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Cosmology is the study of the structure and evolution of the universe as a whole. Prior to the twentieth century this was the domain of religion and speculative philosophy, but it is now a serious branch of physics, and one with spectacular implications. The transition from myth to science was brought about, in large measure, by two empirical findings: Hubble's observation (in 1929) that the Universe is expanding, and the discovery by Penzias and Wilson of Cosmic Microwave Background radiation (in 1965). Taken together, they indicate that the Universe as we know it began with a gigantic explosion 13.7 billion years ago – the Big Bang. In the next two sections we will explore these developments.
But first I would like to alert you to a fundamental assumption that informs almost all modern thinking about cosmology. Before Copernicus, most people assumed that the Earth is at the center of the Universe. The Sun, the Moon, the planets, and the stars orbit around us. Copernicus showed, to the contrary, that while the Moon orbits the Earth, the planets (including the Earth) orbit the Sun. And we now know that the Sun itself is just one of billions and billions of stars. The Earth is not at the center of the Universe – nor is the Sun. Modern cosmology takes the Copernican revolution to its logical conclusion: The Universe has no center, or any kind of “preferred” location – at a given time, it is the same in all places and in all directions. This is known as the Cosmological Principle.
Classical physics, some aspects of which we discussed in Chapter 1, is – for the most part – comforting to our intuitions. You probably wouldn't have come up with Newton's second law (F = ma) on your own (after all, nobody did before Newton), but once it is on the table it feels right. It seems consistent with our everyday experience. Classical physics refines and perfects our intuitions, but it doesn't upset them. By contrast, the four revolutions in twentieth-century physics are wildly counterintuitive; they seem to contradict everything we thought we understood – everything we took for granted about the world. That is, in part, what makes them so interesting. But it also raises a recurring question: “If this is really true, how come I never noticed it before?” I hope you will keep a skeptical eye on that subtext, as we go along.
Einstein's postulates
Einstein published his Special Theory of Relativity in 1905. The special theory is not an account of any particular physical phenomenon; rather, it is a description of the arena in which all phenomena occur. It is a theory of space and time themselves. As such, it takes precedence over all other theories. If you were to propose a new model of elementary particles, say, the first thing to ask would be, “Is it consistent with special relativity?” If not, you have some fast talking to do. As Kant would say, special relativity is a prolegomenon to any future physics.
On our way we passed many countries and put in at the Morning Star, which was just being colonised. We landed there and procured water. Going aboard and making for the Zodiac, we passed the Sun to port, hugging the shore. We did not land, though many of my comrades wanted to; for the wind was unfavourable. But we saw that the country was green and fertile and well-watered, and full of untold good things.
Sailing the next night and day we reached Lamp-town toward evening, already being on our downward way. This city lies in the air midway between the Pleiades and the Hyades, though much lower than the Zodiac. On landing, we did not find any men at all, but a lot of lamps running about and loitering in the public square and at the harbour. Some of them were small and poor, so to speak: a few, being great and powerful, were very splendid and conspicuous. Each of them has his own house, or sconce, they have names like men, and we heard them talking. They offered us no harm, but invited us to be their guests. We were afraid, however, and none of us ventured to eat a mouthful or close an eye. They have a public building in the centre of the city, where their magistrate sits all night and calls each of them by name, and whoever does not answer is sentenced to death for deserting. They are executed by being put out. We were at court, saw what went on, and heard the lamps defend themselves and tell why they came late. There I recognised our own lamp: I spoke to him and enquired how things were at home, and he told me all about them.
Lucian, A True Story, trans. A. M. Harmon, parallel English and Greek
The war of the worldviews
Astrobiology has some backstory.
The tale of our changing view of the possibility of life beyond the Earth is one that begins in ancient times, during the grandeur that was the ancient Greek world. In fact, the intellectual and cultural brilliance of the Greeks will have a profound effect on the rest of our narrative. But exactly what historical root does the plurality of worlds debate have in antiquity? What precursors in philosophy and literature, what flights of imaginative phantasy in fact and fiction? And what bearing did these ideas have on the science and culture of ‘astrobiology’ that was to develop over the coming millennia? Such are the questions that this first chapter seeks to address.
. . . we regard the cosmos as very beautiful. Yet it is also very terrible. For ourselves, it is easy to look forward with equanimity to our end, and even to the end of our admired community; for what we prize most is the excellent beauty of the cosmos. But there are the myriads of spirits who have never entered into that vision. They have suffered, and they were not permitted that consolation. There are, first, the incalculable hosts of lowly creatures scattered over all the ages in all the minded worlds. Theirs was only a dream life, and their misery not often poignant; but none the less they are to be pitied for having missed the more poignant experience in which alone spirit can find fulfilment. Then there are the intelligent beings, human and otherwise; the many minded worlds throughout the galaxies, that have struggled into cognizance, striven for they knew not what, tasted brief delights and lived in the shadow of pain and death, until at last their life has been crushed out by careless fate. In our solar system there are the Martians, insanely and miserably obsessed; the native Venerians, imprisoned in their ocean and murdered for man’s sake; and all the hosts of the forerunning human species. A few individuals no doubt in every period, and many in certain favoured races, have lived on the whole happily. And a few have even known something of the supreme beatitude. But for most, until our modern epoch, thwarting has outweighed fulfilment; and if actual grief has not preponderated over joy, it is because, mercifully, the fulfilment that is wholly missed cannot be conceived.
Olaf Stapledon, Last and First Men
Our position in the material universe is special and probably unique, and . . . it is such as to lend support to the view, held by many great thinkers and writers today, that the supreme end and purpose of this vast universe was the production and development of the living soul in the perishable body of a man.
Alfred Russel Wallace, Man’s Place in the Universe, as Indicated by the New Astronomy
Unlike special relativity, quantum mechanics was not the inspired product of one mind. It developed, in fits and starts, over a period of 25 years (1900–1925), and many cooks stirred the broth. Even now, a century later, quantum mechanics raises profound conceptual questions. Every competent physicist can “do” quantum mechanics, and the predictions it makes are in spectacularly good agreement with experimental results; there can be little doubt that quantum mechanics is “right.” On the other hand, as Richard Feynman once remarked, “nobody understands quantum mechanics.”
I'll tell the story in two parts. First the history (how the essential pieces of the puzzle were assembled), and then the implications (what the finished picture has to say about our world).
Photons
Like relativity, quantum mechanics began with the study of light. In both cases, this historical association is misleading. Relativity, really, has nothing to do with light – to be sure, it involves that magic speed limit, c, and of course the most familiar thing that travels at speed c is light. So light is a convenient vehicle for introducing the subject. But it is perfectly possible to imagine a universe in which there is no such thing as light, and yet relativity might still be valid.
Quantum mechanics, too, starts with light – in this case the “quantum” of light (the photon). But, as it turns out, this is just one rather specialized example of much more general principles – in fact, a rather bad example, because photons are by their nature relativistic (they travel at the speed of light), and it would be more natural to begin with the quantum mechanics of slow-moving objects, in analogy with Newtonian mechanics.
‘Sir,’ I replied to him, ‘the majority of men, who only judge things by their senses, have allowed themselves to be persuaded by their eyes, and just as the man on board a ship which hugs the coastline believes that he is motionless and the shore is moving, so have men, revolving with the Earth about the sky, believed that it was the sky itself which revolved about them. Added to this there is the intolerable pride of human beings, who are convinced that nature was only made for them – as if it were likely that the Sun, a vast body four hundred and thirty-four times greater than the Earth, should only have been set ablaze in order to ripen their medlars and to make their cabbages grow heads!’
‘As for me, far from agreeing with their impudence, I believe that the planets are worlds surrounding the Sun and the fixed stars are also suns with planets surrounding them; that is to say, worlds which we cannot see from here, on account of their smallness, and because their light, being borrowed, cannot reach us. For how, in good faith, can one imagine these globes of such magnitude to be nothing but great desert countries, while ours, simply because we, a handful of vainglorious ruffians are crawling about on it, has been made to command all the others? What! Just because the Sun charts our days and years for us, does that mean to say it was only made to stop us banging our heads against the walls? No, no, if this visible god lights man’s way it is by accident, as the King’s torch accidentally gives light to the passing street-porter.’
Cyrano de Bergerac, Les États et Empiresde la Lune, trans. G. Strachan
Behind every man now alive stand thirty ghosts, for that is the ratio by which the dead outnumber the living. Since the dawn of time, roughly a hundred billion human beings have walked the planet Earth. Now this is an interesting number, for by a curious coincidence there are approximately a hundred billion stars in our local universe, the Milky Way. So for every man who has ever lived, in this Universe there shines a star. But every one of those stars is a sun, often far more brilliant and glorious than the small, nearby star we call the Sun. And many – perhaps most – of those alien suns have planets circling them. So almost certainly there is enough land in the sky to give every member of the human species, back to the first ape-man, his own private, world-sized heaven – or hell. How many of those potential heavens and hells are now inhabited, and by what manner of creatures, we have no way of guessing; the very nearest is a million times farther away than Mars or Venus, those still remote goals of the next generation. But the barriers of distance are crumbling; one day we shall meet our equals, or our masters, among the stars. Men have been slow to face this prospect; some still hope that it may never become a reality. Increasing numbers, however, are asking: ‘Why have such meetings not occurred already, since we ourselves are about to venture into space?’ Why not, indeed? Here is one possible answer to that very reasonable question. But please remember: this is only a work of fiction. The truth, as always, will be far stranger.
Arthur C. Clarke, 2001: A Space Odyssey
We don’t want to conquer the cosmos, we simply want to extend the boundaries of Earth to the frontiers of the cosmos . . . we don’t want to enslave other races, we simply want to bequeath our values and take over their heritage in exchange. We are only seeking Man. We don’t know what to do with other worlds . . . We are searching for an ideal image of our own world: we go in quest of a planet, of a civilization superior to our own but developed on the basis of a prototype of our primeval past.
Astronomy is an observational science, renewed and even revolutionized by new developments in instrumentation. With the resulting growth of multiwavelength investigation as an engine of discovery, it is increasingly important for astronomers to understand the underlying physical principles and operational characteristics for a broad range of instruments. This comprehensive text is ideal for graduate students, active researchers and instrument developers. It is a thorough review of how astronomers obtain their data, covering current approaches to astronomical measurements from radio to gamma rays. The focus is on current technology rather than the history of the field, allowing each topic to be discussed in depth. Areas covered include telescopes, detectors, photometry, spectroscopy, adaptive optics and high-contrast imaging, millimeter-wave and radio receivers, radio and optical/infrared interferometry, and X-ray and gamma-ray astronomy, all at a level that bridges the gap between the basic principles of optics and the subject's abundant specialist literature. Color versions of figures and solutions to selected problems are available online at www.cambridge.org/9780521762298.
Modern astronomical research is beset with a vast range of statistical challenges, ranging from reducing data from megadatasets to characterizing an amazing variety of variable celestial objects or testing astrophysical theory. Linking astronomy to the world of modern statistics, this volume is a unique resource, introducing astronomers to advanced statistics through ready-to-use code in the public domain R statistical software environment. The book presents fundamental results of probability theory and statistical inference, before exploring several fields of applied statistics, such as data smoothing, regression, multivariate analysis and classification, treatment of nondetections, time series analysis, and spatial point processes. It applies the methods discussed to contemporary astronomical research datasets using the R statistical software, making it invaluable for graduate students and researchers facing complex data analysis tasks. A link to the author's website for this book can be found at www.cambridge.org/msma. Material available on their website includes datasets, R code and errata.
This exciting tour of our Universe explores our current knowledge of exoplanets and the search for another Earth-like planet. Beginning with the basic concepts of planet formation and the composition of the Universe, Stuart Ross Taylor summarises our knowledge of exoplanets, how they compare with our planets and why some stars have better habitable zones. Further sections provide a detailed study of our Solar System, as a basis for understanding exoplanetary systems, and a detailed study of the Earth as our only current example of a habitable planet. The book concludes with a philosophical and historical discussion of topics surrounding planets and the development of life, including why our chances of finding aliens on exoplanets is very low. This is an engaging and informative read for anyone interested in planetary formation and the exploration of our Universe.