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When two black holes merge or two neutron stars collide, gravitational waves can be generated. They spread at the speed of light and cause tiny distortions in space-time. Albert Einstein predicted their existence, and the first direct experimental observation dates from 2015. Now, Prof. Ralf Schützhold, theoretical physicist at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), is going one step further. He has conceived an experiment through which gravitational waves can not only be observed but even manipulated. Published in the journal Physical Review Letters (DOI: 10.1103/xd97-c6d7), the idea could also deliver new insights into the hitherto only conjectured quantum nature of gravity.
“Gravity affects everything, including light,” says Schützhold. 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. By doing so, 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,” explains 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,” Schützhold continues. In this case, the gravitational wave dispenses an energy package to the light wave. It should be possible to measure both effects, that is, the stimulated emission and absorption of gravitons, albeit with considerable experimental effort. Schützhold has calculated the huge dimensions of such an experiment: potentially, laser pulses in the visible or near-infrared spectral range could be reflected back and forth between two mirrors up to a million times. In a set-up about a kilometer long, this would produce an optical path length of around one million kilometers. Such an order of magnitude is sufficient to conduct the desired measurement of the energy exchange caused by the absorption and emission of gravitons when light and a gravitational wave meet.
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.
Experiment could also deliver insights into quantum properties of the gravitational field
“It can take several decades from initial idea to experiment,” says Schützhold. 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 Schützhold’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 Schützhold, 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 Schützhold. 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 Schützhold’s concept for the manipulation of gravitational waves is meeting with great interest among his colleagues.
Publication:
R. Schützhold: Stimulated Emission or Absorption of Gravitons by Light, in Physical Review Letters, 2025 (DOI: 10.1103/xd97-c6d7)
Additional information:
Prof. Ralf Schützhold | Director
Institute of Theoretical Physics at HZDR
Tel.: +49 351 260 3618 | E-mail: r.schuetzhold@hzdr.de
Media contact:
Simon Schmitt | Head
Communications and Media Relations at HZDR
Phone: +49 351 260 3400 | Mob.: +49 175 874 2865 | Email: s.schmitt@hzdr.de
The Helmholtz-Zentrum Dresden-Rossendorf (HZDR) performs – as an independent German research center – research in the fields of energy, health, and matter. We focus on answering the following questions:
• How can energy and resources be utilized in an efficient, safe, and sustainable way?
• How can malignant tumors be more precisely visualized, characterized, and more effectively treated?
• How do matter and materials behave under the influence of strong fields and in smallest dimensions?
To help answer these research questions, HZDR operates large-scale facilities, which are also used by visiting researchers: the Ion Beam Center, the Dresden High Magnetic Field Laboratory and the ELBE Center for High-Power Radiation Sources.
HZDR is a member of the Helmholtz Association and has six sites (Dresden, Freiberg, Görlitz, Grenoble, Leipzig, Schenefeld near Hamburg) with almost 1,500 members of staff, of whom about 700 are scientists, including 200 Ph.D. candidates.
Prof. Ralf Schützhold | Director
Institute of Theoretical Physics at HZDR
Tel.: +49 351 260 3618 | E-mail: r.schuetzhold@hzdr.de
R. Schützhold: Stimulated Emission or Absorption of Gravitons by Light, in Physical Review Letters, 2025 (DOI: 10.1103/xd97-c6d7)
https://www.hzdr.de/presse/light_graviton
Sketch of the interferometric set-up for light under the influence of a gravitational wave.
Source: B. Schröder
Copyright: B. Schröder/HZDR
Criteria of this press release:
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Sketch of the interferometric set-up for light under the influence of a gravitational wave.
Source: B. Schröder
Copyright: B. Schröder/HZDR
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