Seeing Further (36 page)

When the Royal Society was founded 350 years ago, the Copernican revolution was only a few decades old. Before Copernicus, many people believed the Earth lay at the centre of the universe and mankind was the pinnacle of creation. The discovery that Earth is but one planet among several orbiting the Sun came as a shock and forced human beings to drastically re-evaluate their place in the universe. It is a lesson that has been repeated often in the centuries that followed. The pivotal change that occurred with Copernicus was so far-reaching that scientists refer to ‘the Copernican principle’ quite generally to mean that our situation in the universe should not be in any way special or privileged. Expressed simply, the Copernican principle asserts that we are typical. Some of the deepest unanswered questions in cosmology and astrobiology in the twenty-first century concern whether and when that principle might break down.

The Copernican principle has been a remarkably reliable guide when applied to astronomy and cosmology, although it got off to a bad start. In the seventeenth century it was widely believed that the other planets and moons in the solar system resembled Earth, even to the extent of being inhabited by plants, animals and sentient beings. Kepler, for example, wrote a treatise about the denizens of Earth’s moon. Galileo pioneered the use of the telescope to study the heavens, and it soon became clear that the other planets differ in many respects from Earth; within the solar system, then, Earth turns out to be a very atypical planet. But Galileo also discovered that the Sun is an undistinguished star among a vast number that collectively make up the Milky Way galaxy. Later measurements established that the galaxy contains about four hundred billion stars in total, arranged in a disc shape and embellished by spiral arms sprouting from a central spherical bulge. The entire assemblage is about one hundred thousand light years across.

At the turn of the twentieth century, it was widely believed that the Copernican principle might soon fail in two key respects. The first concerned the distribution of stars in the universe. The Dutch astronomer Jacobus Kapteyn made a painstaking analysis and concluded that the Sun lay in a privileged position near the centre of the Milky Way, with the galaxy a sort of ‘island universe’ surrounded by a seemingly limitless void. But within a decade or two this model was refuted. As far as we can tell, there is after all nothing very special about the location of the solar system. It actually resides in one of the spiral arms about twenty-five thousand light years from the galactic centre – middle suburbia, if you like.

Related to the question of the structure of the galaxy was a controversy concerning the wispy patches of light painstakingly catalogued in the eighteenth century by Frenchman Charles Messier. Some astronomers maintained they were far-flung galaxies in their own right – other ‘Milky Ways’. The alternative view was that these nebulae were clouds of glowing gas located within the Milky Way. The dispute was finally settled when telescopes became powerful enough to image individual stars in some of the nebulae, revealing them to be other ‘island universes’, or galaxies, in their own right, many very similar to the Milky Way. We now know that the Milky Way is in fact a typical galaxy, just as the Sun is a typical star, so the Copernican principle works on an extra-galactic scale too.

At the same time as the true nature of extra-galactic nebulae was being established, similar observations revealed that the other galaxies are in motion with respect to ours and each other, a feature that could readily be deduced from the Doppler shift in the spectral lines of their light. Edwin Hubble in the USA found a systematic pattern to this motion, which can be summarised by saying that the entire universe is expanding: the galaxies
are, on average, moving away from each other. Running ‘the great cosmic movie’ backwards suggests that, some billions of years ago, the matter in the universe was compressed into a small volume of space and was expanding very rapidly, a state of affairs now called the big bang.

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 10
⁴⁰
. 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 10
⁴⁰
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 late 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.