In Douglas Adams's sci-fi classic, *The Hitchhiker's Guide to the Galaxy*, Earth is a gigantic computer constructed to calculate the meaning of life. Following Adams's lead, a researcher has taken the entire universe as a computer--a fanciful, but fascinating assumption--and calculated just how much number crunching it may have done since the big bang.

The thought experiment is based on some real principles. Researchers developing quantum computers are devising ways to use individual atoms, electrons, or photons to store bits of information and perform calculations at the speed limits set by the laws of physics. Their efforts could someday lead to computers that perform huge numbers of calculations at once. That raises a question: Just how much computation can be squeezed out of a given chunk of matter?

In the past, Seth Lloyd, a quantum-mechanical engineer at the Massachusetts Institute of Technology, has employed the laws of quantum mechanics, relativity, and thermodynamics to estimate the ultimate capacity of a computer with a mass of a kilogram (it's big, really big). He even worked out the potential analytical talent of a black hole (*Science*Now, 30 August 2000). Now Lloyd has calculated the capacity of the entire universe, as he reports in a paper to be published in *Physical Review Letters*.

The speed at which the universe can do calculations depends on its density, and Lloyd finds that the universe can have performed about 10^{120} operations during the roughly 15 billion years since the big bang. As for disk space, the entropy of the universe determines how many bits it holds, and Lloyd figures the universe can store roughly 10^{120} bits of information. (So far, humans have performed about 10^{31} operations on about 10^{21} bits, he calculates.)

What to make of it all? Well, the numbers can be interpreted as the fewest operations and bits required to run a computer simulation that describes the universe in every detail. Another way to look at it is as the total computations that could possibly have been done by any sort of computer in the history of the universe. But most intriguingly, the universe might be a gigantic computer of sorts and the numbers might represent the calculations it has done so far. If the universe is a computer, it might be programmed by random quantum fluctuations, Lloyd says, and that might explain complex phenomenon such as life. "If the universe is doing a computation," he says, "then that might explain its complexity, because computers can do really interesting things with little prompting."

The universe's ability to compute may explain why complex phenomena are possible, says Charles Bennett, a physicist at IBM's T. J. Watson Research Center in Yorktown Heights, New York, but it doesn't explain why they are stable. Noise and random fluctuations disrupt such systems and initiate a competition between complex and simple states, Bennett says: "Why is the competition won by something that's complicated instead of something simple?"

**Related site**

The preprint of Lloyd's paper