The test of the equivalence principle
1 January 2018
According to the Weak Equivalence Principle (EP) holding true within the General Relativity, all the bodies irrespective of their composition fall in the gravitational field with equal acceleration, but some theories predict EP violation. Researchers from the French Aerospace Research Center ONERA and the Cote d'Azur Observatory carried out an experiment on the MICROSCOPE satellite to test EP with a record precision of ≈ 10−14. On board the satellite there are two hollow coaxial cylinders suspended freely in a gravity-free state. The inner cylinder is made of platinum-rhodium alloy and the outer one of the titanium-aluminum-vanadium alloy. For the test, another such system existed but with cylinders made of the same material (Pt and Rh alloy). The forces necessary for keeping the inner and outer cylinders motionless relative to each other were measured by electrostatic sensors. The existence of a signal modulated with a satellite rotation frequency would testify to EP violation. No such violation has been revealed up to now, and the Eotvos parameter was limited to the value δ (Ti,Pt)=[-1 ± 9(stat.) ± 9(syst.)]×10−15. This result improves the preceding limit by an order of magnitude. The result obtained by V.B. Braginskii with colleagues in MSU had remained the best in precision (at a level of ≈ 10−12) for many years (see Sov. Phys. Usp. 14 811 (1972), Physics-Uspekhi 52 1 (2009)).
Source: Phys. Rev. Lett. 119 231101 (2017)
Search for axions
1 January 2018
The axion field was initially proposed to explain the absence of CP violation in strong interactions, but the quanta of this field – axions – appeared to be good candidates for the role of dark matter particles. In the interaction with a coherently oscillating axion field, the electric dipole moments of nucleons will oscillate, which will affect the character of their spin precession. C. Abel (the University of Sussex, Great Britain) with colleagues reported new results of the search for axions from the data of the Sussex-RAL-ILL experiment which was carried out in 1998-2002 at the Institute Laue-Langevin in France. Measured was the frequency ratio of ultracold neutrino and 199Hg atom spin precession in a trap with magnetic and electric fields. The axion field was assumed to constitute dark matter in the Galaxy, and therefore its local density and velocity are known. No anomalous precession was revealed, but limits were obtained on the axion masses ma and the coupling constants of the axion field with gluons and nucleons. In particular, the masses 10−24 eV ≤ ma ≤ 10−17 eV were excluded. The new laboratory limits on the coupling constants are better by three orders of magnitude then the astrophysical limits.
Source: Phys. Rev. X 7 041034 (2017)
Photonuclear reactions in a lightning discharge
1 January 2018
T. Enoto (KyotoUniversity, Japan) et al. reported observation of gamma rays generated by photonuclear reactions and annihilation of positrons e+ by lightning discharges. Lightnings are natural particle accelerators (see the review of A.V. Gurevich and K.P. Zybin in Physics-Uspekhi 44 1119 (2001)). The avalanches of runaway e- that develop in strong electric fields generate bremsstrahlung gamma-ray emission. Such gamma-ray bursts have already been registered by ground- and airplane-based detectors and have also been observed by cosmic telescopes as terrestrial gamma-ray bursts. The energy of these gamma rays has theoretically been predicted to suffice for photonuclear reaction generation in atmosphere. Such reactions have not earlier been observed with statistical confidence, although the registration of neutrons and e+ presumably born in these reactions has been reported. In particular, the occurrence of n upon lightning was noticed at Tyan Shan mountain station (see Physics-Uspekhi 55 532 (2012)). On February 6, 2017, T. Enoto with colleagues registered at a distance of 0.5-1.7 km from the lightning discharge a gamma-ray burst less than 1 ms long simultaneously with lightning. Then gamma-ray emission was observed which was fading out exponentially during 200 ms and an additional retarding pulse with a spectral maximum at 0.511 MeV. These observations are interpreted by following chain of events. The gamma-ray burst simultaneous with the lightning was induced by bremsstrahlung e- emission in the discharge channel. The gamma-ray photons interacted with the air molecules with the formation of unstable 13N and n nuclei in the reaction 14N+γ → 13N+n. The neutrons interacted with 14N with the formation of 14C and 15N nuclei in an excited state, and upon their transition to the ground state gamma-ray photons with energies <10,8 MeV were emitted and were observed after the lightning. Then happened β+-decays of 13N nuclei into stable 13C nuclei with the emission of e+. The e+e- annihilation yielded the observed gamma-ray signal with the energy of 0.511 MeV corresponding to the mass e+. One minute’s delay after the lightning corresponds to the direction and velocity of the wind that carried over the cloud with 13N nuclei to the detectors. The described observations testify convincingly to the presence of photonuclear reactions in lightnings.
Source: Nature 551 481 (2017)
Gravitational field variations under the earthquake
1 January 2018
An earthquake induces the motion of large masses of soil, which results in gravitational field variations. As distinct from elastic P-waves propagating in the Earth crust and in the upper mantle at a velocity of 6 to 10 km s−1, the gravitational field perturbations move at the velocity of light. M. Vallee (Paris Diderot University, France) with colleagues registered indirectly these perturbations during the 2011 Tohoku (Japan) earthquake 9.1 in magnitude. The data of 11 seismic stations located in Asia 1-2 thousand kilometers from the seismic center were used. The gravitational-field earthquake-induced perturbations produced secondary seismic waves near these stations. The gravitational and secondary seismic signals in sum caused acceleration variations of ≈ 1 nm s−2 which were registered by the seismic stations before primary P-waves. The registration of gravitational perturbations opens a new important channel of information about strong earthquakes.
Source: Science 358 1164 (2017)
Quasar at a redshift z=7.54
1 January 2018
E. Banados (Carnegie Observatory in Pasadena, USA) et al. revealed a quasar at red shift z= 7.54 when the Universe was only 690 million years old (the previous record was z=7.09).The bolometric luminosity of the quasar corresponds to the black hole (BH) mass 8×108M☉. If this BHis assumed to increase constantly its mass under accretion in the Eddington regime, then at a redshift z ≈ 40 black holes with mass ≥104M☉ must have existed that had an enigmatic origin. The singularities of the spectrum show that the quasar was observed when the surrounding gas was largely neutral, i.e., the reionization process has not yet been finished.
Source: Nature, online publication of December 6, 2017
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.
Permanent editor is Yu.N. Eroshenko.
It is compiled from a multitude of Internet sources.