So, Where Are They All Then?COLONISATION | VON NEUMANN PROBES | MASS MIGRATION | EXPANSION TIMESCALES | DETECTABLE EVIDENCE | THE IMMORTAL NETWORK | THE FERMI PARADOX | PHYSICAL DIFFICULTIES | BIOLOGICAL DIFFICULTIES | SOCIOLOGICAL REASONS | TEMPORAL REASONS | AGAINST ET Despite the increasing complexity of search strategies and processing power available, no verified evidence of any extra-terrestrial intelligence has been detected so far. This is not entirely unreasonable in light of the requirements of a thorough search (SETI’s 100 billion channels) and it may well be some time before we can honestly say that we have searched long and hard enough to say yay or nay to the question. However, there are many scientists who believe that SETI is a fruitless task because there are other expectations that would accompany the rise of intelligent civilisations throughout the galaxy, expectations that would provide more direct evidence on our doorstep. Interstellar colonisationIf we are taking our first tentative steps towards the stars, is it reasonable to expect that other civilisations have followed the same route and with greater success? True, we are extrapolating from a statistical case where n = 1, but we may feel justified in thinking that on the grounds that most processes in life science do not scatter over too large a range, and whether we are average or on the edge of a probability distribution, given the large number of prospective civilisations given by the Drake Equation, then at least a few should follow the steps towards colonisation. What are these steps? We can speculate that in the future, provided we don’t extinguish ourselves in a nuclear conflagration, a biological contagion or disruption of the global ecosystem, then exploration of the solar system will continue with larger and larger numbers of manned flights. Global resources will be strained and energy will be at a premium. Under these conditions we may decide to mine asteroids and moons for their raw materials. Perhaps we will build a large sphere all the way around the Sun to trap its energy - a construct known as a "Dyson Sphere" (Freeman Dyson, 1960). The technology will develop to transport large amounts of men and materials over large distances, presumably to the point where an interstellar journey becomes feasible. Once this point is reached, then exploration of nearby star systems, perhaps using "generation ships" will take off. These spacefaring colonies will travel between the stars until they reach suitable planets, whereupon they will settle and eventually build more ships. From then on colonisation will proceed at an exponential rate. Once free of the parent planet, then the process will be independent of that planet and its fate. Von Neumann probes as interstellar phagesOne possibility for exploration which avoids the biological problems of interstellar travel is to send machines instead. The ideal type of machine for doing this exploration is known as the von Neumann probe: This is a hypothesised self-replicating machine, which can harvest raw materials from comets, asteroids or planetary bodies to produce a second generation of probes when it reaches another solar system. The parental machine stays to explore this solar system, whilst its offspring head off for more distant stars. The advantages of this system are:
Stellar evolution and the motivation for mass interstellar migrationsAlthough we know nothing about any alien civilisation, we can assume that, given a large number of possible species, some of these would share the human drive to exploration and adventure - possibly as a result of a similar evolution as a territorial creature. There is, however, another motivation towards interstellar travel. Given the range of acceptable stars and their formation rates, in the history of the galaxy there might well have arisen intelligent races fairly soon after the galaxy itself was formed. Even if the evolutionary process were to take as long as it has on Earth, then this could have happened some 5 billion years ago, before the solar system itself was born. In fact such civilisations should now be finding that their parent star is nearing the end of its life on the main sequence. Even for a small number of civilisations at any one time (10-100), the statistics of stellar evolution indicate that at least one of these civilisations would find itself living around a dying star. The options are stark. Either adjust to the engorgement, rapid pulsations and subsequent collapse of the star to a feeble white dwarf radiating 1000 times more weakly then the original star - or migrate. Estimates of expansion timescalesIn the relatively short space of just 2 million years, from the origins of the first recognisable humans in the African Rift, we have spread across the face of the Earth and planted permanent, self-sufficient colonies on all but the most inhospitable lands. The only places remaining to be settled are the seafloor, and space. Once on the route to interstellar colonisation, we might ask how quickly, given conceivable technology, the process could continue. Eric Jones makes several assumptions to estimate this:
A modified diffusion equation is applied (it has been successfully used
to model population dispersal for many plant and animal species (Newman
1986)) Terms must be included to account for the source rate of population,
and the model is appropriate as long as the average distance between the
place of birth of a parent and the place of birth of offspring is large.
However it fails in the case of interstellar migration because the local
growth rate greatly exceeds the emigration rate. The assumptions made in this calculation must be regarded with some suspicion, in fact if a colony is able to live in space without the need to settle at any one place, then the diffusion equation will apply and the civilisation will permeate the galaxy quickly, but with no permanent location. Evidence to look forThe manifestations of technology discussed above could well be signs of intelligence to search for in our own or nearby solar systems. In particular, suggestions have been made to search for the following:
The immortal networkAs a species we are on the brink of a new age of communication. For the first time in history it is possible to share information in the form of text, pictures, movies and sound almost instantaneously with anyone else in the world possessed of an internet connection. This revolution allows rapid and low cost access to a wealth of human wisdom and experience from the comfort of your own home. The World Wide Web will continue to evolve to be the preferred method of trading information across the globe within the next few years. If highly advanced civilisations have sprung up in the history of the galaxy, then such an interstellar network would be a logical way to share information and expertise efficiently. Once started, the immortal network would be available for any subsequent civilisations to tap and contribute to. The proximity and accessibility of the nearest "terminal" would be a test of the potential users ability to accept the information. So, where are they all then?The great physicist Enrico Fermi is reputed to have asked this question in the 1950s. A symposium based on this question was organised in 1980 by Ben Zuckerman and Michael Hart. Most scientists believe in the empirical fact that there are no intelligent beings from outer space on Earth now. Bearing in mind that absence of evidence is not necessarily evidence of absence, if other civilisations exist there are 5 possible explanations for this:
Let us now look at these points in more detail: The problems of interstellar travel split up into (i) physical and technological problems, (ii) biological and sociological problems. Physical and technological problemsThe Voyager and Pioneer probes discussed above are already on an interstellar journey travelling at ~ 10km/s (1/30 000 speed of light). The nearest star Proxima Centauri is about 4 light years away (just over 1 parsec). If they were going that way, these probes would take 120 000 years to make this simple journey. In practice they will travel for much longer until they encounter the gravitational clutches of a star. Herein lies one of the basic problems of interstellar travel. The distances between the stars are vast. Even in the galactic centre where stars are packed closer together the average separation of stars is ~ 1 ly or ~ 9 trillion km. The centre of the galaxy is 30 000 ly away, taking the Voyager probe 900 million years to make the journey! Quite clearly the chemical propulsion systems we rely on at the moment are wholly inadequate to undertake such journeys within human timescales, although this is no barrier to a von Neumann type colonisation process. What speeds would be acceptable for manned travel to the nearest stars? The faster the better, it would seem. Surely the speed of light, c (299 792 km/s) or faster would offer acceptable journey times? There is, however, a fundamental barrier in this respect indicated by the Theory of Relativity. It is currently accepted that the speed of light is nature’s top speed (bar exotic but unproven particles named tachyons). Light (and all other waves in the electromagnetic spectrum) may travel at this speed simply because the wave packets that make it up (photons) are massless. In fact if light did not travel at this speed it would not exist. Relativity shows that anything that has mass (i.e. anything composed of ordinary matter; protons, neutrons, electrons) would require infinite energy to accelerate it to the speed of light. In our particle accelerators we pour tremendous energies into subatomic particles to reach velocities of 0.9999999c, but not one of them will ever attain c. There obviously has to be a compromise between energy availability and travel time. One interesting consequence of such high velocities is the time dilation effect whereby the passage of time as measured in such a particle’s reference frame is much shorter then that measured in an external observers frame. This effect is observed daily in decay times that are much longer than would be expected for particles at rest (e.g. muons produced in the upper atmosphere by cosmic ray interactions). This effect would mean that the experience of journey time on board a high velocity spaceship would be shorter than the journey time as measured from Earth. Sagan et al used an example ship capable of accelerating at 10 m/s2 (equivalent to Earth gravity) for half of the journey then decelerating at the same rate to reach the destination. A journey to the centre of the galaxy (30 000 ly) would only have a ship time of 20 yr, a trip to the Andromeda galaxy a ship time of 30 yr, despite the fact that as measured from Earth the crew would take some 3 million years to get there. The Daedalus Project (British Interplanetary Society 1973-8) proposed a flight to the Alpha Centauri system at 0.12c taking ~50 years (at these speeds relativistic time dilation is not significant). What energy sources and propulsion systems could be used to achieve this? As indicated previously, chemical reactions have specific impulses that are far too low for such a task. NASA has recently launched its Deep Space probe as a test bed for an ion engine – charged particles accelerated to high exhaust velocities using electric fields. Although they have potential to build up to very high velocities, their thrust is very low (6 grammes in the case of Deep Space!) and long acceleration periods are required. Nuclear propulsion systems of various types have been proposed, both fission and fusion based. The Daedalus Project proposed a system based on the fusion of deuterium and tritium pellets, with the reaction products being directed by magnetic fields in the manner of a conventional rocket exhaust. An estimated 50 000 tonnes of fuel would be needed for a payload of 1000 tonnes. Unfortunately the abundance of tritium on Earth is low and it would need to be "mined" from the gas giants. The most efficient conversion of mass to energy is the annihilation of matter/antimatter, more specifically protons and their antiproton counterparts. This process produces charged pions which then decay into gamma rays and neutrinos after ~ 3 x 10 -8 sec. Over 40% of the rest mass of the protons/antiprotons can be converted into kinetic energy this way. This process can be used to expel reaction products within a magnetic containment system (very high field strengths required) or used to heat up a quantity of working fluid that may be ejected as an exhaust. Calculations indicate that the optimum fuel/payload ratio for a journey at 0.12c is about 4. So a 1 tonne capsule would need 4 tonnes of reaction fluid heated by some 10kg of antimatter fuel. The Daedalus mission would require 620 tonnes of fluid and 1.6 tonnes of antimatter. To date, only tiny amounts of antimatter have been produced on Earth in particle accelerators like CERN’s antiproton collector and FermiLab’s antiproton source. The latter produces about 50 billion antiprotons an hour and the cost of this averaged over a year amounts to some $48 million for a total of 300 000 billion particles. This sounds like a lot of energy, but you would need over 100 000 such sources to power a single light bulb (Krauss). The production of 1.6 tonnes of antimatter is beyond the financial and technical capabilities of any government on Earth for now and the foreseeable future. Basically it takes many times more energy to make antimatter than you can get back from its annihilation. Sadly, despite searches for natural sources in the Universe, all the matter we see "out there" is normal ordinary protons, neutrons electrons etc. with only tiny amounts evident in cosmic rays - Big Bang Theory puts an upper limit of 1 part in 10 billion. There is also a significant problem in the storage of such large amounts of antimatter within a matter spacecraft. Other proposed methods of getting to the stars are light sails, laser powered rockets and interstellar pellet streams, but all of these suffer from the difficulties of transmitting and capturing light beams/particle steams over the vast distances necessary. One approach has been to suggest the use of an interstellar "ramjet", whereby a large magnetic field sweeps up hydrogen from the interstellar medium in front of the spacecraft and uses this as fuel. The density of the interstellar medium is so small that vast fields would have to be generated at considerable energy cost. It must be emphasised that apart from the basic barriers offered by physics, many of these problems are of a technological nature and may well be surmounted by future developments and discoveries. Even if a way was found to travel at appreciable fractions of the speed of light there are other factors that might prevent successful interstellar journeys. One of these is the ever present interstellar medium, composed mainly of hydrogen and helium, but also in some regions containing enhanced quantities of heavy elements and particles of carbonaceous/silicate dust. The collision of even the tiniest dust particle at these speeds could be catastrophic (bearing in mind that a fleck of paint travelling in low earth orbit at ~40 km/s shattered the outer window of the Space Shuttle). Obviously collision with any larger chunk of ice/planetoid would generate significant quantities of energy. A 1000 tonne spacecraft travelling at 0.1c impacting on a planet would release some 1020J of energy - sufficient to cause a major catastrophe on the scale of global extinction scenarios in Earth’s past. Maybe other civilisations have realised this and decided to stay at home in a "paranoid Universe". Biological and sociological problems.If it is a requirement that a space vehicle should be manned then possible ways of dealing with the extended time periods involved in travel between the stars are:
Freezing has a variety of mechanical and biological problems. Firstly the freezer system has to "live" through the voyage, even if the crew are hibernating. At the bare minimum, it would require power for the initial freezing, and power for the final thaw, even if in between the ship saved power by letting itself equilibrate with the temperature of space (a few degrees above absolute zero). Although it is know that sperm, ova, antibodies and embryos can survive several years whilst frozen, the effects of prolonged freezing may be too severe. For instance, a common factor to all the frozen mammoths in Siberia, bison in Alaska and Otzi the Iceman, is that their bodies have become mummified. Over time, the water in their cells has been drawn out in a process akin to freeze-drying. Also, different types of cells may have different requirements when frozen: bull semen can be frozen without any difficulty, but horse semen requires glycerol to be added to it before freezing, or it will not survive. If a person’s heart requires one treatment, his skin another and his brain a third, then obviously freezing is not a viable option for long term survival! Generation ships avoid the problem of keeping specific individuals alive long enough to complete the journey to another star, but have many other problems of their own. Firstly there is the matter of supplies. Everything - air, food, clothing, tools, spare parts - will have to be (a) recycled, (b) enough to last the whole voyage, or (c) manufactured from a stock of raw materials carried on the journey. Air and water recycling is tricky - as seen in the problems of both the Russian and American "biosphere" experiments - but is possible in a closed system with large reservoirs of all the required constituents (the life support in Star Trek noticeably lacks these reservoirs - people start to suffocate as soon as the lights go off unexpectedly!) Food production will also require an energy source, e.g. light for photosynthesis of plants, power source for industrial manufacture of artificial foods. The ship will also have to have sufficient shielding to keep the crew safe from the effects of cosmic radiation. Another problem presented by a lifetime in space is lack of gravity. Living in zero gravity causes severe decalcification of the bones, loss of muscle tone in the legs and heart, and sometimes a drop of efficiency in the immune system. Astronauts returning to Earth after a few months in space may need several weeks to fully recover, despite taking daily exercises when in orbit. Lack of gravity may also have other unwanted effects - for instance a baby assumes the best position for birth by orientating itself head downwards in the hours prior to labour starting. Gravity also assists the mother to push during contractions. Thus births in zero gravity may be difficult. A crew that has spent many generations in zero g may not be able to tolerate the transition to an environment with significant g, or - at the very least - would have to spend a long time learning basic skills such as walking and have to adjust psychologically to an environment which has a clear up and down, and where things (including people) accelerate when they fall. A ship which simulates gravity by spinning would counteract these problems, but would have to be very large, otherwise there would be a significant difference in the gravity experienced by your head and your feet. If there is sufficient thrust available to power the ship, then both the acceleration and deceleration phases will produce substantial artificial gravity. This disappears when the ship coasts at constant speed. Any ship will have a minimum crew complement needed to keep it functional and also a maximum crew its "life support" systems (air recycling, food production, etc.) can support. On a generation ship this therefore imposes very strict rules on reproduction. Dependent on the exact balance between these two factors, the resulting situation may be (a) compulsory reproduction by all the crew or (b) a fraction of crew members each generation are forbidden to have children. If the journey lasts a significant number of generations, then inbreeding will become a problem, particularly if the original crew complement is small. The solution to this is either strict controls on who breeds with whom, or carrying a supply of frozen sperm, eggs or embryos to allow the crew to have test tube babies. And a final problem with small populations is that by chance all the children in one generation may turn out to be the same sex. An all female generation would require cross generational breeding to keep the population going. An all male generation is obviously doomed. A further ethical problem with generation starships is that the second or subsequent generations may decide that they would much rather go back to Earth, than complete the journey that they were sent on. Sociological ReasonsSome typical reasons given for absence of ET on Earth are:
There are many other sociological explanations, but most suffer from a logical flaw in that to work, they must hold for a civilisation for the duration of its lifetime, and for all civilisations across the galaxy. If just one of these departed from this philosophy we might see some evidence of visitation, especially if the civilisation had a long lifetime. All of these hypotheses are not amenable to empirical testing, and we rely on our own history to look for trends. What is certain from this source is that a tremendous variety of outlook flourishes and no single philosophy or set of behaviours lasts indefinitely, even on the relatively short human scale. Temporal ReasonsOne of the most obvious explanations is that perhaps many civilisations may exist, but they haven’t had time to reach us yet. How realistic is this scenario? From discussions of expansion timescales (above), if we extrapolate human colonisation into the future, we find that it would be possible to settle the entire galaxy in the relatively short space of 60 million years. By reversing this, we might assume that other civilisations would have broadly similar expansion rates, and since they would have originated at many different times in the history of the galaxy, then we should expect to see some evidence already on our doorstep. The only way this could not be the case is if space exploration on the whole only started 60 million years ago. This seems improbable given the age of the galaxy (~1010 years). The second hypothesis is that visitors from space arrived here in the fairly recent past, but did not settle here permanently. This is the "Erich von Daniken" school of thought, and various interesting archaeological finds have been cited as evidence of this idea. The flaw in this argument is that, assuming the visitors arrived as soon as they could, the fact that an intelligent civilisation developed such technology within such a short time as our own space age requires an incredible temporal coincidence, given the age of the galaxy. Maybe, the visitors arrived at a much earlier epoch, say 500 million years ago, and no temporal coincidence need be invoked. If this is the case, we still have to explain why, given the short expansion timescales, they have never returned, or we have no evidence of this visitation to hand. Looking for evidence on Earth is not necessarily the best strategy, since erosion and tectonic activity, together with biological processes could render artefacts down within only hundreds of thousands of years. Far better to search for such evidence on the Moon, or other bodies in the solar system free of these problems. This idea was the cornerstone of Arthur.C.Clarke’s 2001- A Space Odyssey, and would certainly qualify as our "right of passage" into the spacefaring community. Of course there are many people who believe that not only have extraterrestrials arrived on Earth in the past, but they are still here. This is the UFO hypothesis and is discussed later. Arguments against ETOf course one of the simplest explanations for the absence of any evidence is that there are no other civilisations, have never been any other civilisations and we are indeed the first intelligent species in the galaxy. We might be desperately alone. Interestingly enough, whilst physical scientists seem very optimistic that this is not the case, life scientists seem to veer the other way, finding it difficult to conceive that such an intricate and complex entity as the human brain, or its equivalent could evolve more than once in the lifetime of the galaxy. However to many scientists this smacks of the very anthropocentrism that has clouded human philosophy for hundreds of years, and they will invoke the Copernican Principle to infer we are more likely to be a typical product of the Universe than a special one. Nevertheless, many scientists believe that none of the hypotheses stated above are sufficient to explain the absence of ETI and therefore SETI is a waste of time and money. |