#### The Shortcomings of Intuition

The more physicists discover about the inner workings
of the universe, the more difficult it is for people to intuitively
understand their discoveries. Most discoveries of the twentieth
century have run exactly counter to what people believe in their
hearts about the world around them. The examples are too numerous
to mention, but one of the best-known intuition-foiling insight
of the twentieth century is Einstein’s special theory of relativity.
Einstein showed, through a series of complicated mathematical proofs,
that only the speed of light is constant, and that all other motion
is relative. What one person perceives to be happening in the world
around him might differ dramatically from what another person perceives—and it’s
likely that both people are perfectly correct. Special relativity, Greene
warns, is “not in our bones.” The idea that different observers
experience space and time differently is not one that we experience
in our daily lives. We cannot *feel* special relativity.
We must just study the equations, read the explanations, and take
scientists’ word for it.

Einstein’s revision of Newton’s particle model of light
is another phenomenon that seems, at an intuitive level, too strange
to apply to the real world. Einstein and others proved that light
has both wavelike *and* particle-like properties,
a duality that ran counter to scientists’ long-standing beliefs
that light must have one property or the other. Louis de Broglie
later showed that all matter shares this dual nature. With macroscopic
objects, the wave nature is almost impossible to detect, and the
duality only becomes important on small-distance scales. Here we
encounter another counterintuitive set of laws. From a perspective
of day-to-day observation, nothing makes less sense than particles’
behavior on an ultramicroscopic level.

Quantum physics is even harder to grasp viscerally. Refining
a suggestion made by Erwin Schrödinger, Max Born was the first to argue
that an electron wave can only be described from a standpoint of
probability. We can never know precisely what will come next in the
cosmos, just what is likeliest to happen. Schrödinger worked from
de Broglie’s conception of dual-nature matter to develop an equation
that guides the shape of probability waves (or *wave functions*).
Einstein himself—the same man who trampled centuries of human intuition
about the speed of light, motion, and gravity—was appalled by this
seemingly random and senseless hypothesis. He said, famously, “God
does not play dice with the universe.”

Interestingly, after detailing instance after instance
of intuitive lapses, Greene offers an intuition-based explanation
of string theory’s development and necessity. He says that string
theory caught on because physicists have an intuitive objection
to the idea that the universe operates according to two contradictory
sets of laws. They embrace string theory because it offers the possibility
of one coherent Theory of Everything—and this appeals to their intuitive
desire for one system.

#### Theory of Everything

It is possible, physicists believe, to uncover a single
theory that explains every single phenomenon in the universe seamlessly,
with no contradictions or “cosmological constants” or other evidence
of scientists’ incomplete understanding. Einstein spent the last
thirty years of his life seeking a unified field theory that could
explain all four forces—the strong force, the weak force, electromagnetism, and
gravity—and all of matter within a single theoretical framework.
String theorists hope to realize Einstein’s disappointed ambition.
They hope to uncover a single quantum mechanical theory that explains
the entire universe, and even multiple universes, should they exist.

It is necessary to find a single theory to establish the
inevitability of the universe’s composition. For an understanding
to be thorough, it can contain no gaps or glitches or internal inconsistencies
(like the divide between general relativity and quantum mechanics).
String theorists were troubled when they discovered five equally
valid mathematical explanations of string theory. This multiplicity
of explanations directly undercut the dream of unity. But when Witten showed
that these five versions might be five variations on the same formula,
string theorists realized once again that arriving at a unified
theory will require drastic revisions of what they consider reality.

Perhaps a unified theory will emerge after scientists
find, for example, that the universe is composed of a multitude
of dimensions invisible to the eye. Or perhaps they will find that
the universe humans inhabit is just one of a multitude. Cosmology—the
study of the universe’s origins—has generated some eyebrow-raising
speculations about the universe’s essential symmetry and coherence. Should
any of these speculations be proved, scientists will be that much
closer to unveiling the Theory of Everything—the ambition that Greene
describes as the “Holy Grail” of modern physics.

#### Elegance

String theorists seek to explain a wide range of phenomena
with a very small number of versatile, far-reaching ideas—a single
set of universal laws. String theory is the most elegant theory
yet conceived because it is predicated on the unity of all particles
and nuclear forces. The idea of tiny strings in constant vibration
is an extremely simple notion, but it may well prove the basis of
every event in the universe, and that, to Greene, is the definition
of elegance. Throughout the book, Greene uses words like *elegant*, *aesthetic*,
and *beautiful* when describing string theory—especially
in contrast to the chaotic principles of quantum mechanics. The
more physicists learn about the universe, the more elegant and concise their
theories will likely be.

#### Theory versus Experiment

Ultimately, the efficacy of a physical theory resides
in its ability to decipher and predict physical phenomena. Theories
have little value until they can be tested, proved, and applied
to the real world beyond the blackboard and lab. In the 1980s, Sheldon
Glashow reminded string theorists of this crucial aspect of the
scientific method. Glashow and other detractors were eager to point
out that string theory, in its current formulation, contains far
too many gaps and question marks to be experimentally accessible.

Greene regrets this problem, which he believes reflects
the sweeping ambition of string theory. Most of the time, theory
tends to follow observation and experiment. In string theory, the
order of operations is reversed. The elementary particles of matter
are simply too small—or, as has recently been suggested, too large—to
be detected outside a nuanced theoretical framework. String theorists must
know exactly what they are looking for before they can hope to find
it. For this reason, they are working feverishly to discover which
Calabi-Yau shape is correct. Only after resolving this matter can
physicists begin to verify string theory experimentally. Even with
such specific information, the challenge of verifying string theory
experimentally remains formidable. Locating matter at Planck length
is, at the moment, beyond the capabilities of the most advanced
technological equipment. But though it may take years or even generations,
Greene believes that one day an accelerator will manage to probe
that tiny scale. In the meantime, physicists must continue working
out the mathematical underpinnings of the theory.

#### The Uncertainty Principle

The uncertainty principle, which is the key feature of
quantum mechanics, proclaims that some features of the universe—namely the
position and velocity of a particle—cannot be known with absolute
precision, or at least not at the same time. This uncertainty becomes
more exaggerated as distance and time shrink, indicating the chaos
and frenzy that are prevalent in the microscopic realm. In the absence
of concrete predictions about what occurs at these small-distance
scales, physicists must settle for probability equations.

Heisenberg’s uncertainty principle applies specifically
to the laws of quantum mechanics. But, in other realms of physics,
the element of uncertainty is both the chief motivator and the major
stumbling block. For every statement of fact in *The Elegant
Universe*, Greene offers an unresolved speculation. Though
our knowledge of the universe has advanced incalculably over the
last hundred years, we are still surrounded on all sides by mysteries
beyond our grasp. But not yet knowing the answers should not prevent
us from continuing to ask the questions. Cosmological certainty
is a most elusive goal, but Green believes it is a goal worth pursuing.