If we look at the Drake Equation and ponder the Fermi Paradox, we should expect that, out there in the blackness, there must be advanced civilizations, with hopes, dream, aspirations – and questions – similar to our own.
Of course we wonder whether they are trying to signal us. But what if they echo one of our fondest dreams? To travel to the stars!
When our best scientists imagine ways to get to the nearest star, they get a plethora of technologies worth looking at, and most of them use enormous amounts of energy. This is bad news for us, if we dream of putting something like this together.
But the good thing about this consumption of energy, it guarantees some kind of unmistakable signature. An alien starship zipping from system to system would stand a very good chance of being detectable.
Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft, by Dr. Robert Zubrin
A digest version: To Detect a Starship
For the goal of traveling to the nearest star, many spacecraft propulsion systems have been proposed. The systems that appear to be capable of producing the most thrust, require the use of antimatter.
When anti-matter meets matter, it’s expected that this releases colossal amounts of energy, which can be harnessed as thrust. Awesome as this might be, most estimates place the velocity available as somewhere around 10% of the speed of light.
Some think that positrons can be substituted for anti-matter, with a similar production of thrust.
Not surprisingly, a huge amount of ink has been spilt regarding this largely theoretical technology:
New and improved antimatter spaceship for Mars missions
Antimatter propulsion – the beginnings of true interstellar flight
Re-thinking the antimatter rocket
Matter–antimatter gigaelectron volt gamma ray laser rocket propulsion
Fear not, I am not interested in developing a starship engine, or even exploring the possibilities past a point. All we need to know is that the leading candidate for interstellar propulsion, would involve antimatter or positrons, and would inevitably emit HUGE amounts of gamma-rays in the opposite direction.
Even fusion rockets, fission rockets and magsails, in fact almost any effective propulsion system, seems to emit lots of gamma radiation. Zubrin and others have even visualized and calculated how large a starship would have to be in order to make this worthwhile. To my mind, this is interesting, but not necessary, since we are not building the thing – we are just watching the skies.
Certainly other technologies are being considered, including solar sails, wormholes, Bussard ramjet, Alcubierre drive or whatever. But among millions and millions of spacefaring civilizations, some are bound to try antimatter propulsion.
Such an gamma-ray signature would be a much higher-powered extraterrestrial signal, much easier for us to see than a directed or incidental radio signal.
So my suggestion is that we should just look for gamma-ray emissions out there in the blackness. And don’t be surprised that astronomers are familiar with a number of different types of gamma-ray sources in the universe.
I don’t have all of the answers to this, of course. But going from a model of “assume no ETs” to a model of “they could be out there”, then we need to consider whether we are looking at exhaust from an ET’s engine today. Are there characteristics we need to be looking for that would suggest an artificial origin?
A gamma-ray burst can last from milliseconds to several minutes. The theories for the source of these events? The bursts that are longer than a few seconds? A supernova, a hypernova or a magnetar.
The shorter ones, from milliseconds to a couple of seconds? That’s unknown! They are not usually near any obvious stellar source, such as supernovas. Possibly from the merger of binary neutron stars?
The energy emitted is at a very narrow angle, and at a speed nearly at the speed of light. Unless the gamma rays are emitted as two equal and opposite jets, then the source would be impelled at a very high rate of speed. And if that’s true, then we would see a red-shifted signal – and we do occasionally!
The source for a short gamma-ray burst, whatever it is, must be a relatively small object, based on these observations. Possibly a mothership built to sustain large numbers of travelers for many years.
Once we get a candidate signal, Zubrin, in his paper, suggests using this data to calculate the putative starship’s trajectory: where it came from and where it’s going.
Our astronomers may actually be looking down the tailpipe of an interstellar craft right now, without knowing it. Realizing that could be truly history-making.