The Universe Might Last Forever, Astronomers Say, but Life Might Not

	Barney Taxel for The New York Times Dr. Freeman Dyson, above, and Dr. Lawrence M. Krauss and Dr. Glenn D. Starkman, top, have investigated the future.

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Infinity on Trial

Although Dr. Dyson agrees with this gloomy view of life in an accelerating universe, he and Dr. Krauss and Dr. Starkman are still arguing about whether life is also doomed in a universe that is not accelerating, but just expanding and getting slower and colder.

Quantum theory, the Case Western authors point out, limits how finely the energy for new thoughts can be shaved. The theory decrees that energy is emitted and absorbed in tiny indivisible lumps called "quanta." Any computation must spend at least this much energy out of a limited supply. Each new thought is a step down an energy ladder with a finite number of steps. "So you can only have a finite number of thoughts," said Dr. Krauss.

"If you want to stare at your navel and not think any new thoughts, you won't dissipate energy, " he explained. But that would be a boring way to spend eternity. If life is to involve more than the eternal reshuffling of the same data, he and Dr. Starkman say, it cannot be eternal.

Dr. Dyson, however, says this argument applies only to so-called digital life, in which there is a fixed number of quantum states. Creatures like the black cloud, which could grow along with the universe, he said, would have an increasing number of quantum states, and so there would always be more rungs of the ladder to step down. So the bottom need never be reached and life and thought could go on indefinitely.

But nobody knows whether such a life form can exist, said Dr. Krauss.

Compared with the sight of the World Trade Center towers collapsing or the plight of a sick child, this future extinction may seem a remote concern. Dr. Allan Sandage, an astronomer at Carnegie Observatories in Pasadena, Calif., who has spent his life investigating the expansion and fate of the universe, said: "Life on this earth is going to vanish in 4.5 billion years. I wouldn't get hung up on the fact that the lights are all going out in 30 billion years."

Dr. Dyson said he was still an optimist. It is too soon to start panicking, he counseled in an e-mail message. The observations could be wrong.

"At present all possibilities are open," he wrote. "The recent observations are important, not because they answer the big questions about the history of the universe, but because they give us new tools with which to explore the history."

Even in an accelerating universe, Dr. Dyson said, humans or their descendants might one day be able to rearrange the galaxies and save more of them from disappearing. Another glimmer of hope comes from the deadly and chilling Hawking radiation itself, said Dr. Raphael Bousso, from the Institute of Theoretical Physics at the University of California at Santa Barbara. Since that radiation is produced by unpredictable quantum fluctuations, he pointed out, if you wait long enough anything can appear in it, even a new universe. "Sooner or later one of those quantum fluctuations will look like a Big Bang," he said.

In that case there is the possibility of a future, if not for us, at least for something or somebody. In the fullness of time, after all, physics teaches that the improbable and even the seemingly impossible can become the inevitable. Nature is not done with us yet, nor, as Dr. Dyson indicates, are we necessarily done with nature.

We all die, and it is up to us to decide who and what to love, but, as Dr. Weinberg pointed out in a recent article in The New York Review of Books, there is a certain nobility in that prospect.

"Though aware that there is nothing in the universe that suggests any purpose for humanity," he wrote, "one way that we can find a purpose is to study the universe by the methods of science, without consoling ourselves with fairy tales about its future, or about our own."