GLOSSARY 341
Dark matter Invisible matter
that can only be detected by its
gravitational effect on visible
matter. Dark matter holds
galaxies together.
Diffraction The bending of waves
around obstacles and spreading out
of waves past small openings.
DNA Deoxyribonucleic acid. A
large molecule in the shape of a
double helix that carries genetic
information in a chromosome.
Doppler effect The change in
frequency of a wave experienced
by an observer in relative motion
to the wave’s source.
Ecology The scientific study of
the relationships between living
organisms and their environment.
Electric charge A property of
subatomic particles that causes
them to attract or repel one another.
Electric current A flow of
electrons or ions.
Electromagnetic force One
of the four fundamental forces of
nature. It involves the transfer
of photons between particles.
Electromagnetic radiation A
form of energy that moves through
space. It has both an electrical and
a magnetic field, which oscillate at
right-angles to each other. Light is a
form of electromagnetic radiation.
Electroweak theory A theory
that explains the electromagnetic
and weak nuclear force as one
“electroweak” force.
Electron A subatomic particle
with a negative electric charge.
Electrolysis A chemical change
in a substance caused by passing
an electric current through it.Element A substance that
cannot be broken down into other
substances by chemical reactions.Endosymbiosis A relationship
between organisms in which one
organism lives inside the body or
cells of another organism to their
mutual benefit.Energy The capacity of an object
or system to do work. Energy can
exist in many forms, such as
kinetic energy (movement) and
potential energy (for example, the
energy stored in a spring). It can
change from one form to another,
but never be created or destroyed.Entanglement In quantum
physics, the linking between
particles such that a change in one
affects the other no matter how
far apart in space they may be.Entropy A measure of the
disorder of a system. Entropy
is the number of specific ways a
particular system may be arranged.Ethology The scientific study of
animal behavior.Event horizon A boundary
surrounding a black hole within
which the gravitational pull of the
black hole is so strong that light
cannot escape. No information
about the black hole can cross its
event horizon.Evolution The process by which
species change over time.Exoplanet A planet that orbits a
star that is not our Sun.Fermion A subatomic particle,
such as an electron or a quark, that
is associated with mass.Field The distribution of a force
across space-time, in which each
point can be given a value for that
force. A gravitational field is an
example of a field in which the force
felt at a particular point is inversely
proportional to the square of the
distance from the source of gravity.Force A push or a pull, which moves
or changes the shape of an object.Fractal A geometric pattern in
which similar shapes can be seen
at different scales.Gamma decay A form of
radioactive decay in which
an atomic nucleus gives off
high-energy, short-wavelength
gamma radiation.Gene The basic unit of heredity of
living organisms, which contains
coded instructions for the formation
of chemicals such as proteins.General relativity A theoretical
description of space-time in which
Einstein considers accelerating
frames of reference. General
relativity provides a description
of gravity as the warping of
space-time by mass. Many
of its predictions have been
demonstrated empirically.Geocentrism A model of the
universe with Earth at its center.Gravity A force of attraction
between objects with mass.
Massless photons are also
affected by gravity, which
general relativity describes
as a warping of space-time.