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


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.


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.


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.


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


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.


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.


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.


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.


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.


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.


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.


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.


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


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.