DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ADM mass
is the extrinsic curvature tensor of the surfaces
of constant time t , | denotes covariant differentiation
with respect to the three-dimensional
metric γ ,K=Kij γ ij , and (3) R is the intrinsic
scalar curvature of t . From the above form of
the action, it is apparent that α and β i are not
dynamical variables (no time derivatives of the
lapse and shifts functions appear). Further, the
extrinsic curvature of t enters in the action to
build a sort of kinematical term, while the intrinsic
curvature plays the role of a potential. See
Arnowitt–Deser–Misner(ADM)decomposition
of the metric.
ADM mass According to general relativity,
the motion of a particle of mass m located in
a region of weak gravitational field, that is far
away from any gravitational source, is well approximated
by Newton’s law with a force
F=G mMADM
r 2
wherer isaradialcoordinatesuchthatthemetric
tensor g approaches the usual flat Minkowski
metric for large values ofr. The effective ADM
massMADM is obtained by expanding the timetime
component of g in powers of 1/r,
gtt =−1 + 2MADM
r
,
1
+O
r2
.
Intuitively, one can think of the ADM mass as
the total (matter plus gravity) energy contained
in the interior of space. As such it generally
differs from the volume integral of the energymomentum
density of matter. It is conserved if
no radial energy flow is present at large r.
More formally, M can be obtained by integratingasurfacetermatlarger
intheADMform
of the Einstein–Hilbert action, which then adds
to the canonical Hamiltonian. This derivation
justifies the terminology. In the same way one
can define other (conserved or not) asymptotical
physical quantities like total electric charge and
gauge charges. See ADM form of the Einstein–
Hilbert action, asymptotic flatness.
Adrastea Moon of Jupiter, also designated
JXV. Discovered by Jewitt, Danielson, and Synnott
in 1979, its orbit lies very close to that
of Metis, with an eccentricity and inclination
© 2001 by CRC Press LLC
that are very nearly 0 and a semimajor axis of
1.29 × 10 5 km. Its size is 12.5 × 10 × 7.5 km,
its mass, 1.90 ×10 16 kg, and its density roughly
4 gcm −3 . It has a geometric albedo of 0.05 and
orbits Jupiter once every 0.298 Earth days.
ADV(AcousticDopplerVelocimeter) Adevice
that measures fluid velocity by making use
of the Doppler Effect. Sound is emitted at a
specific frequency, is reflected off of particles in
the fluid, and returns to the instrument with a
frequency shift if the fluid is moving. Speed of
the fluid (along the sound travel path) may be
determined from the frequency shift. Multiple
sender-receiver pairs are used to allow 3-D flow
measurements.
advance of the perihelion In unperturbed
Newtonian dynamics, planetary orbits around a
spherical sun are ellipses fixed in space. Many
perturbations in more realistic situations, for instance
perturbations from other planets, contribute
to a secular shift in orbits, including a
rotation of the orbit in its plane, a precession of
the perihelion. General relativity predicts a specific
advance of the perihelion of planets, equal
to 43 sec of arc per century for Mercury, and
this is observationally verified. Other planets
have substantially smaller advance of their perihelion:
for Venus the general relativity prediction
is 8.6 sec of arc per century, and for Earth
the prediction is 3.8 sec of arc per century. These
are currently unmeasurable.
The binary pulsar (PSR 1913+16) has an observable
periastron advance of 4.227 ◦ /year, consistent
with the general relativity prediction. See
binary pulsar.
advection The transport of a physical property
by entrainment in a moving medium. Wind
advects water vapor entrained in the air, for instance.
advection dominated accretion disks Accretion
disks in which the radial transport of
heat becomes relevant to the disk structure. The
advection-dominated disk differs from the standard
geometrically thin accretion disk model because
the energy released by viscous dissipation
is not radiated locally, but rather advected toward
the central star or black hole. As a conse-