# Transfer of energy from gravity back to other “more familiar” forms of energy?

In this question I've mentioned an account of the recently reported 2nd observation of gravitational waves, LIGO and Virgo Collaborations, Phys. Rev. Lett. 116, 241103, 15 June 2016, where 1 of the 22 solar masses is said to have been converted into pure energy - gravitational waves.

My question here is in two parts:

1. Is there any standard theoretical framework where this energy could - in any way - transfer back to more "familiar" forms of energy, where "familiar" means mechanical, electrical, thermal... things less exotic than the energy being stored in the vibration of space itself.
2. Is there any discussion of one way this might actually come about. Sometimes a theory that says something is possible doesn't by itself make it very obvious how it would be possible.

I'm looking for something carefully worked out and published, and not interested in any discussion of practicality.

• Possible duplicate of One solar mass radiated away as pure energy? (Gravitational waves) – Kyle Oman Jun 16 '16 at 10:30
• Technically some of the wave's energy was converted to kinetic energy of the detectors. But I don't think you could ever get a useful amount of energy unless the sun were to merge with a big black hole or something like that. – Javier Jun 16 '16 at 11:22
• You appear to be overthinking the issue - so that you "cannot see the wood for the trees." – sammy gerbil Jun 16 '16 at 11:44
• @KyleOman Ive asked both questions and have linked them, but they're simply different questions. One asks for confirmation of a value, the other asks for a theoretical basis. Drive-by "possible duplicate"s don't make stakexchange better. – uhoh Jun 16 '16 at 11:55
• @uhoh I felt they were close enough to cast a vote, though as a bit of an edge case. We'll see what the rest of the community thinks. One vote means nothing, and it's not supposed to either. – Kyle Oman Jun 16 '16 at 12:27

## 3 Answers

Feynman gave an argument of beads on a string or rod. The passage of a gravitational wave would cause the beads to move in a way similar to the arms of the LIGO interferometer. He argued that the motion might have friction on the string. We might think of this as magnets on a solonoid. If there are magnets at different places on the solonoid their motion would induce EMF by induction, and their motion would result in a net current and voltage across the solonoid.

Don't expect gravitational radiation to become any serious energy source. The coupling constant of gravitation is $8\pi G/c^4~=~4.12\times 10^{-45}N^{-1}$, which is very small. The Einstein field equation $G_{\mu\nu}~=$ $(8\pi G/c^4)T_{\mu\nu}$. For the right hand side the density of energy or momentum and the left curvature it is clear you need large curvatures to get large energy densities.

• Thus the last sentence: "... and not interested in any discussion of practicality." – uhoh Jun 16 '16 at 11:57
• Great - thanks! Can you share a link or at least a reference to where I might see Feynman's argument or read more about it? – uhoh Jun 16 '16 at 12:07
• Sorry - I removed the "accepted" status just until I can get a link or reference to where Feynman wrote this so I can read it in more thoroughly. It wasn't a private communication was it? :) – uhoh Jun 16 '16 at 12:23
• The Wikipedia article on the sticky bead argument contains several references, including the foreword to the Feynman Lectures on Gravitation and contemporary papers by Bondi, Weber, and Wheeler. – Michael Seifert Jun 16 '16 at 13:41
• @MichaelSeifert thank you for that - just what I needed (and possibly others as well) – uhoh Jun 16 '16 at 13:44

This is from the Wikipedia article mentioned. I think this is really worth reading, as it not only describes some of the physics associated with the accepted answer and it's citation of Feynman's argument about a bead a stick, but perhaps gives a little insight into the development of the field as well.

Feynman's argument

Later in the Chapel Hill conference, Richard Feynman — who had insisted on registering under a pseudonym to express his disdain for the contemporary state of gravitational physics — used Pirani's description to point out that a passing gravitational wave should in principle cause a bead on a stick (oriented transversely to the direction of propagation of the wave) to slide back and forth, thus heating the bead and the stick by friction. This heating, said Feynman, showed that the wave did indeed impart energy to the bead and stick system, so it must indeed transport energy, contrary to the view expressed in 1955 by Rosen.

In two 1957 papers, Bondi and (separately) Joseph Weber and John Archibald Wheeler used this bead argument to present detailed refutations of Rosen's argument.(5)(6)

(5) Bondi, Hermann (1957). "Plane gravitational waves in general relativity". Nature 179 (4569): 1072–1073. Bibcode:1957Natur.179.1072B. doi:10.1038/1791072a0.

(6) Weber, Joseph & Wheeler, John Archibald (1957). "Reality of the cylindrical gravitational waves of Einstein and Rosen". Rev. Mod. Phys. 29 (3): 509–515. Bibcode:1957RvMP...29..509W. doi:10.1103/RevModPhys.29.509.

In order to be detected, the gravity waves have to be converted to some other form of energy (eg mechanical, electrical).

So the answer to question 1 is Yes. The answer to question 2 is any article about gravity-wave detectors/detection.

• Are you absolutely sure? One can only ever detect things that are converted to energy? – uhoh Jun 16 '16 at 12:10
• @uhoh : Do you know of a counter-example? – sammy gerbil Jun 16 '16 at 12:23
• I'm thinking about it actually. This is not an ordinary measurement, it's space that is changing. There aren't a lot of previous experiments sensitive to the metric expansion of space, are there! I think there's something in neutron interferometry that allows you to measure something without necessarily exchanging energy with it, but that will take some digging, or a new SE question! – uhoh Jun 16 '16 at 12:33
• So I've asked. This is interesting! – uhoh Jun 16 '16 at 12:47
• It might well be true that gravitational waves can only be detected by at least partially converting them into other forms of energy, but this is not at all self- evident and should be backed by appropriate evidence. (As an example, gravity waves (the ones in water and other fluids) can easily be detected without altering their energy content by simply looking at them. Gravitational waves may well be different but if so this difference needs to be suitably explained.) – Emilio Pisanty Jul 23 '16 at 23:57