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 b...

Physicist Proposes Experiment to Actively Influence Gravitational Waves

Ralf Schützhold, a theoretical physicist at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), has designed an experiment that would go beyond detecting gravitational waves and actually manipulate them by swapping energy with laser light. The proposal appears in Physical Review Letters.

Laser light and the quantum nature of gravity Image: nanotechnologyworld - Laser light and the quantum nature of gravity

The Core Idea

According to nanotechnologyworld.org, Schützhold’s scheme transfers tiny energy packets between a light wave and a gravitational wave when the two cross paths. The light wave loses a small amount of energy; the gravitational wave gains exactly the same amount, becoming slightly more intense. The process also runs in reverse, with the gravitational wave giving up energy to the light.

“Gravity affects everything, including light,” Schützhold told nanotechnologyworld.org. The energy quanta exchanged correspond to one or several gravitons, the hypothetical exchange particles of gravity.

How the Experiment Would Work

The setup resembles LIGO on steroids. Laser pulses in the visible or near-infrared range would bounce between two mirrors up to a million times inside a roughly one-kilometre apparatus, producing an effective optical path of around a million kilometres. That length is what the energy exchange needs to leave a measurable trace.

The detection trick uses a purpose-built interferometer. Two light waves pick up different frequency shifts depending on whether they absorb or emit gravitons. When recombined, they produce an interference pattern that reveals the frequency change, and therefore the graviton transfer.

LIGO already proves the underlying sensitivity is achievable: its four-kilometre arms detect length changes of a few attometres (10⁻¹⁸ m) caused by passing gravitational waves, per nanotechnologyworld.org.

Why It Matters for Quantum Gravity

The theoretical backbone for GW–light coupling is well established. Work on arxiv.org shows that a passing gravitational wave shifts the frequency of light it traverses, with “the change in radar distance and the frequency shift… actually two facets of the same GW-photon interaction.” Schützhold’s proposal turns that passive shift into an active exchange.

The bigger prize is a window onto the quantum nature of gravity. Using entangled photons could sharpen the interferometer enough to probe the quantum state of the gravitational field itself. As Schützhold puts it in nanotechnologyworld.org, “Then we could even draw inferences about the quantum state of the gravitational field itself.” A null result, where the predicted interference fails to appear, would itself be significant, disproving current graviton-based theory.

Schützhold cautions that “it can take several decades from initial idea to experiment,” though LIGO’s architectural similarity may shorten the path.

The primary write-up at nanotechnologyworld.org is the dominant source here and worth reading in full; the original paper, “Stimulated Emission or Absorption of Gravitons by Light,” is available from Physical Review Letters.

Sources: nanotechnologyworld.org, Physical Review Letters, arxiv.org