M. Gasperini

Causal horizons, accelerations and strings

All the points of a string are always causally connected provided their relative acceleration is smaller than the critical valueac= (mαá)−1 wherem is the mass and 1/α′ the string tension. It is pointed out that this limiting acceleration characterizes the transition to an unstable regime, in which an approximate description of the string motion around the classical path of a point particle is no longer consistent.

On the Maxwell equations in a Riemann-Cartan space

Abstract Considering the vacuum polarization effect in the presence of torsion, the modifications of the Maxwell equations are explicitly computated to first order in the spin-torsion coupling constant.

Strong gravity with torsion: Some cosmological deduction

Recently i t has been pointed out (1.~) tha t the s t rong-gravi ty theory (3) provides the physical basis for the large-number hypothesis of Dirac (4). The purpose of the present paper is to show that , introducing spin and torsion into the s t rong-gravi ty equations according to the Einstein-Cartan formalism (5), the Dirac law can be extended also to the angular momentum, an4 the total spin of the Universe may be evaluate4 and related to the IIubble radius. If one considers the Universe and a hadron as two physical systems internal ly governed by similar laws, differing only for a scale-factor which carries the Newton gravi ta t ional field into the s trong-gravity field (6.s), one is led to assume tha t the hadronic s trong-gravity potent ia l q~(h)~kfm/rc 2 is of the same order of magnitude as the gravi ta t ional potcnt ial of the Universe ~ ( u ) ~ GM/Rc 2, and the following relat ion is deduced (1.2,6,7):

Constraints on unified theories from the experimental tests of the equivalence principle

The predictions of a general unified theory for the gravitational, electromagnetic and scalar field are compared with the results of the experimental tests of the equivalence principle. It is shown that the theoretical predictions do not disagree with experimental data provided that the coupling of the scalar to the electromagnetic field is suppressed by a factork ∼ 10−3, or, alternatively, the scalar field is massive; in this case, a lower limit for its mass is obtained.

The angular momentum of celestial bodies and the fundamental dimensionless constants of nature

SummaryIt is suggested a possible connection between two empirical astrophysical laws concerning the angular momentum of celestial bodies. It is shown that both relations seem to indicate that the fine-structure constant α is the typical dimensionless parameter not only of atomic physics, but also of gravitational physics.

Propagating torsion and electromagnetic gauge invariance

Abstract A photon-torsion interaction is proposed, preserving local gauge invariance in its usual form. Deviations from geodesical motion at a macroscopical level are predicted in the case of gravitational interactions between two polarized bodies.

A « semi-minimal » coupling principle for the electromagnetic field in a space with torsion

In order to preserve local gauge invar iance , i t is well known (1-7) tha t a gauge field cannot be min imal ly coupled to the geome t ry of a space wi th a nonsymmet r i c connect ion such as a R iemann-Car tan space U 4 I f we t ry to make compat ible min imal coupling wi th gauge invar iance in the case of the e lec t romagnet ic (e.m.) field, we are compelled to impose a rb i t ra ry constraints upon torsion (s), or to modify the usual concept of gauge t ransformat ion and to allow torsion propaga t ion in v a c u u m (9). Since in this w a y we get over the l imits of the Eins te in-Car tan theory, where torsion is s t r ic t ly de te rmined by spin dis t r ibut ions and cannot p ropaga te outside ma t t e r (1), i t seems t h a t the only way to recover the e.m. gauge invar iance, in the f ramework of this theory, is to replace the min imal coupl ing principle (MCP) wi th some other empir ical prescription. The s implest hypothesis to preserve gauge invar iance is to assume tha t there is no coupling at all be tween torsion and the e.m. field, i.e. t h a t the Maxwell equat ions , wr i t ten in a ,U4, are exact ly the same as if t hey were wr i t t en in the R iemann space V 4 obta ined f rom U 4 by pu t t ing torsion to zero. This prescr ipt ion finds a ve ry s t rong and na tura l just i f icat ion in the f ramework of the Poincard gauge field theories of g rav i ta t ion (lo), where the gauge fields (like the e.m. potent ia l Ak) are t rea ted as Poineard scalars and then do no t couple to the connect ion; one can say, then, t ha t (1,5.6).

The role of spin in a Hermitian theory of gravity

SummaryWe propose a Hermitian gravitational theory in which the skew part of the metric tensor is generated by the spin density of matter, and we investigate the gauge invariance of the Lagrangian in the weak-field approximation. We find a close analogy between our second-order Lagrangian and the linearized Lagrangian of an alternative off-shell formulation ofN=1 supergravity.

Gravitation Without Lorentz Invariance

As is well known, general relativity is a theory of gravity based on the requirement of local Lorentz symmetry.

Gravitational waves and massive tensor particles in thef-g theory of gravity

Starting from the generally covariant version of the Pauli-Fierz mass term, it is stressed that the tensor fields representing spin-2 particles, eigenstates of strong and gravitational interactions, are linear combinations of one massive and one massless state. This implies that any hadronic reaction, in which massive tensor particles are produced, can be regarded, at least in principle, also as an effective source of gravitons which may become very important in the early stages of the universe; conversely, any process in which gravitational radiation of sufficiently high energy is emitted, should be a source of strongly interacting tensor particles which decay into photons and neutrinos. These two effects could be used for producing and detecting gravitational waves.