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Unit 7: Astrophysics Page 9

Key Vocabulary

Lesson 3: Relativity

Absolute motion: Motion that exists, undeniably, without reference to
anything else. The relativity principle denies the possibility of absolute
motion.

Black hole: An object so small yet so massive that escape speed exceeds
the speed of light. General relativity predicts the possibility of black holes,
and modern astrophysics has essentially confirmed their existence.

Color force: The very strong force that acts between quarks, binding them
together to form hadrons and mesons.

Dark matter: Matter in the cosmos that is undetectable because it doesn’t
glow. Dark matter, some of it in the form of as-yet-undiscovered exotic
particles, is thought to comprise most of the universe.

Electromagnetic wave: A structure consisting of electric and magnetic
fields in which each kind of field generates the other to keep the structure
propagating through empty space at the speed of light, c. Electromagnetic
waves include radio and TV signals, infrared radiation, visible light,

ultraviolet light, x rays, and gamma rays.

Electroweak force: One of the three fundamental forces now identified, the
electroweak force subsumes electromagnetism and the weak nuclear force.
Elsewhere: A region of spacetime that is neither past nor future. The
elsewhere of a given event consists of those other events that cannot

influence or be influenced by the given event—namely, those events that are
far enough away in space that not even light can travel between them and
the given event.

Escape speed: The speed needed to escape to infinitely great distance from
a gravitating object. For Earth, escape speed from the surface is about 7
miles per second; for a black hole, escape speed exceeds the speed of light.

Ether: A hypothetical substance, proposed by nineteenth century physicists
and thought to be the medium in which electromagnetic waves were
disturbances.

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