The Earth, at first glance, seems a pretty unique place. It’s the only known planet to support life. However, anyone with a basic grasp of arithmetic can see how that shouldn’t be the case; there are somewhere in the region of 300,000,000,000 (three hundred billion) stars in our galaxy, and most are thought to have planets in orbit around them. Furthermore, our galaxy itself is only one of roughly 1,000,000,000,000 (a trillion) that are known to exist in the universe. If only one in a million stars had a planet around it, and only one in a million of those planets supported life, and one in a million of those living planets had intelligent life, there would be literally hundreds of thousands of civilisations in the universe.
Our human civilisation is really very noisy. We leak communications into space. The Wi-Fi box in your living room doesn’t aim its signal at your computer, it broadcasts in every direction; your mobile phone is the same, broadcasting in every direction. Some of the signal created by your conversations goes straight up into space. Of course, telephones and internet routers are small, low powered devices. The real loud-mouths are the television and radio masts, which boom their signal into the cosmos. So if alien civilisations exist, and if they are as noisy as us, why haven’t we heard them?
Firstly, if millions of civilisations do exist in the universe, they exist very far apart. There is a spherical bubble of Earthly radio waves about 200 lightyears wide, expanding into space at the speed of light. This sounds huge, but in the context of the Milky Way, it is not. In all likelihood, it would take many millennia of steady broadcasting for communications to reach the nearest neighbour. We have no way of knowing how long advanced civilisations survive, as we have barely reached adolescence.
Secondly, as broadcast technology advances, communications get more efficient. In the early days of television, broadcasters used a small number of extremely powerful analogue antennas to air programmes; nowadays, we have a preference for low-power, local, digital antennas, which are much quieter.
Thirdly, and probably most importantly, is the concept of path attenuation. Imagine that expanding shell – two radio waves emanating from the same source do not travel in parallel. Over great distances, the radio waves diverge, they radiate, much like the rays of the Japanese Naval Flag (right). Put simply, the further away from a radio source you are, the weaker the signal.
It is unlikely that the background noise of a civilisation ticking over would be audible to its nearest neighbour; it would simply be too quiet. However, that is not to say that a civilisation at the height of its technological competence would not want to deliberately send out a message to say “Hello, we’re here.” It is signals like this that are the focus of the Search for Extra-Terrestrial Intelligence (SETI). So what are the results so far?
An early promising signal was LGM-1 (standing for ‘little green man’), detected in 1967 by a British observatory. The signal was peculiar in that it oscillated with perfect regularity, peaking every 1.3373 seconds. No natural phenomenon known at the time could have caused this, and so it was thought to be alien in origin. Further work determined that the source was a new class of star, a pulsar. Pulsars spin rapidly, sending a beam of radiowaves into space; much like a lighthouse does. A few years prior, the Soviets had detected CTA-102, which too was thought to be alien, only until it was found to be an exotic stellar remnant: a quasar.
Later, in 1977, a SETI researcher observed what was later named the ‘Wow! signal’. The recording system represented signal strength as a multiple of the normal background noise of space. A two was considered unremarkable; a seven was a notably high reading. Originating in Sagittarius, the Wow! signal topped out at U, representing signal-to-noise ratio of 31. It rose steadily, peaked, and decreased over 71 seconds. To date, no one has any idea what caused it, and it has never been heard from again.