A theoretical proposal would use laser light to exchange tiny amounts of energy with a passing gravitational wave.Repeated reflections could give light an effective million-kilometer path, allowing minute frequency shifts to build into a measurable interference signal.The experiment has not been demonstrated, and observing the predicted energy transfer would not by itself prove gravitons exist.<\/p>\n
A laser pulse could do more than reveal a gravitational wave passing through a detector. Under a proposed experiment, it could give the wave a tiny amount of energy, or take some away. The resulting change in light would offer a way to examine that exchange.<\/p>\n
\u201cGravity affects everything, including light,\u201d Sch\u00fctzhold said. That familiar interaction is the starting point for an attempt to move from detecting gravitational waves toward actively changing their energy.<\/p>\n
The idea also touches a deeper question: whether gravity obeys quantum rules. But the paper draws a firm distinction between measuring an energy exchange and proving that gravitational energy comes in discrete packets called gravitons.<\/p>\n
Sketch of the interferometric set-up for light under the influence of a gravitational wave. (CREDIT: B. Schr\u00f6der\/HZDR) Working with a wave that already exists<\/p>\n
Gravitational waves are traveling distortions of spacetime. Black hole mergers and neutron star collisions can generate them, and LIGO directly detected a gravitational wave for the first time in 2015. Those observations established a way to study powerful events far beyond Earth.<\/p>\n
Producing detectable gravitational radiation in a laboratory is much harder. Gravity\u2019s weakness makes even the emission of a single hypothetical graviton<\/a> extraordinarily difficult with ordinary moving masses. Sch\u00fctzhold therefore considers light interacting with a gravitational wave that already exists.<\/p>\n In the quantum description assumed by the paper, each graviton carries an energy determined by the wave\u2019s frequency. A light pulse could transfer energy to that wave through stimulated emission, or gain energy through absorption.<\/p>\n \u201cIt would make the gravitational wave a tiny bit more intense,\u201d Sch\u00fctzhold said of the emission process. The light would lose the same amount of energy. In the reverse process, the gravitational wave would supply energy to the light.<\/p>\n The exchange changes the light\u2019s frequency by an extremely small amount. Detecting that change, rather than measuring a visibly stronger gravitational wave, is the proposed route to an experimental signal.<\/p>\n Sketch (not to scale) of a possible geometry. The initial laser pulse is split up by a half silvered mirror (dotted black line on bottom left) into two pulses (red lines), first propagating in the \ud835\udc65 and \ud835\udc66 directions, respectively. (CREDIT: Ralf Sch\u00fctzhold et al, Physical Review Letters) Two light pulses take different routes<\/p>\n The experiment begins by splitting a laser pulse into two pulses that initially travel in perpendicular directions. A passing gravitational wave affects the energy exchange differently along those directions. Mirrors then redirect the pulses so that each travels along the other direction.<\/p>\n Timing matters. Ideally, the change in direction occurs at the appropriate point in the gravitational wave\u2019s cycle. That allows one pulse to gain energy while the other loses it, rather than having successive portions of the interaction cancel.<\/p>\n The design resembles a Mach-Zehnder or Sagnac interferometer, devices that compare light traveling along different paths. Here, however, the pulses do not immediately recombine after interacting with the gravitational wave<\/a>.<\/p>\n Instead, both enter a further optical path. Their slightly different frequencies gradually produce a difference in phase, or position within their oscillation cycles. When the pulses finally overlap, that accumulated difference changes their interference pattern.<\/p>\n This separates the brief interaction with the gravitational wave from the longer period used to read its consequences. The latter stage could continue after the wave has passed.<\/p>\n Sketch of the experiment proposed by Sch\u00fctzhold to test the emission or absorption of gravitons. In the scheme, a high-power laser pulse is split into two pluses that interact with a passing gravitational wave. (CREDIT: B. Schr\u00f6der\/HZDR) A million-kilometer journey inside a smaller instrument<\/p>\n The proposal\u2019s scale is substantial. Sch\u00fctzhold considers an effective optical path of roughly one million kilometers, achieved through repeated reflections rather than a physical structure spanning that distance. Around a million reflections between mirrors separated on kilometer scales would provide the required order of magnitude.<\/p>\n For visible or near-infrared light, the calculation gives a lasting frequency shift on the order of one ten-millionth of a hertz. After a few seconds of propagation, that could produce a phase difference around one ten-millionth of a radian.<\/p>\n The example uses millijoule-scale pulses containing roughly 10 quadrillion photons<\/a>. Combining a large photon number with the long accumulation path could make the tiny effect measurable under the paper\u2019s assumptions.<\/p>\n These estimates do not amount to a completed instrument. Maintaining the necessary optical performance through so many reflections would be a major challenge. Imperfect timing with the incoming gravitational wave would also reduce the effect.<\/p>\n Sch\u00fctzhold suggests a train of pulses could ease that timing problem. Measurements coordinated with LIGO could help connect the proposed energy-exchange signal with an independently detected gravitational wave.<\/p>\n Entangled light offers a harder extension<\/p>\n Ordinary coherent laser pulses are one starting point. More unusual quantum states of light could improve the precision of the phase measurement. The paper examines a highly entangled state called a NOON state.<\/p>\n