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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
27
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
20
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;
24
when we talk about the actual life we know and love, evolution plays a cen-
25
tral role.
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The “correct” definition of life, one that we’re going to discover through
27
careful research, doesn’t exist. The life- forms with which we are familiar
28
share a number of properties, each of which is interesting and many of
29
which are remarkable. Life as we know it moves (internally if not exter-
30
nally), metabolizes, interacts, reproduces, and evolves, all in hierarchical,
31
interconnected ways. It’s obviously a uniquely important part of the big
32
picture.
33
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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•
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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-
20
ilar circumstances.
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Schrödinger is focusing on the “ self- sustaining” part of the NASA def-
23
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
27
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
30
might maintain its shape for a long time, but it will never repair itself. A
31
rock can be in motion, for example, if an avalanche starts it rolling down-
32
hill; but once it gets to the bottom, it will stop moving and just sit there. It
33
won’t brush itself off and climb back up the hill, like an animal might.
34
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
02
come into being as organized structures; they then are able to maintain that
03
order over long periods of time.
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As with the formation of complexity in the first place, the truth is the
05
converse of our most naïve expectation. Complex structures can form, not
06
despite the growth of entropy but because entropy is growing. Living organ-
07
isms can maintain their structural integrity, not despite the second law but
08
because of it.
09
•
10
11
Everyone knows that the sun provides a useful service to life here on Earth:
12
energy, in the form of photons of visible light. But the really important
13
thing we get from the sun is energy with very low entropy— so-called free
14
energy. That energy is then put to use by biological organisms, and returned
15
to the universe in a highly degraded form. “Free energy” is a confusing term
16
that actually means “useful energy”— think “free” as in “free to do some-
17
thing.” It has nothing to do with “energy for free”— the total amount of
18
energy is still constant.
19
The second law says that the entropy of an isolated system will increase
20
until the system reaches maximum entropy, after which it will sit there in
21
equilibrium. In an isolated system, the total amount of energy remains
22
fixed, but the form that energy takes goes from being low- entropy to being
23
higher- entropy. Think of burning a candle. If we kept track of all the light
24
and heat generated by the candle, the total amount of energy would stay the
25
same over time. But the candle can’t burn forever; it goes for a while and
26
then stops. The energy locked inside has been transformed from a low-
27
entropy form to a high- entropy form, and there’s no going back.
28
Free energy can be used to do what physicists call work. If we take some
29
macroscopic object and move it around, we are doing work on it. The defi-
30
nition of “work” is simply the force we exerted to get the thing going, times
31
the distance over which it moved. It requires work to lift a stone from the
32
bottom of a hill up to the top. Essentially everything useful that you can do
33
with energy is some kind of work, whether it’s getting a rocket into orbit or
34
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-
01
tem times its entropy. The flow of heat from one system to another increases
02
the amount of useless disordered energy. Indeed, one way of formulating
03
the second law is to say that, in an isolated system, free energy is converted
> 04
into disordered energy as time passes.
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Free
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.
25
Over time, energy is converted from “free” (available to do work) to
“disordered” (dissipated, useless).
26
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Schrödinger’s idea was that biological systems manage to keep moving
28
and maintaining their basic integrity by taking advantage of free energy in
29
their environments. They take in free energy, use it to do whatever work
30
they need it to do, then return the energy to the world in a more disordered
31
form. (In the first edition of his book he went to great lengths not to use the
32
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
34
readers.)
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•
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Whether a certain amount of energy is “free” or “disordered” depends on
04
its environment. If we have a piston full of hot gas, we can use it to do work
05
by letting it expand and push the piston. But that’s assuming that the piston
06
isn’t surrounded by gas of equal temperature and density; if it is, there’s no
07
net force on the piston, and we can’t do any work with it.
08
The light we get from the sun is low- entropy relative to its environment,
09
and therefore contains free energy, available to do work. The environment
10
is just the rest of the sky, dotted with starlight and suffused with the cosmic
11
microwave background radiation, at a few degrees above absolute zero. A
12
typical photon emitted by the sun has 10,000 times the energy of a typical
13
photon in the microwave background.
14
Imagine there were no sun. The entire sky would look like the night sky
15
does now. Here on Earth, we would quickly equilibrate, and come to the
16
same cold temperature as the night sky. There would be no free energy; life
The Big Picture Page 41