In the early 1960s, Princeton physicist Robert Dicke invoked the anthropic principle to explain the age of the universe. He argued that this age must be compatible with the evolution of life, and, for that matter, with sentient, conscious beings who wonder about the age of the universe. In a universe that is too young for life to have evolved, there are no such beings. Over the decades, this argument has been extended to other parameters of the universe we observe around us, and thus to questions such as: Why is the mass of the electron 1,836.153 times smaller than that of the proton? Why are the electric charges of the up and down quarks exactly 2/3 and -1/3, respectively, on a scale in which the electron's charge is -1? Why is Newton's gravitational constant, G, equal to 6.67384 x 10^-11? And, the question that has deeply puzzled so many physicists for a century (since its discovery in 1916): Why is the fine structure constant, which measures the strength of electromagnetic interactions, so tantalizingly close to 1/137 ---the inverse of a prime number? (We now know it to far greater accuracy: about 1/137.035999.) Richard Feynman wrote: "It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the 'hand of God' wrote that number, and 'we don't know how he pushed his pencil'" (QED: The Strange Theory of Light and Matter, page 131, Princeton, 1985). The great British astronomer Arthur Eddington (who in 1919 proved Einstein's claim that spacetime curves around massive objects by making observations of starlight grazing the Sun during a total solar eclipse) built entire numerological theories around this number; and there is even a joke that the Austrian physicist and quantum pioneer Wolfgang Pauli, who throughout his life was equally obsessed with the number 137, asked God about it when he died (in fact: in a hospital room number 137) and went up to heaven; God handed him a thick packet and said: "Read my preprint, I explain it all here." But if constants of nature are simply what they are, nothing more can be said about them, right? Well, our viewpoint may suddenly change if a startling new finding should be confirmed through independent research by other scientists. Recently, astrophysicist John Webb of the University of New South Wales in Sydney, Australia, and colleagues published new findings that indicate that the fine structure constant may not be a constant after all---it may vary through space or time. Through observations of galaxies that lie 12 billion light-years roughly to the north with those at the same distance lying to the south, the team discovered variations in the fine structure constant amounting to about 1 part in 100,000. It is not clear whether quantum effects would drastically change when a fundamental constant such as the fine structure constant varies by such minute amounts. But if they do, and the change in the constant is significant, it could mean that there are universes---or distant parts of our own universe---where matter as we know it, and hence life, could not exist. Such a conclusion would greatly amplify the weight of the anthropic principle as a powerful argument for why we observe and measure the physical parameters we do. It is important to note that there is still skepticism about the finding, expressed for example in this post from Sean Carroll last year. But the possibility that this result is real cannot be discounted. The American physicist and Nobel Laureate Steven Weinberg has always been ahead of his time. In 1998, just a few months before the announcement of the stunning astronomical discovery that the universe is accelerating its expansion---leading to the conclusion that "dark energy" permeates space---which is being rewarded by this year's Nobel Prize in physics, Weinberg and colleagues at the University of Texas published a paper about the "dark energy." (This paper was an extension of Weinberg's 1987 paper on the anthropic principle and the cosmological constant.) They argued that if "dark energy" (the energy of the vacuum, also called the "cosmological constant," as first proposed by Einstein) exists, its value must fall within a very narrow range: otherwise the energy would be too high for galaxies to coalesce through the gravitational force, or too low, in which case the gravitational force among all matter would win out---leading to a gravitational collapse before galaxies and life had time to evolve. If we are here to observe the universe we do, the "dark energy," this cosmological constant that counters gravity, must have a very finely tuned value (as would also have to happen with Newton's constant, the masses and charges of the quarks and the electron, the fine structure constant, the parameters governing the strong and weak nuclear forces, and so on). Thus Weinberg and his colleagues used the anthropic principle to actually predict a discovery that would be made a few months later. Some time ago, I interviewed Weinberg about his work and the anthropic principle, which led him to this striking prediction. "The universe," he told me, "could well be like a giant Schrödinger's Cat. There are parts of the universe where the cat is alive, where the cosmological constant is just the right level and there are scientists there observing it and asking questions. And there are parts of the universe where the cat is dead---where the cosmological constant is too small or too large and therefore there is no life and no scientists asking questions about the universe." To me, this image of the giant cat is the best analogy for a universe governed by the powerful, and yet perhaps disturbing, idea of the anthropic principle.
Amir D. Aczel is a researcher at the Center for the Philosophy and History of Science at Boston University and the author of 18 books about mathematics and physics, as well as numerous research articles. He is a Guggenheim Fellow and a frequent commentator on science in the media. See more at his website or follow him on Twitter: @adaczel
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