In the last decade and a half, astronomy has finally realised one of its long-term goals, that of identifying planets in other star systems, and indeed the number of extra-solar planets now known far exceeds the roll-call in our own solar system. Although many of these planets are “hot Jupiters” – gas giants in close orbit around their primaries – advances in observational techniques have now located potentially-Earthlike worlds such as Gliese 581 d, a large terrestrial (rocky) planet orbiting near the outer edge of the habitable zone of Gliese 581, a red dwarf located approximately twenty light years from Earth.
If – as astronomers have long believed – Earthlike planets are fairly common in the universe, what of life and by extension, what of what are colloquially known as aliens, or more accurately as Extra Terrestrial Intelligences (ETIs)? This is a topic that has attracted so much attention from flying saucer enthusiasts and conspiracy theorists that mainstream academics tend to shun it, but there is one question that refuses to go away:
“Where are they?”
This is the question asked in 1950 by the Nobel Prize-winning Italian physicist Enrico Fermi, and it is a very good question. Assuming we make it through the next few centuries, there is on the face of it no reason why humans shouldn’t visit Gliese 581 d or the many other Earthlike planets that will doubtless have been discovered by then. But if that is the case, why have ETIs living on these planets not visited us? If intelligent life is common, by the law of averages some of these alien civilizations must be more technologically advanced than us and already capable of crossing interstellar space.
So why are they not here? Here, in ascending order of probability, are some possibilities:
i. They visited us in antiquity, using their advanced technology to build the Pyramids, Stonehenge etc.
ii. They are here now, but there is an X-Files or Dark Skies-style government conspiracy afoot to keep the public from finding out.
iii. We have been placed out of bounds under a Star Trek-style Prime Directive, prohibiting interference in our natural development as a civilisation.
iv. Aliens are aware of us, but after monitoring our radio broadcasts, they are keeping well clear.
v. We are alone and there are no ETIs, at least in our galaxy and its surrounds.
vi. Any technological civilisation will invariably wipe itself out before interstellar travel can develop.
vii. Even with the most advanced technology, interstellar travel is extremely difficult and rarely undertaken.
We can safely dismiss possibility one: the construction of these ancient monuments was a stupendous feat, but well within the range of the Neolithic and Bronze Age technologies of Homo sapiens without any assistance from ET. And while one cannot categorically rule out the second and third possibilities, my personal feeling is that both are extremely unlikely! Possibility four is slightly more believable: Bush, Blair and Mugabe are three excellent reasons for giving humanity a wide berth, and of course that is just the tip of the iceberg.
Far more plausible is the idea that Man is alone, but my feeling is that this, too, is unlikely. Over the last few centuries, Science has demoted the Earth from the very centre of Creation to a small planet circling a medium-sized star, far from the centre of a perfectly ordinary galaxy; a planet now known to have at least one analogue in Gliese 581 d only a few light-years away. If intelligent life could emerge in such an undistinguished place, it seems perfectly reasonable to suppose it has also done so elsewhere. Indeed it seems inconceivable that it has not done so.
The sixth and gloomiest possibility is bound to have its supporters. There are of course many ways in which our present civilization could wipe itself out. At the moment, the hot (pun intended) favourite is global warming, but there are other strong contenders. Even though the threat of global thermonuclear war has now receded, the possibility of a regional nuclear exchange such as one involving India and Pakistan is a very real possibility and would have consequences ranging far beyond the Indian subcontinent. Those other Horsemen of the Apocalypse, Pestilence and Famine cannot be discounted either, as fears over pandemic bird flu and the alarming recent jump in world food prices show.
However, even if global civilization does collapse, it is difficult to believe that there would be no survivors, and that the descendents of these could not eventually rebuild civilization, albeit after the passage of many centuries. Even if global warming had completely altered climatic conditions, Homo sapiens is a highly adaptable species and there is no reason to suppose the survivors could not adjust to the new conditions. There is evidence to suppose that modern humans have already survived one such “bottleneck”, occurring 73,000 years ago after the explosion of the Toba super-volcano caused a “volcanic winter” during which temperatures plummeted across the globe, and conditions for life must have become extremely harsh.
Even if mankind were to become extinct here on Earth, does this imply the same must inevitably happen whenever and wherever intelligent life arises?
One possibility that suggests it might is the so-called Doomsday Argument, proposed by astrophysicist Brandon Carter in 1983, which uses purely statistical methods to predict the likely lifespan of the human race. The basic premise is that if the total number of humans that will ever exist is N, then we are most likely to find ourselves at a point in time where roughly ½ N humans have already been born. However, given that the world population is now increasing at a very rapid rate, it will only be a few centuries before the remaining ½ N have also been born, and mankind’s day will be done.
This argument can be generalized to state that once an intelligent species develops the technology to rapidly increase its numbers, its days are numbered. In the case of our own species – and almost certainly any alien species - the crucial technology was agriculture, which permitted food to be produced rather than obtained from the wild, and thus permitted far larger populations to be sustained than was possible with hunter gathering. Complex societies soon followed: chiefdoms, then city-states and then empires, all with ever-increasing populations. Eventually mechanization of agricultural methods coupled with advances in medical science led to the rapid population increases of the last century.
If Carter is correct, then the doomsday clock for mankind began ticking as the last Ice Age ended, 11,000 years ago, and the same fate would befall any alien species, once it discovered agriculture.
But is Carter correct? Personally I am highly sceptical of any argument not backed up by hard science, however hard it may be to refute on purely logical or mathematical grounds. Philosophy, for example, is full of such arguments which can be used to prove the existence of souls, God, etc. Plato’s Theory of Forms is a wonderfully elegant theory, but few would argue that it relates to the universe we live in.
There is also the problem that the argument makes no attempt to define what is meant by a “human”. Does this include the Neanderthals, Homo erectus, etc, or just Homo sapiens? If the argument simply applies to just one species, then even if it is correct a small population of surviving humans might, through what is known as the “founder effect”, become a biologically-distinct type, i.e. evolve into a new human species. (In biology, this is thought to be one way by which new species evolve.) Thus although Homo sapiens would be extinct, humans in the broader sense would not.
(This idea of a new human species arising from a small founding population isn’t new and featured in Olaf Stapledon’s classic 1930 work Last and First Men, in which the Second Men arose from a small group of survivors of the First Men (us) who were aboard a polar research vessel at the North Pole and escaped the effects of a nuclear chain reaction that wiped out everybody else.)
My personal view is that regardless of the long-term future of mankind, the notion that no species survives long enough to achieve interstellar travel must be rejected.
We turn, therefore, to the last of the possibilities listed above. Could interstellar travel be impossible or, at any rate be so difficult as to be rarely undertaken? What is known as the "technological ceiling" of the Universe defines the fundamental limits of what even the most advanced technology can accomplish. The best-known example of such a limit is Einstein’s prohibition on any object or signal exceeding the speed of light.
It may, though, one day be possible to build a vehicle capable of approaching light speed. Even so, an interstellar voyage to even the nearer stars would be a monumental undertaking. Such a spacecraft could travel from London to New York in one-fiftieth of a second, but would require over four years to reach Proxima Centauri. Or put it another way, the distance from Earth to Proxima is roughly six-and-a-half billion times the distance from London to New York. And that’s only the nearest star.
However, one consequence of the Theory of Relativity is that time would slow down for the crew of a spacecraft travelling at close to light-speed, and from their point of view a voyage to Proxima Centauri would take less than four years – possibly much less, maybe even days. In Star Trek, the warp drive would frequently be too badly damaged even for Scotty to repair, and the USS Enterprise would be forced to head back to the nearest starbase on impulse power only. Nevertheless it could still reach near-light speed in a matter of seconds and (though the point was never made in the show) take advantage to time dilation to reach its destination with comparative ease.
Unfortnately, it might not be as simple as that. It could be that our Universe has a low "technological ceiling". On this picture, our scientists could very likely understand the underlying principles behind the technology of a civilisation tens or even hundreds of millions years older than our own. Arthur C. Clarke’s well-known Third Law stating that “any sufficiently-advanced technology is indistinguishable from magic” might turn out to be wrong.
The spacecraft featured in Clarke’s later novels couldn’t travel faster than light, but they did have some fairly exotic forms of propulsion which might in fact turn out to be impossible. Another highly speculative (and underappreciated) technology is the “inertial damper” that prevents Kirk and co being turned into strawberry jam by the enormous accelerations the Enterprise uses when getting up to speed.
Faced with these difficulties, even an ETI equivalent of Alexander the Great or Genghis Khan might be forced to curb his ambitions, and there never will be a Galactic Empire or a United Federation of Planets.
But this doesn’t entirely answer the question. Even if interstellar travel will always be extremely difficult, why has the long-running SETI program failed to pick up evidence of alien radio transmissions? The explanation for this is, I suspect, very simple. Even here on Earth, the idea of broadcasting in the strict sense of the world will soon be a thing of the past. Much television is now delivered through tight-beam satellite transmission or via cable; radio broadcasting will eventually be replaced by what is oxymoronically referred to as “internet radio”; and much communication goes via short-ranged cellular networks. None of this could be detected from space.
In other words, a SETI type program could only be successful in two situations. The first would be to listen into a civilisation during the brief interval – probably not much more than a century – between radio and broadband. This would require the incredible coincidence of a civilisation being at almost exactly the same stage of technological development (allowing for the time for its transmissions to reach Earth) as ourselves, a possibility which we can safely dismiss. The second situation would require ETIs to deliberately target Earth with a transmission.
It’s not so much a case of ET phoning home, it’s more a case of waiting for him to phone us.
© Christopher Seddon 2008