By Keir Liddle
For many the face of science due to his TV appearances in the likes of the Simpsons, esteemed physicist Stephen Hawking is famous for many things, particularly his Brief History of Time and his research on black holes as well as being almost completely paralysed by the neuro-muscular dystrophy associated with amyotrophic lateral sclerosis (ALS). Recently, however, it has not been physics that has seen Hawking appear in the public eye; rather, his view on aliens as noted recently by the press (here, here and here, for example). Hawking is reported as saying:
“We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet. I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach.”
How much credence we can lend the claims, which appear to be part of a promotional campaign for a new Discovery channel documentary, is debatable: PR companies are not necessarily famed for their restraint! However, it is interesting to see how the media have portrayed the story – especially the Daily Mail (whose article is the third linked to) with the headline “Earth could be at risk of an invasion by aliens living in massive ships” which, dare I say it – wearing my prejudices on my sleeve – fits with the Mail’s editorial stance on immigration, at least!
But how worried should we be that there will be Klingons on the starboard bow, or that V risks becoming a reality?
In 1961 Frank Drake, American astronomer and astrophysicist and founder of SETI, devised the Green Bank equation (more commonly known as Drake’s equation) for calculating the likelihood of extraterrestrial civilisations existing. The equation is as follows:
- N = the number of civilizations in our galaxy with which communication might be possible;
- R* = the average rate of star formation per year in our galaxy
- fp = the fraction of those stars that have planets
- ne = the average number of planets that can potentially support life per star that has planets
- fℓ = the fraction of the above that actually go on to develop life at some point
- fi = the fraction of the above that actually go on to develop intelligent life
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time such civilizations release detectable signals into space
There is naturally considerable disagreement, given our current knowledge of the Universe, about what values should be used to fill these values – Wikipedia lists the current estimates as those given below:
R* = the rate of star creation in our galaxy: Latest calculations from NASA and the European Space Agency indicate that the current rate of star formation in our galaxy is about 7 per year.
fp = the fraction of those stars that have planets: It is now known from modern planet searches that at least 40% of sun-like stars have planets,and the true proportion may be much higher. Infra-red surveys of dust discs around young stars imply that 20-60% of sun-like stars may form terrestrial planets.
ne = the average number of planets (satellites may perhaps sometimes be just as good candidates) that can potentially support life per star that has planets: Since about 200 planetary systems are known, this implies ne > 0.005. It has also been determined that about 10% of star systems in the Galaxy are hospitable to life, by having heavy elements, being far from supernovae and being stable themselves for sufficient time.
fl = the fraction of the above that actually go on to develop life: In 2002, Charles H. Lineweaver and Tamara M. Davis (at the University of New South Wales and the Australian Centre for Astrobiology) estimated fl as > 0.13 on planets that have existed for at least one billion years using a statistical argument based on the length of time life took to evolve on Earth.
fi = the fraction of the above that actually go on to develop intelligent life: This value remains particularly controversial. Pessimists point out that of tens of millions of species on Earth, only one has become intelligent and infer a tiny value for fi. Optimists note the generally increasing complexity of life and conclude that the eventual appearance of intelligence might be inevitable, meaning fi=1. Skeptics point out that the large spread of values in this term and others make all estimates unreliable.
fc = the fraction of the above that are willing and able to communicate: There is considerable speculation why a civilization might exist but choose not to communicate, but there is no hard data.
L = the expected lifetime of such a civilization for the period that it can communicate across interstellar space: In an article in Scientific American, Michael Shermer estimated L as 420 years, based on compiling the durations of sixty historical civilizations. Using twenty-eight civilizations more recent than the Roman Empire he calculates a figure of 304 years for “modern” civilizations. David Grinspoon has argued that once a civilization has developed it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for L potentially billions of years. If this is the case, then the galaxy has been steadily accumulating advanced civilizations since it formed
Despite telling us how many civilisations are out there, many observers have pointed out that Drake’s equation is a very simple model that does not include potentially relevant parameters. David Brin states:
[Drake’s equation] merely speaks of the number of sites at which ETIs spontaneously arise. It says nothing directly about the contact cross-section between an ETIS and contemporary human society.
So Drake’s equation may be able, one day perhaps, to tell us how many aliens there are out there, but it can’t tell us if they are likely to come calling, letting us know when to jump behind the sofa and pretend to be out. In short, we can say that, given the scale of the Universe, it is unlikely that there isn’t life out there, nor perhaps unlikely that there isn’t intelligent life– but close enough to reach us? The jury is very much out on that.
As for Hawking’s statement that we shouldn’t attempt to contact alien civilisations there is one major problem with this: we simply can’t avoid it. We have been spamming the Universe with radio waves for over a hundred years now. If aliens as far away as Aldebaran have the relevant technology, then they will just be hearing about the events of the Second World War as a whole century of popular radio and TV programming will be bombarding them– so maybe it’s not surprising that they haven’t been in touch!
If Hawking is right, then perhaps we should consider measures to counteract the broadcasting of ITV further into space, lest Ant and Dec inspire intergalactic war!
Of course, it works both ways: some day we might wake up to be greeted by the news that we have just received the first broadcast of some alien world. Assuming that we as a species will live long enough to experience it, we could one day enjoy the dramas, documentaries, history and culture of another galactic civilization.