THE ultimate future of life may seem a strange thing for physicists to be debating, but the behaviour of the universe can tell us about more than just time and space, galaxies and supernovae. Our future in the cosmos is not just a matter of chance circumstances or our own ingenuity. It is circumscribed by the most fundamental laws of nature, written into the fabric of the universe during the big bang.
The first serious analysis of our likely fate was carried out by the Princeton physicist Freeman Dyson. In 1979, Dyson published a study on the thermodynamics of life, using physics to argue that life could last forever in the universe. His argument is really about life that has consciousness: life that thinks. At some level, thought must be like computation, he says, involving the physical processing of information. Any practical computation depletes its energy source. Dyson assumed that the universe doesn鈥檛 hold infinite sources of energy, so any life would eventually face an energy crisis. But he argued that an organism can stretch out a dwindling energy supply by slowing its metabolism, which is equivalent to operating at a lower temperature, and slowing down the rate at which it performs computations. In other words, it can save energy by thinking more sluggishly.
But even cooling down 鈥� and thus slowing down 鈥� can鈥檛 eke out a finite energy resource forever. That鈥檚 because there鈥檚 a limit on how cold an organism can get.
Advertisement
Every computation produces heat that has to be radiated away. In order to do this, the organism must remain warmer than its environment, because heat can only flow from a hot object to a cold one. The cosmological data of Dyson鈥檚 time indicated that the temperature of the cosmos was dropping more quickly than would the operating temperature of any organism trying to keep thinking. So far, so good.
Death by overheating
However, Dyson also realised that, because radiating away heat relies in general on the properties of electrons, quantum mechanics dictates a fundamental limit to how fast the heat can be dissipated. If the organism produces heat faster than its electrons can dissipate it, it is doomed to death by overheating.
The answer, said Dyson, is to 鈥渉ibernate鈥�. In Dyson鈥檚 definition, a hibernating organism essentially stops its metabolism entirely, which means it must stop thinking. Yet it continues to radiate away accumulated waste heat. He showed that a judicious combination of periods of ever-slower activity and spells of hibernation make it possible to perform an infinite number of computations with a finite amount of energy: the organism can go on thinking forever. So, the outlook for life 鈥� albeit a strange, sluggish and cold kind of life 鈥� is good. Or so we thought.
What Dyson didn鈥檛 know was that bad news was on the way. Recent observations of distant supernovae have shown that the expansion of the universe, which has been going on since the big bang, is speeding up (快猫短视频, 11 April 1998, p 26). That鈥檚 a problem, as it means our future energy sources may be slipping out of our grasp. In an expanding universe, the further away an object is from us, the faster it is heading into the distance. At a particular distance, known as the de Sitter horizon, objects are receding at the speed of light. Anything beyond the de Sitter horizon is forever out of reach. So, because of cosmic acceleration, distant parts of the universe will eventually reach the point of no return. Every galaxy beyond our Local Group 鈥� which gravity keeps bound together 鈥� is moving inexorably towards our de Sitter horizon. Once they pass it, their light can never reach us.
That means the galaxies will wink out one by one. The space beyond our galactic neighbourhood will begin to appear cold and dark, and we鈥檒l no longer be able to find out anything about it (see 鈥淪hrinking horizons鈥�). That鈥檚 not just a problem for astronomers of the year two trillion. It means that when a galaxy disappears over the de Sitter horizon, we lose a potential source of energy.
The same is true everywhere. All life, wherever it is in the universe, faces a dwindling energy supply. As Dyson showed, it seems that life can survive such a setback by slowing down its metabolism and hibernating periodically. But the de Sitter horizon does more than create an energy crisis. In the wrong circumstances, it can set a limit to how cold the universe can get. And if you are a creature trying to operate at ever-lower temperatures, this is very bad news indeed.
The problem stems from a suggestion first made by Stephen Hawking. Whenever there is a horizon beyond which one cannot see or travel 鈥� be it a black hole鈥檚 event horizon or a de Sitter horizon 鈥� this boundary emits a small but significant amount of radiation. The radiation comes from quantum fluctuations that occur in the vacuum of space. These fluctuations continually create pairs of particles and antiparticles. Ordinarily, these particles annihilate one another immediately. But if a pair pops into being close to a de Sitter horizon, one particle can wander over the horizon and became irrevocably separated from its partner before they can cancel each other out. And so one half of the pair is left, adding a contribution to the heat energy in the accessible universe on our side of the de Sitter horizon. This means that space in an accelerating universe can never get cooler than a particular temperature, known as the Hawking temperature.
Nowhere to go
It鈥檚 hardly a balmy glow: working from what the supernova data reveals, it鈥檒l be something of the order of 10鈭�29 kelvin. Nevertheless, as we lower our operating temperature to slow down our metabolism, we are eventually going to hit a point where we reach the same temperature as our environment. Then we won鈥檛 be able to radiate away waste heat. In other words, although we might be extremely cold, we鈥檒l still fry the moment we try to think.
Cosmologists John Barrow and Frank Tipler were the first to point out this disastrous scenario in their book The Anthropic Cosmological Principle. And then, in 2000, Lawrence Krauss and Glenn Starkman of Case Western Reserve University in Cleveland, Ohio, published a much more detailed analysis. Their conclusion was stark: given Dyson鈥檚 scenario, nothing 鈥� not mining the universe for its energy resources, not even hibernation 鈥� could preserve life forever in an accelerating universe. Hibernation will not work, they say, because to lower its energy, an organism must be able to descend onto a lower rung of the ladder of quantum energy states. But, say Krauss and Starkman, the rungs cannot go on forever. Eventually an organism must reach its quantum ground state, and then it has nowhere to go. Life is doomed.
No one is panicking yet. The possible extinction of life in the universe is not going to be a pressing issue for at least 1040 years. 鈥淭he timescale in question is immense,鈥� says Krauss. 鈥淚t鈥檚 nothing to sell your stocks about.鈥�