Schutzhold notes that gravity acts on all forms of energy, including electromagnetic radiation, so light and gravitational waves should influence each other when they cross. In the scenario he describes, a light wave can transfer a small energy packet to a gravitational wave, slightly amplifying the gravitational signal while reducing the light's energy by exactly the same amount. This loss appears as a minute shift in the light's frequency and corresponds to the emission of one or several gravitons into the gravitational wave.

The process can also run in reverse, with the gravitational wave passing energy to the light so that the wave of spacetime loses a quantum of energy while the laser field gains it. Schutzhold calculates that detecting such stimulated emission and absorption of gravitons would demand a very large interferometric setup using intense laser pulses in the visible or near-infrared range. In one configuration, pulses would bounce between two mirrors up to a million times in a system about a kilometer long, generating an effective optical path length around one million kilometers.

However, the change in the frequency of the light wave caused by the absorption or release of the energy of one or more gravitons in interaction with the gravitational wave is extremely small. Nevertheless, by using a cleverly constructed interferometer it should be possible to demonstrate these changes in frequency. In the process, two light waves experience different changes in frequency - depending on whether they absorb or emit gravitons. After this interaction and passing along the optical path length they overlap again and generate an interference pattern. From this, it is possible to infer the frequency change that has occurred and thus the transfer of gravitons.

"It can take several decades from initial idea to experiment," says Schutzhold. But perhaps it might happen sooner in this case as the LIGO Observatory - acronym for the Laser Interferometer Gravitational-Wave Observatory - that is dedicated to detecting gravitational waves, shows strong similarities. LIGO consists of two L-shaped vacuum tubes approximately four kilometers long. A beam splitter divides a laser beam onto both arms of the detector. As they pass through, incoming gravitational waves minimally distort space-time, which causes changes of a few attometers (10-18 meters) in the originally equal length of the two arms. This tiny change in length alters the interference pattern of the laser light, generating a detectable signal.

In an interferometer tailored to Schutzhold's idea, it could be possible not only to observe gravitational waves but also to manipulate them for the first time by stimulated emission and absorption of gravitons. According to Schutzhold, light pulses whose photons are entangled, that is, quantum mechanically coupled, could significantly increase the sensitivity of the interferometer further. "Then we could even draw inferences about the quantum state of the gravitational field itself," says Schutzhold. While this would not be direct proof of the hypothetical graviton, which is the subject of intense debate among physicists, it would at least be a strong indication for its existence. After all, if the light waves did not exhibit the predicted interference effects when interacting with gravitational waves, the current theory based on gravitons would be disproved. It is thus hardly surprising that Schutzhold's concept for the manipulation of gravitational waves is meeting with great interest among his colleagues.

Research Report:Stimulated Emission or Absorption of Gravitons by Light

Related Links
Helmholtz-Zentrum Dresden-Rossendorf
The Physics of Time and Space