The Sciences

Number Crunching the Cosmos

By Kathy A SvitilSep 1, 2002 5:00 AM


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How big is the universe? nor-mally you would expect scientists to give an answer expressed in light-years, or maybe even in miles. Seth Lloyd, a quantum mechanical engineer at the Massachusetts Institute of Technology, proposes a completely different way to take the measure of the cosmos: how much data it generates. Every physical event in cosmic history represents a piece of information, he says. So rather than thinking of the universe as just a collection of stars and galaxies, he considers it as a giant computer and tries to determine the amount of data that has passed through it since the beginning of time.

"The idea that the universe processes information has been around for decades," Lloyd says. The challenge lies in defining and quantifying this notion. In a conventional computer, binary digits flip from 0 to 1, or vice versa, to perform computations. In the universal analog, each flip in the "computation" occurs when subatomic particles interact and change their quantum energy states, the most fundamental unit of physical change. The cosmos contains roughly 1080 particles, which represent 10^90 bits of information. By factoring in the amount of available energy and the speed of light, Lloyd estimates the universe could have performed at most 10^120 logical operations since the Big Bang.

Illustration by Fred Rix

"If we wanted to simulate the universe on a computer, this is how powerful it would need to be," he says. But such a computer would have to be as powerful as the universe itself. In reality, we can only make crude approximations, because all the computers that ever existed on Earth have carried out just 10^31 operations.

So what is the function of the universe computer? Well, it isn't creating spreadsheets or searching the Web (although our PCs exist within the universe, so by definition those tasks are a part of what the universe does). Mostly, the cosmic computations involve the movements and interactions of "force" particles, such as photons and gravitons, with the building blocks of matter— electrons, quarks, and the like— whose output is their physical behavior. "The universe is computing its own dynamical evolution— including, among other things, us," Lloyd says.

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