DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

space-like infinity
space-like infinity The endpoint i 0 of all
space-like geodesics of a space-time.
space-like vector An element t of a linear
space with a Lorentzian metric g of signature
(−, +, +, +) such that the norm is g(t, t) =
gabt a t b > 0. The norm of a space-like four
vector in the theory of relativity represents the
spatial distance of two events that appear simultaneous
to some observer.
spacetime The collection of all places and all
moments of time in the whole universe. For various
problems simplified models of our physical
spacetime are constructed. For example, cosmology
often ignores details of the geometry
and matter distribution in the universe that are
smaller than a group of galaxies. In investigating
planetary motions, typically all stars other
than the sun are ignored, and it is assumed that
the empty space around the sun extends to infinite
distances. A mathematical model of spacetime
is a four-dimensional space (one of the four
coordinates is time) with a given metric; two
spacetimes are in fact identical if their metrics
can be transformed one into the other by a coordinate
transformation. Points of the spacetime
are called events. Two events p and q can
be in a timelike, lightlike (null), or spacelike
relation. The metric makes it possible to calculate
the lapse of time between the events p
and q when they are in a timelike relation or
the distance between them when they are in a
spacelike relation. In fact the metric contains all
the relevant information about the geometry and
physics in its underlying spacetime, although
some of the information may be technically difficult
to extract. In general relativity, Einstein’s
equations relate the matter distribution to the
geometry of the spacetime. Geodesics can be
calculated to determine the trajectories of particles,
and of light rays, moving under the influence
of gravitational forces. If the properties
of the spacetime are found to correspond
to part of the observed world, then the mathematical
spacetime model is considered realistic
in the appropriate range of phenomena. Very
few such analytically exact realistic models exist.
Among them are the flat Minkowski spacetime
that is the geometrical arena of special relativity,
the spacetime of the Schwarzschild solu-
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tion that describes spherical black holes, and the
sun’s dominant, spherically symmetric gravitational
field in the solar system (but without taking
into account the planets’ own gravitational
fields or the rotation and rotational deformation
of the star), the spacetimes of the Friedmann–
Lemaître and Robertson–Walker cosmological
models that are used to model the whole universe,
the Kerr spacetime describing the gravitation
field of a stationary rotating black hole, and
a few more spacetimes corresponding to simple
patterns of gravitational waves and isolated
structures in the universe (the Lemaître–Tolman
cosmological model is among the latter). However,
computational modeling is beginning to
provide a much longer list of accessible realistic
spacetimes.
space weather The conditions and processes
occurring in space which have the potential to
affect the near Earth environment and, in particular,
technological systems. Space weather
processes include the solar wind and interplanetary
magnetic field, solar flares, coronal mass
ejections from the sun, and the resulting disturbances
in the Earth’s geomagnetic field and
atmosphere. The effects can range from the unexpected
(e.g., disruption of power grids, damage
to satellites) to the common (e.g., failure
of HF systems). Although space weather effects
have been recognized and studied for many
years, it has only recently developed as a recognized
field of unified activity which attempts to
forecast solar flares, magnetic storms and other
space-related phenomena.
spallation A nuclear reaction in which an
atomic nucleus is struck by an incident high energy
particle. As a result, particles typically
heavier than an α-particle are ejected from the
nucleus. Astrophysical amounts of the lightly
bound isotopes 6 Li, 9 Be, 10 B, and 11 B are believed
to have been formed by spallation by energetic
cosmic rays.
special relativity A description of mechanical
and electromagnetic phenomena involving
sources and observers moving at velocities close
to that of light, but in the absence of gravitational
effects. Maxwell’s theory describes the dynamics
of electric and magnetic fields and predicts