Gravitational wave telescopes work in a very different way than optical or radio telescopes, but they do have one thing in common: they are tuned to a specific range of frequencies.
With optical telescopes, we have detectors for different frequencies, or colors, of light. It's similar to the way the digital camera on your phone works, except that rather than having a mix of red, green, and blue sensors, optical telescopes have sensors that detect a range of colors and use filters to look at particular colors. Likewise, radio telescopes have detectors that observe different ranges of frequencies from near-infrared to very low frequencies with long wavelengths. Since we can't observe radio light with our eyes, astronomers call these ranges "bands" rather than colors.
Gravitational wave astronomy is still a young field, and the detectors we have aren't nearly as sensitive as optical telescopes. Current observatories such as LIGO, Virgo, and KAGRA are really only good at detecting high-frequency gravitational waves. Particularly the high-frequency chirps that occur when black holes merge. At the other extreme, astronomers have searched for very low-frequency gravitational waves by observing small shifts in the radio ticks of pulsars. These low-frequency waves could have been triggered by the earliest moments of the Big Bang, but we don't yet have enough evidence to be sure.
Between these two extremes is the mid-band range. Specifically, the range of gravitational waves with frequencies ranging from a few hertz to less than a millihertz. There are proposed space-based observatories such as LISA, which will be able to detect mid-band waves, but it will be decades before LISA is built and operational. Recently, however, a team has proposed a different kind of detector that could see mid-band gravitational waves in the near future.
*An illustration of a proposed optical cavity detector and how an array of detectors could be used. Credit: Barontini, et al*
Traditional gravity telescopes use an interferometer with a baseline of several kilometers. A beam of laser light is split and sent down two long vacuum chamber arms, bounces off mirrors, and then comes back together. As a gravitational wave passes through the telescope, the lengths of the arms shift relative to each other, causing the laser to create a shifting interference pattern. It's essentially the same design used by Michelson and Morley a century ago, just on a larger scale. The arms of the telescope have to be very long because the gravitational effect is so small.
In this new work, the authors propose an approach similar to atomic clocks instead. Atomic clocks measure time by tuning a laser beam to an optical transition—officially the hyperfine transition of caesium-133—using a precise optical cavity. Since the atomic transition has a constant frequency, the lasers are tuned with extreme precision. Rather than detecting gravitational waves by looking at variances in the laser itself, the authors consider looking for shifts relative to the atomic standard. This would allow for mid-range gravitational wave detectors that could fit on an optical lab bench.
It's an interesting idea on paper but might not be practical. While this new approach wouldn't be affected by the kind of vibrational noise that makes current observatories so tetchy, it would be affected by things such as thermal noise, which may drown out any gravitational signal. But the good news is that we can build one with current technology. If it works, we might not need to wait decades to explore the mid-band of gravitational waves.
Reference: Barontini, Giovanni, et al. "Detecting milli-Hz gravitational waves with optical resonators." *Classical and Quantum Gravity* 42.20 (2025): 20LT01.
