A universe of infinite universes

According to a controversial notion known as the anthropic principle, certain otherwise baffling features of the universe can only be understood by including ourselves in the equation. The universe must be suitable for life, otherwise we would not be here to wonder about it.

The features in question are mysterious numbers in the equations of physics and cosmology, denoting, say, the amount of matter in the universe or the number of dimensions, which don’t seem predictable by any known theory — yet.

They are like the knobs on God’s control console, and they seem almost miraculously tuned to allow life. A slight tweak one way or another from the present settings could cause all stars to collapse into black holes or atoms to evaporate, negating the possibility of biology.

If there were only one universe, theorists would have their hands full trying to explain why it is such a lucky one. But supporters of the anthropic principle argue that there could be zillions of possible universes, many different possible settings ruled by chance.

Their view has been bolstered in recent years by a theory of the Big Bang, known as inflation. It implies our universe is only one bubble in an endless chain of them, and by string theory — the so-called theory of everything — whose equations seem to have an almost uncountable number of solutions, each representing a different possible universe.

Only a few of these will be conducive to life, the anthropic argument goes.

Scientists agree that the name ‘‘anthropic principle’’, is pretentious, but that’s all they agree on. Some regard the idea as more philosophy than science. Others regard it as a betrayal of the Einsteinian dream of predicting everything about the universe.

David Gross regards it as a virus. ‘‘Once you get the bug you can’t get rid of it,’’ he complained at the conference. Gross, director of the Kavli Institute for Theoretical Physics in Santa Barbara, California, had agreed to lead a panel discussion on the notorious principle.

Often found puffing on a cigar, he is not known to be shy about expressing his opinion. ‘‘I was chosen because I hate the anthropic principle,’’ he said.

But playing a central role in defending the need for what he called ‘‘anthropic reasoning’’ was Steven Weinberg, a Nobel laureate from the University of Texas.

Like Gross, Weinberg is a particle physicist. He is still famous for writing in his 1977 book, The First Three Minutes, ‘‘The more the universe seems comprehensible, the more it also seems pointless.’’

Weinberg is among the most prominent of theorists who have reluctantly accepted, at least provisionally, the anthropic principle as a kind of tragic necessity in order to explain the gnarliest knob of all.

Called the cosmological constant, it is a number that measures the amount of cosmic repulsion caused by the energy in empty space. That empty space should be boiling with such energy is predicted by quantum theory, and astronomers in the past few years have discovered that some cosmic repulsion seems to be accelerating the expansion of the universe.

But theoretical attempts to calculate this constant, also known as lambda, result in numbers 1,060 times as high as those astronomers have measured.

IN 1989, before any cosmological constant had been discovered, Weinberg used the anthropic principle to set limits on the value of the constant. Suppose instead of being fixed by theory, it was random from universe to universe.

In his paper, Weinberg argued that lambda in our universe could not be too big or the repulsive force would have prevented the formation of galaxies, stars and us. Since we are here, the constant should be small.

But when his own turn came, Gross questioned whether the rules of the anthropic game were precise enough. What were the parameters that could vary from universe to universe? How many could vary at once? What was the probability distribution of their values, and what was necessary for ‘‘life’’?

Ascribing the parameters of physics to mere chance or vagaries of cosmic weather is defeatist, discouraging people from undertaking the difficult calculations that would actually explain why things are they way they are.

It is also dangerous, Gross declared to ringing applause. ‘‘It smells of religion and intelligent design,’’ he said, referring to a variety of creationism that argues the universe is too complex to have evolved by chance.(New York Times)