Is there anyone out there?
Are there other stars than the Sun in the galaxy that are orbited by planets that support intelligent life?
Brian Cox doesn’t think so.
In his BBC tv series on the human race’s place in the universe – Human Universe – he gives his reasons.
I’m not sure that I agree with him, but here’s his thinking.
In the 1940s Hungarian/American mathematician John von Neumann came up with the concept of self replicating machines (which he called universal constructors) – machines that could reproduce themselves. (Having just stated that von Neumann came up with the concept I’d like to qualify that statement with the observation that most concepts are thought up independently by several people, it’s just that only one of them usually gets the credit).
If such machines were to be launched into space they could land on asteroids, planets etc and mine the mineral resources there, using the minerals to construct replicants of themselves. By a process of asteroid/planet hopping they could thus multiply and spread out through the galaxy. It’s estimated that by such a process the whole galaxy could be colonised in ten million years.
If an intelligent alien race had at some point over ten million years ago (that is, for most of the history of the galaxy) decided to create such machines they’d have spread throughout the galaxy by now and we’d be able to see evidence of their existence.
But we can’t, so they didn’t.
This, according to Brian Cox, is the clincher when it comes to deciding whether advanced, space-going civilisations have existed before. He thinks that because the concept of self-replicating galaxy-colonising machines constructed by advanced alien civilisations is possible in principle we have to construct an argument for why we don’t see them – and he can’t think of any such argument.
He concludes that therefore it’s probable that there never have been advanced space-going civilisations in the galaxy up until now – until we came along.
Personally, I’m slightly uncomfortable with the reasoning here. The lack of evidence of self-replicating alien technology in our vicinity seems like a rather tenuous reason to dismiss the concept of intelligent alien life-forms.
In the tv programme Brian Cox stated that he couldn’t think of any reasons why where wouldn’t be evidence of alien self-replicating machinery, so I assume that he’s weighed up the arguments and has dismissed the ones that give reasons why advanced civilisation may exist and yet may not send out self-replicating machines. Unfortunately he didn’t give us any examples, which is a shame.
I can think of a few off the top of my head. They’re probably all flawed, but here are three of them.
Reason one: advanced alien intelligences have existed that are capable of making self-replicating galaxy-colonising machines, but they decided not to. They may have realised that a galaxy that was full of their machines, self-replicating endlessly – long after the civilisation itself had died out – was not a good idea. They’d probably have tried prototype self-replicating machines on their home planet first, with either inconvenient or dire consequences.
Reason two: the advanced alien lifeforms just weren’t interested in galactic colonisation, for either psychological or practical reasons.
Reason three: self-replicating machines have indeed spread throughout the galaxy, but the machines are hidden from our perception so that they don’t disturb us.
The whole television programme was about the possibility of life elsewhere in the universe (and was actually subtitled “Are We Alone?”), culminating in a discussion about intelligent alien life, so to dismiss the possibility of alien intelligent life on the basis of ‘no self-replicating machines’ without giving his reasons seems a bit lax.
Maybe he thought that the audience for such a programme wouldn’t be interested in arguments that were made in order to be dismissed, or maybe he just ran out of time (Time that could have been found, may I suggest, by cutting the silly sequence in which school children lined up with coloured lanterns to depict, very badly, the Hertzsprung-Russell diagram perhaps. Anyone who could make sense of that sequence probably already knew what the Hertzsprung-Russell diagram was).