The Big Picture

Home > Other > The Big Picture > Page 41
The Big Picture Page 41

by Carroll, Sean M.


  07

  is not because physics is so hard— it’s because we understand so much of it

  08

  that there’s a lot to learn, and that’s because it’s fundamentally pretty

  09

  simple.

  10

  Our goal is to offer a plausibility sketch that the world can ultimately be

  11

  understood on the basis of naturalism. We don’t know how life began, or

  12

  how consciousness works, but we can argue that there’s little or no reason

  13

  to look beyond the natural world for the right explanations. We can always

  14

  be wrong in that belief; but then again, we can always be wrong about any

  15

  belief.

  16

  Asking that our understanding of human life be compatible with what

  17

  we know about the underlying physics places some interesting constraints

  18

  on what life is and how it operates. Knowing the particles and forces of

  19

  which we are made allows us to conclude with very high confidence that

  20

  individual lives are finite in scope; our best cosmological theories, while

  21

  much less certain than the Core Theory, suggest that “life” as a broader

  22

  concept is also finite. The universe seems likely to reach a state of thermal

  23

  equilibrium. At that point it won’t be possible for anything living to sur-

  24

  vive; life relies on increasing entropy, and in equilibrium there’s no more

  25

  entropy left to generate.

  26

  Those swirls in the cream mixing into the coffee? That’s us. Ephemeral

  27

  patterns of complexity, riding a wave of increasing entropy from simple be-

  28

  ginnings to a simple end. We should enjoy the ride.

  29

  30

  31

  32

  33

  34

  35S

  36N

  236

  Big Picture - UK final proofs.indd 236

  20/07/2016 10:02:48

  01

  02

  29

  03

  04

  Light and Life

  05

  06

  07

  08

  09

  10

  11

  12

  I

  13

  talian astronomer Giovanni Schiaparelli will go down in history as the

  14

  discoverer of the “canals on Mars.” In 1887, after observing our plane-

  15

  tary neighbor through his telescope, Schiaparelli reported that its sur-

  16

  face was crisscrossed with long, straight lines he labeled canali. The idea

  17

  captured the imagination of people around the world, including American

  18

  astronomer Percival Lowell, who oversaw the construction of a new obser-

  19

  vatory in Arizona and performed countless observations of Mars. Based on

  20

  what Lowell thought he saw— a system of interlocking oases connected by

  21

  the canals, which seemed to change with the passage of time— he developed

  22

  elaborate ideas about life on the Red Planet, featuring an advanced civiliza-

  23

  tion struggling to survive in an environment with precious little water. He

  24

  popularized this idea in a series of books that became very influential, help-

  25

  ing to inspire H. G. Wells’s The War of the Worlds.

  26

  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

  28

  there were any canals there. The Italian word “canali” should have been

  29

  translated into English as “channels,” not “canals.” Channels occur natu-

  30

  rally, while canals are artificially constructed. The second problem is that

  31

  Schiaparelli didn’t observe any channels either. The features he described

  32

  were artifacts of the difficulty involved in observing a faraway planet with

  33

  relatively primitive instruments.

  34

  Today, we have examined Mars quite closely, including with a number

  S35

  of orbiters and landers sent by the United States, the Soviet Union, Europe,

  N36

  237

  Big Picture - UK final proofs.indd 237

  20/07/2016 10:02:48

  T H E B IG PIC T U R E

  01

  and India. (As of this writing, Mars is the only known planet to be inhab-

  02

  ited solely by robots.) We haven’t found any decaying cities or ancient archi-

  03

  tectural landmarks, but the search for life continues. Perhaps not in the

  04

  form of Lowell’s dying civilization or Wells’s malevolent tripods, but there

  05

  is certainly a chance of eventually finding microscopic life-forms elsewhere

  06

  in the solar system— if not on Mars, then possibly in the oceans of Jupiter’s

  07

  moon Europa (which has more liquid water than all the oceans on Earth),

  08

  or on Saturn’s moons Enceladus and Titan.

  09

  The question is, will we know it when we see it? What is “life” anyway?

  10

  Nobody knows. There is not a single agreed- upon definition that clearly

  11

  separates things that are “alive” from those that are not. People have tried.

  12

  NASA, which is heavily invested in looking for life outside the Earth,

  13

  adopted a working definition of a living organism: a self- sustaining chemi-

  14

  cal system capable of Darwinian evolution.

  15

  We could quibble with the bit about “Darwinian evolution.” That’s a

  16

  feature of how living organisms here on Earth have in fact come to be, but

  17

  not a characterization of what each organism is. When you come across an

  18

  injured squirrel and ask, “Is it alive?” nobody answers, “I don’t know, let’s

  19

  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

  21

  someday construct an artificial life-form. By this criterion, such a beast

  22

  would automatically be judged nonliving without further thought, which

  23

  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.

  26

  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-

  34

  cific origin of life here on Earth; from there we can once again expand our

  35S

  view, to see how living creatures evolve and interact with one another.

  36N

  238

  Big Picture - UK final proofs.indd 238

  20/07/2016 10:02:48

  l Ig h t A n d l I F E

  •

  01

  02

  One of the many suggested definitions of life was put forward by none

  03

  other than Erwin Schrödinger, who helped formulate the fundamental

  04

  principles of quantum mechanics. In his book What Is Life? , Schrödinger

  05

  examined the question from a physicist’s point of view. The fundamental

  06

  problem, as he saw it, was one of balance. On the one hand, living things

  07

  are constantly changing and moving. Whether it’s a cheetah chasing after a

  08

  gazelle, or sap moving slowly through the branches of a redwood tree, some-

  09

  thing is always happening inside living organisms. On the other hand, liv-

  10

  ing things also maintain their structure; throughout their changes they

  11

  preserve some basic integrity. What kind of physical process, he wondered,

  12

  could manage to consistently straddle the line between stasis and change?

  13

  This question prompted Schrödinger to put forward a definition of life

  14

  that seems very different from NASA’s:

  15

  16

  When is a piece of matter said to be alive? When it goes on

  17

  “doing something,” exchanging material with its environment,

  18

  and so forth, and that for a much longer period than we would

  19

  expect an inanimate piece of matter to “keep going” under sim-

  20

  ilar circumstances.

  21

  22

  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

  24

  self- sustaining: waterfalls, oceans, and for that matter the inanimate rock

  25

  on which William Paley stubbed his toe.

  26

  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

  28

  presuming to offer a once- and- for- all definition of a precise concept; he’s

  29

  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

  S35

  N36

  239

  Big Picture - UK final proofs.indd 239

  20/07/2016 10:02:48

  T H E B IG PIC T U R E

  01

  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.

  04

  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.

  35S

  Free energy is energy in a potentially useful form. The high- entropy

  36N

  2 40

  Big Picture - UK final proofs.indd 240

  20/07/2016 10:02:48

  l Ig h t A n d l I F E

  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.

  05

  06

  07

  08

  Free

  Free

  09

  Energy

  Energy

  10

  11

  12

  13

  14

  15

  16

  17

  18

  19

  Disordered

  Disordered

  20

  Energy

  Energy

  21

  22

  23

  24

  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

  27

  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.

  33

  I’m asking a little more of you than Schrödinger was willing to ask of his

  34

  readers.)

  S35

  N36

  2 4 1

  Big Picture - UK final proofs.indd 241

  20/07/2016 10:02:48

  T H E B IG PIC T U R E

  01

  •

  02

  03

  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

 

‹ Prev