Rainbow in the Dark

How thinking about changing colors could help us catch signals from extraterrestrial intelligence that would otherwise slip through the cracks

Let’s do a thought experiment.

Imagine you’re out in space with your friend and they shine a green light at you. If your friend starts travelling towards you at a constant speed, that green light will appear a little bluer (called a blueshift) as the wavelengths of light get compressed. If they instead start travelling away from you at a constant speed, the green light will appear a little closer to yellow (called a redshift).

Now let’s take this a little further: imagine your friend starts at a standstill with their green light, and then starts accelerating towards you, moving faster and faster. What will you see? The light will start as green, and shift bluer and bluer as time goes on. This effect is called a Doppler acceleration, and it happens to any electromagnetic wave whose source is accelerating towards or away from you - including radio waves.

But let’s continue this analogy in visible wavelengths of light. Let’s say you’re looking for a flashlight from an extraterrestrial intelligence (ETI). We don’t know what color to expect - red, yellow, blue, purple, who knows what color the ETI have chosen. So we tell our computers* to search for bright spots in every color that our telescope can collect. But we have to be careful - if the color is changing throughout the observation, because of Doppler acceleration, the computer might miss it because it’s just looking for signals of a single color!

Okay, but why would the ETI’s transmitter (the radio equivalent of our flashlight from before) be accelerating in the first place?

Turns out, space is full of things that are constantly accelerating towards and away from us (radial acceleration). A planet orbiting a star is accelerating. A transmitter on the surface of a rotating planet is accelerating. A transmitter that’s orbiting a planet like a satellite is accelerating. Earth is accelerating around its rotation axis and around the sun as we try to take our data, causing the same issues via symmetry!

All of these effects stack, and cause a transmitter that is sending out a single wavelength of light to appear to change drastically over time.

In our new paper, we wanted to calculate exactly how drastically the wavelengths would change over time. We can tell our computers to search for these drifting signals through time, but we have to give them a maximum limit of how much drift to expect. So what’s the maximum?

To answer that question, we considered every planet in our solar system, every known exoplanet, all of the asteroids and comets in our solar system, orbits around main sequence stars, neutron stars, and even black holes. We calculated the fastest acceleration we would expect in each case, and what its expected Doppler drift would be.

In the end, we found that the searches for extraterrestrial intelligence in the past have used a maximum Doppler drift rate that was too small, leading to the potential for missed signals. Luckily, with the new guideline (200 Hz/s at 1 GHz, for those of you who want units) we should be able to catch signals from ETI no matter what the acceleration is like in their home system!

The new paper has been accepted to the Astrophysical Journal and is also available on the arXiv preprint server.

*there’s waaaay too much data for our team to go through it all by eye, so we have to write algorithms to find interesting signals for us

Post by Sofia Sheikh, former Berkeley SETI undergraduate intern - now a PhD student in Astrobiology at Penn State University