DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

tension head (ψ)
is the energy-momentum tensor computed from
the microscopic field configuration, in the particular
case of a straight string. For an ordinary
relativistic string we haveT tt +T zz = 0; hence
we recover the equation of state for a structureless
Goto–Nambu string, where U (the energy
per unit length) and T are constant and equal
to each other. For more general vortex-forming
field theories, the corresponding cosmic string
model will be characterized by variable tension
and energy per unit length which, in a generic
state, will be related by an inequality of the form
T ≤ U. See duality in elastic string models, energy
per unit length (cosmic string), equation of
state (cosmic string), Goto–Nambu string.
tension head (ψ) There is tension on pore
water in the unsaturated zone because water is
held to the mineral grains by surface-tension
forces. It is conventional to measure pressure
(p) relative to atmospheric pressure, thus in the
unsaturated zone p0. Negative
pressure is often called suction or tension, and
ψ is called tension head when p