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Unit 7: Astrophysics Page 10

Event horizon: A spherical surface surrounding a black hole and marking

the “point of no return” from which nothing can escape.
Field: A way of describing interacting objects that avoids action at a
distance. In the field view, one object creates a field that pervades space; a
second object responds to the field in its immediate vicinity. Examples

include the electric field, the magnetic field, and the gravitational field.
Frame of reference: A conceptual framework from which one can make

observations. Specifying a frame of reference means specifying one’s state
of motion and the orientation of coordinate axes used to measure positions.
General theory of relativity: Einstein’s generalization of special relativity

that makes all observers, whatever their states of motion, essentially
equivalent. Because of the equivalence principle, general relativity is
necessarily a theory about gravity.

Gravitational lensing: An effect caused by the general relativistic bending
of light, whereby light from a distant astrophysical object is bent by an
intervening massive object to produce multiple and/or distorted images.

Gravitational time dilation: The slowing of time in regions of intense
gravity (large spacetime curvature).

Gravitational waves: Literally, “ripples” in the fabric of spacetime. They
propagate at the speed of light and result in transient distortions in space
and time.

Gravity: According to Newton, an attractive force that acts between all
matter in the universe. According to Einstein, a geometrical property of
spacetime (spacetime curvature) that results in the straightest paths not
being Euclidean straight lines.

Hadron: A “heavy” particle, made up of three quarks. Protons and neutrons
are the most well known hadrons.

Length contraction: The phenomenon whereby an object or distance is
longest in a reference frame in which the object or the endpoints of the
distance are at rest. Also called the Lorentz contraction and Lorentz-
Fitzgerald contraction.

Mass-energy equivalence: The statement, embodied in Einstein’s
2
equation E=mc , that matter and energy are interchangeable.

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