Prof. Ralf Schützhold at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has devised an experiment that allows for not only the observation of gravitational waves but also their manipulation. The study was published in the journal Physical Review Letters.
Sketch of the interferometric set-up for light under the influence of a gravitational wave. Image Credit: B. Schröder/HZDR
When two black holes merge or when two neutron stars collide, gravitational waves may be produced. These waves propagate at the speed of light and induce minute distortions in the fabric of space-time. Albert Einstein was the first to predict their existence, with the initial direct experimental detection occurring in 2015.
The study has the potential to provide new insights into the previously speculated quantum characteristics of gravity.
Gravity affects everything, including light.
Ralf Schützhold, Professor and Theoretical Physicist, Helmholtz-Zentrum Dresden-Rossendorf (HZDR)
And this interaction also occurs when gravitational waves and light waves meet. Schützhold’s idea is to transfer tiny packets of energy from a light wave to a gravitational wave. The energy of the light wave is reduced slightly, and the energy of the gravitational wave is increased by the same amount. This energy is equal to that of one or several gravitons, the exchange particles of gravity that have been postulated in theoretical models, but not yet proven.
“It would make the gravitational wave a tiny bit more intensive,” explained the physicist.
The light wave, on the other hand, loses exactly the same amount of energy, which leads to a minute change in the light wave’s frequency.
The process can work the other way around, too.
Ralf Schützhold, Professor and Theoretical Physicist, Helmholtz-Zentrum Dresden-Rossendorf (HZDR)
The gravitational wave transmits an energy packet to the light wave. It should be feasible to observe both phenomena, namely, the stimulated emission and absorption of gravitons, although this would require significant experimental effort. Schützhold has estimated the vast scale of such an experiment: potentially, laser pulses within the visible or near-infrared spectral range could be reflected back and forth between two mirrors as many as one million times.
In a configuration approximately one kilometer in length, this would yield an optical path length of about one million kilometers. Such a magnitude is adequate to perform the intended measurement of the energy transfer resulting from the absorption and emission of gravitons when light interacts with a gravitational wave.
The alteration in the light wave's frequency due to the absorption or emission of energy from one or more gravitons interacting with the gravitational wave is exceedingly minor. However, by employing a well-designed interferometer, it should be feasible to demonstrate these frequency changes.
During this process, two light waves undergo varying frequency changes based on whether they absorb or emit gravitons. Following this interaction and traversing the optical path length, they converge once more and produce an interference pattern. From this pattern, one can deduce the frequency change that has taken place, thereby indicating the transfer of gravitons.
Experiment Could Also Deliver Insights into Quantum Properties of the Gravitational Field
“It can take several decades from initial idea to experiment,” said Schützhold.
However, it is possible that this may occur sooner in this instance, as the LIGO Observatory, which stands for the Laser Interferometer Gravitational-Wave Observatory, is specifically designed to detect gravitational waves and exhibits significant similarities. LIGO is composed of two L-shaped vacuum tubes, each approximately 4 km in length.
A beam splitter directs a laser beam into both arms of the detector. As the gravitational waves traverse through, they cause minimal distortions in space-time, resulting in alterations of a few attometers (10-18 m) in the initially equal lengths of the two arms. This minute variation in length modifies the interference pattern of the laser light, producing a signal that can be detected.
In an interferometer designed in accordance with Schützhold’s concept, it may be feasible not only to detect gravitational waves but also to control them for the first time through stimulated emission and absorption of gravitons. Schützhold suggests that light pulses containing entangled photons, which are quantum mechanically linked, could greatly enhance the sensitivity of the interferometer even further.
Then we could even draw inferences about the quantum state of the gravitational field itself.
Ralf Schützhold, Professor and Theoretical Physicist, Helmholtz-Zentrum Dresden-Rossendorf (HZDR)
Although this may not serve as direct evidence for the theoretical graviton, a topic that is heavily contested among physicists, it would nonetheless provide a compelling indication of its existence. Indeed, if the light waves failed to demonstrate the anticipated interference effects when interacting with gravitational waves, the existing theory that relies on gravitons would be invalidated. Therefore, it is not unexpected that Schützhold's idea for manipulating gravitational waves is garnering significant interest from peers.
Journal Reference:
Schützhold, R. (2025) Stimulated Emission or Absorption of Gravitons by Light. Physical Review Letters. DOI:10.1103/xd97-c6d7. https://journals.aps.org/prl/abstract/10.1103/xd97-c6d7