I remember vividly the first time the illusion of immortality confronted me directly. I was in Paris to give a lecture and, having a free afternoon, took advantage of the moment to wander down through the 7th arrondissement to the grand 18th-century estate that now comprises the Rodin Museum.
Rodin has long been my favorite sculptor, not merely for the sensual beauty of his pieces but because he juxtaposes form and formlessness, having his magnificent characters arise directly out of rough, unhewn stone.
I was pondering precisely this emergence of form and formlessness in our universe while looking at the two lovers frozen in an eternal embrace in Rodin’s famous “The Kiss.” All at once, it struck me that the embrace was far from eternal.
The marble from which the piece was formed is a fairly young rock—at most, hundreds of millions of years old, but likely much younger, far younger than our 4.5-billion-year-old Earth. And how long will the sculpture last? Even barring human disasters that might lead to its destruction, Earth itself has a finite future. Two billion years from now, as the sun heats up, the surface of the Earth will resemble that of Venus, reaching perhaps 1,000 degrees. A few billion years after that, the sun will grow in size as it flames out, encompassing the Earth’s orbit.
Even the very atoms that make up the rock have a finite lifetime. Most of them emerged from a supernova explosion perhaps 5 billion years ago, which led to the formation of our solar system. And after the demise of Earth, many of them will again be strewn into the interstellar cosmos, with some impinging upon a nascent star to be recycled and reformed once again.
Indeed, our scientific knowledge tells us, in every sense of the word—and, in my opinion, thankfully—that immortality is an illusion, even for those things that seem eternal on a human timescale.
Several years ago, I read a beautiful 1979 article by the physicist Freeman Dyson about life in the far future of the universe, and it, too, struck a chord. I had recently speculated that the dominant energy in the universe resided in empty space, and this speculation was validated by a series of striking observations in 1998. Faced with this new picture of the universe, I began to think about what implications this “dark energy”—which is gravitationally repulsive and will cause the expansion of the universe to accelerate over time—would have for the future of life. Dyson had previously argued that eternal life was possible, not for an individual, but for a civilization of conscious beings, and I wondered if that was really true.
Dyson, being (forgive the pun) the eternal optimist, was thinking of possibilities, not practicalities. He didn’t worry about little challenges, like the death of our own sun and other stars, or about the fact that even protons and neutrons, the building blocks of atomic nuclei, may themselves ultimately decay. He imagined that as intelligent life gets starved of energy by depleting all of the local resources around it, it might transfer consciousness to a different physical system, like a computer or something similar that might use less than the 80 to 100 watts that a human being utilizes even in sleep. Anyone who has seen a “Star Trek” episode is familiar with this kind of scenario, where Kirk discovers a computer encoding all of the experiences of some civilization and then accidentally, or intentionally, destroys it.
After considerable effort, my colleague Glenn Starkman and I showed that even in an infinite universe that itself expands forever, any specific civilization has access to only a finite amount of energy. As the universe expands, it carries away the available energy too quickly to allow any one location to harvest an infinite amount.
Dyson was not daunted, however. He argued that even with access to only a finite amount of energy, eternal life for a civilization might be possible as long as the individual thought processors in that civilization ‘hibernate’ for ever longer intervals, waking sparingly to think and, then, managing a sleep far more sound than anything we experience, during which the complexity of their normal thought processes goes to zero.
Is such deep hibernation possible? And would such a life be worth living anyway? No one really knows. But it turns out that even if one allows such a possibility, the laws of physics may still make eternal life impossible.The culprit is quantum mechanics.
It seems hard to imagine that quantum mechanics, which governs the behavior of the universe on small atomic scales, would be relevant to determining the future of macroscopic, thinking objects, but it is.
The problem comes down, once again, to what it means to think. Our human brains use about 20 watts of power. (Truly amazing when you think about it: A computer with the processing power of the human brain and made using current architecture would utilize as much energy as all of humanity currently uses—about 20 terawatts. Somehow evolution did a more efficient job by at least a factor of a million million!)
It turns out, however, that it is possible to design a computer which requires no energy to do a computation. Such a reversible computer, as it is called, could do a computation over and over again for no energy cost whatsoever. However, the laws of physics tell us that to erase the computer registers and perform a new computation dissipates energy.Thus, after such an erasure, the computer is in a lower energy state.
Now, the laws of quantum mechanics tell us that energy can be lost only in finite amounts. If the spacing of energy levels in a finite-sized system remains finite and constant, then an infinite number of computations—an infinite number of thoughts, Dyson’s definition of what would comprise eternal life for a civilization—would require an infinite amount of energy. Only if the energy spacing between quantum levels in the system somehow decreased with time as computations continued would a civilization with access to only a finite amount of energy be able to go on thinking.
Dyson has referred to this as the “analog vs. digital” question. If the finite energy bits dissipated by living systems are significant (i.e., the energy is clearly dissipated in discrete lumps) on the scale of energy accessible to the system as a whole, then this system must lose consciousness eventually. On the other hand, if somehow the energy losses are kept forever smaller (i.e., the discrete lumps are so small as to seem effectively continuous, like an analog rather than a digital signal) than the energies required to keep the systems alive, then eternal life for a civilization becomes possible.
That is where the current debate lies. Starkman and I have argued that quantum mechanics puts an ultimate limit on eternal life for a civilization, and Dyson still hopes that somehow a system can be devised to beat these limits.
For those who find depressing the notion that the laws of physics preclude eternal life, there are two sources of solace. First, the limits we are talking about, when the universe becomes starved of energy, will not occur for a colossally long time, order and orders of magnitude longer than the current age of the universe.
Second, it turns out that even if any single civilization must die out, life in the universe need not end. For even if civilizations are not eternal, nothing stops the universe itself from continuing to expand forever, and in an infinite universe that lasts an infinitely long time, small statistical fluctuations will continue to occur, due both to thermal effects and to quantum mechanics. It is, therefore, possible that a very rare fluctuation will produce a living, thinking organism once again.This organism may thrive for a while and then die out. A long time in the future, another fluctuation will produce another thinking organism, and so on.
Indeed, infinity is a long time. In an infinite universe, statistical fluctuations will actually recreate you an infinite number of times, allowing you an infinite number of different lives. And if there are an infinite number of infinite universes, as some current ideas suggest, the number of times you will be replicated will be infinitely greater. In each universe, life may play out differently, for better or worse, so that while life might eventually end even in all these universes, in another universe, at least, the essay I am writing will not end here.