The Grand Design

Alternative histories • a formulation of quantum theory in which the probability of any observation is constructed from all the possible histories that could have led to that observation.

Anthropic principle • the idea that we can draw conclusions about the apparent laws of physics based on the fact that we exist.

Antimatter • each particle of matter has a corresponding anti-particle. If they meet, they annihilate each other, leaving pure energy.

Apparent laws • the laws of nature that we observe in our universe—the laws of the four forces, and the parameters such as mass and charge that characterize the elementary particles—in contrast to the more fundamental laws of M-theory that allow for different universes with different laws.

Asymptotic freedom • a property of the strong force that causes it to become weaker at short distances. Hence, although quarks are bound in nuclei by the strong force, they can move within nuclei almost as if they felt no force at all.

Atom • the basic unit of ordinary matter, consisting of a nucleus with protons and neutrons, surrounded by orbiting electrons.

Baryon • a type of elementary particle, such as the proton or neutron, that is made of three quarks.

Big bang • the dense, hot beginning of the universe. The big bang theory postulates that about 13.7 billion years ago the part of the universe we can see today was only a few millimeters across. Today the universe is vastly larger and cooler, but we can observe the remnants of that early period in the cosmic microwave background radiation that permeates all space.

Black hole • a region of space-time that, due to its immense gravitational force, is cut off from the rest of the universe.

Boson • an elementary particle that carries force.

Bottom-up approach • in cosmology, an idea that rests on the assumption that there’s a single history of the universe, with a well-defined starting point, and that the state of the universe today is an evolution from that beginning.

Classical physics • any theory of physics in which the universe is assumed to have a single, well-defined history.

Cosmological constant • a parameter in Einstein’s equations that gives space-time an inherent tendency to expand.

Electromagnetic force • the second strongest of the four forces of nature. It acts between particles with electric charges.

Electron • an elementary particle of matter that has a negative charge and is responsible for the chemical properties of elements.

Fermion • a matter-type elementary particle.

Galaxy • a large system of stars, interstellar matter, and dark matter that is held together by gravity.

Gravity • the weakest of the four forces of nature. It is the means by which objects that have mass attract each other.

Heisenberg uncertainty principle • a law of quantum theory stating that certain pairs of physical properties cannot be known simultaneously to arbitrary precision.

Meson • a type of elementary particle that is made of a quark and an anti-quark.

M-theory • a fundamental theory of physics that is a candidate for the theory of everything.

Multiverse • a set of universes.

Neutrino • an extremely light elementary particle that is affected only by the weak nuclear force and gravity.

Neutron • a type of electrically neutral baryon that with the proton forms the nucleus of an atom.

No-boundary condition • the requirement that the histories of the universe are closed surfaces without a boundary.

Phase • a position in the cycle of a wave.

Photon • a boson that carries the electromagnetic force. A quantum particle of light.

Probability amplitude • in a quantum theory, a complex number whose absolute value squared gives a probability.

Proton • a type of positively charged baryon that with the neutron forms the nucleus of an atom.

Quantum theory • a theory in which objects do not have single definite histories.

Quark • an elementary particle with a fractional electric charge that feels the strong force. Protons and neutrons are each composed of three quarks.

Renormalization • a mathematical technique designed to make sense of infinities that arise in quantum theories.

Singularity • a point in space-time at which a physical quantity becomes infinite.

Space-time • a mathematical space whose points must be specified by both space and time coordinates.

String theory • a theory of physics in which particles are described as patterns of vibration that have length but no height or width—like infinitely thin pieces of string.

Strong nuclear force • the strongest of the four forces of nature. This force holds the protons and neutrons inside the nucleus of an atom. It also holds together the protons and neutrons themselves, which is necessary because they are made of still tinier particles, quarks.

Supergravity • a theory of gravity that has a kind of symmetry called supersymmetry.

Supersymmetry • a subtle kind of symmetry that cannot be associated with a transformation of ordinary space. One of the important implications of supersymmetry is that force particles and matter particles, and hence force and matter, are really just two facets of the same thing.

Top-down approach • the approach to cosmology in which one traces the histories of the universe from the “top down,” that is, backward from the present time.

Weak nuclear force • one of the four forces of nature. The weak force is responsible for radioactivity and plays a vital role in the formation of the elements in stars and the early universe.