Demystifying Graviton Detection

Abstract

Detecting single gravitons is possible through the extraction of discrete energy quanta from passing gravitational waves. For decades, this was considered hopeless, with estimates suggesting a detector the size of Jupiter. Here, I explain how these earlier estimates relied on the highly inefficient electron ionization process, while the absorption through the full mass quadrupole of a macroscopic system can bring graviton detection within experimental reach. Operating in the LIGO frequency band solves the challenges of graviton sources and noise suppression, allowing gravitons to be extracted from known passing gravitational waves and detection events to be heralded by classical gravitational-wave observations. Combined with the ability to resolve single energy quanta in macroscopic systems, such as in quantum acoustics, this opens a realistic path toward graviton detection. I also discuss the general limitations of particle detection in establishing an underlying quantum theory, illustrated by an energy-nonconserving semiclassical alternative. Nevertheless, the semiclassical alternative is not standard classical wave physics, and extracting $E=\hbarω$ from a passing wave constitutes bona fide particle detection. This opens the way not only to the first detection of individual gravitons, but also to experimentally probing linearized quantum gravity and the graviton counting statistics of gravitational waves.