【诺奖得主Wilczek科普专栏】延迟宇宙热寂 | 蔻享学术
■ 作者：FrankWilczek
Frank Wilczek
The laws of physics say that in the distant future, all change and activity in the cosmos will come to an end. Can that fate be postponed?
One very hot day this summer, during a morning swim, my mind wandered to a different version of climate change: the “heat death” of the universe. Ironically, though it remains a plausible outcome for cosmic history, it’s a less distressing subject than our own warming planet.
The idea of heat death arose with the scientific understanding of heat itself in the 19th century. The core idea is simple: Physical systems tend to settle toward equilibrium. For example, heat will tend to flow from a hot body into an adjacent cold body, cooling the former and warming the latter, until both reach the same intermediate temperature, after which heat is no longer exchanged.
Ultimate equilibrium and stasis can be postponed by the injection of energy, but only temporarily. Engines can be refueled, animals fed, batteries recharged; but engines wear down, animals die, and batteries lose their juice.
These common observations are generalized and sharpened in the science of thermodynamics. The capstone of thermodynamics is its so-called Second Law, first formulated mathematically by Rudolf Clausius in 1865, which states that entropy, a measure of disorder, increases over time—distinctive structure erodes. Featureless equilibrium is the state of maximum entropy, toward which the Second Law drives us.
The inexorable logic of the Second Law leads, in the long run, to a bland universe wherein nothing changes-that is, heat death. Modern physical cosmology has fleshed out that conclusion. Gravity wants matter to clump, but in the early universe, matter’s distribution was extremely uniform, so gravity was way out of equilibrium. Over time, gravity has sought to come into equilibrium, notably by condensing stars out of gas clouds. The high density and pressure found inside stars ignites nuclear fuel.
Nuclear burning injects heat and powers a dynamic universe. But this is a temporary reprieve. After a few tensof billions of years, stars everywhere will have exhausted their fuel and winked out.
There are several ways that our distant descendants, or other embodiments of mind in the universe, might resist heat death. Here are some ideas that occurred to me as I swam:
First, it is probably possible to burn matter further than stars do. Stars rearrange protons and neutrons but do not change their overall number. Burning those particles would give access to hundreds of times more energy. Another (barely) conceivable form of fuel is “dark matter.” At present, nobody knows what it is, but there’s lots of it in the universe.
Second, future engineers also might be able to arrange controlled collisions of planets or dead stars, to tap into the energy the crashes liberate.
Third, future minds themselves might be able to run on very limited power. Recent theoretical work on reversible and quantum computers, and on time crystals, has shown that there’s no lower limit to how little energy they need to keep making progress, or at least to keep moving.
Fourth, since we don’t really understand what triggered the Big Bang, it’s conceivable that someday we’ll be able to engineer something similar, and thereby rejuvenate the universe.
It was a good swim. I had fun saving the universe by inventing speculative technological fixes and adaptations, spiced up with wishful thinking. The long-term future of mind in the universe is desperate, but not serious.
Unfortunately, when I emerged from the lake, it was even hotter than before. Here and now on Earth the situation is dead serious—though maybe not yet utterly desperate.
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