Gribbin: The Birth of Time
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John Gribbin, The Birth of Time : How Astronomers Measured the Age of the Universe. New Haven : Yale University Press, 1999. 237 pages; 8 pages of black-and-white plates; with "Further Reading" and index.
It's a good question: "How do we know how big—or how old, since they go together—the universe is?" Answering it with precision and care, as Gribbin does in this book, turns out to be a fascinating story that takes in a big chunk of scientific history, the development of quite a few explosive ideas, and not a few colorful characters. Gribbin tells the story very well, with a good pace and at a level that will keep most readers engaged and satisfied.
The very idea that the universe might even have an age, that there might have been a beginning, started to seep into the scientific consciousness in the middle of the nineteenth century. Indeed, Darwin's ideas about evolution by means of natural selection were held hostage for some time by mistaken ideas about how old our Sun could—or could not—be. That question didn't get answered correctly until well into the twentieth century after Einstein discovered the equivalency of mass and energy, and the discovery of fusion and the subsequent realization that fusion powered the stars.
And so understanding our Sun was at the core of the mystery, at least at the start. Nowadays most people realize that the Sun fuses prodigious amounts of hydrogen fuel and that it can burn at its present rate for billions of years. Still, I was fascinated by this brief description that establishes some sense of what those numbers all mean, taken together.
Inside the Sun, just one proton in every hundred million is travelling fast enough to do that trick [of approaching near enough another proton to fuse]. The quantum calculations show that on average it would take an individual proton fourteen billion years to find a partner able to join it in forming a deuteron through a head-on collision. Some will take longer than average, some will find partners more quickly. The Sun is only about 4.5 billion years old, which is why most of its protons have yet to find partners in this way (in any case, only protons in the core of the Sun have any hope of taking part in the p-p chain[, the principle fusion process in the Sun]; in the cooler outer layers of the Sun, nuclear fusion cannot occur at all.) Overall, just one collision in every ten billion trillion (1 in 1022) initiated the p-p chain. But there are so many protons inside the Sun, and so many collisions, that even at this incredibly slow rate, and even though only 0.7 percent of the mass of each set of four protons is released as energy whenever a nucleus of helium-4 is formed, about 5 million tonnes of mass are converted into pure energy every second at the heart of the sun. In round numbers (to the nearest hundred million tonnes), 600 million tonnes of hydrogen are converted into 595 million tonnes of helium every second in the heart of the Sun, with the other 5 million tonnes or so being converted into pure energy. And even at this rate, so far the Sun has processed only about 4 percent of its original stock of hydrogen [...] into helium, even though the p-p chain has been operating for 4.5 billion years. [pp. 47—48]
The idea that the universe actually had a beginning really came to the fore after astronomers Hubble and Humason published a paper with measurements they had been able to make of the red-shift of distant galaxies. It may seem obvious to say it, but before science could work on the question of the age of the universe, the idea that it had a beginning had to take definite form.
But what did it all mean? If the galaxies are moving apart today, the implication is that they were closer together in the past. Go back far enough into the past, and they must have been on top of one another—there must have been, in some sense, a beginning to the expanding Universe. If the expansion has been going on at the same rate all the time, it is easy to calculate, from the constant of proportionality in the redshift-distance relation, how long it has been since all the galaxies in the visible Universe were squashed together in one lump. Using a value of about 525 kilometres per second per Megaparsec for the constant (which became known as Hubble's Constant, and is now denoted by the letter H, although Hubble himself used K), this "age of the Universe" comes out as about two billion years.
By the 1930s, as we have seen, radioactive dating techniques had already established that the Earth and the Solar system must be much older than this. Either those measurements were wrong (unlikely), or the Universe had not always been expanding at the same rate (quite plausible...), or there was something wrong with Hubble's interpretation of Humason's redshift measurements. But the important point is that such questions were being asked in the early 1930s, not that there were yet any definite answers. Before the discovery of the redshift-distance relation, Hubble's Law, nobody seriously considered the possibility that the universe itself had been born at a definite moment in time and had a measurable age. After the 1931 paper by Hubble and Humason, the idea of measuring the age of the Universe became part of the scientific investigation of the world. [pp. 134—135]
After that idea took shape, scientists could move on to improving measurements, improving understanding, and resolving apparent discrepancies between different methods for measuring the size and age of the universe. To Gribbin's credit he does not try to over simplify or talk around measurements and theories that don't match up at first (or, sometimes, ever), results whose meaning and importance take time to establish and appreciate. This is how science advances and the author navigates the process honestly but without losing sight of his ultimate goal.
A later chapter in the book is given over to the author's very worthwhile discussion of his own contribution to research on the question and he elucidates this philosophy of revealing the process.
Although my own contribution to the "age of the Universe debate" was by no means definitive, and represents just one minor brick in the scientific edifice, I shall go into some detail about the work I was involved in during the mid-1990s for two reasons. First, it is a chance to describe from the inside a piece of scientific research as it was carried out, false starts, blind alleys, and all. Too often, popular accounts of the scientific endeavour give a distorted version of the truth, in which the march of science seems to be just that—an inexorable forward progression. But that is seldom even close to giving a true vision of what goes on at the cutting edge of research. [pp. 198—199]
This was an exciting book that brought together quite a few ideas and many scientific results in an interesting and scientifically honest narrative that actually answered the question posed in the book's subtitle. What a rare accomplishment!
-- Notes by JNS