In the same way as radio telescopes split up incoming radio waves into their constituent channels, spectrometers attached to optical telescopes split up incoming light into a rainbow of frequencies. Just like radio, light is a form of electromagnetic wave, but whereas FM radio transmissions are at frequencies around 100 MHz, and Breakthrough Listen’s radio search takes place at frequencies between 1 and 10 GHz, visible light is made up of electromagnetic waves with frequencies of a few hundred THz. Different technology is needed to perform our observations at these frequencies, but the principle is the same: look for signals that are too narrow in frequency to have been formed by natural processes.
Earthlings have used lasers for communication for half a century, but the technology is still in its infancy. In principle, however, civilizations not much more advanced than our own might choose to use lasers to communicate over interstellar distances. The ability of lasers to encode information in a narrow beam, over a narrow range of frequencies, means that they might appear as narrow-band signals, perhaps also in the form of short pulses, at high contrast against natural broad-band astronomical sources of light such as stars.
Somewhat surprisingly, even with the technology available to humans in the early 21st century, we can in principle detect lasers as faint as 100 Watts (the same power as an average household bulb) from planets orbiting nearby stars. As with radio searches, there are potential contaminating signals here too, however. Rather than radio frequency interference, we must successfully identify and remove cosmic rays - bursts of energy deposited in our detectors by high energy subatomic particles.
Breakthrough Listen is using the Automated Planet Finder at Lick Observatory to undertake the world’s deepest and broadest search for optical laser transmissions from nearby stars. APF datasets are much smaller than those from GBT so we can analyze them using computer workstations rather than relying on the distributed computing power available through SETI@home. We are also releasing APF data into the Breakthrough Listen public archive.