Dear Seth Shostak
27th April 2000
It is 400 years since Giordano Bruno was burnt at the stake for heresy. He believed that the earth was not at the centre of the universe and that life on Earth was not unique. He was right about the former, but what about the latter?
Asked the question "are we alone in the Universe?" most people believe that the answer is "no," even when the question specifies that life elsewhere must be able to communicate with us in an intelligible form. Supporters of the "life is unique" brigade have a big hill to climb. Let me start the ascent.
The argument of those who believe that we are not alone runs as follows. There are many stars in the galaxy (many more in the Universe) and many have planets. A large fraction of these will have the conditions necessary for our kind of life: oxygen, water, and so on. It is then only one step to the evolution of intelligent life, et voilà: "Life is common." End of problem.
True, planets are quite common around other stars, and their presence has been demonstrated by a number of scientists using the techniques of optical astronomy; it should not be long before planets with the ingredients of life are detected. Moreover, the recent work on the Mars meteorite, with the possible detection of bacteria-style fossils, is intriguing. Although there is considerable argument about the accuracy of these findings, free water was found on the early Martian surface and there may well be elementary life beneath the surface now. It would not be surprising if elementary life-Martian-style-were common.
It is the next step-a step which took 3 billion years on Earth-which worries me: the transition from elementary biological systems (perhaps extending as far as lower animals) to the polished intelligent beings who now inhabit Earth. My worries are twofold.
The first concerns the factors which can snuff out life before it reaches the intelligent category. Setting aside naturally occurring problems of climate, fertility and so on, there are astronomical hazards associated with the impact of extraterrestrial bodies, enhanced emissions from the Sun, and effects from other stars. This problem is arousing considerable interest, not least because the US military has found a new enemy to replace the Russians. Although it looks as though Earth could last a billion years before the doomsday comet arrives, this period could be much shorter if smaller comets, or even near-misses, were seriously to disturb the climate. The situation may be much worse on other "earths" if the comet rain were denser, which is quite likely because the Earth is protected by Jupiter. Also our Sun is, fortunately, very quiescent-which is why we are here-although it is likely that dramatic solar flares occur every 1m years. Many other stars are much less stable, with prospects for life on their planets correspondingly reduced.
When allowance is made for these factors, the odds on "life is common" are considerably reduced. But what clinches the argument is my second difficulty. This concerns the answer to the question: where are they? We, on Earth, have probably only about 1 billion years to go before the solar radiation level renders life unbearable and we have to leave. Certainly, in about 4 billion years, the Sun will run out of fuel and swell alarmingly. When we ask "where are they?" we are not asking "why have we not detected any signals from them?" but rather, "why are they not here in person?" The point is that, if intelligent life were common, there should have been colonisation by the inhabitants of planets around the many other stars which have come to the end of their lives. If the "people" leaving the other planets were similar to us, Earth would have seemed an appealing place to settle. But there is no evidence of their presence.
The simplest conclusion is that these other civilisations do not exist. What, then, is the point of spending large sums searching for their signals using radio receivers (Jodrell Bank, Arecibo, and so on)? My own view is that there is a good case to be made for such spending and for the work of your own institute, dedicated to the Search for Extraterrestrial Intelligence (Seti). For a start, the "we are alone" argument could be wrong. And something unexpected in another area-perhaps of purely astronomical interest-might be discovered. Furthermore, there is the stimulus to technology from the development of your listening equipment. But I remain sceptical about hearing "them" out there.
Arnold Wolfendale
Dear Arnold Wolfendale
28th April 2000
You do me a disservice. You don't believe the intelligent extraterrestrials are out there, and yet you think that I and my colleagues should continue to bang away searching for them-not so much because we might succeed, but because we could stumble upon some intriguing spin-off. This is like encouraging James Cook to sail to the South Pacific by appealing to his possible contribution to ship design.
You begin with the currently accepted view that planets capable of incubating life are probably common. I agree, of course. Indeed, estimates of the numbers of planets which populate our galaxy is usually tallied in tens of billions. You then concede that biology will likely spring up on many of these other worlds; once more, I concur. But you then argue (along with Peter Ward and Donald Brownlee in their recent book Rare Earth) that while life may be commonplace, intelligent life-the kind which could make its presence known via radio signals hurled across the vast spaces between the stars-is rare, and possibly absent altogether. In this view, we inhabit an enormous galactic zoo populated by lesser creatures. Humans, you suggest, are the smartest things in the Milky Way. This is a nice point of view, at least for humans. But given the enormity of the Universe, and the lesson against cosmic hubris first taught by Copernicus, we should be suspicious of this proposition.
You offer two reasons why we should not expect extraterrestrial companions. The first is that the slow progression from simple to sentient life will often be stopped in its evolutionary tracks. Intelligence on Earth was slow in coming. It has been 3.6 billion years since life arose on our planet, and 600m years since the Cambrian explosion of complex, multicellular creatures. This long R&D phase before nature produced humans does suggest that the whole process is vulnerable to even a rare catastrophe. But one didn't happen here-and Earth is not likely to be an astronomical rarity.
Certainly there have been mass extinctions, the most famous being the destruction of three-fourths of all species-most notably the dinosaurs-65m years ago. This disaster was probably caused by a rock from space-an asteroid or comet-about 10km in size. But neither this nor any of the other calamities to befall our planet (including at least one period of global glaciation) have succeeded in terminating biology. The chain of life has snaked unbroken through every disaster for over 3.5 billion years.
Life is durable. And, as you say, we should probably thank Jupiter for keeping the inner solar system relatively free of large comets (although Jupiter also pulls some of these missiles our way). But there is no reason to think that large planets able to deflect lethal rubble from the cradles of life are scarce. Indeed, planet-hunters have found that at least 3 per cent of Sun-like stars have Jupiter-like planets.
Your second argument is that aliens should be in the neighbourhood-driven to colonisation by imploding stars-but they are not. This argument, as you know, dates from a remark by the physicist Enrico Fermi in 1950. Indeed, it provoked a cottage industry of research intended to explain how aliens might be plentiful, but poor travellers. Many of these explanations are reasonable. For example, Frank Drake has noted the daunting cost of interstellar travel and concluded that few civilisations would do it, thus limiting the number of colonies. Others have pointed out that colonisation efforts always run out of steam. Even insects which specialise in "colonising" vegetation by chewing it full of holes don't devour the entire forest.
In other words, it could be that the lack of apparent alien presence in our neighbourhood tells us nothing. After all, homo sapiens has been wandering the globe for 100,000 years and yet there are still places where I can go and be out of sight of all humans and their artefacts. Within 200kms of my home is barren desert, devoid of any sign of the hundreds of millions of people who populate the continent. A native of Nevada might logically-but incorrectly-infer that his family was the lone clot of humans in America. You are like this putative Nevada native.
It was not long ago-a matter of decades-that the existence of life elsewhere was considered a radical idea among astronomers. Nearby planets seemed brutally inhospitable, and the complexity of life suggested that even a simple bacterium was an improbable project that nature would seldom complete. But you now admit that biology may not be such a rare phenomenon after all. I would suggest that evolution to intelligence is a far less daunting proposition than creating the initial living cells. And if the first has taken place on a huge scale, then the second will often occur.
Seth Shostak
Dear Seth
2nd May 2000
You say that I do you a "disservice" by pointing out that there may be spin-offs from your search. Clearly you're a member of the "all or nothing" brigade-a risky stance if you fail to find those signals. In this context, you mention my hero, Captain Cook. Well, one of his key contributions to knowledge was the testing of a John Harrison chronometer. (Harrison was after the Longitude Prize of 1714-?20,000-for devising a method of determining longitude at sea to a high degree of accuracy. He took on the astronomical establishment, who wanted to use stellar, planetary and lunar positions to do the job, and beat them with his superb horological technique.)
Back to the aliens. You refer to the tens of billions of planets in our galaxy. You are right, but the number in the right range of size, which have gentle stars, not subject to alarming disturbances, with appropriate climate and so on, is probably no more than 1m-and could be many fewer. So, if the likelihood of intelligent life evolving is less than one in a million, we are alone-in our galaxy, at least. "One in a million" sounds like a small probability, but it would be a remarkably high probability for life to evolve into intelligent beings capable of communicating with us in a manner we can understand. And if you abandon the idea of "life like us," how will you identify its signals?
Now to Fermi's question: where are they? I'm waiting to hear some better "cottage industry explanations" for avoiding the horns of this dilemma. Even the best example you quote-Frank Drake's point about the cost of interstellar travel-is a non-starter. If there are many civilisations in the galaxy, as you suggest, then quite a large fraction will be ahead of us technically, and quite able to send out large numbers of their "people." After all, the drawing to an end of the possibility of life on a planet should be a great spur to innovation; I hope you can recognise SOS calls in extraterrestrial languages.
Arnold
Dear Arnold
3rd May 2000
You are overly impressed with Fermi's throwaway comment. The galaxy is an enormous place, with hundreds of billions of stars splayed across formidable tracts of unexplored real estate. You have strolled down to the local beach, found no whales or walruses, and concluded that it is improbable that the ocean could be home to large mammals.
I have already suggested several reasons why sophisticated alien societies might be common, but still undiscovered by us (more can be found in my book, Sharing the Universe). In particular, the galaxy might be "urbanised," but we could be located in an uninteresting rural area. In addition, I would urge caution in assuming that tomorrow's technology (which the aliens presumably have) will solve the difficulties inherent in interstellar travel, à la Star Trek. Physics, not engineering, is the real problem. Gallivanting from star to star at any reasonable speed is stupendously costly in terms of the required energy. To draw any conclusions from our isolation is premature. We have not done much hunting for signs of nearby alien intelligence, and it is unclear whether we would recognise them anyway.
But to the nub of your argument: you grant the probable existence of tens of billions of planets in the Milky Way. You then make the odd statement that only 1m or fewer of these worlds will enjoy the conditions necessary for spawning biology. Where, in heaven's name, do you get this number? Sun-like stars, which account for one in ten of the galaxy's stellar complement, are incredibly stable producers of light. True, we don't know what fraction of the Milky Way's billions of planets will have the conditions required for life, but in our own solar system three worlds are believed to be capable of cooking up a bit of biology: Earth, Mars, and one of Jupiter's moons, Europa. That's three among nine planets which could have produced life. So how do you justify the claim that only 1m of the galaxy's tens of billions of worlds might be carpeted with biology?
I suspect that life is common, and that proof will come in the form of discoveries to be made on Mars and Europa within the next 20 years. But you have a valid point when you question whether a sizeable fraction of living worlds will produce intelligent life, capable of building the powerful transmitters which would make them detectable by our radio telescopes. Maybe that fraction is small. Only once (twice, if you count the Neanderthals) has nature evolved an intelligent species on this planet, and this high IQ experiment may have been an unlikely accident. But note that there has been a substantial increase in relative brain size for several classes of earthly mammals in the last 50m years. Most of today's furry creatures are a lot brainier than the dinosaurs who stomped over our planet not so long ago. This at least suggests that sentience has real survival value, and nature will often produce it.
Intelligence may or may not frequently raise its cerebral head, but I suspect that it is a durable phenomenon when it does so. The whole point of Seti is to find out if this is the case. We hunt for simple, narrow-band signals-a sure sign of beings with a good grasp of physics and engineering-which keeps the search as culturally non-specific as possible. We don't worry too much about how the aliens are constructed.
Seth
Dear Seth
4th May 2000
You talk about the galaxy being "an enormous place, with hundreds of billions of stars splayed across formidable tracts of unexplored real estate." But we're trying to work out statistical probabilities-and even one billion stars with a probability of intelligent life per star (or solar system) of much less than one in a billion makes us almost certainly unique.
You also admit that "it's unclear whether we would recognise them [alien signals] anyway." You are a glutton for punishment! Your life is dedicated to looking for an unrecognisable needle in a giant haystack. To recap: the odds against extraterrestrial intelligence-capable of communicating with us in an intelligible way -are very high. But because of the likelihood of something astronomically interesting turning up-and "just in case"-please keep at it.
Dear Arnold,
6th May 2000
Of course we're trying to work out probabilities. My point is that if, as you say, we are alone in the Milky Way, then the odds against the evolution of intelligent life must be more than an imposing thousand to one, or a daunting million to one-they must be billions to one. This seems unrealistically high odds against intelligent life when we know that there are around half a trillion stars in our galaxy alone. If our Sun were extraordinary, or if planets were rare, then there would be a rationale for such irrational odds. Neither is the case. Indeed, I have yet to hear of one peculiarity of our situation which could so sharply distinguish us. If astronomy has taught us anything in the past 400 years, it is that every time we thought we enjoyed a unique cosmic situation, we were wrong.
Seth
