Can We Know the Universe?
The following excerpt was published in Broca's Brain (1979).
by Carl Sagan
"Nothing is rich but the inexhaustible wealth of nature. She shows us
surfaces, but she is a million fathoms deep." Ralph Waldo Emerson
cience is a way of
thinking much more than it is a body of knowledge. Its goal is to find out how the
world works, to seek what regularities there may be, to penetrate the connections
of thingsfrom subnuclear particles, which may be the constituents of all
matter, to living organisms, the human social community, and thence to the cosmos
as a whole. Our intuition is by no means an infallible guide. Our perceptions may
be distorted by training and prejudice or merely because of the limitations of our
sense organs, which, of course, perceive directly but a small fraction of the
phenomena of the world. Even so straightforward a question as whether in the
absence of friction a pound of lead falls faster than a gram of fluff was answered
incorrectly by Aristotle and almost everyone else before the time of Galileo.
Science is based on experiment, on a willingness to challenge old dogma, on an
openness to see the universe as it really is. Accordingly, science sometimes
requires courageat the very least the courage to question the conventional
Beyond this the main trick of science is to really think
of something: the shape of clouds and their occasional sharp bottom edges at the
same altitude everywhere in the sky; the formation of the dewdrop on a leaf; the
origin of a name or a wordShakespeare, say, or "philanthropic"; the reason
for human social customsthe incest taboo, for example; how it is that a lens
in sunlight can make paper burn; how a "walking stick" got to look so much like a
twig; why the Moon seems to follow us as we walk; what prevents us from digging a
hole down to the center of the Earth; what the definition is of "down" on a
spherical Earth; how it is possible for the body to convert yesterday's lunch into
today's muscle and sinew; or how far is updoes the universe go on forever, or
if it does not, is there any meaning to the question of what lies on the other side?
Some of these questions are pretty easy. Others, especially the last, are mysteries
to which no one even today knows the answer. They are natural questions to ask.
Every culture has posed such questions in one way or another. Almost always the
proposed answers are in the nature of "Just So Stories," attempted explanations
divorced from experiment, or even from careful comparative observations.
But the scientific cast of mind examines the world critically as
if many alternative worlds might exist, as if other things might be here which are
not. Then we are forced to ask why what we see is present and not something else.
Why are the Sun and the Moon and the planets spheres? Why not pyramids, or cubes,
or dodecahedra? Why not irregular, jumbly shapes? Why so symmetrical worlds? If you
spend any time spinning hypotheses, checking to see whether they make sense, whether
they conform to what else we know, thinking of tests you can pose to substantiate
or deflate your hypotheses, you will find yourself doing science. And as you come
to practice this habit of thought more and more you will get better and better at
it. To penetrate into the heart of the thingeven a little thing, a blade of
grass, as Walt Whitman saidis to experience a kind of exhilaration that, it
may be, only human beings of all the beings on this planet can feel. We are an
intelligent species and the use of our intelligence quite properly gives us
pleasure. In this respect the brain is like a muscle. When we think well, we feel
good. Understanding is a kind of ecstasy.
But to what extent can we really know the universe around
us? Sometimes this question is posed by people who hope the answer will be in the
negative, who are fearful of a universe in which everything might one day be known.
And sometimes we hear pronouncements from scientists who confidently state that
everything worth knowing will soon be knownor even is already knownand
who paint pictures of a Dionysian or Polynesian age in which the zest for
intellectual discovery has withered, to be replaced by a kind of subdued languor,
the lotus eaters drinking fermented coconut milk or some other mild hallucinogen.
In addition to maligning both the Polynesians, who were intrepid explorers (and
whose brief respite in paradise is now sadly ending), as well as the inducements
to intellectual discovery provided by some hallucinogens, this contention turns
out to be trivially mistaken.
Let us approach a much more modest question: not whether we can
know the universe or the Milky Way Galaxy or a star or a world. Can we know,
ultimately and in detail, a grain of salt? Consider one microgram of table salt, a
speck just barely large enough for someone with keen eyesight to make out without
a microscope. In that grain of salt there are about 1016 sodium and chlorine atoms. That is a 1 followed by 16
zeros, 10 million billion atoms. If we wish to know a grain of salt we must know at
least the three-dimensional positions of each of these atoms. (In fact, there is
much more to be knownfor example, the nature of the forces between the
atomsbut we are making only a modest calculation.) Now, is this number more
or less than a number of things which the brain can know?
How much can the brain know? There are perhaps
1011 neurons in the brain, the
circuit elements and switches that are responsible in their electrical and chemical
activity for the functioning of our minds. A typical brain neuron has perhaps a
thousand little wires, called dendrites, which connect it with its fellows. If, as
seems likely, every bit of information in the brain corresponds to one of these
connections, the total number of things knowable by the brain is no more than
1014, one hundred trillion. But this
number is only one percent of the number of atoms in our speck of salt.
So in this sense the universe is intractable, astonishingly
immune to any human attempt at full knowledge. We cannot on this level understand
a grain of salt, much less the universe.
But let us look a little more deeply at our microgram of salt.
Salt happens to be a crystal in which, except for defects in the structure of the
crystal lattice, the position of every sodium and chlorine atom is predetermined.
If we could shrink ourselves into this crystalline world, we would rank upon rank
of atoms in an ordered array, a regularly alternating structuresodium,
chlorine, sodium, chlorine, specifying the sheet of atoms we are standing on and
all the sheets above us and below us. An absolutely pure crystal of salt could
have the position of every atom specified by something like 10 bits of information.
This would not strain the information-carrying capacity of the brain.
If the universe had natural laws that governed its behavior to
the same degree of regularity that determines a crystal of salt, then, of course,
the universe would be knowable. Even if there were many such laws, each of
considerable complexity, human beings might have the capability to understand
them all. Even if such knowledge exceeded the information-carrying capacity of the
brain, we might store the additional information outside our bodiesin books,
for example, or in computer memoriesand still, in some sense, know the
Human beings are, understandably, highly motivated to find
regularities, natural laws. The search for rules, the only possible way to
understand such a vast and complex universe, is called science. The universe
forces those who live in it to understand it. Those creatures who find everyday
experience a muddled jumble of events with no predictability, no regularity, are
in grave peril. The universe belongs to those who, at least to some degree, have
figured it out.
It is an astonishing fact there are laws of nature,
rules that summarize convenientlynot just qualitatively but
quantitativelyhow the world works. We might imagine a universe in which
there are no such laws, in which the 1080 elementary particles that make up a universe like
our own behave with utter and uncompromising abandon. To understand such a
universe we would need a brain at least as massive as the universe. It seems
unlikely that such a universe could have life and intelligence, because beings
and brains require some degree of internal stability and order. But even if in
a much more random universe there were such beings with an intelligence much
greater than our own, there could not be much knowledge, passion or joy.
Fortunately for us, we live in a universe that has at least
important parts that are knowable. Our common-sense experience and our
evolutionary history have prepared us to understand something of the workaday
world. When we go into other realms, however, common sense and ordinary
intuition turn out to be highly unreliable guides. It is stunning that as we go
close to the speed of light our mass increases indefinitely, we shrink towards
zero thickness in the direction of motion, and time for us comes as near to
stopping as we would like. Many people think that this is silly, and every week
or two I get a letter from someone who complains to me about it. But it is a
virtually certain consequence not just of experiment but also of Albert
Einstein's brilliant analysis of space and time called the Special Theory of
Relativity. It does not matter that these effects seem unreasonable to us. We
are not in the habit of traveling close to the speed of light. The testimony
of our common sense is suspect at high velocities.
Or consider an isolated molecule composed of two atoms
shaped something like a dumbbella molecule of salt, it might be. Such a
molecule rotates about an axis through the line connecting the two atoms. But
in the world of quantum mechanics, the realm of the very small, not all
orientations of our dumbbell molecule are possible. It might be that the
molecule could be oriented in a horizontal position, say, or in a vertical
position, but not at many angles in between. Some rotational positions are
forbidden. Forbidden by what? By the laws of nature. The universe is built in
such a way as to limit, or quantise, rotation. We do not experience this
directly in everyday life; we would find it startling as well as awkward in
sitting-up exercises, to find arms out stretched from the sides or pointed up
to the skies permitted but many intermediate positions forbidden. We do not
live in the world of the small, on the scale of 10-13 centimeters, in the realm where there are twelve
zeros between the decimal place and the one. Our common-sense intuitions do
not count. What does count is experimentin this case observations from
the far infrared spectra of molecules. They show molecular rotation to be
The idea that the world places restrictions on what humans
might do is frustrating. Why shouldn't we be able to have intermediate
rotational positions? Why can't we travel faster than the speed of
light? But so far as we can tell, this is the way the universe is constructed.
Such prohibitions not only press us toward a little humility; they also make
the world more knowable. Every restriction corresponds to a law of nature, a
regulation of the universe. The more restrictions there are on what matter and
energy can do, the more knowledge human beings can attain. Whether in some
sense the universe is ultimately knowable depends not only on how many natural
laws there are that encompass widely divergent phenomena, but also on whether
we have the openness and the intellectual capacity to understand such laws.
Our formulations of the regularities of nature are surely dependent on how the
brain is built, but also, and to a significant degree, on how the universe is
For myself, I like a universe that includes much that is
unknown and, at the same time, much that is knowable. A universe in which
everything is known would be static and dull, as boring as the heaven of some
weak-minded theologians. A universe that is unknowable is no fit place for a
thinking being. The ideal universe for us is one very much like the universe
we inhabit. And I would guess that this is not really much of a coincidence.
( Carl Sagan, "Can We Know the Universe?: Reflections on a Grain
of Salt;" from Broca's
Brain: Reflections on the Romance of Science, New York: Random House,
1979, pp. 13-18. )
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