The Big Bang

THE CHINESE TELL OF A TIME during the Hsia dynasty (ca. 2205—ca. 1782 BC) when our cosmic environment suddenly changed. Ten suns appeared in the sky. The people on earth suffered greatly from the heat, so the emperor ordered a famous archer to shoot down the extra suns.

The archer was rewarded with a pill that had the power to make him immortal, but his wife stole it. For that offense she was banished to the moon.

The Chinese were right to think that a solar system with ten suns is not friendly to human life.

Today we know that, while perhaps offering great tanning opportunities, any solar system with multiple suns would probably never allow life to develop. The reasons are not quite as simple as the searing heat imagined in the Chinese legend.

In fact, a planet could experience a pleasant temperature while orbiting multiple stars, at least for a while. But uniform heating over long periods of time, a situation that seems necessary for life, would be unlikely. To understand why, let’s look at what happens in the simplest type of multiple-star system, one with two suns, which is called a binary system.

About half of all stars in the sky are members of such systems. But even simple binary systems can maintain only certain kinds of stable orbits, of the type shown below.

In each of these orbits there would likely be a time in which the planet would be either too hot or too cold to sustain life. The situation is even worse for clusters having many stars.

Our solar system has other “lucky” properties without which sophisticated life-forms might never have evolved. For example, Newton’s laws allow for planetary orbits to be either circles or ellipses (ellipses are squashed circles, wider along one axis and narrower along another).

The degree to which an ellipse is squashed is described by what is called its eccentricity, a number between zero and one. An eccentricity near zero means the figure resembles a circle, whereas an eccentricity near one means it is very flattened. Kepler was upset by the idea that planets don’t move in perfect circles, but the earth’s orbit has an eccentricity of only about 2 percent, which means it is nearly circular.

As it turns out, that is a stroke of very good fortune.

Seasonal weather patterns on earth are determined mainly by the tilt of the earth’s axis of rotation relative to the plane of its orbit around the sun.

During winter in the Northern Hemisphere, for example, the North Pole is tilted away from the sun. The fact that the earth is closest to the sun at that time—only 91.5 million miles away, as opposed to around 94.5 million miles away from the sun in early July—has a negligible effect on the temperature compared with the effect of its tilt.

But on planets with a large orbital eccentricity, the varying distance from the sun plays a much larger role. On Mercury, for example, with a 20 percent eccentricity, the temperature is over 200 degrees Fahrenheit warmer at the planet’s closest approach to the sun (perihelion) than when it is at its farthest from the sun (aphelion).

In fact, if the eccentricity of the earth’s orbit were near one, our oceans would boil when we reached our nearest point to the sun, and freeze over when we reached our farthest, making neither winter nor summer vacations very pleasant. Large orbital eccentricities are not conducive to life, so we are fortunate to have a planet for which orbital eccentricity is near zero.

We are also lucky in the relationship of our sun’s mass to our distance from it.

That is because a star’s mass determines the amount of energy it gives off. The largest stars have a mass about a hundred times that of our sun, while the smallest are about a hundred times less massive.

And yet, assuming the earth-sun distance as a given, if our sun were just 20 percent less or more massive, the earth would be colder than present-day Mars or hotter than present-day Venus.

Traditionally, given any star, scientists define the habitable zone as the narrow region around the star in which temperatures are such that liquid water can exist. The habitable zone is sometimes called the “Goldilocks zone,” because the requirement that liquid water exist means that, like Goldilocks, the development of intelligent life requires that planetary temperatures be “just right.”

The habitable zone in our solar system, pictured above, is tiny. Fortunately for those of us who are intelligent life-forms, the earth fell within it!

Newton believed that our strangely habitable solar system did not “arise out of chaos by the mere laws of nature.” Instead, he maintained, the order in the universe was “created by God at first and conserved by him to this Day in the same state and condition.” It is easy to understand why one might think that.

The many improbable occurrences that conspired to enable our existence, and our world’s human-friendly design, would indeed be puzzling if ours were the only solar system in the universe.

But in 1992 came the first confirmed observation of a planet orbiting a star other than our sun. We now know of hundreds of such planets, and few doubt that there exist countless others among the many billions of stars in our universe.

That makes the coincidences of our planetary conditions—the single sun, the lucky combination of earth-sun distance and solar mass—far less remarkable, and far less compelling as evidence that the earth was carefully designed just to please us human beings. Planets of all sorts exist.

Some—or at least one—support life. Obviously, when the beings on a planet that supports life examine the world around them, they are bound to find that their environment satisfies the conditions they require to exist.

It is possible to turn that last statement into a scientific principle: Our very existence imposes rules determining from where and at what time it is possible for us to observe the universe. That is, the fact of our being restricts the characteristics of the kind of environment in which we find ourselves.

That principle is called the weak anthropic principle. (We’ll see shortly why the adjective “weak” is attached.) A better term than “anthropic principle” would have been “selection principle,” because the principle refers to how our own knowledge of our existence imposes rules that select, out of all the possible environments, only those environments with the characteristics that allow life.

Though it may sound like philosophy, the weak anthropic principle can be used to make scientific predictions. For example, how old is the universe?

As we’ll soon see, for us to exist the universe must contain elements such as carbon, which are produced by cooking lighter elements inside stars. The carbon must then be scattered through space in a supernova explosion, and eventually condense as part of a planet in a new-generation solar system.

In 1961, physicist Robert Dicke argued that the process takes about 10 billion years, so our being here means that the universe must be at least that old.

On the other hand, the universe cannot be much older than 10 billion years, since in the far future all the fuel for stars will have been used up, and we require hot stars for our sustenance. Hence the universe must be about 10 billion years old. That is not an extremely precise prediction, but it is true—according to current data the big bang occurred about 13.7 billion years ago.

As was the case with the age of the universe, anthropic predictions usually produce a range of values for a given physical parameter rather than pinpointing it precisely.

That’s because our existence, while it might not require a particular value of some physical parameter, often is dependent on such parameters not varying too far from where we actually find them.

We furthermore expect that the actual conditions in our world are typical within the anthropically allowed range.

For example, if only modest orbital eccentricities, say between zero and 0.5, will allow life, then an eccentricity of 0.1 should not surprise us because among all the planets in the universe, a fair percentage probably have orbits with eccentricities that small.

But if it turned out that the earth moved in a near-perfect circle, with eccentricity, say, of 0.00000000001, that would make the earth a very special planet indeed, and motivate us to try to explain why we find ourselves living in such an anomalous home. That idea is sometimes called the principle of mediocrity.

The lucky coincidences pertaining to the shape of planetary orbits, the mass of the sun, and so on are called environmental because they arise from the serendipity of our surroundings and not from a fluke in the fundamental laws of nature.

The age of the universe is also an environmental factor, since there are an earlier and a later time in the history of the universe, but we must live in this era because it is the only era conducive to life.

Environmental coincidences are easy to understand because ours is only one cosmic habitat among many that exist in the universe, and we obviously must exist in a habitat that supports life.

The weak anthropic principle is not very controversial. But there is a stronger form that we will argue for here, although it is regarded with disdain among some physicists.

The strong anthropic principle suggests that the fact that we exist imposes constraints not just on our environment but on the possible form and content of the laws of nature themselves.

The idea arose because it is not only the peculiar characteristics of our solar system that seem oddly conducive to the development of human life but also the characteristics of our entire universe, and that is much more difficult to explain.