Seeking Signatures of Graviton Emission and Absorption
A major milestone in human development was the transition from passively observing electromagnetic phenomena, such as electric discharges and magnetism, to actively manipulating them. This shift led to a plethora of applications—from power plants to modern electronics. The exquisite control of electromagnetic fields and of their interaction with matter has also yielded deep insights into the fundamental laws of nature, allowing us to test modern theories with remarkable precision. Now Ralf Schützhold of the Helmholtz-Zentrum Dresden-Rossendorf in Germany argues that a similar turning point may be reached for gravity [1]. His approach for manipulating gravity relies on experiments that can control the emission or absorption of gravitons, the hypothetical elementary particles mediating the gravitational interaction in a quantized theory of gravity. To this end, Schützhold has proposed an experiment in which a gravitational wave hitting an optical interferometer causes a measurable phase shift in a light pulse traveling through the interferometer. Such a shift could reveal whether gravitons are absorbed or emitted in the process. The realization of this proposal would offer tantalizing prospects for manipulating gravity and for testing its quantum nature.
Our current understanding of gravity comes from Einstein’s general theory of relativity. There, gravity is not described as a force but rather as a consequence of the curvature of spacetime caused by matter. Within this paradigm, one can describe a wide range of gravitational phenomena, from the motion of planets and stars to the evolution of the Universe. Moreover, the theory has led to the discovery of previously unknown aspects of the cosmos, such as the existence of black holes and of gravitational waves.
Gravitational waves are ripples in spacetime that travel at the speed of light. Although they were predicted over a century ago [2, 3], it took about 50 years to find indirect evidence of their existence [4, 5], and another 40 until the LIGO experiment directly detected them for the first time [6]. Hundreds of other gravitational-wave events have since been reported, opening a new window on the cosmos. However, the gravitational waves observed so far originated from extreme astrophysical events, such as the merger of two black holes. At lower energies (below the Planck energy of about 1019 GeV) gravity might behave similarly to electromagnetism, with its energy quantized in gravitons (units of , where is the frequency of the gravitational wave). This possibility raises a natural question: Can the emission or absorption of a graviton be detected in experiments using available or near-future technology?
Unfortunately, owing to the feebleness of the gravitational interaction, the answer is no, at least if the approach is that of using moving objects to generate the gravitational field. Considering setups with realistic masses, velocities, and lengths, one can show that it is virtually impossible to produce even a single graviton. This challenge led Schützhold to take a different approach. Instead of using dynamical masses, his proposal involves spotting gravitons by measuring the energy transferred between a laser beam and a passing gravitational wave. If such an energy exceeded the single-graviton energy , it could be interpreted as a “fingerprint” of the (stimulated) emission or absorption of gravitons.
Schützhold considers a Mach-Zehnder-type laser interferometer, in which a light pulse is split into two pulses traveling over different, perpendicular paths. This geometry is akin to that of gravitational interferometers such as LIGO, Virgo, and KAGRA. But unlike those interferometers, Schützhold’s scheme has a second stage, in which the pulses propagate over an identical path and are then recombined. Their interference on a detector produces a signal proportional to their phase difference. In the absence of gravitational waves, this phase difference would vanish. However, if a polarized gravitational wave similar to those routinely detected by LIGO passes through the interferometer, the induced spacetime distortions would modulate the frequency of the photons—a consequence of a tiny transfer of energy between the photons and the wave. As a result, the frequency of the photons would increase along one path and decrease along the other, leading to a nonzero phase difference.
Although this phase shift is expected to be minuscule, it could be amplified thanks to a feature of the proposed setup. Namely, after the two split pulses undergo frequency modulation, their shifted frequencies would lead to the accumulation of a sizeable phase difference in the setup’s second stage, amplifying the detected signal (Fig. 1). It is worth stressing an important difference between Schützhold’s scheme and detectors such as LIGO. In the new scheme, the phase-accumulation length can theoretically reach millions of kilometers. But in LIGO, the interferometer’s length is instead limited by the frequency of the gravitational wave: The travel time of light through the interferometer must be shorter than the wave’s half period to prevent the wave’s opposite cycle from canceling the signal. Practically, this constrains the effective optical path length to roughly 1000 km, which LIGO achieves through hundreds of reflections between mirrors a few kilometers apart.
To assess the feasibility of his experiment, Schützhold considered commercially available, tabletop laser technology (a high-power pulsed laser at visible or near-infrared frequencies) and a passing gravitational wave with a strain (amplitude of spacetime distortion) comparable to that of the waves so far detected by LIGO. Schützhold estimated that a 1-million-km phase-accumulation path (achievable, for instance, with a million reflections between kilometer-distant mirrors) would produce a phase difference of about 10–7, which would be measurable with available technology. Realizing this setup would be a massive undertaking—requiring, in particular, the attainment of a stable, 1-million-km phase-accumulation stage and the use of ultrastable lasers—but wouldn’t face fundamental barriers.
Observing a phase difference for an energy transfer larger than would strongly support the graviton hypothesis. It would not, however, amount to conclusive proof of the existence of gravitons, as such a phase shift could also result from a semiclassical interaction between the photons and the gravitational wave. Conversely, a nondetection in conjunction with a detection by LIGO, Virgo, and KAGRA would cast doubt on the graviton picture.
Finding signatures of graviton emission or absorption would be a first step toward the manipulation of gravitons and of gravitational waves. It would also open the tantalizing possibility of probing the quantumness of the gravitational field, which could guide the development of a theory of quantum gravity. Lastly, variations of the proposed experiment could test detailed quantum properties of the gravitational field. As Schützhold points out, if the light fed into the interferometer is in a highly nonclassical quantum state (such as a so-called NOON state) instead of a classical, coherent state, then measurements of the photon field could be used to distinguish specific quantum states of the gravitational field. All these features would contribute to the growing efforts to tackle one of nature’s deepest secrets—the quantumness of gravity emerging at low energies that may be experimentally accessible in the lab [7–10].
References
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