A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Glossary

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big bang theory: (n) the speculated event during which time and space began and atoms formed in our universe. Physicists speculate that all matter in the universe was originally concentrated in a single point of infinite density that was scattered in an explosion, the effects of which we experience as the expansion of the universe.

big crunch: (n) the opposite of the big bang. The big crunch is one possible ending of the universe in which all matter will cease to move apart and instead collapse in on itself.

black hole: (n) an area in the universe where the force of gravity is so strong that no light or matter can escape it. For that reason, a black hole can only increase in size but never decrease.

Chandrasekhar limit: (n) designated as 1.4 solar masses, or a mass just above that of our sun. The limit determines whether a collapsing star (under the limit) will settle into a white dwarf or neutron star, or whether it (over the limit) forms a black hole.

complete unified theory: (n) a theory, also known as unified field theory, speculated to exist that brings together all the laws governing the universe. Such a theory would reconcile the principles of general relativity and quantum mechanics.

cosmology: (n) in scientific terms, the study of the composition of the universe. This aspect of astronomy speculates on the origin and interrelationships of all matter that exists.

entropy: (n) the degree of progressive disorder that occurs in an ordered system. In our universe, it is the tendency of ordered matter and energy to disintegrate toward a state of randomness.

event horizon: (n) the boundary of a black hole. It is that point of proximity to a black hole at which matter and energy are either absorbed or remain trapped in its two-dimensional rim.

fractal: (n) a finite figure with irregular curves or shapes that has identical curves/shapes at large and small scales. Fractals occur either in nature or as formulas, such as the Mandelbrot "snowman."

fundamental forces: (n) forces that act on matter in different ways and in varying strengths. The four fundamental forces are gravity, electromagnetic force, weak nuclear force, and strong nuclear force.

general theory of relativity: (n) a theory to express overall mass and energy relationships between celestial objects. Einstein's theory of relativity includes the force of gravity, but this theory has not been reconciled with theories in quantum mechanics.

geocentric model: (n) a cosmological model that illustrates the position of Earth as fixed in the center, with the sun, moon, and other planets moving around it. This model was believed to be true until Copernicus pointed out that it did not fit the truth of observation.

grand unified theories (GUTs): (n) a collection of overlapping theories about how our universe works. These theories are termed "partial" because no one theory is true in all instances.

heliocentric model: (n) a cosmological model that places the sun in the center, with Earth and the other planets orbiting around it. This model replaced the geocentric model.

light-year: (n) the distance traveled by light in one (Earth) year. It is measured at about 5.88 trillion miles.

Planck's constant: (n) a constant that relates the amount of energy of a photon with the frequency of a electromagnetic wave. This energy is measured in a quantum of electromagnetic radiation.

primordial: (adj) original to or starting before all others. A primordial black hole is one that was formed at the beginning of the universe.

quanta: (n, quantum, n, pl) tiny packets of energy . Although Newtonian physics holds that quantities are continuous, this is not possible on the very small scale of subatomic particles, so charges are expressed in tiny bursts having discrete rather than continuous values, or quanta.

quantum mechanics: (n) a branch of physics that examines phenomena of matter at a very small-scale level. Quantum mechanics does not obey the law of gravitation.

quark: (n) a charged particle at the subatomic level that responds to the strong force. Three quarks together make either a proton or a neutron in an atom.

red shift: (n) the Doppler effect of very distant stars moving away from us, which displays as "red" light (the "stretching" of light toward the longer wavelengths of the visible light spectrum). It is by studying the red shift of stars that astrophysicists can then estimate the rate of expansion of the universe.

relativity: (n) a theory in physics that explains the relationships between time and space and between energy and mass. Relativity states that the speed of light does not depend on the state or position of the observer.

singularity: (n) a point at which the curvature of space and time becomes infinite. It is speculated that the center of a black hole is a singularity.

special theory of relativity: (n) Einstein's special theory of relativity is based on two assumptions: 1) the speed of light is constant in a vacuum; and 2) the laws of physics do not vary in inertial systems, where a body stays at rest or moves with constant linear velocity unless acted on by an outside force.

spin: (n) an internal property of elementary particles. Spin is measured by how the observer perceives different views of a given particle.

string theory: (n) the theory that particles can be described as waves along a one-dimensional "string." Such strings can be open (unconnected end to end) or closed (joined at each end or joined to another string).

uncertainty principle: (n) the inability to determine both the position and the velocity of a tiny particle. This principle was developed by Werner Heisenberg (1901–76) and is a foundation of quantum mechanics.

wave-particle duality: (n) the tendency of tiny particles to behave sometimes like a particle and sometimes like a wave, depending on what an observer is measuring. This duality makes it difficult (if not entirely impossible) to reconcile quantum physics with relativity.

wormhole: (n) a possible avenue for time travel or movement from one position in the universe to another many light-years away in a shorter amount of time. Wormholes may be links to universes parallel to our own or other much smaller ones.

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