1,010 research outputs found
On the Bel radiative gravitational fields
We analyze the concept of intrinsic radiative gravitational fields defined by
Bel and we show that the three radiative types, N, III and II, correspond with
the three following different physical situations: {\it pure radiation}, {\it
asymptotic pure radiation} and {\it generic} (non pure, non asymptotic pure)
{\it radiation}. We introduce the concept of {\em observer at rest} with
respect to the gravitational field and that of {\em proper super-energy} of the
gravitational field and we show that, for non radiative fields, the minimum
value of the relative super-energy density is the proper super-energy density,
which is acquired by the observers at rest with respect to the field. Several
{\it super-energy inequalities} are also examined.Comment: 15 pages; no figures; v2: minor change
Creatable Universes
We consider the question of properly defining energy and momenta for non
asymptotic Minkowskian spaces in general relativity. Only spaces of this type,
whose energy, linear 3-momentum, and intrinsic angular momentum vanish, would
be candidates for creatable universes, that is, for universes which could have
arisen from a vacuum quantum fluctuation. Given a universe, we completely
characterize the family of coordinate systems for which one could sensibly say
that this universe is a creatable universe.Comment: 19 pages; v2: we have added an Appendix to make explicit the proof of
the quasi-uniqueness of the defined energy and momenta of the Universe, for
every space-like 3-surfac
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