Fri, 16. April 2010
How Long Have We Got?
To estimate the number of other communicative civilisations in the Milky Way galaxy, scientists use a simple formula called the 'Drake Equation', which consists of a number of quantifiable variables which, when multiplied together, provide an 'answer'.
It's not hard maths to do and I still find it quite astonishing that one of the biggest human questions of all – whether we are we alone in the Galaxy - is actually reduced to such a simple equation. In science, mathematics rules.
Bear with me, its only seven numbers multiplied together. The potential number of other communicative civilisations in our galaxy (N) is estimated as follows:
N = R * x fp x ne x fl x fi x ft x L , where:
R*= Rate of formation of suitable stars per year
fp = Fraction of these stars with planets (0-1)
ne= Number of suitable planets per planetary system
fl = Fraction of these planets where life develops (0-1)
fi = Fraction of these where intelligent lifeforms evolve (0-1)
ft = Fraction of these where technology develops (0-1)
L = Lifetime of communicating civilisations in years
Unfortunately, most of the factors which form this famous equation cannot be pinned down to any real degree of accuracy, although astronomers are able to make a decent estimate of some of them. (For instance, the rate of formation of stars in the galaxy is generally thought to be close to seven per year). So, due to these uncertainties, currently the resulting 'answer' to the equation might be as low as zero other civilisations or there may be hundreds of thousands of them teeming all over the Milky Way. It depends on the scale of estimates for each variable.
I want to turn the spotlight on the last of these variables, the lifetime of a communicating civilisation, because I see this as the pivotal part of the equation. Given optimistic numbers for the first six variables but a pessimistic number for the lifetime of a civilisation, there could be very few communicating civilisations out there and we may even be alone at this time. Put simply, the number of civilisations currently in our galaxy is very highly dependent on the longevity of such civilisations. So how do we put a figure on longevity?
The only civilisation that we can study is our own. So first, how long have we been a communicating civilisation? In this 13.7 billion year old Universe, on this 4.5 billion year old planet, humans have been around for perhaps only 150,000 years. Human civilisation is just a few thousand years old at most and we only developed the ability to communicate using electro-magnetic waves 115 years ago. The ability to communicate on an inter-stellar scale was only developed in the 1960's, so we have been a communicating civilisation for a mere half a century. During that time we have listened but rarely sent communications powerful enough for other civilisations to hear, so that only barely qualifies us as a 'communicating civilisation'
One certainty is that our civilisation cannot and will not last forever. It will end one day. The most tantalising question is how long will it last for - and maybe that needs another equation of variables in itself. When the sun begins to expand into a red giant in four billion years time, will advanced humans watch with horror as the oceans start to evaporate away? I doubt if anyone will be around then!
Could our civilisation last another million years? Surely during the course of that time, the odds are high that some terrorist or tyrannical leader of a rogue country will have us all nuked, leaving survivors to fend off a long nuclear winter, abandoning any inter-stellar communications?
What about 100,000 years? That's short by geological and astronomical standards but still a very long time by human standards. Do we have the resources to last that long? When we've mined all the energy and polluted the atmosphere and oceans beyond redemption or caused a runaway greenhouse effect, will we retain the ability to communicate?
Maybe 10,000 years? Can the world feed everybody for so long? What are the chances of a catastrophic collision with a large asteroid over such a long period? Or a devastating supernova or gamma ray burst in our galactic neighbourhood? If we survive global warming, what would another ice age do to civilisation?
Can we survive a thousand years as a 'communicating civilisation'? What will the world's population be by then? It's growing exponentially now and is already 6.8 billion. How can the world feed hundreds of billions of people? Where would the water come from? Can a way be found to limit the population to a sustainable number? Will racial and religious disharmony send us back to the dark ages? What will we do after we run out of oil resources? Would our problems be so overwhelming that radio astronomy falls by the wayside?
Another hundred years? Will there be a devastating global pandemic? Could civilisation already be on the downward spiral? Is there any chance that world leaders can agree on a way to deal with climate change issues?
The answers to those questions are beyod the scope of this article but if I were forced to make a personal guess at what maximum age our own communicating civilisation could reach, it would not be a very optimistic one and it would certainly be to the lower end of the time scale.
That's a depressing thought in it's own right but the point I am making is if all the galactic civilisations that ever existed (assuming there have been any) live short life-spans of maybe just a few hundred years, then the Drake Equation makes it quite clear that there will not be very many other civilisations out there at any particular single point of time during the lifetime of our galaxy. Just to put it in context, if the average lifespan of a communicating civilisation is only (say) 500 years, then over the course of the last 13 billion years, there may statistically have been 26 million successive civilisations in the Milky Way galaxy without any of them overlapping in time - and only one would be around right now (us).
It might explain why scientists have not detected any intelligent galactic signals yet from our galactic vicinity. After decades of searching for radio signals, there may be no other such civilisation at this moment in time. That's all the more reason to keep looking.
The image shows the search area for exo-planets. The search for radio signals is likely to come from the same small region - maybe less than 1% of the galaxy. The rest is likely beyond our reach.
The original verson of this article was first published in 'Prime Focus' magazine (April 2010), the journal of Macarthur Astronomical Society.