Since mankind first looked up at the night sky, one of the biggest questions has been "Are we alone in the Universe?". Philosophers and theologians have debated this issue for centuries, while astronomers and astrobiologists continue to look for evidence and explore the data. When NASA sent probes to the edge of our solar system and beyond, they included information on our location in the galaxy and our culture in case other life forms one day discover them. Projects such as SETI have spent decades processing radio signals, and enlisted thousands of volunteers from around the globe to help search for any signal that is not natural. So far there has been nothing.

Another question that has received slightly less discussion is whether or not we should even try to find alien life!

We really do not know what exists in the Universe, or even in our local region of the Milky Way galaxy. We might discover that our nearest neighbours are actually militant conquerors who we would be better off hiding from. Some scientists have even suggested that we should limit attempts to send messages and probes into deep space until we have a better idea of who might receive them. 

For that reason it is important to discuss the probability that other civilizations on other worlds might be hostile towards us. Unfortunately this is very difficult to do, as we really have only a single example of an advanced civilization - our own. And a single data point is not very useful in making accurate predictions.

However one model that is gaining popularity (at least on the forums I frequent) is the Rhodes Model of Alien Aggression. First published about three years ago, it provides a very simple but generic model of how civilizations on another planet might develop and evolve, and by running this simulation thousands of times an estimate can be obtained of the chance that an alien able to travel across interstellar distances might be hostile towards us.

In the Rhodes Model, a planet is assigned five random variables. These represent the speed at which the species gains knowledge, the amount of communication between different civilizations on the planet, the overall health of creatures living on this planet (ie is it a world that is easy for life, or a very difficult and nearly uninhabitable world), a medical factor that represents the ability of the creatures to heal themselves, and finally a random number of distinct societies on the world (between ten and twenty). In addition, each society or civilization on the planet is assigned five more random numbers, corresponding to aggression, diplomacy, general health, general intelligence and knowledge, and the overall strength of the society to defend itself or to attack its neighbours.

The simulation then progresses in discrete time steps. With each time step, an algorithm is used to determine from the properties given above which civilizations will declare war on each other, which will trade information with each other, and which will focus on advancing their own knowledge and technology. In this way, the simulation is able to determine which societies on the planet will survive, and which will die out, as well as determining which societies will eventually have sufficient technological skill to travel to other planets. It is important to understand whether spacefaring cultures are more likely to be diplomatic and peaceful, or to be militant conquerors. And while this model uses very simple descriptions of each society, its simplicity allows for even a basic desktop computer to simulate thousands of planets in a very short time.

And the results are quite interesting. In the original paper, five runs involving a total of fifty thousand inhabited planets were simulated and allowed to evolve according to these simple rules. And although each planet was allowed a wide range of random properties, the result of each of the five runs was very similar. In the Rhodes Model, which is fairly generic and should be a reasonable approximation to the real Universe, it was found that less than 18% of technologically advanced species will be aggressive. The remaining 82% are friendly explorers who are not going to be a threat to humanity at all.

That is pretty good news for our future as a species. Of course a 1 in 5 chance of meeting invaders who will destroy us is still pretty scary, but considering that the remainder are friendly and already survived in the Universe suggests we might not be in as much danger as some have worried about.

It must be stressed of course that this is a very simple model and does not include much detail about real biological and cultural evolution of species, or include other properties that might affect how a society changes over time. But considering how little we know about alien species and life in general, it is reassuring that such a generic model gives a positive result and suggests that we might not be in as much danger as some have suggested.

Regardless of what is lurking in the galaxy, it will be a most exciting and perhaps startling moment in our own history when we finally meet other advanced lifeforms. For now we can only guess and speculate on who else is out there...