The Big Picture
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is not because physics is so hard— it’s because we understand so much of it
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that there’s a lot to learn, and that’s because it’s fundamentally pretty
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simple.
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Our goal is to offer a plausibility sketch that the world can ultimately be
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understood on the basis of naturalism. We don’t know how life began, or
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how consciousness works, but we can argue that there’s little or no reason
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to look beyond the natural world for the right explanations. We can always
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be wrong in that belief; but then again, we can always be wrong about any
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belief.
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Asking that our understanding of human life be compatible with what
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we know about the underlying physics places some interesting constraints
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on what life is and how it operates. Knowing the particles and forces of
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which we are made allows us to conclude with very high confidence that
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individual lives are finite in scope; our best cosmological theories, while
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much less certain than the Core Theory, suggest that “life” as a broader
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concept is also finite. The universe seems likely to reach a state of thermal
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equilibrium. At that point it won’t be possible for anything living to sur-
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vive; life relies on increasing entropy, and in equilibrium there’s no more
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entropy left to generate.
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Those swirls in the cream mixing into the coffee? That’s us. Ephemeral
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patterns of complexity, riding a wave of increasing entropy from simple be-
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ginnings to a simple end. We should enjoy the ride.
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Light and Life
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I
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talian astronomer Giovanni Schiaparelli will go down in history as the
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discoverer of the “canals on Mars.” In 1887, after observing our plane-
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tary neighbor through his telescope, Schiaparelli reported that its sur-
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face was crisscrossed with long, straight lines he labeled canali. The idea
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captured the imagination of people around the world, including American
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astronomer Percival Lowell, who oversaw the construction of a new obser-
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vatory in Arizona and performed countless observations of Mars. Based on
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what Lowell thought he saw— a system of interlocking oases connected by
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the canals, which seemed to change with the passage of time— he developed
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elaborate ideas about life on the Red Planet, featuring an advanced civiliza-
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tion struggling to survive in an environment with precious little water. He
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popularized this idea in a series of books that became very influential, help-
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ing to inspire H. G. Wells’s The War of the Worlds.
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There were two problems. The first was that Schiaparelli, although he
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was also interested in the possibility of life on Mars, had never claimed that
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there were any canals there. The Italian word “canali” should have been
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translated into English as “channels,” not “canals.” Channels occur natu-
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rally, while canals are artificially constructed. The second problem is that
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Schiaparelli didn’t observe any channels either. The features he described
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were artifacts of the difficulty involved in observing a faraway planet with
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relatively primitive instruments.
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Today, we have examined Mars quite closely, including with a number
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and India. (As of this writing, Mars is the only known planet to be inhab-
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ited solely by robots.) We haven’t found any decaying cities or ancient archi-
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tectural landmarks, but the search for life continues. Perhaps not in the
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form of Lowell’s dying civilization or Wells’s malevolent tripods, but there
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is certainly a chance of eventually finding microscopic life-forms elsewhere
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in the solar system— if not on Mars, then possibly in the oceans of Jupiter’s
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moon Europa (which has more liquid water than all the oceans on Earth),
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or on Saturn’s moons Enceladus and Titan.
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The question is, will we know it when we see it? What is “life” anyway?
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Nobody knows. There is not a single agreed- upon definition that clearly
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separates things that are “alive” from those that are not. People have tried.
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NASA, which is heavily invested in looking for life outside the Earth,
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adopted a working definition of a living organism: a self- sustaining chemi-
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cal system capable of Darwinian evolution.
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We could quibble with the bit about “Darwinian evolution.” That’s a
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feature of how living organisms here on Earth have in fact come to be, but
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not a characterization of what each organism is. When you come across an
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injured squirrel and ask, “Is it alive?” nobody answers, “I don’t know, let’s
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see if it’s capable of Darwinian evolution.” The usefulness of a definition is
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that it should help us decide difficult cases, such as when scientists might
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someday construct an artificial life-form. By this criterion, such a beast
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would automatically be judged nonliving without further thought, which
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isn’t especially helpful. For our present purposes, this is indeed quibbling;
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when we talk about the actual life we know and love, evolution plays a cen-
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tral role.
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The “correct” definition of life, one that we’re going to discover through
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careful research, doesn’t exist. The life- forms with which we are familiar
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share a number of properties, each of which is interesting and many of
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which are remarkable. Life as we know it moves (internally if not exter-
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nally), metabolizes, interacts, reproduces, and evolves, all in hierarchical,
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interconnected ways. It’s obviously a uniquely important part of the big
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picture.
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We can start with general principles, working our way toward the spe-
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cific origin of life here on Earth; from there we can once again expand our
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One of the many suggested definitions of life was put forward by none
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other than Erwin Schrödinger, who helped formulate the fundamental
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principles of quantum mechanics. In his book What Is Life? , Schrödinger
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examined the question from a physicist’s point of view. The fundamental
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problem, as he saw it, was one of balance. On the one hand, living things
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are constantly changing and moving. Whether it’s a cheetah chasing after a
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gazelle, or sap moving slowly through the branches of a redwood tree, some-
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thing is always happening inside living organisms. On the other hand, liv-
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ing things also maintain their structure; throughout their changes they
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preserve some basic integrity. What kind of physical process, he wondered,
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could manage to consistently straddle the line between stasis and change?
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This question prompted Schrödinger to put forward a definition of life
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that seems very different from NASA’s:
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When is a piece of matter said to be alive? When it goes on
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“doing something,” exchanging material with its environment,
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and so forth, and that for a much longer period than we would
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expect an inanimate piece of matter to “keep going” under sim-
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ilar circumstances.
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Schrödinger is focusing on the “ self- sustaining” part of the NASA def-
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inition, which most of us just breeze over. After all, many things seem to be
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self- sustaining: waterfalls, oceans, and for that matter the inanimate rock
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on which William Paley stubbed his toe.
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The crucial idea here is that a living being “keeps going” for “a much
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longer period than we would expect.” That’s a bit vague; Schrödinger isn’t
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presuming to offer a once- and- for- all definition of a precise concept; he’s
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trying to capture something of our intuition about what life is. A rock
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might maintain its shape for a long time, but it will never repair itself. A
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rock can be in motion, for example, if an avalanche starts it rolling down-
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hill; but once it gets to the bottom, it will stop moving and just sit there. It
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won’t brush itself off and climb back up the hill, like an animal might.
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This is another way in which living organisms seem to— but don’t
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actually— violate the second law of thermodynamics. Not only do they
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come into being as organized structures; they then are able to maintain that
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order over long periods of time.
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As with the formation of complexity in the first place, the truth is the
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converse of our most naïve expectation. Complex structures can form, not
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despite the growth of entropy but because entropy is growing. Living organ-
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isms can maintain their structural integrity, not despite the second law but
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because of it.
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Everyone knows that the sun provides a useful service to life here on Earth:
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energy, in the form of photons of visible light. But the really important
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thing we get from the sun is energy with very low entropy— so-called free
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energy. That energy is then put to use by biological organisms, and returned
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to the universe in a highly degraded form. “Free energy” is a confusing term
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that actually means “useful energy”— think “free” as in “free to do some-
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thing.” It has nothing to do with “energy for free”— the total amount of
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energy is still constant.
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The second law says that the entropy of an isolated system will increase
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until the system reaches maximum entropy, after which it will sit there in
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equilibrium. In an isolated system, the total amount of energy remains
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fixed, but the form that energy takes goes from being low- entropy to being
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higher- entropy. Think of burning a candle. If we kept track of all the light
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and heat generated by the candle, the total amount of energy would stay the
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same over time. But the candle can’t burn forever; it goes for a while and
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then stops. The energy locked inside has been transformed from a low-
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entropy form to a high- entropy form, and there’s no going back.
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Free energy can be used to do what physicists call work. If we take some
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macroscopic object and move it around, we are doing work on it. The defi-
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nition of “work” is simply the force we exerted to get the thing going, times
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the distance over which it moved. It requires work to lift a stone from the
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bottom of a hill up to the top. Essentially everything useful that you can do
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with energy is some kind of work, whether it’s getting a rocket into orbit or
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gently lifting your eyebrow to indicate skepticism.
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Free energy is energy in a potentially useful form. The high- entropy
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remainder is the “disordered energy,” equal to the temperature of the sys-
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tem times its entropy. The flow of heat from one system to another increases
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the amount of useless disordered energy. Indeed, one way of formulating
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the second law is to say that, in an isolated system, free energy is converted
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into disordered energy as time passes.
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Free
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Energy
Energy
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Disordered
Disordered
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Energy
Energy
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Another way of thinking about the second law of thermodynamics.
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Over time, energy is converted from “free” (available to do work) to
“disordered” (dissipated, useless).
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Schrödinger’s idea was that biological systems manage to keep moving
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and maintaining their basic integrity by taking advantage of free energy in
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their environments. They take in free energy, use it to do whatever work
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they need it to do, then return the energy to the world in a more disordered
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form. (In the first edition of his book he went to great lengths not to use the
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phrase “free energy,” because he thought the concept would be confusing.
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I’m asking a little more of you than Schrödinger was willing to ask of his
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readers.)
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Whether a certain amount of energy is “free” or “disordered” depends on
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its environment. If we have a piston full of hot gas, we can use it to do work
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by letting it expand and push the piston. But that’s assuming that the piston
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isn’t surrounded by gas of equal temperature and density; if it is, there’s no
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net force on the piston, and we can’t do any work with it.
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The light we get from the sun is low- entropy relative to its environment,
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and therefore contains free energy, available to do work. The environment
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is just the rest of the sky, dotted with starlight and suffused with the cosmic
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microwave background radiation, at a few degrees above absolute zero. A
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typical photon emitted by the sun has 10,000 times the energy of a typical
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photon in the microwave background.
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Imagine there were no sun. The entire sky would look like the night sky
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does now. Here on Earth, we would quickly equilibrate, and come to the
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same cold temperature as the night sky. There would be no free energy; life