Einstein’s work was crucial for the current understanding of gravitational waves and the development of stimulated radiation that culminated in the invention of lasers. Dr Jing Liu, from the University of Chinese Academy of Sciences, has combined the two into an intriguing proposal: it is possible to create the gravitational equivalent of a laser.

Let’s start with the basics. The word laser stands for Light Amplification by Stimulated Emission of Radiation. A laser is made of light all with roughly the same frequency (or, in other words, it is monochrome) and it is coherent, so it can be focused to a tight spot or can be used to create ultrashort pulses. By stimulating a quantum mechanical energy transition, it is possible to get light out all with the same frequency.

Natural lasers exist and they are called masers – with the “m” standing for microwaves. These astrophysical masers come from a bunch of sources, including comets, stellar atmosphere, and even the aurorae of Jupiter. So if light can make a laser, could gravity as well?

Gravitational radiation shares similar properties with light behavior. Gravitational waves have frequencies and move at the speed of light, so in principle, you could make a laser with them. This would require a source that produces stimulated gravitational waves with a specific frequency. Anything that has mass and moves creates gravitational waves, but you are not getting that specific energy transition you encounter with atoms.

But maybe, there could be something like a gravitational atom – a structure where the gravitational interactions supersede the electromagnetic ones. The theoretical idea of a gravitational atom is recent and Liu exploited that hypothetical to test if a gravitational laser is possible. Liu’s gravitational atom is a rotating black hole surrounded by a cloud of axions, incredibly light hypothetical particles that are a leading candidate for dark matter.

Though it is yet to be peer-reviewed, the work suggests that theoretically speaking, it is possible to generate resonant energy transition in clouds of axions. Those transitions, akin to an electron in an atom losing or gaining energy, would release gravitational waves of the same energy and direction. That would be a gravitational laser.

So are we getting ready to find these gravitational lasers from these axions? Not quite yet. There are a lot of hypotheticals, but understanding what gravitational signals might look like is key to actually discovering them. And the laser signal would not look like anything we have encountered so far, so it matters to know what it is.