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In a recent paper (arXiv:1610.02973 [gr-qc]) written by two U.S University Professors (from California State University Fresno and Embry Riddle Aeronautical University), theoretically, is has been shown that massless particles, such as photons, could be produced by the gravitational waves.

In particular they wrote:

"If the production mechanism of electromagnetic radiation fr om the gravitational wave background proposed here occurs and is significant, then we would predict that the gravitational wave signal should also be accompanied by an electromagnetic signal. However, in our process this electromagnetic signal should have roughly the same frequency as that of the gravitational wave. Thus we would predict that the electromagnetic wave coming from the gravitational wave would have extremely long wavelengths , on the order of 100s of kilometers i.e. the associated electromagnetic wave would have very large wavelengths. These wavelengths are of such a length that they could easily have g one undetected up to now."

Two particular questions which we may ask are:

1- How such large wavelength electromagnetic waves could be discovered?

2- Can discovering these sorts of electromagnetic waves be another proof for the existence of gravitational waves?

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Two answers for your two questions, but first let me say that I have not read the paper to ascertain its veracity or accuracy, nor the parameters besides wavelength (such as power and directivity). Perhaps you can expand on that.

1) detecting radio waves with wavelengths (WL) of hundreds of kilometers

It's done already for communications with submerged submarines, and detections of other atmospheric and solar perturbations. The simplest antenna is just a long cable dragged behind the sub. It's obviously not hundreds of Kms long, more like maybe 100's of meters long. It'll detect, but it is not a high gain, or even zero gain antenna, rather it is a lossy antenna. So it all depends on how much power is received.

Note that 100 km WL is 3 KHz. That frequency range and even lower is called VLF (very low freq), and also low enough ULF. See the wiki article about ULF waves, it also has a picture of what is needed to generate them to communicate with the subs. There's been other detectors or antennas for VLF and ULF, usually a distributed set of antennas over a large area.

The whole question is what is the power level that it must detect, and how does it make sure that it is from gravitational waves and not from the sun, or magnetic effects on the earth etc, i.e., how is it unique. The power level will be the critical parameter.

Since EM waves interact more strongly with matter than gravitational waves, even if they are some orders of magnitude weaker (I power, at the antenna), it might still be easier to detect them. The uniqueness is another question.

2)another proof of gravitational waves? Don't need it, it's already been detected indirectly (which this would also be) and directly. But it would be additional evidence, and it might also tell us something about the gravitational wave parameters that would supplement what we observe directly.

Specifically, if we can get the direction of arrival from a combination of 3-4 detectors, we could locate the source, but that's also going to be a similar issue than what we need for that from gravitational waves because they have a similar wavelength (for direction finding) and the same speed (for time difference of arrivals), so about the same accuracies. Still, that could be used to correlate them, in direction or location and time.

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    $\begingroup$ Totally agree on 1) and 2); Especially on 2): evidence for the emission mechanism would be the real deal here, not the evidence for the waves them self. On 1) I agree that detecting those signals will be very hard but maybe with many antennas, coincidence measurements and very good models for what one expect to see it might be possible. $\endgroup$ – N0va Oct 14 '16 at 0:07
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    $\begingroup$ Agreed, after looking at the paper the mechanism proposed is similar to the Schwinger mechanism for electron positron pairs from a static strong electric field. The paper did not calculate the power, or indeed the graviton production rate, but it gave a partial estimate of the reduction in the grav wave amplitude due to it, which to get a produced power estimate has to be integrated. Also, the calculation is for 'scalar' photons, i.e., a calculation still needs to be done for real photons. If it really happens and gets detected, detecting the Schwinder effect would be a new finding $\endgroup$ – Bob Bee Oct 14 '16 at 1:16
  • $\begingroup$ Note that GW have been detected directly now. $\endgroup$ – DilithiumMatrix Oct 14 '16 at 14:55
  • $\begingroup$ The new possible finding would be if the mechanism by Schwinger. happen. The paper referred to a variation they called a dynamic Schwinger effect, induced by GWs. Of course gravitational waves have been previously detected, and it is clear that that is no confirmation of that dynamic Schwinger effect. Whether that quantum effect has a GR equivalent I don't know, but even then experimental confirmation of that, classically or quantum, would be a new detection, and be confirmations of the quantum effect, and if classically also in the equations, another confirmation of GR – Bob Bee 7 min $\endgroup$ – Bob Bee Oct 15 '16 at 19:29

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