DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

longshore sediment transport Transport of
sediment in a direction parallel to the trend of
the coast. See littoral transport.
long slit spectroscopy A technique employed
to obtain spectra of extended objects,
such as galaxies or planetary nebulae. The spectrograph
aperture on the focal plane of the telescope
is limited by a slit, whose width is typically
a few seconds of arcs or less, and whose
height may cover an angular size of several minutes
of arc. Only light coming from the narrow
strip defined by the slit is allowed to enter
the spectrograph to avoid contamination by
adjacent strips: nearby sources could produce
spectra that would overlap spatially on the detector.
Long slit spectroscopy has been employed
in the measurement of continuum, absorption,
and emission lines from every extended object.
An example is the construction of radial velocity
and rotation curves of galaxies. See velocity
curve.
look-back time The finite speed of light
means that objects are seen as they were at some
time prior to the observer’s time of observation.
Thus T , the look-back time, is T=d/c, where
d is the distance to the observed object, andc is
the speed of light.
Lorentz boost Lorentz transformation of
space-time coordinates from one system of reference
to another moving at a constant velocity
with respect to each other. These transformations
distinguish themselves from general
Lorentz transformations in that they do not include
rotation of spatial coordinates. Named after
Hendrik Lorentz (1853–1928). See Lorentz
transformation.
Lorentz factor, γ The quantity
γ=
1
1 − v2
c 2
wherev is the speed andc is the speed of light. γ
is an indicator of special relativistic effects and
entersintolengthcontractionandtimedilitation,
for instance.
Lorentz–Fitzgerald contraction The decrease
of the length of a physical body when
© 2001 by CRC Press LLC
Lorentz transformation
measured in a uniformly moving reference system
rather than in the reference system of the
body, as calculated by the Lorentz Transformations
in the Special Theory of Relativity.
Named after Hendrik Lorentz and George Francis
FitzGerald. See coordinate transformation
in special relativity.
Lorentz force equation Equation describing
the force on a charged particle moving in specified
electric (E) and magnetic (B) field, with
velocity v:
F = q(E + v × B),
where this is a vector equation; the charge is in
coulombs, the electric field has units of Voltmeters,
and the magnetic field is measured in
Tesla. The presence of the cross product × indicates
that the magnetic force is orthogonal to
both the direction of the magnetic field, and to
the direction of motion of the charged particle.
Lorentzian metric The metric of a fourdimensional
manifold with one negative and
three positive eigenvalues, thus of signature
(−,+,+,+) (or one negative and three positive).
An example is Minkowski space-time,
having the metricds 2 =−c 2 dt 2 +dx 2 +dy 2 +
dz 2 written in Cartesian coordinates, wherec is
the speed of light. The space-time model of
general relativity is also one with a Lorentzian
metric.
Lorentz invariance The invariance of physical
expressions under Lorentz transformations
from one coordinate system to another coordinate
system moving uniformly with respect to
the first. In special relativity all physical laws
must be Lorentz invariant. Named after Hendrik
Lorentz (1853–1928). See coordinate transformation
in special relativity.
Lorentz transformation In special relativity,
the coordinate transformations relating distance
and time measurements in two reference
systems (“frames”) in relative motion. If written
in rectangular coordinates {t,x,y,z} =
{x 0 ,x 1 , x 2 ,x 3 } the relation between two
285