Astroparticle Physics

134 6 Primary Cosmic Rayslow interaction probabilityof gravitational radiationimaging with gravity waves?quadrupole radiationaround 1950. At that time nobody really doubted the existenceof the neutrino, but there was no strong neutrino sourceavailable to test the prediction. Only the oncoming nuclearreactors were sufficiently powerful to provide a large enoughneutrino flux to be observed. The problem was related to thelow interaction cross section of neutrinos with matter.Compared to gravitational waves the neutrino interactionwith matter can be called ‘strong’. The extremely lowinteraction probability of gravitational waves ensures thatthey provide a new window to cataclysmic processes in theuniverse at the expense of a very difficult detection. For neutrinosmost astrophysical sources are almost transparent. Becauseof the feeble interaction of gravitational waves theycan propagate out of the most violent cosmological sourceseven more freely compared to neutrinos.With electromagnetic radiation in various spectral rangesastronomical objects can be imaged. This is because thewavelength of electromagnetic radiation is generally verysmall compared to the size of astrophysical objects. Thewavelength of gravitational waves is much larger so thatimaging with this radiation is almost out of question. In elec-tromagnetic radiation time-dependent electric and magneticfields propagate through space-time. For gravitational wavesone is dealing with oscillations of space-time itself.Electromagnetic radiation is emitted when electriccharges are accelerated or decelerated. In a similar waygravitational waves are created whenever there is a nonsphericalacceleration of mass–energy distributions. Thereis, however, one important difference. Electromagnetic radiationis dipole radiation, while gravitational waves presentquadrupole radiation. This is equivalent to saying that thequantum of gravitational waves, the graviton, has spin 2¯h.Electromagnetic waves are created by time-varying dipolemoments where a dipole consists of one positive andone negative charge. In contrast, gravity has no charge, thereis only positive mass. Negative mass does not exist. Even an-timatter has the same positive mass just as ordinary matter.Therefore, it is not possible to produce an oscillating massdipole. In a two-body system one mass accelerated to theleft creates, because of momentum conservation, an equaland opposite action on the second mass which moves it tothe right. For two equal masses the spacing may change butthe center of mass remains unaltered. Consequently, there isno monopole or dipole moment. Therefore, the lowest orderof oscillation generating gravitational waves is due tooscillation modes of antennae