# Gravitational waves vs. electromagnetic for telecommunications

Assuming it would be possible, what would be the advantages/disadvantages of manipulating gravitational waves for telecommunications versus using electromagnetic?

• There's a public JASON report here that in part addresses use of gravitational waves for communication. – GPhys Jul 26 '16 at 4:49
• Thanks @GPhys! I am curious on this subject because it seems China is working (pending decision-maker approval) towards using GW in some aspect for communications. There has also been rumors of military application. Just trying to weigh the credibility. – Keola Sawyer Jul 26 '16 at 4:54
• As you can see in the report, it is the opinion of JASON that this is in no way credible. – GPhys Jul 26 '16 at 6:20
• Thanks @GPhys. Didn't realize some make believe company proposed to do that. A quick reading of the report seems to be comprehensive, intuitively right, and with enough details to destroy the credibility of the proposals. The author concludes the proposals are off by 25 or so orders of magnitude, with statements like it would take all the energy produced on earth over a number of universe lifetimes. The author totally debunked an otherwise extraordinary, and false, technical proposal. They proposed to build reasonably sized (orders of meters) gravitational wave detectors and even generators. – Bob Bee Jul 27 '16 at 1:38

The whole thing is much simpler. Communicating at a distance (tele) with gravitational waves would require astronomical or astrophysical bodies to generate those waves. The gravitational force is much weaker than electromagnetic, so it requires much more energy to generate and much bigger 'antennas' to detect them.

The grav to EM force ratio is about $10^{-33}$, by one measure, specifically the grav force of two protons vs the EM force at the same distance. It is basically impossible except for astrophysical massess.

The advantage of gravitational waves is that they interact with matter so weakly that they don't get absorbed by all the matter or energy in intergalactic space much, and we can detect them when we can not detect EM waves. For instance, all the maelstrom during the first 380,000 years of the universe we can not see electromagnetically, since before recombination it was all scattered and absorbed in the electron proton plasma then existing, mostly. That and seeing inside neutron stars or supernova will be real useful.

But it is also worse. Because they require large bodies their wavelengths are very long, thus requiring large 'Antennas' (from a few Kms to say space based interferometers a million or 100 million light years long). You'd then have to get the detected signal to your 'user', probably electromagnetically. Sounds architecturally poor. But it is worse: the large wavelength means small bandwidths, and thus small Shannon capacity (bits/sec). Too small for anything of use other than science or science fiction: the wave we detected in 2015 was basically sound frequencies. And we barely detected it.

BTW, to beam it you'd have to beamforming it, typically having a bigger transmit antenna, thus even more miles of mass to form the even larger size antenna.

No free lunch, no free physics

Although you didn't mention, I suspect under "it would be possible" you understand that there are no engineering advantages/disadvantages. These influence the actual engineering investitions often more as we would think.

So, we have only a single difference between them, which is its largest advantage and also largest disadvantage, too:

Unshieldability.

There is no way to shield or deflect gravitational waves, while the electromagnetic radiation shows a very diverse picture in this sense.

This results, GW sources would be detectable in the whole Earth.

A little remark: the practical unshieldability of the GW and that we can't create them, are coming unfortunately from the same reason: we can't really interact with them. Consider the recently found gravity waves of a black hole merge. It radiated away 3 solar masses in some tenths of seconds. For this short time, it was much more power as the whole visible Universe. And we need to build a billion \$ device to detect it, because its only effect on us was, that it changed the length of a km-length laser beam with a minor part of a proton length.

• One can't shield them, but one could always interfere with them... so there isn't much of an advantage there, either. – CuriousOne Jul 26 '16 at 3:58
• @CuriousOne Advantage is that you can send waves directly to New Zealand and they will arrive directly. Interfering with negative intent would be, on my opinion, too specualtive in this view (although maybe the whole question is too speculative). Maybe directed GW beams could avoid this problem. – peterh Jul 26 '16 at 4:02
• If someone can send X, then someone can interfere with X. You can argue (successfully), that one can't even send X (at least not with any known technical mechanism), but if you are speculating that this can be overcome, then you have to allow for interference. – CuriousOne Jul 26 '16 at 4:09
• @CuriousOne Yes, and this interference is also the largest disadvantage. A similar problem already happened in the 60s, as both the USA and the CCCP used ultra low frequency radio waves for the communication with their military submarines. – peterh Jul 26 '16 at 4:17
• @peterh, I believe they still use extremely low freq and very low freqradio waves in submarines (not in the Navy so don't know for sure). Would GW be better for penetrating seawater? – Keola Sawyer Jul 26 '16 at 4:27