bob
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    Electromagnetic radiation is also subject to the inverse square law and it dominates our global communications infrastructure, so I don't think this is an insurmountable challenge. – Gibybo Nov 20 '13 at 15:34