If There’s an Alien Race Living on our Doorstep, Why Can’t We Hear Them?

As I was watching Battlestar Galactica last night, I was thinking about the lack of alien civilizations in the show. To be honest, I tire easily of humanoid alien beings with curiously shaped heads synonymous with Star Trek et al., so I’m loving the fact a far-off human colony created their own evil race, the Cylons. So far, so good, I’m getting sucked into BSG (will it be as good as, or even better than Bablyon 5? That has yet to be seen, but it looks promising).

These thoughts took me back to an Astroengine article I wrote in November with my usual gripe about our obsession for looking under rocks on Mars (The Search For Life, What’s the Point?). I reached the conclusion that I’d much rather be pottering around in an empty cosmos, devoid of life, than bumping into an angry neighbour who wants to probe/assimilate/hybridize me. Science fiction musings I know, but it isn’t that far from some of the conclusions that could arrive from using the famous Drake equation that underpins our incessant search for intelligent extraterrestrial life.

Today, I was referred to some research addressing the Fermi Paradox, although we haven’t heard from our extra-terrestrial neighbours yet, doesn’t mean they’re not transmitting. The galaxy could be teeming with intelligent aliens, but we just can’t hear them…

In a nutshell, the Fermi paradox is the contradiction between the comparatively high number of alien civilizations predicted to exist and the low number of alien civilizations we have found evidence for (i.e. zero). Enrico Fermi posed this question in 1950, and it has endured ever since (nope, still no aliens). At the time, Fermi was referring to the lack of evidence of alien spacecraft or other alien artefacts, and today, the Fermi paradox is used to question why the search for extraterrestrial life has still turned up no evidence of ET transmissions. This fact is also known as the “Great Silence”. Surely, if there are any alien civilization out there (as predicted by the Drake equation), they would have transmitted a radio signal of some form (an accidental leaking alien radio/TV show, or deliberate “hey neighbour!“, METI-like signal)?

At first glance, we could say that the Drake equation really is nonsense (after all, how can any equation predict more than one intelligent civilization in our galaxy, when we only have experience of one: us), and that we are the only kids on the Milky Way block. But, say if the equation is actually correct; there are more extraterrestrial civilizations living in our galaxy, but we just haven’t detected them yet. After all, the galaxy is a big place, it measures 100,000 light years across. It would therefore take 100,000 years for any electromagnetic transmission to travel from one end to the other.

In new research published by a researcher at the Bouchet-Franklin Institute (Rochester, NY), Reginald Smith presents a thought provoking answer to the Fermi paradox: it’s all about the number density of transmitting civilizations in our galaxy.

Life is short, we are acutely aware of that, but if we take a look at the human race, our radio transmissions seem like a mere spark when compared with cosmic time scales. In this case, the Earth’s radio transmission horizon is only 100 light years (as radio communications were invented only 100 years ago). We effectively have a tiny sphere of detection probability. Within 100 light years from our solar system, there’s estimated to be under 15,000 stars, and only a few hundred are considered to be life–as we know it–sustaining G-type stars (like our Sun). Compare that with the estimated 200 to 400 billion stars that are thought to exist in our galaxy, and you’ll rapidly see as a race, humans have a very small sphere of influence.

Example:
Signals from Earth will have travelled approximately 100 light years since early radio communications. Only 15,000 stars will have been able to receive our signal in that time. What percentage of the Milky Way’s total number of stars have we been able to transmit to in the last 100 years?

Assuming the galaxy contains 200 billion stars, we have only transmitted to 7.5×10^-6% of all the stars in our galaxy. That’s a minuscule 0.0000075% of the total number of stars in our galaxy have had the chance to listen into our transmissions. A small fraction of that number could even entertain the thought of being suitable for life, so already, the chances of any extraterrestrial civilization tripping over us are slim, unless they already live very close to us.

Already, for a “young” communicating civilization such as ourselves, we can assume the chances are slim as to whether any alien race will be able to hear us. However, eventually, many thousands of years into the future, if they are out there, aliens will have the opportunity to hear us (whether we are still here or not in galactic timescales would be open to debate). However, as Smith points out, just because communicating civilizations might be close enough to another transmitting race, it doesn’t mean that they will be heard. Their signal may be too weak to be distinguished from background noise. Galactic neighbours could be transmitting at each other for thousands of years but never know they are there.

So, to overcome the weakening of communicating civilizations signals, there needs to be a minimum density of transmitting aliens within a certain volume.

What is most interesting about this analysis is that it demonstrates it can be possible for many CCs [communicating civilizations] in the same galaxy to never contact one another. For example, even assuming the average CC has a lifetime of 1,000 years, ten times longer than Earth has been broadcasting, and has a signal horizon of 1,000 light-years, you need a minimum of over 300 CCs in the galactic neighborhood to reach a minimum density. For example, if there were only 200 CCs in our galactic neighborhood roughly meeting these parameters, probabilistically they will never be aware of each other.

This finding can give pause to both those who predict no other CCs or those who predict a high number of CCs in our galactic neighborhood. Arguing that the lack of contact signifies the lack of CCs may be tempered with the fact that if there is a signal horizon, even a galaxy replete with life may have relatively isolated CCs in the absence of interstellar travel or extremely power signals. On the other hand, high estimates of CCs in our galactic neighborhood does not guarantee that there will ever be contact between them, especially reciprocal. — Reginald D. Smith, 2009.

In conclusion, unless our galaxy has over 300 civilizations, each one transmitting (for over 1000 years), there is a very high chance that (unless some of these advanced alien civilizations lived very close to one another) no two civilizations may ever know of anyone else’s existence. The huge galactic distances coupled with signal degradation leads to a situation where, if communicating civilizations remain static, within their home star system, without interstellar travel, the likelihood of forging alien relations will forever remain unlikely.

The Milky Way could be a galactic ecosystem, and we may never know it…

Getting back to my next episode of Battlestar Galactica, I’m wondering: Are there really no aliens in the BSG Universe? Or is it just that the galactic communicating civilization density is too low…?

Source: arXiv:0901.3863v1 [physics.gen-ph], arXiv blog via @jorgecandeias