Direct registration of gravitational waves
1 March 2016
The gravitational waves from merger of two black holes were for the first time registered directly by the gravitational wave detector LIGO. Gravitational waves were predicted by A. Einstein in 1916, but their existence has been up to now revealed only indirectly by the change in the orbit of binary pulsar PSR B1913+16. The first direct registration took place on September 14, 2015 by two independent laser interferometers located in USA at a distance of 3 thousand kilometers from each other. The signal designated as GW150914 was registered by two detectors with a time lag of 7 ms caused by the gravitational wave propagation at the velocity of light. With allowance for this time shift and different orientations of the detectors the signal shape in the two detectors coincides up to an accuracy of measurement noises. The principle outline of the interferential experiment for gravitational wave detection realized in LIGO detectors was proposed by M.E. Gertsenstein and V.I. Pustovoit in 1962 and the fundamental contribution to elaboration of the method of recording superweak signals was made by V.B. Braginskii, K. Thorne and their colleagues (see, e.g., the reviews in
Sov. Phys. Usp. 8 513 (1966), Phys. Usp. 43 691 (2000), Phys. Usp. 44 1 (2001)). The detectors are modified Michelson interferometers with 4-km arms isolated thoroughly from seismic and other noises. A gravitational wave changes the arm length thus causing a phase sift of the laser signals propagating along them, which results in a shift of the observed interference pattern. The sensitivity of LIGO interferometers makes noticeable the extension of their arms by mere 10-17 cm. The GW150914 signal frequency during the observation time (∼0.2 s) increased from 35 to 250 Hz and had a peak amplitude h=1.0×10-21. The shape of GW150914 corresponds exactly to the prediction of General Relativity for the last stage before the merging of two black holes rotating around each other along the orbit and also for the signal from damping oscillations of the rotating black hole formed after merging. It was revealed that merged had been black holes with masses 36 M☉ and 29 M☉ at a distance of 410+160-180 Mpc from the Earth. In these observations, the signal-to-noise ratio reached 24 and the statistical significance of signal recording made up 5.1σ. Thus, the existence of binary systems was demonstrated that consist of black holes with nearly maximum masses which might have occurred in explosions of supernovae collapsing in the course of the standard star evolution. It is not excluded, however, that black holes were born as a result of collapses of massive pre-galactic stars or the merging of smaller-mass black holes in dense star clusters. As a concomitant result the restriction was obtained on the graviton mass mg<1.2×10-22 eV. This restriction is however smaller than that obtained earlier from the dynamics of galactic clusters and weak lensing. Localization of GW150914 on the sky according to the LIGO data had the shape of a band 600 square degrees in area. A weak gamma-ray burst from that band was registered by the Fermi Gamma-ray burst Monitor (GRM) in cosmic observatory 0.4 s after GW150914. No ground-based or astrophysical sources of the burst have been observed. It is not yet clear how a gamma-ray signal could be generated upon the merging of two black holes if it was not an accidental projection. The observation of GW150914 opens the era of gravitational-wave astronomy and will make it possible to verify the modifications of General Relativity in future. Russian researchers from MSU and the Institute of Applied Physics (Nizhnii Novgorod) participate in LIGO collaboration.
Sources: Phys. Rev. Lett. 116 061102 (2016), arXiv:1602.03920 [astro-ph.HE]
1 March 2016
The possibility of the existence of stable or short-lived bound states of four neutrons — tetraneutrons 4n has long been studied both theoretically and experimentally, but it is only in one experiment (F.M. Marques et al. 2002) that the bound state 4n was observed in the reaction 14Be → 10Be+4n. However, this result has not been confirmed in the subsequent studies. K. Kisamori (The University of Tokyo and the Institute of Physical and Chemical research RIKEN, Japan) with colleagues carried out a new experiment and possibly registered the birth of the resonant state 4n in the reaction 4He(8He,8Be). The 8He ion beam was obtained in the interaction of an 18O beam with a beryllium target after which the beam collided in turn with a liquid helium target. 8He and the final reaction products, namely, two alpha particles into which 8Be decayed, were registered by the coincidence method. The 4n systems were not registered directly but were revealed on the basis of conservation laws. The discovered maximum in the spectrum of the reaction products is likely to correspond to the resonant state 4n, although the stable state cannot be excluded. Four 4n production events were registered and the statistical significance of the result was estimated to be at the level of 4.9σ. The results of the experiment are consistent with the data of F.M. Marques et al., but independent experiments are needed for their verification.
Source: Phys. Rev. Lett. 116 052501 (2016)
Quantum limited long distance heat conduction
1 March 2016
Quantum mechanics sets a fundamental limit (quantum of heat conduction) on the maximum heat flux that can be transferred through one channel. In the previous experiments, heat near the quantum limit could only be transferred through small distances less than 100 µm. M. Mottonen (Aalto University, Finland) and his colleagues were the first to demonstrate heat transfer with a flux approaching the quantum limit through macroscopic distances up to 1 meter. Photons of microwave radiation transferred through a superconducting volute waveguide were used as heat carriers. Photons are electrically neutral, and therefore their interaction with the surrounding substance is weaker, which reduces the loss compared to electrons. Superconducting tunnel contacts were used as sensitive thermometers. The results of the experiment agree well with theoretical calculations and can be applied to cool elements in nanoelectronics.
Source: Nature Physics, online publication of February 1, 2016
RadioAstron observations with angular resolution of 21 µas.
1 March 2016
The first results of interferometric observations with a very long base (VLBI) are presented which were performed using a 10-meter space radio telescope RadioAstron and 15 ground-based radio telescopes. RadioAstron on board the Russian Spektr-R satellite makes up the base 7.9 Earth diameters long, which allowed observations with the now record angular resolution of 21 µas. The jet structure was observed in BL Lacertae — an active-nucleus galaxy which gave the name of the whole class of galaxies — blazars. At distances of 40, 100 and 250 µas from the jet base nodes can be seen that could be due to shock waves. Two polarized radiation components are observed at a distance of 0.5µas from the nucleus. A gradient in the measure of Faraday rotation was revealed in ground-based space observations at different frequencies. This indicates the presence of a spiral magnetic field in the jet. It was concluded that the magnetic field and the radiating particles show no energy equipartition because the brightness temperature in the nucleus exceeds 3×1012 K. The RadioAstron project is implemented by research workers of Astro Space Center of P.N. Lebedev Physical Institute (ASC LPI) in collaboration with Russian and foreign colleagues.
Source: Astrophysical journal 817 96 (2016)
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