Gravitational Grating

TL;DR

This paper proposes a novel method to detect gravitational waves by observing diffraction patterns and modulation in quasar light curves caused by gravitational wave interactions, especially in strongly lensed quasars.

Contribution

It introduces the concept of gravitational diffraction as a new observational technique for gravitational wave detection using quasar light curves.

Findings

Diffraction angle of gravitational grating is approximately the ratio of electromagnetic to gravitational wavelengths.

Modulation in quasar light curves with periods of a few hours can indicate gravitational wave interactions.

Optical depth for observing this phenomenon in lensed quasars is estimated to be around 0.2.

Abstract

In this work, we study the interaction of the electromagnetic wave (EW) from a distant quasar with the gravitational wave (GW) sourced by the binary stars. While in the regime of geometric optics, the light bending due to this interaction is negligible, we show that the phase shifting on the wavefront of an EW can produce the diffraction pattern on the observer plane. The diffraction of the light (with the wavelength of $\lambda_e$) by the gravitational wave playing the role of {\it gravitational grating} (with the wavelength of $\lambda_g$) has the diffraction angle of $\Delta\beta \sim \lambda_e/\lambda_g$. The relative motion of the observer, the source of gravitational wave and the quasar results in a relative motion of the observer through the interference pattern on the observer plane. The consequence of this fringe crossing is the modulation in the light curve of a quasar with the period of few hours in the microwave wavelength. The optical depth for the observation of this phenomenon for a Quasar with the multiple images strongly lensed by a galaxy where the light trajectory of some of the images crosses the lensing galaxy is $\tau \simeq 0.2$. By shifting the time delay of the light curves of the multiple images in a strong-lensed quasar and removing the intrinsic variations of a quasar, our desired signals, as a new method for detection of GWs, can be detected.