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.

At present there is a popular consensus in favor of intelligent life elsewhere in the universe. These notions are at least 2500 years old and rest on several assumptions, that have run as a common thread throughout history. Many lie outside the realm of scientific enquiry coming under the heading of “must be” arguments. The first is that the universe is infinite and so must contain planets identical to the Earth. This is often phrased as the “Big Numbers” argument. There are so many stars with planets that somewhere out there must be a replica of us. The second is that because life exists here, it must be common elsewhere. The third is that the development of intelligence is inevitable and happens elsewhere concurrently with, or more commonly in advance of, the evolution of life on Earth. These themes are addressed below under several headings.

THE PLURALITY OF WORLDS [2]

The discovery of many planets orbiting other stars, free-floating objects and the widespread occurrence of dusty circum-stellar disks, some with gaps in which planets are lurking, has raised once again in a dramatic fashion, the ancient question posed amongst others by Albertus Magnus in the thirteenth century: “since one of the most wondrous and noble questions in Nature is whether there is one world or many, a question that the human mind desires to understand, it seems desirable for us to enquire about it” [3]. One of the favorite current quotations of astrobiologists comes from Metrodorus of Chios (350 BCE). It is usually stated as “it is unnatural in a large field to have only one ear of corn and in the infinite universe, only one living world”. These questions have been discussed under many headings for the past 25 centuries since Democritus, Epicurus and Metrodorus favored a multitude of worlds.

AN EXPANDING VIEW

In order to obtain some perspective on the place of planets in the scheme of things, it is useful to contemplate the scale of the universe, as we perceive it at present. Everything in the universe is very isolated. The nearest star to us is Proxima Centauri, a red dwarf and the faintest member of a triple star system of which Alpha Centauri is the brightest. This star is familiar to dwellers in the southern hemisphere, as it forms one of the Pointers to the Southern Cross. Light from this nearest star takes 4.2 years to reach us. Although Proxima Centauri is the nearest star at present, the dwarf star Ross 248 will succeed to the title in about 33,000 Earth years. Because of the slow relative movements of the stars, our familiar constellations, such as Orion the Hunter and his companion, the Great Dog, will be rearranged and replaced by other groupings in the future. Edmund Halley (1656–1742), of comet fame, seems to have been one of the first to realize this, by observing that the positions of many stars in the early eighteenth century differed from those recorded in the catalogue of Hipparchus in the second century BCE.

We live in remarkable times replete with technical advances, a consequence of the great intellectual advances of the seventeenth and eighteenth centuries in Europe. Destiny or Chance in 1998 looked at the solar system to examine the question whether our planets were likely to be reproduced elsewhere. From the evidence then available, this was judged to be very unlikely, while the possibility of intelligent life resembling Homo sapiens [1] elsewhere was assessed to be zero. In the succeeding dozen years, major improvements in technology have resulted in the discovery of thousands of exoplanets. Has the situation changed? Yes, in the sense that it has gotten worse. Not only are the exoplanets “Strange New Worlds” as a popular book title has it, but our familiar solar system itself, with its tidy circular orbits, appears to be a rarity. The very architecture of the solar system, familiar to every schoolchild, appears to have arisen through chance collisions and migrations half a millennium after it formed.

Destiny or Chance was written following a close look at our solar system. The numerous planets, satellites, TNOs, asteroids, centaurs and other assorted debris that surround our Sun provided no evidence of design. The resulting array, strange enough when looked at objectively, was clearly the result of a series of chance events. Halfway through writing Destiny or Chance, the first exoplanets were discovered. These “Hot Jupiters” were totally unexpected by astronomers, although less surprising to students of the solar system. Lurking in the background is the expectation that something like the Earth, complete with its set of interesting inhabitants, might be discovered.