Films such as Contact, one of my personal favorites, depict astronomers searching the skies using sophisticated radio telescopes, hoping to hear from intelligent life elsewhere in the Universe. In fact, virtually all contact with aliens seems to arrive as a radio signal from outer space, at least according to Hollywood. But, is this necessarily what we should expect?
In one sense, this seems to be contrary to what science would suggest. Specifically, the amount of information that can be transmitted in a fixed amount of time — known as bandwidth — is proportional to the frequency of light used. Radio waves, being the lowest frequency light, would then seem an odd choice.
X-rays and gamma rays would seem more logical as they reside in the opposite end of the electromagnetic spectrum. And strictly speaking this is certainly true, but there is more to consider than just bandwidth.
Specifically, we must also consider the ability to transmit and then detect the information, and finally, whether or not it would even be able to travel across the galaxy.
Radio waves are relatively easy to transmit and receive. We use them to send out radio and television signals, and communicate between all matters of communication devices. They also have the useful property of being able to propagate through our atmosphere without being absorbed or deflected.
In contrast, our atmosphere will absorb X-ray, gamma ray and even optical light to some extent. And the higher energy radiation is quite difficult to create, and even more challenging to detect. Therefore, the highest energy photos would be a peculiar choice as a signal transmitter by alien races.
But supposing an alien race of superior intellect, they may not be so concerned about our ability to accurately detect their gamma ray signals. Maybe they are only interested in communicating with us if we are of sufficient technological ability that we can manipulate gamma rays with ease. Certainly a reason to take a closer look at the data from the Fermi Gamma Ray Telescope, if you are so inclined. (The data is free to download and analyze.)
Given, though, that it is such a challenge to produce and receive high-energy radiation, we’ll assume for a moment that aliens would use other wavelengths of light. So, would optical be preferential to radio?
Probably not. The main reason is that optical light is easily scattered and absorbed by our atmosphere. Moreover, it is easily absorbed by interstellar gas and dust, making it difficult to penetrate various parts of our galaxy. This would severely limit the percentage of the galaxy that we, or other alien race, could potentially talk to.
So, while it seems strange, on one hand, that radio is the default choice for communications from potential alien races, the alternatives pose greater problems. Besides if we are really looking for intelligent life, wouldn’t we expect them to be about to get around the bandwidth problem?