#### Key Figures

Greene mentions a number of contemporary physicists—Gabriele
Veneziano, Pierre Ramond, and Shing-Tung Yau among them—who have
made important contributions to the advancement of string theory.
The following list focuses primarily on the predecessors of string
theory: scientists and mathematicians from earlier eras who laid
the groundwork for what is now the cutting edge of physics.

**Niels Bohr (1885–1962)** -
A Danish physicist and contemporary of Einstein.
Bohr developed quantum mechanics and was the first to apply the
quantum theory to the problem of atomic structure. He received the
Nobel Prize in 1922.

**Max Born (1882–1970)** -
A German physicist. In 1926, Born introduced one
of the most bizarre—but still experimentally verifiable—aspects
of quantum theory: the idea that an electron wave must be interpreted
from the standpoint of probability. Born’s reinterpretation of Schrödinger’s
wave equation led to a new theory of quantum mechanics.

**Prince Louis de Broglie (1892–1987)** -
A French nobleman. In 1923, de Broglie suggested
that Einstein’s conception of the wave-particle duality of light
also applied to matter. For discovering the wave-nature of electrons, Broglie
was awarded the 1929 Nobel Prize for Physics.

**Sir Arthur Eddington (1882–1944)** -
An English physicist. Eddington tested Einstein’s
theory of general relativity during a 1919 total solar eclipse and
found that the bending of light rays that Einstein predicted actually occurred.
(Eddington’s conclusions were later called into question, but at
the time they turned Einstein into an international celebrity.)

**Albert Einstein (1879–1955)** -
A German-American physicist. Einstein formulated
both the theories of special and of general relativity. His theory
of gravitation marked a profound revision of Newton’s ideas.

**Leonhard Euler (1707–1783)** -
A Swiss mathematician and physicist. Euler is considered
one of the founders of pure mathematics. His studies of strongly
interacting particles influenced many physicists throughout the twentieth
century.

**Richard Feynman (1918–1988)** -
An American theoretical physicist. Feynman reinvented
quantum electrodynamics in the years following World War II. He
advanced a powerful new way to think of Born’s probability theory,
and many consider him the most important theoretical physicist since
Einstein.

**Murray Gell-Mann (1929–)** -
An American physicist. In 1969, Gell-Mann won the
Nobel Prize for his classification systems of atomic and subatomic
particles, and the ways in which they interact. It was Gell-Mann
who coined the term

*quark*, which he borrowed from
James Joyce’s

*Finnegans Wake*, to describe the building
blocks of matter.

**Sheldon Glashow (1932–)** -
An American theoretical physicist. Glashow, along
with Steven Weinberg and Abdus Salam, was awarded the 1979 Nobel
Prize for Physics for his revolutionary formulation of electroweak theory,
which explains the unity of electromagnetism and the weal force.

**Samuel Goudsmit (1902–1978)** -
A Dutch-American physicist. Goudsmit, along with
George Uhlenbeck, proposed the concept of electron spin, which posits
that electrons rotate on an axis. This insight led to many revisions
in theories about atomic structure and quantum mechanics.

**Stephen Hawking (1942–)** -
An English theoretical physicist. Hawking’s black
hole theory combines quantum mechanics and general relativity. Hawking
is the author of the bestseller

*A Brief History of Time:
From the Big Bang to Black Holes *(1988), an explanation
of the cosmos intended for the general public. He has also received
the Albert Einstein Award, which is the most important award in
theoretical physics.

**Werner Heisenberg (1901–1976)** -
The first proponent of the uncertainty principle,
which has remained the key feature of quantum mechanics since its
introduction in 1927.

**Heinrich Hertz (1857–1894)** -
A German physicist. In 1887, Hertz found that when
electromagnetic radiation (light) shines on certain metals, they
release electrons. From his studies of James Clerk Maxwell’s electromagnetic theory,
Hertz established that light and heat are both electromagnetic forces.

**Edwin Hubble (1889–1953)** -
An American astronomer. Hubble proved that the universe
is expanding.

**Theodor Kaluza (1885–1954)** -
A German mathematician. In 1919, Kaluza proposed
that the universe might contain more than three spatial dimensions.
Kaluza’s theory was considered outlandish, and it took Einstein
several years to consider Kaluza’s theory seriously, but string theorists
today find it remarkably prescient.

**Oskar Klein (1894–1977)** -
A Swedish physicist. In 1926, Klein refined Theodor
Kaluza’s notion of an extradimensional universe.

**Pierre-Simon de Laplace (1749–1827)** -
A French mathematician, astronomer, and physicist.
Laplace is best known for applying Newton’s theory of gravitation
to the solar system.

**James Clerk Maxwell (1831–1879)** -
A Scottish physicist. Maxwell developed the set of
four equations that became the basis of electromagnetic theory,
the single force unifying electricity and magnetism. Maxwell’s work had
a huge influence on twentieth-century physics, and he is ranked
alongside Isaac Newton and Albert Einstein for the scope of his
contributions. Maxwell’s field equations prompted Max Planck to
formulate the quantum hypothesis—the theory that radiant-heat energy
is emitted only in finite quantities, or quanta.

**Max Planck (1858–1947)** -
A German theoretical physicist. Planck pioneered
quantum theory.

*Planck’s constant,* *Planck tension*,
and

*Planck mass* are all named after him. His work
revolutionized physicists’ understanding of atomic and subatomic
particles. Planck won the Nobel Prize in 1918.

**George Bernhard Riemann (1826–1866)** -
A German mathematician. Riemann’s geometrical studies
were foundational to Einstein’s theory of relativity.

**Abdus Salam (1926–1996)** -
A Pakistani physicist. Salam was awarded the 1979
Nobel Prize, along with Sheldon Glashow and Steven Weinberg, for
his work developing electroweak theory.

**Erwin Schrödinger (1887–1961)** -
An Austrian physicist. Schrödinger argued that waves
were really “smeared” electrons. He objected to the then-universally
accepted description of matter in terms of waves and particles, and
instead advanced a quantum mechanical wave equation. Schrödinger
shared the Nobel Prize of 1933.

**Karl Schwarzchild (1873–1916)** -
A German astronomer and physicist. Schwarzchild worked
out Einstein’s field equations of general relativity while stationed
on the Russian front during World War I.

**George Uhlenbeck (1900–1988)** -
A Dutch physicist. Uhlenbeck, along with Samuel Goudsmit,
proposed the concept of electron spin, which posits that electrons
rotate on an axis. This insight led to many revisions in theories about
atomic structure and quantum mechanics.

**Steven Weinberg (1933–)** -
An American nuclear physicist. Weinberg shared the
1979 Nobel Prize with Sheldon Glashow and Abdus Salam for the formulation
of electroweak theory. Weinberg showed that photons and bosons actually
belong to the same particle family.

**Edward Witten (1951–)** -
An American physicist. Witten instigated the second
superstring revolution in 1995. It was Witten who first proposed
that the five versions of string theory were really just five interpretations
of the same theory. He also introduced the important possibility
that string theory encompasses far more than just strings.

**Thomas Young (1773–1829)** -
An English physicist. Young disproved Newton’s conception
of light as a stream of particles. By allowing light to pass through
two pinholes onto a screen, he found that the light beams spread
apart and overlapped. In the area of overlap, Young saw bands of
bright light alternating with bands of darkness. With this demonstration,
he revived the century-old wave theory of light and established
the principle of interference of light.

#### Key Terms

**Antimatter** -
Matter
with the same gravitational properties as regular matter, but with
an opposite electric charge and opposite nuclear force charges.

**Antiparticle** -
A
particle of antimatter.

**Big bang** -
The
widely accepted theory concerning the origin of the universe. The
big bang theory posits that the universe evolved approximately 10
to 15 billion years ago from the explosion of an incredibly dense,
hot substance that was contained at one point. The universe has
been expanding since the first fraction of a second after the big
bang occurred.

**Big crunch** -
The
term referring to what some physicists believe will happen when
the expanding universe stops and implodes. When the big crunch occurs,
according to the theory, all space and matter will collapse together.

**Black hole** -
A
region of space formed when a giant star collapses and all of its
mass compresses to a single point, forming a gravitational field
so overpowering that it traps anything that comes close to it, including
light.

**Boson** -
A
pattern of string vibration with an amount of spin measurable in
whole numbers. A boson is most often a messenger particle.

**Bosonic string theory** -
The first version of string theory. Bosonic string
theory, which dealt with string’s vibrational patterns, emerged
in the 1970s. This version was later revised and replaced by supersymmetrical
string theory.

**Calabi-Yau shape/space** -
A theoretical configuration that many physicists
believe might contain the additional dimension string theory requires.
Many thousands of such possible configurations exist, but string
theory has yet to verify the correct one.

**Electromagnetism/electromagnetic force** -
One of the four fundamental forces, along with gravity,
the strong force, and the weak force. Electromagnetism determines
all types of electromagnetic radiation, including light, X-rays,
and radio waves.

**Electroweak theory** -
A relativistic quantum field theory that describes
the weak force and the electromagnetic force within a single framework.

**Elegance** -
To
Greene, string theory defines elegance because it is extremely simple,
but it may explain every event in the universe.

**Elementary particle** -
The indivisible or “uncuttable” unit found in all
matter and forces. Elementary particles are now categorized by quarks
and leptons, and their antimatter counterparts.

**Equivalence principle** -
The basic tenet of general relativity. The equivalence
principle states that accelerated motion is indistinguishable from
gravity. It generalizes the theory of relativity by showing that
all observers, regardless of their state of motion, can say that
they are at rest, provided they take the presence of a gravitational
field into account.

**Flop transitions** -
Also called

*topography-changing transitions*.
Flop transitions are the act of Calabi-Yau space ripping and repairing
itself.

**Force carrier particle** -
A particle that transmits one of the four fundamental
forces. The strong force is associated with gluon; electromagnetism
with the photon; the weak force with W and Z; and graviton (which
hasn’t yet been discovered) with gravity.

**Fundamental force** -
There are four fundamental forces : electromagnetism,
strong force, weak force, and gravity.

**General theory of relativity** -
Albert Einstein’s formulation that gravity results
from the warping of spacetime. Through this curvature, space and
time communicate the gravitational force.

**Graviton** -
Physicists
believe that graviton—which has not yet been proven to exist—is
the particle carrier of the gravitational force.

**Gravity** -
The
weakest and most mysterious of the four fundamental forces. Gravity
acts over an infinite range, and gravitation describes the force
of attraction between objects containing either mass or energy.

**M-theory** -
The
theory under which all five previous versions of string theory fall.
The most recent synthesis of string theory ideas, M-theory predicts
eleven spacetime dimensions and describes “membranes” as a fundamental
element in nature.

**Mirror symmetry** -
A precept of string theory that demonstrates how two
different Calabi-Yau shapes have identical physics.

**Newton’s laws of motion** -
Laws of motion based on an absolute and unchanging
notion of space and time. Newton’s laws of motion were later replaced
by Einstein’s theory of special relativity.

**Particle accelerator** -
A machine that speeds up the movement of particles
and then either shoots them out at a fixed target or makes them
collide. Particle accelerators allow physicists to study the movement
of particles in extreme conditions.

**Perturbation theory** -
A formal framework for making approximate calculations.
Perturbation theory is a linchpin of string theory in its current
form. The approximate solution will be refined later as more details
fall into place.

**Photon** -
The
smallest bundle of light. Photons are the messenger particles of
the electromagnetic force.

**Photoelectric effect** -
The action of electrons shooting from a metallic
surface when light is shone onto that surface.

**Planck energy** -
The
energy required to probe Planck-length-scale distances.

**Planck length** -
Planck
length—approximately 10–33 centimeters—is the
scale at which quantum fluctuations occur. Planck length is also
the size of a typical string.

**Planck mass** -
Planck
mass is roughly equal to the mass of a grain of dust, or ten billion
billion times the mass of a proton.

**Planck’s constant** -
Planck’s constant is also known (and written) as the
“h-bar.” It is a fundamental component of quantum mechanics.

**Planck tension** -
About
10 (to the 39th power) tons. Planck tension is equal to the tension
of a typical string.

**Quanta** -
According
to the laws of quantum mechanics, the smallest physical unit that
something can be broken into. Photons are the quanta of the electromagnetic field.

**Quantum field theory** -
Also known as

*relativistic quantum field theory*.
Quantum field theory describes particles in terms of fields, as
well as how particles can be created or annihilated, and how they
scatter.

**Quantum foam** -
Also
known as

*spacetime foam*. Quantum foam is the violent
turbulence of spatial fabric at an ultramicroscopic scale. Its existence
is one of the chief reasons that quantum mechanics is incompatible
with general relativity.

**Quantum mechanics** -
The framework of laws that describe matter on atomic
and subatomic scales. The uncertainty principle is a pillar of quantum
mechanics.

**Quarks** -
A
family of elementary particles (matter or antimatter) that make
up protons and neutrons. There are many types of quarks: up, charm,
top, down, strange, and bottom. Quarks are acted upon by the strong
force. Murray Gell-Mann named quarks after he read James Joyce’s

*Finnegans
Wake*.

**Special theory of relativity** -
Einstein’s description of particle motion, which
hinges on the constancy of the speed of light. The theory of relativity
states that even if an observer is moving, the speed of light never
changes. Distance, time, and mass, however, all depend on the observer’s
relative motion.

**Spin** -
The
theory that all particles have an intrinsic amount of spin in either
whole- or half-integer denominations.

**Standard model** -
A
quantum model that explains three of the fundamental forces—electromagnetism,
the strong force, and the weak force—but does not take gravity into
consideration.

**String** -
Miniscule
one-dimensional vibrating strands of energy. String theories posit
that these filaments are the basis of all elementary particles.
The length of a string is 10–33 cm; strings
have no width.

**Strong force** -
So
called because it is the strongest of the four fundamental forces.
It holds quarks together and keeps protons and neutrons in the nuclei
of atoms.

**Superstring theory** -
A theory that describes resonant strings as the most
elementary units in nature.

**Supersymmetry** -
A
principle of symmetry relating the properties of particles with
a whole-number quantity of spin (bosons) to those with half a whole
number of spin (fermions). Supersymmetry posits that all elementary matter
particles have corresponding superpartner force carrier particles.
No one has yet observed these theoretical superpartners, which are
thought to be even larger than their counterparts.

**Tachyon** -
A
particle that has a negative mass when squared. The existence of
a tachyon usually indicates a problem with a theory.

**Topology** -
The
study of geometric figures’ properties that exhibit ongoing transformations
and are unchanged by stretching or bending.

**Uncertainty principle** -
Heisenberg’s uncertainty principle is the crux of
quantum mechanics. It proclaims that you can never know both the
position and the velocity of a particle simultaneously. To isolate
one, you must somehow blur the other.

**Unified field theory** -
A theory describing all four fundamental forces and
all of matter within a single framework.

**Weak force** -
One
of the four fundamental forces. Weak force operates over a short
range.