The Anthropic Principle

Einstein famously said "I want to know God's thoughts; the rest are details." And what he meant was that he wanted to learn how the universe was put together. He wanted to discover the physical laws that bring us the reality we observe around us. Building on the earlier work of Newton and Maxwell, and others who had come before him, Einstein succeeded in his life's quest by bringing us both the special and the general theories of relativity, which explain motion and gravity in a far deeper and more general way than had been earlier understood. Einstein's equation of general relativity is concise and precise and it captures much about nature using a parsimony of notation. Einstein's work is therefore considered the paradigm for "elegance" in physics and cosmology today. The goal of modern physics has been to come up with a more comprehensive understanding of nature, following Einstein's equation as an example, to result in a theory that would explain everything about the universe--from the very small, where quantum mechanics rules, to the very large, the realm of general relativity.

But despite many great achievements, including the Standard Model of particle physics, a theory that explains the behavior of elementary particles and whose latest great success is the recent discovery of the Higgs boson, modern physics and cosmology are surprisingly moving away from Einstein's dream. Let me explain this troubling aspect of modern science.

The Standard Model, which combines quantum mechanics with Einstein's special (but not general) theory of relativity, is one of the most important theories in science today. It explains many aspects of particle theory, including the interactions among elementary particles. For example, the theory shows how quarks are held together inside protons and neutrons, which make up the nuclei of all matter in the universe. The theory also explains how matter interacts with light; how the electron is bound to the nucleus; and how certain particles can decay into others, for instance through radioactive decay. And now, with the Higgs discovery--the crowning glory of the Standard Model--we also know how mass came about in the early universe. But the theory has deep "holes" inside it: It has too many parameters that need to be added into the formulas--they are not dictated, or predicted, by the theory itself, as Einstein and his modern followers would have hoped.

For example, neither the Standard Model nor any other theory in physics can explain why the proton is exactly 1836.1526 times heavier than the electron. No one has any idea why this is so. Physicists' underlying reasoning has been that an "elegant," Einstein-like theory should be powerful enough to provide us with a theoretical reason for why this must be the case. We understand that this ratio is an important parameter and that without its having this exact value perhaps atoms of matter could not exist. But, hard as we try, we cannot explain this number!

And there are many such fundamental constants of nature that resist all attempts at a theoretical derivation and which no theory has ever been able to shed light on. They remain deep puzzles--conundrums that physicists break their heads trying to solve. One of the most interesting stories in the history of physics is about the long-standing quest to explain the value of another parameter, the so-called fine structure constant. The fine structure constant is a unit-less number discovered in 1916 in relation to the spectral lines of atoms ("fine structure" thus refers to the structure of lines in a spectrum). This number governs the strength of all electromagnetic interactions and it is equal to 1/137.035999. Note that it is very close to 1/137, which is the inverse of a prime number--and hence it has titillated many to try to explain it. Richard Feynman described it as: "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'." The great quantum pioneer Wolfgang Pauli spent a lifetime trying to understand it, and was so obsessed with this quest that he discussed his dreams about it with the famous psychologist Carl Jung (both lived in Zurich at the time). The eminent British scientist Arthur Eddington built numerological--almost mystical--theories about the fine structure constant (which he wrongly thought was 1/136) and why nature might have chosen it. All for naught.

In the face of such daunting challenges to force theories to predict the values of numerical constants, many physicists and cosmologists have taken a different tack. Alan Guth of MIT, who in the early 1980s came up with the theory of cosmic inflation to explain aspects of the aftermath of the Big Bang, gave a talk on November 1 of this year in which he repeated the argument that many physicists consider the only one to explain the values of many of nature's key numbers. "We can only observe a universe in which the constants of nature support our existence," Guth said, adding: "This is called the selection effect--also known as the anthropic principle."

The idea, which seems to have been first proposed by the cosmologist Robert Dicke in 1961, is as follows. We can only live in a part of the universe that can support life. This means that all the parameters of nature we observe around us have to be what they are, or else we wouldn't be here to ask this question. It now seem, in the twenty-first century, that in some cases we cannot go beyond this kind of explanation. Of course this is disappointing to many people: We would like to know why things are the way they are. After all, this is what science is all about. But for lack of a better explanation for the values of certain constants we end up conceding that something has to be a certain way, because it can't be otherwise if we are here to observe it.

The anthropic principle has had some successes. In 1998, a few months before the dramatic announcement of the astronomical discovery that the universe is accelerating its expansion and hence that the vacuum of space must be permeated with "dark energy" pushing the universe to expand faster, Steven Weinberg and colleagues at the University of Texas published a paper in which they predicted that if dark energy does exist, its numerical strength would have to fall within some bounds. The idea was that if the energy was too high, our universe would have expanded too fast for galaxies to form, and hence we wouldn't be here. Equally, if the dark energy was too weak, the universe would have re-collapsed on itself because there wouldn't be enough of a force to push it outwards. In this case, too, we wouldn't be here to observe the universe. Since we are here, the numerical value of the dark energy (also called the "cosmological constant") has to fall within acceptable bounds for a stable universe that supports curious beings asking questions. The prediction of the bounds on the dark energy came from no actual theory, but simply through anthropic reasoning!

So the anthropic principle seems to work, despite its being inelegant. And with the new assumption in cosmology that we may live in a multiverse: a (possibly infinite) collection of many universes, the anthropic principle sits well here: It can tell us that we must live in that one, out of the many universes out there, that can support life. And since our universe does, its parameters must be just what they are. Einstein would not have liked it. But ultimately--or at least until we have better theories--maybe this is the best we can do.