Seeing Further

by Bill Bryson

  With the discovery of other galaxies, the scale of the universe leapt once more. Since the time of Copernicus, the sheer size of the cosmos has dazzled people again and again. The solar system is a few light hours across. The nearest large galaxy, Andromeda, is about two million light years away. Hubble observed galaxies ten times further away than this, but saw no end in sight. Hubble’s eponymous Space Telescope can now image galaxies more than twelve billion light years away, a volume of space encompassing trillions of galaxies in all. Remarkably, even on the largest scale of size, the Copernican principle again comes through with flying colours. Deep space surveys reveal clusters of galaxies spread with surprising uniformity throughout the universe. It seems we not only live in a typical galaxy, but even our extra-galactic neighbourhood is typical.

  The large-scale uniformity of the cosmos is confirmed in another way. The big bang that started off the universe as we know it was intensely hot, and filled space with heat radiation. As the universe expanded so the radiation cooled, but it remains as a fading afterglow of the fiery cosmic birth, detectable today in the form of a background of microwaves coming from all directions of space. The cosmic microwave background radiation has been travelling more or less undisturbed since about 380,000 years after the big bang, which occurred 13.7 billion years ago. It thus carries an imprint of what the universe was like at a very early epoch. Measurements show that, to one part in a hundred thousand, matter and radiation were distributed smoothly throughout space at that time.

  The second potential failure of the Copernican principle around 1900 concerned the formation of planets. A popular theory at that time was the so-called encounter hypothesis, according to which the Sun suffered a close approach by another star, which caused blobs of matter to be sucked off and flung into orbit round the Sun. Since such close encounters are highly improbable, the theory predicted that planetary systems will be very rare. In other words, the Sun may be a typical star, but its retinue of planets might be very exceptional.

  The problem of the solar system’s typicality had to wait far longer for a resolution. It was only in the 1990s that astronomers observed the first extra-solar planets, and with improving techniques the tally has grown to about four hundred. To date, no earthlike planets have shown up, but that is no surprise, because the current instrumentation isn’t sensitive enough to detect them. Space-based planet-finding systems should be able to detect other earths, however. There is no good reason why earthlike planets should not exist in abundance throughout our galaxy and others. Although it is not yet quite certain, it seems therefore that the solar system, and planet Earth, are fairly typical. The Copernican principle may have failed when Earth is compared to our sister planets in the solar system, but within the larger class of all planets, it is probably successful. Of course, success or failure of a typicality hypothesis depends on the level of detail we are interested in. For example, Earth’s moon was probably created when a Mars-size body slammed into the proto-Earth shortly after the solar system formed. This cataclysm produced a moon that is unusually large for the size of the planet. It will surely be very rare to find another earthlike planet with a similar-sized moon.

  Although the Copernican principle has no basis in physical law – it is more a rule of thumb – it is nevertheless tempting to apply it to other aspects of our circumstances. For example, Earth is host to abundant life. Is that typical of most earthlike planets? Many scientists think so; indeed, the subject of astrobiology is founded on the expectation that life is widespread in the universe. However, there is an obvious complication. We can observe the universe only from a location that supports life, which means we have in a sense selected where we are (or rather, our location has been selected for us). If there was only one planet in the universe with life, we would have to be on it. So we must be cautious in using the typicality argument. In fact, some scientists prefer to invert the reasoning and apply an atypicality, or anti-Copernican, principle.

  To illustrate the issues involved, let me discuss not our location in space, but our location in time. In the 1930s, the physicist Paul Dirac and the astronomer Arthur Eddington were struck by a strange relationship in basic physics and cosmology. The hydrogen atom is held together by an electromagnetic force between the proton and electron. There is also a tiny gravitational force of attraction between them. The ratio of these forces is a staggering 1040. How, wondered Dirac and Eddington, did such a large number come out of fundamental physics? (It remains a mystery today.) But the peculiar twist is that the same very large number crops up in a completely different context. The age of the universe – that is, the time since the big bang – is also about 1040 when expressed as a ratio using basic atomic units of time. Surely these two very large numbers are not the same by coincidence? Dirac at least thought not. He reasoned that they had to be linked deep down by some law of physics. However, because the age of the universe is not a fixed number – it gets bigger every day! – if there is such a linkage it implies that the ratio of forces must also increase with time, with gravity growing relatively weaker as the universe ages. Dirac developed an elaborate mathematical theory to incorporate this effect, and astronomers set about testing whether the force of gravity is indeed time-dependent.

  Dirac’s argument, however, contained a hidden Copernican assumption: it supposed that the cosmic epoch at which we find ourselves living isn’t special. Therefore an observer seven billion years ago would have found gravity to be twice as strong as it is for us, and an observer fourteen billion years from now would find gravity to be about half as strong as it is today, but in both cases the big number concordance would be the same as it is for us. Clearly the typicality assumption is questionable in this case. In the 1960s, the astrophysicist Robert Dicke pointed out how. The existence of intelligent observers like Homo sapiens has two basic prerequisites: suitable chemical elements and a star like the Sun that burns steadily for billions of years while evolution does its stuff. The key element for all earthlife, and probably any form of life, is carbon. Carbon was not coughed out of the big bang; rather, it was made in the cores of massive stars, which then exploded as supernovae and laced the interstellar gases with life-encouraging material. It follows that life would not have been possible until at least one generation of stars had lived and died. On the other hand, after several generations of star burning, the raw material needed for new star formation will dwindle, and stable stars will become a rarity. These considerations therefore bracket the epoch at which life is likely to arise in the universe, to between one and, say, ten stellar lifetimes. Dicke spotted that the lifetime of a star depends on both gravitation and electromagnetism. If by some magic we could make gravity suddenly stronger, the Sun would shrink and get hotter, burn its nuclear fuel faster and die quicker. The strength of the electromagnetic force controls the rate at which heat can diffuse from the energy source (nuclear fusion reactions) in the core of the star, reach the surface, and flow away into space. The balance between these two forces thus turns out to be the dominant factor in determining the star’s lifetime. A rough calculation shows that the lifetime of the star, when expressed in atomic units, depends on precisely the ratio of electromagnetic to gravitational forces flagged by Dirac and Eddington. So the big number ‘coincidence’ is convincingly explained as a consequence of an observer selection effect. The cosmic epoch at which we are living is indeed typical enough within the range permitted – the solar system is 4.5 billion years old, placing us in the middle range of the ‘habitability window’ before stars get scarce. However, assuming the universe endures for trillions of years and is not overtaken by a big crunch or similar cosmic catastrophe, the era of ‘observership’ (at least for observers who evolve naturally) occupies an atypical sliver of cosmic history.

  How does the Copernican principle play out for the distribution of life across the galaxy and beyond? Until the turn of the twentieth century there was a general belief among scientists that many other life-harbouring worlds existed. Even as l
ate as 1906, the astronomer Percival Lowell was convinced that Mars not only hosted life, but intelligent Martians, who had built a network of canals. During the twentieth century, the mood began to swing against the idea that life is common. Hopes of finding life elsewhere in the solar system began to fade as better telescopes, and then interplanetary space probes, revealed hostile conditions on our sister planets. This mood of scepticism extended to all extraterrestrial life, so that by the 1970s the Nobel Prize-winning biologist Jacques Monod felt able to proclaim in his book Chance and Necessity, ‘Man at last knows that he is alone in the unfeeling immensity of the universe.’ The grounds for this scepticism stemmed from advances in molecular biology, and the growing understanding of life’s extraordinary complexity, suggesting to many that its origin must have involved a statistical fluke of stupendous proportions, unlikely to have happened twice. These sentiments were reinforced when, in 1977, two Viking space probes landed on Mars with the express intention of testing for microbes in the soil. Nothing definitive resulted (and certainly no canals were found!). It began to seem as if life on Earth was in fact highly atypical, even unique, in the universe.

  Today, the pendulum has swung back again in favour of the idea that life is widespread in the universe. One reason for the renewed optimism is the discovery that terrestrial organisms can flourish under a much wider range of conditions than assumed hitherto. Microbes have been found near deep ocean volcanic vents living at temperatures above 120 ?C. Others have been found thriving in acid strong enough to burn human flesh, in the strongly saline waters of the misnamed Dead Sea and in the radioactive waste pools of nuclear reactors. Even the inner core of the Atacama Desert, where the rainfall is essentially zero, supports a low level of bacteria. These discoveries have given hope that microbial life at least might be possible on planets previously thought to be too hostile. In addition, clear evidence for liquid water – thought to be essential for life as we know it – on Mars and Europa (a moon of Jupiter) has rekindled hopes that primitive organisms might yet be found elsewhere in our solar system.

  In spite of this new-found optimism, we still lack an accepted theory of life’s origin. In 1859, Charles Darwin gave a convincing theory of how life has evolved over billions of years from simple microbes to the richness and diversity of the biosphere we see today, but he pointedly left out of his account how life got started in the first place. ‘One might as well speculate about the origin of matter,’ he quipped. Nevertheless, he did outline the germ of an idea, by referring to ‘a warm little pond’ in which all manner of chemicals might accumulate and, driven by the energy of sunlight, would react to form ever more complex molecules. Over an immense period of time sufficient chemical complexity might eventuate that the ‘soup’ would make the transition from non-living to living (whatever that transition may be – nobody knows).

  Darwin’s casual suggestion became the ‘primordial soup’ theory of life’s origin, developed by J.B.S. Haldane and Alexander Oparin in the 1920s. The theory was put to an interesting experimental test in 1952, when Stanley Miller, then a student of Harold Urey at the University of Chicago, sought to re-create the conditions on the primeval Earth by putting methane, ammonia, hydrogen and water in a flask and sparking electricity through it for a week. Miller was delighted to discover a red-brown sludge of organic gunk in the flask, from which many amino acids were identified. Amino acids are the building blocks of proteins, and some scientists saw the Miller–Urey experiment as the first step on the road to life down which a simple chemical mixture would be inexorably conveyed by the passage of time. Many pre-biotic soup experiments have since been performed under various conditions (we now know that the early Earth did not have an atmosphere quite like that assumed by Miller). It turns out to be easy to make amino acids; in fact, they are even found in meteorites. Much harder, however, is to produce long proteinous chains (peptides), or the building blocks of RNA and DNA. Some scientists are still hopeful that ‘more of the same’ would create life given enough time, but others are sceptical that simply zapping chemicals willy-nilly with energy will turn a non-living mixture into a living cell. It is often remarked that we may soon be able to make life in the laboratory using existing microbes as a blueprint and reconstructing a new organism piecemeal. (Viruses have already been made that way, but viruses do not satisfy some definitions of life because they lack the ability to reproduce unaided.) While that may be true, and is clearly possible in principle, it would not solve the problem of how Mother Nature performed the trick without fancy equipment, trained biochemists and a clear plan of action.

  From the point of view of the Copernican principle, we do not need to know the details of biogenesis, only how probable it is given plausible pre-biotic conditions. Is life on Earth the result of a freak chemical accident, or are there general principles that favour the emergence of organised complexity, and thereby facilitate the formation of life ‘against the raw odds’ computed from random shuffling of building blocks? Such a ‘life principle’ (essentially Copernicus’ principle for biological systems) is often mooted, but there is no hint of how it might be derived from the known laws of physics and chemistry. Nevertheless, the science of complexity is in its infancy, and it may be that there are general principles of complex organisation that are not yet understood. It is frequently pointed out that the elements needed for life – primarily carbon, but also oxygen, nitrogen, hydrogen, phosphorus and sulphur – are common in the universe, and that even simple organic molecules have been found in interstellar clouds. Sometimes this is used to argue that life must therefore also be common, but that is to confuse a necessary with a sufficient condition. To be sure, these substances are necessary for life, but it may require all sorts of other materials, and special conditions, before the basic building blocks self-assemble into the hugely elaborate structure of a living cell. It’s easy to make bricks, but making houses requires far more than throwing a pile of bricks in the air.

  If life were discovered on another planet, it would offer support for a life principle. There is, however, a caveat. By common consent, Mars offers the best hope for finding extraterrestrial life in the near future. Unfortunately, it may not settle the matter. Mars and Earth trade rocks blasted off their surfaces by asteroid and comet impacts, and hurled into orbit. A couple of dozen Mars meteorites have been found on Earth so far. During geological history, a prolific traffic of material has taken place between the two planets, mostly Mars to Earth on account of Mars’ lower gravity, but some the other way too. It has become clear in recent years that microbes could hitch a ride this way. Cocooned within a rock, a microbe would be shielded from the harsh conditions of interplanetary space, especially the radiation, and could remain viable even after a sojourn of some millions of years orbiting the Sun. It seems inevitable that living terrestrial microbes will have been delivered to Mars this way, especially before 3.5 billion years ago when the bombardment by cosmic debris was far higher than it is today. Conversely, if there was once life on Mars, it will have spread to Earth. The intermingling of the two biospheres complicates the story of life’s history though. It may be that life started on Mars and later came to Earth, or vice versa, or that life started from scratch independently on both planets, but became cross-contaminated. Only if there is clear evidence for two independent origins would the discovery lend support to the life/Copernican principle.

  While we wait (possibly a very long time) for Mars to be explored for life, and perhaps evidence for a second genesis, there is a way that the life principle can be tested right here on Earth. No planet is more earthlike than Earth itself, so if life does pop up on cue in earthlike conditions, it should have emerged many times over on our home planet. Biologists have long assumed that all life on Earth has descended from a single common origin. Gene sequencing confirms that all known organisms are genetically linked and can be positioned on a universal tree of life. However, the vast majority of species are microbes, and only a tiny fraction of these has even been characte
rised, let alone sequenced. You can’t tell by looking what they are made of. It is entirely possible that some terrestrial microbes are the products of different biogenesis events, in effect ‘alien organisms’, constituting a type of shadow biosphere. The universal tree of life on Earth might actually be a forest. The identification of a single microbe that is sufficiently alien for us to rule out a common origin with standard life, would have sweeping consequences. It would establish the Copernican principle for biology and point to a universe teeming with life.

  And that brings me to the tantalising question of whether we are alone in the universe, as Monod claimed. When it comes to intelligent life, the status of the Copernican principle is very uncertain indeed. Even if life has got going on many planets, there is no known law or principle that compels it to evolve intelligence or sentience. The Darwinian mechanism implies that evolution is blind; nature cannot ‘look ahead’ and strive for the goal of intelligence, or any other trait. So there will be no progressive trend towards sentient beings like ourselves, unless it comes about because natural selection strongly favours certain features and structures, or if there are yet-to-be-discovered principles of organisation at work in nature.

  Nevertheless, as always experiment must be the arbiter, and fifty years ago that experiment began with the inception of SETI – the Search for Extraterrestrial Intelligence. A small band of astronomers have been sweeping the skies with radio telescopes in the hope of stumbling across a radio signal from an alien civilisation elsewhere in the galaxy, so far without success. At the time SETI began in 1960, the general feeling was that life, let alone intelligent life, was exceedingly atypical for a planet. The sentiment was summed up by the biologist George Gaylord Simpson in a 1964 article entitled ‘On the non-prevalence of humanoids’, in which he described SETI as ‘a gamble at the most adverse odds with history’. Today, SETI receives far more scientific backing, although the basic facts have changed little since Simpson wrote his article. We still don’t know whether the origin of life on Earth was a freak event and whether the evolution of human intelligence was a statistical fluke.