**Glossary**

**absolute space:**Newton's view of space; envisions space as unchanging and independent of its contents.

**absolute spacetime:**View of space emerging from special relativity; envisions space through the entirety of time, from any perspective, as unchanging and independent of its contents.

**absolutist:**Perspective holding that space is absolute.

**acceleration:**Motion that involves a change in speed and/or direction.

**accelerator, atom smasher:**Research tool of particle physics that collides particles together at high speed.

**aether, luminiferous aether:**Hypothetical substance filling space that provides the medium for light to propogate; discredited.

**arrow of time:**Direction in which time seems to point — from past to future.

**background independence:**Property of a physical theory in which space and time emerge from a more fundamental concept, rather than being inserted axiomatically.

**big bang theory/standard big bang theory:**Theory describing a hot, expanding universe from a moment after its birth.

**big crunch:**One possible end to the universe, analogous to reverse of the big bang in which space collapses in on itself.

**black hole:**An object whose immense gravitational field traps anything, even light, that gets too close (closer than the black hole's event horizon).

**braneworld scenario:**Possibility within string/M-theory that our familiar three-spatial dimensions are a three-brane.

**Casimir force:**Quantum mechanical force exerted by an imbalance of vacuum field fluctuations.

**classical physics:**As used in this book, the physical laws of Newton and Maxwell. More generally, often used to refer to all nonquantum laws of physics, including special and general relativity.

**closed strings:**Filaments of energy in string theory, in the shape of loops.

**collapse of probability wave, collapse of wavefunction:**Hypothetical development in which a probability wave (a wavefunction) goes from a spread-out to a spiked shape.

**Copenhagen interpretation:**Interpretation of quantum mechanics that envisions large objects as being subject to classical laws and small objects as being subject to quantum laws.

**cosmic microwave background radiation:**Remnant electromagnetic radiation (photons) from the early universe, which permeates space.

**cosmic horizon, horizon:**Locations in space beyond which light has not had time to reach us, since the beginning of the universe.

**cosmological constant:**A hypothetical energy and pressure, uniformly filling space; origin and composition unknown.

**cosmology:**Study of origin and evolution of the universe.

**critical density:**Amount of mass/energy density required for space to be flat; about 10^-23 grams per cubic meter.

**D-branes, Dirichlet-**A

*p*-branes:*p*-brane that is "sticky"; a

*p*-brane to which open string endpoints are attached.

**dark energy:**A hypothetical energy and pressure, uniformly filling space; more general notion than a cosmological constant as its energy/pressure can vary with time.

**dark matter:**Matter suffused through space, exerting gravity but not emitting light.

**electromagnetic field:**The field which exerts the electromagnetic force.

**electromagnetic force:**One of nature's four forces; acts on particles that have electric charge.

**electron field:**The field for which the electron particle is the smallest bundle or constituent.

**electroweak theory:**The theory unifying the electromagnetic and the weak nuclear forces into the electroweak force.

**electroweak Higgs field:**Field that acquires a nonzero value in cold, empty space; gives rise to masses for fundamental particles.

**energy bowl:**See

*potential energy bowl.*

**entropy:**A measure of the disorder of a physical system; the number of rearrangements of a system's fundamental constituents that leave its gross, overall appearance unchanged.

**entanglement, quantum entanglement:**Quantum phenomenon in which spatially distant particles have correlated properties.

**event horizon:**Imaginary sphere surrounding a black hole delineating the points of no return; anything crossing the event horizon cannot escape the black hole's gravity.

**field:**A "mist" or "essence" permeating space; can convey a force or describe the presence/motion of particles. Mathematically, involves a number or collection of numbers at each point in space, signifying the field's value.

**flat space:**Possible shape of the spatial universe having no curvature.

**flatness problem:**Challenge for cosmological theories to explain observed flatness of space.

**general relativity:**Einstein's theory of gravity; invokes curvature of space and time.

**gluons:**Messenger particles of the strong nuclear force.

**gravitons:**Hypothetical messenger particles of the gravitational force.

**grand unification:**Theory attempting to unify the strong, weak, and electromagnetic forces.

**Higgs field:**See

*electroweak Higgs field.*

**Higgs field vacuum expectation value:**Situation in which a Higgs field acquires a nonzero value in empty space; a Higgs ocean.

**Higgs ocean:**Shorthand, peculiar to this book, for a Higgs field vacuum expectation value.

**Higgs particles:**Finest quantum constituents of a Higgs field.

**horizon problem:**Challenge for cosmological theories to explain how regions of space, beyond each other's cosmological horizon, have nearly identical properties.

**inertia:**Property of an object that resists its being accelerated.

**inflationary cosmology:**Cosmological theory incorporating a brief but enormous burst of spatial expansion in the early universe.

**inflaton field:**The field whose energy and negative pressure drives inflationary expansion.

**interference:**Phenomenon in which overlapping waves create a distinctive pattern; in quantum mechanics, involves seemingly exclusive alternatives combining together.

**Kaluza-Klein theory:**Theory of universe involving more than three spatial dimensions.

**Kelvin:**Scale in which temperatures are quoted relative to absolute zero (the lowest possible temperature, —273° on the Celsius scale).

**luminiferous aether:**See

*aether.*

**M-theory:**Currently incomplete theory unifying all five versions of string theory; a fully quantum mechanical theory of all forces and all matter.

**Mach's principle:**Principle that all motion is relative and that the standard of rest is provided by average mass distribution in the universe.

**Many Worlds interpretation:**Interpretation of quantum mechanics in which all potentialities embodied by a probability wave are realized in separate universes.

**messenger particle:**Smallest "packet" or "bundle" of a force, which communicates the forces' influence.

**microwave background radiation:**See

*cosmic microwave background radiation.*

**negative curvature:**Shape of space containing less than the critical density; saddle shaped.

**observable universe:**Part of universe within our cosmic horizon; part of universe close enough so that light it emitted can have reached us by today; part of universe we can see.

**open strings:**Filaments of energy in string theory, in the shape of snippets.

**Ingredient of string/M-theory with p-spatial dimensions. See also D-brane.**

*p*-brane:**Planck length:**Size (10^-33 centimeters) below which the conflict between quantum mechanics and general relativity becomes manifest; size below which conventional notion of space breaks down.

**Planck mass:**Mass (10^-5 grams, mass of a grain of dust; ten billion billion times the proton mass); typical mass of a vibrating string.

**Planck time:**Time (10^-43 seconds) it takes light to traverse one Planck length; time interval below which conventional notion of time breaks down.

**phase transition:**Qualitative change in a physical system when its temperature is varied through a sufficiently wide range.

**photon:**Messenger particle of the electromagnetic force; a "bundle" of light.

**potential energy:**Energy stored in a field or object.

**potential energy bowl:**Shape describing the energy a field contains for a given field value; technically called the field's potential energy.

**probability wave:**Wave in quantum mechanics that encodes the probability that a particle will be found at a given location.

**quantum chromodynamics:**Quantum mechanical theory of the strong nuclear force.

**quantum fluctuations, quantum jitters:**The unavoidable, rapid variations in the value of a field on small scales, arising from quantum uncertainty.

**quantum measurement problem:**Problem of explaining how the myriad possibilities encoded in a probability wave give way to a single outcome when measured.

**quantum mechanics:**Theory, developed in the 1920s and 1930s, for describing the realm of atoms and subatomic particles.

**quarks:**Elementary particles subject to the strong nuclear force; there are six varieties (up, down, strange, charm, top, bottom).

**relationist:**Perspective holding that all motion is relative and space is not absolute.

**rotational invariance, rotational symmetry:**Characteristic of a physical system, or of a theoretical law, of being unaffected by a rotation.

**second law of thermodynamics:**Law that says that, on average, the entropy of a physical system will tend to rise from any given moment.

**spacetime:**The union of space and time first articulated by special relativity.

**special relativity:**Einstein's theory in which space and time are not individually absolute, but instead depend upon the relative motion between distinct observers.

**spin:**Quantum mechanical property of elementary particles in which, somewhat like a top, they undergo rotational motion (they have intrinsic angular momentum).

**spontaneous symmetry breaking:**Technical name for the formation of a Higgs ocean (Higgs Field); process by which a previously manifest symmetry is hidden or spoiled.

**standard candles:**Objects of a known intrinsic brightness that are useful for Measuring astronomical distances.

**standard model:**Quantum mechanical theory composed of quantum chromodynamics and the electroweak theory; describes all matter and forces, except for gravity. Based on conception of point particles.

**strong nuclear force:**Force of nature that influences quarks; holds quarks together inside protons and neutrons.

**string theory:**Theory based on one-dimensional vibrating filaments of energy (see super-string theory), but which does not necessarily incorporate supersymmetry. Sometimes used as shorthand for superstring theory.

**superstring theory:**Theory in which fundamental ingredients are one-dimensional loops (closed strings) or snippets (open strings) of vibrating energy, which unites general relativity and quantum mechanics; incorporates supersymmetry.

**supersymmetry:**A symmetry in which laws are unchanged when particles with a whole number amount of spin (force particles) are interchanged with particles that have half of a whole number amount of spin (matter particles).

**symmetry:**A transformation on a physical system that leaves the system's appearance unchanged (e.g., a rotation of a perfect sphere about its center leaves the sphere unchanged); a transformation of a physical system that has no effect on the laws describing the system.

**time-reversal symmetry:**Property of the accepted laws of nature in which laws make no distinction between one direction in time and the other. From any given moment, the laws treat past and future in exactly the same way.

**time slice:**All of space at one moment of time; a single slice through the spacetime block or loaf.

**translational invariance, translational symmetry:**Property of accepted laws of nature in which the laws are applicable at any location in space.

**uncertainty principle:**Property of quantum mechanics in which there is a fundamental limit on how precisely certain complementary physical features can be measured or specified.

**unified theory:**A theory that describes all forces and all matter in a single theoretical structure.

**vacuum:**The emptiest that a region can be; the state of lowest energy.

**vacuum field fluctuations:**See

*quantum fluctuations.*

**velocity:**The speed and direction of an object's motion.

**W and Z particles:**The messenger particles of the weak nuclear force.

**wavefunction:**See probability wave.

**weak nuclear force:**Force of nature, acting on subatomic scales, and responsible for phenomena such as radioactive decay.

**which-path information:**Quantum mechanical information delineating the path a particle took in going from source to detector.