V. de Sabbata

Conversion of photons into gravitons and vice versa in a static electromagnetic field

SummaryProduction of gravitational waves by photons incident on a static electromagnetic field as well as the production of photons by gravitational waves incident on a static electromagnetic field are studied. The possibility of using these results for the detection of gravitational waves is also examined.RiassuntoSi studia la produzione di onde gravitazionali da fotoni incidenti su un campo elettromagnetico statico e la produzione di fotoni da onde gravitazionali incidenti sempre su un campo elettromagnetico statico. Si esamina inoltre la possibilità di usare i risultati ottenuti per rivelare le onde gravitazionali.РеэюмеИсследуется рождение гравитационных волн фотонами в статическом злектромагнитном поле, а также и рождение фотонов гравитационными волнами в статическом злектромагнитном поле. Также иэучается воэможность испольэования зтих реэультатов для детектирования гравитационных волн.

The importance of spin and torsion in the early universe

SummaryIt is shown the connection between torsion, which is a necessary geometrical feature of the space-time when gravity is considered in the realm of elementary-particle physics as, for instance, in the early universe, and some property of string theory. We emphasize also the role of torsion when one tries to quantize the gravitational field, that is to quantize the space-time itself. Finally it is shown that through the time-temperature uncertainty relation it is possible to arrive at the notion of a minimum acceleration that can be important in connection with observations involving galaxies.

Graviton emission by photons in a gravitational field

SummaryWe calculate the cross-section for the emission of a graviton by a photon interacting with an external gravit ational field using the quantum formulation of the linearized equations of general relativity. After showing that the chosen Lagrangian interaction gives the correct formula for the bending of a light beam as predicted by Einstein, the calculation of the matrix element for the emission of a graviton is carried out in detail. It is shown that this interaction is completely negligible and cannot explain, for instance, the cosmological red-shift of light. A more effective process may be the splitting of a photon into three ones in an external gravitational field.RiassuntoSi calcola la sezione d’urto per l’emissione di gravitoni da parte di fotoni interagenti con un campo gravitazionale, per mezzo della formulazione quantistica della teoria linearizzata della relatività generale. Dopo aver fatto vedere che con la lagrangiana di interazione usata si trova l’incurvamento dei raggi luminosi così come previsto da Einstein, si passa al calcolo esplicito dell’elemento di matrice per il detto processo. Si trova che la sezione d’urto è del tutto trascurabile e che non si può, ad esempio, render conto in questa maniera dello spostamento cosmologico verso il rosso. Un processo più efficace potrebbe essere, però, la separazione di un fotone in tre in un campo gravitazionale.РезюмеМы вычисляем поперечное сечение излучения гравитона фотоном, взаимодействуюшим с внешним гравитационным полем, используя квантовую формулуировку линеаризованных уравнений обшей теории относительности. Показав, что выбранный Лагранжиан взаимодействия дает правильную формулу для искривления светового пучка, как у Эйнштейна, подробно проводим вычисление матричного элемента для излучения гравитона. Показано, что это взаимодействие полностью пренебрежимо и не может, например, обьяснить космологическое красное смешение света. Более сушественным пожет быть расшепление одного фотона в три во внешнем гравитационном поле.

Magnetic fields in the early universe

SummaryWe consider some mechanisms which can be responsible for the formation of a magnetic field in the early universe. It happens that introducing the spin-torsion coupling interaction a magnetic field results, compatible with observation data, which may be important in the process of galaxy formation.

Strong gravity as the connecting link underlying universal relations between angular momenta of celestial bodies and spin of elementary particles

SummaryEmpirical relations between the angular momenta of a wide range of celestial bodies as well as relations between masses and spins of hadrons are shown to have the same fundamental basis with strong gravity as the underlying link. Magnetic moments of celestial bodies and elementary particles are also discussed in this context.RiassuntoSi mostra che le relazioni empiriche tra i momenti angolari di una grande varietà di corpi celesti ed anche le relazioni tra le masse e gli spin degli adroni hanno la stessa base fondamentale, dove la gravità forte costituisce il legame comune. In questo contesto vengono discussi anche i momenti magnetici dei corpi celesti e delle particelle elementari.РезюмеПоказывается, что эмпирические соотношения между утловыми моментами широкого класса небесных тел, а также соотношения между массами и спинами адронов имеют одну и ту же фундаментальную основу, где сильная гравитация представляет соединительное звено. В этом контексте также обсуждаются магнитные моменты небесных тел и элементарных частиц.

Gravitons in the universe

SummaryIn order to calculate the density of gravitons in the universe, we examine various processes that can contribute to the production of gravitons and we give an estimate of the cross-sections for their production by two photons that react in presence of a gravitational field. We give also the cross-sections for the inverse processes, that is for the destruction of gravitons. After having examined other processes of production, in particular the production by close binary systems of stars, we find that in the case of the evolutionary universe, that is, in a universe with an age of ∼1018 s, the situation is far from equilibrium and the gravitons are still accumulating. At the present stage the mean density of gravitons in the whole universe seems to be ∼ 10−19 erg/cm3.RiassuntoSi prendono in esame vari processi di produzione e distruzione dei gravitoni e si fa una stima della densità dei gravitoni nell’universo. In particolare si calcolano le sezioni d’urto di produzione di gravitoni da parte di fotoni in presenza di campi gravitazionali esterni quali quelli forniti dalle stelle, dalle galassie e dalle quasar. Nel caso di un universo di tipo evolutivo a cui si attribuisca l’età di 1018 s, si trova che la densità dei gravitoni non ha raggiunto uno stadio di equilibrio ma è in continuo aumento. Attualmente tale densità media sembra essere di ∼ 10−19 erg/cm3.РеэюмеС целью вычислить плотность гравитонов во Вселенной, мы исследуем раэличные процессы, которые могут дать вклад в рождение гравитонов, и мы даем оценку поперечных сечений для их рождения при столкновении двух фотонов в гравитационном поле. Мы приводим также поперечные сечения для обратных процессов, которые обуславливают раэвал гравитонов. После исследования других процессов рождения, в частности, рождения блиэкими двойными системами эвеэд, мы получаем, что в случае зволюционируюшейся Вселенной, т.е. Вселенной с воэрастом ∼1018 сек, ситуация далека от равновесия, и гравитоны еше аккумулируются. На данном зтапе средняя плотность гравитонов во всей Вселенной равна ∼10−19 зрг/см3.

Splitting of a photon into two photons in the coulomb field of a nucleus

SummaryThe cross-section for the splitting of a photon into two photons in the Coulomb field of a nucleus is determined with the standard techniques of the quantum electrodynamics for the case in which the energy of the incident photon is smaller than two electron masses. Numerical evaluations for some values of the variables are made in this energy range.RiassuntoViene calcolata la sezione d’urto per la conversione di un fotone in due fotoni nel campo Coulombiano di un nucleo, con le tecniche della elettrodinamica quantistica. Il calcolo è fatto per energie del fotone incidente minori di due masse elettroniche. In tale intervallo d’energia sono dati i risultati numerici per alcuni valori delle variabili.

Gravitational scattering of two- and four-component neutrinos

SummaryThe gravitational scattering of neutrinos is considered in the one-graviton-exchange approximation. In this scheme it is found that two- and four-component neutrinos are scattered in the same amount by bosons and in the small-angle limit have the same cross-sections as photons. Only neutrino-neutrino scattering gives two slightly different cross-sections for the two- and the four-component cases.RiassuntoSi prende in esame la diffusione gravitazionale dei neutrini nell’approssimazione in oui viene scambiato un gravitone. In questo schema si trova che i neutrini a due e a quattro componenti sono diffusi nella stessa maniera sia da parte di una massa grande, sia da parte di particelle scalari, sia da parte di fotoni ed inoltre per angoli piccoli la sezione d’urto è identica, nei vari casi detti, a quella di diffusione dei fotoni. Solo nel caso della diffusione gravitazionale di neutrini da parte di neutrini si trova che i due casi, che si riferiscono ai neutrini a due e a quattro componenti, danno sezioni d’urto diverse. Cio potrebbe essere significativo nei problemi cosmologici.РезюмеВ приближении обмена одним гравитоном рассматривается рассеяние нейтрино. В этой схеме получается, что двух-и четырех-компонентны е нейтрино рассеиваются таким же образом, как бозоны, а в пределе малых углов имеют те же поперечные сечения, как фотоны. Только рассеяние нейтрино на нейтрино дает два слегка различных поперечных сечения для случая двух и четырех компонент.

Anyons and torsion

SummaryWe consider, in analogy to electromagnetic anyons as charge magnetic-flux composite, the gravitational anyons as mass-spin )energy-spin) composite. We show that torsion interaction has many features in common with the Chern-Simons term, so supporting the possiblity of a gravitational anyon arising from torsion without introduction of the Chern-Simons term (and so without any arbitrary parameter).

On the quantization of general relativity in anholonomic variables

In discussing Bohr-Sommerfeld-like quantum rules for gravity, it is argued that Einsteins Riemannian theory of general relativity rather leads to a quantum field-mechanics than to a quantum-field theory of gravity. We construct the canonically conjugate coordinates and momenta of this gravito-dynamics in the framework of the Einstein-Cartan teleparallelism.