Narrow Results

Accretion disks around stellar-mass black holes (BHs) are observed to emit radiation peaking in the soft X-rays (at about 1 keV) when the source is in the thermal state. This emission is expected to be polarized, because of electron scattering in the disk. However, photons will experience a rotation of the polarization plane due to General Relativity effects. X-ray polarization measurements can then be crucial in assessing strong gravity effects around accreting stellar-mass BHs, providing independent constraints on their spin. In this work, we present the simulated polarization spectra (obtained according to the model by Dovčiak et al.) for the X-ray, thermal radiation emitted by the accretion disk, as expected to be measured by IXPE, an X-ray polarimeter which will be launched in 2021. We also illustrate the code, currently under development, incorporate the contribution of returning radiation and account for absorption effects by the disk material.

The notion of observers’ and their measurements is closely tied to the Lorentzian metric geometry of spacetime, which in turn has its roots in the symmetries of Maxwell’s theory of electrodynamics. Modifying either the one, the other or both ingredients to our modern understanding of physics, requires also a reformulation of the observer model used. In this presentation we will consider a generalized theory of electrodynamics, so called local and linear premetric, or area metric, electrodynamics and its corresponding spacetime structure. On this basis we will describe an observer’s measurement of time and spatial length. A general algorithm how to determine observer measurements will be outlined and explicitly applied to a first order premetric perturbation of Maxwell electrodynamics. The latter contains for example the photon sector of the minimal standard model extension. Having understood an observer’s measurement of time and length we will derive the relativistic observables time dilation and length contraction. In the future a modern relativistic description of the classical test of special relativity shall be performed, including a consistent observer model.

As a matter of fact, modern physics is not taught in the majority of secondary schools; however, recent researches suggest that there are several advantages in introducing modern physics early in school. This is, for instance, the approach of the Einstein-First project. In the context of this approach, we report here some preliminary results of an intervention in the first year of Italian secondary school.

The AstroCamp is an academic excellence program in the field of astronomy and physics for students in the last 3 years of pre-university education, which often includes a course (or a significant part thereof) on Relativity. After an introduction to the principles, goals and structure of the camp, I describe the approach followed by camp lecturers (myself and others) for teaching Special and General Relativity, and some lessons learned and feedback from the students. I also provide some thoughts on the differences between the physics and mathematics secondary school curricula in Portugal and in other countries, and on how these curricula could be modernized.

Numerical simulations in the relativistic regime have certainly become a powerful investigation tool for High Energy Astrophysics. Here we show a simple method to provide any numerical code with a complete set of diagnostic tools aimed at producing synthetic synchrotron emissivity, polarization, and spectral index maps for comparison with observations. The method is based on the addition to the numerical code of an evolution equation for the maximum energy of the emitting particles, and this takes into account both adiabatic and radiative losses along fluid streamlines. As an application, we consider here the case of Pulsar Wind Nebulae (PWNe), for which the method is applied on top of an axisymmetric MHD model. Results are presented and discussed.

We show that the soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) can be explained as recently proposed highly magnetized white dwarfs (B-WDs). The radius and magnetic field of B-WDs are perfectly adequate to explain energies in SGRs/AXPs as the rotationally powered energy. While the highly magnetized neutron stars require an extra, observationally not well established yet, source of energy, the magnetized white dwarfs, yet following Chandrasekhar’s theory (C-WDs), exhibit large ultra-violet luminosity which is observationally constrained from a strict upper limit.

Physicists’ understanding of relativity and the way it is handled is up to present days dominated by the interpretation of Albert Einstein, who related relativity to specific properties of space and time. The principal alternative to Einstein’s interpretation is based on a concept proposed by Hendrik A. Lorentz, which uses knowledge of classical physics alone to explain relativistic phenomena. In this paper, we will show that on the one hand the Lorentz-based interpretation provides a simpler mathematical way of arriving at the known results for both Special and General Relativity. On the other hand, it is able to solve problems which have remained open to this day. Furthermore, a particle model will be presented, based on Lorentzian relativity and the quantum mechanical concept of Louis de Broglie, which explains the origin of mass without the use of the Higgs mechanism. It is based on the finiteness of the speed of light and provides classical results for particle properties which are currently only accessible through quantum mechanics. NOTE: This paper is a corrected update of the proceedings of the Vigier meeting in Baltimore 2014.

Since 1964, the NASA Goddard Space Flight Center (GSFC) has been using short pulse lasers to range to artificial satellites equipped with passive retroreflectors. Today, a global network of 40 satellite laser ranging (SLR) stations, under the auspices of the International Laser Ranging Service (ILRS), routinely tracks two dozen international space missions with few-millimeter precision using picosecond pulse lasers in support of Earth science. Lunar laser ranging (LLR) began in 1969, shortly after NASA's Apollo 11 mission placed the first of five retroreflector packages on the Moon. An important LLR data product has been the verification of Einstein's equivalence principle and other tests of general relativity. In 1975, the University of Maryland used a laser ranging system to continuously transfer time between two sets of atomic clocks — one set on the ground and the other in an aircraft — to observe the predicted relativistic effects of gravity and velocity on the clock rates. Two-way asynchronous laser transponders promise to extend these precise ranging and time transfer capabilities beyond the Moon to the planets, as evidenced by two successful experiments carried out in 2005 at distances of 24 and 80 million km respectively.

The Gamma-Ray Large Area Space Telescope (GLAST), to be launched in the fall of 2007, will measure the spectra of distant extragalactic sources of high energy γ-rays, particularly active galactic nuclei and γ-ray bursts. GLAST can look for energy-dependent γ-ray propagation effects from such sources as a signal of Lorentz invariance violation (LIV). These sources should also exhibit the high energy cutoffs predicted to be the result of intergalactic annihilation interactions with low energy photons having a flux level as determined by various astronomical observations. Such annihilations result in electron–positron pair production above a threshold energy given by 2m_e in the center-of-momentum frame of the system, assuming Lorentz invariance. If Lorentz invariance is violated, this threshold can be significantly raised, changing the predicted absorption turnover in the observed spectrum of the sources. Stecker and Glashow have shown that the existence of such absorption features in the spectra of extragalactic sources puts constraints on LIV. Such constraints have important implications for some quantum gravity and large extra dimension models. Future spaceborne detectors dedicated to measuring γ-ray polarization can look for birefringence effects as a possible signal of loop quantum gravity.

As shown by Coleman and Glashow, a much smaller amount of LIV has potential implications for possibly suppressing the "GZK cutoff" predicted to be caused by the interactions of cosmic rays having multijoule energies with photons of the 2.7 K cosmic background radiation in intergalactic space. Owing to the rarity of such ultrahigh energy cosmic rays, their spectra are best studied by a UV-sensitive satellite detector which looks down on a large volume of the Earth's atmosphere to study the nitrogen fluorescence tracks of giant air showers produced by these ultrahigh energy cosmic rays. We discuss here, in particular, a two-satellite mission called OWL, which would be suited for making such studies.

Dynamical aspects of quantum Brownian motion at low temperatures are investigated. Exact calculations of quantum entanglement among two Brownian oscillators are given without invoking the Born-Markov or Born approximation widely used for the study of open systems. Our approach is applicable for arbitrary time scale, in particular, suitable to probe short time regime at cold temperatures where many experiments on quantum information processing are performed. We study separability criteria based on the uncertainty relation, negativity, and entanglement of formation. We found a crossover behavior in a disentanglement process between quantum and thermal fluctuation dominated regimes. The fluctuation-dissipation relation which holds for one-particle quantum Brownian particle in the long time limit does not hold for two particles interacting with common environment. The deviation from the relation is originated in the interaction mediated by environment which drives two particles into a steady oscillatory state preventing the system to thermalize. Consequently there is a residual entanglement at low temperatures for arbitrary coupling strength. This entanglement is generated from environment nonperturbatively even when two modes are not entangled initially. When a distance of two oscillators are varied, competition between entanglement induced from environment and modified decoherence due to finite separation leads to the characteristic distance where entanglement is minimized. We also discuss the influence of Unruh effect on entanglement dynamics.

Gamma-Ray Bursts are among the most relativistic objects known. They involve a relativistic motion with large bulk Lorentz factors (⌈ > 100 or even higher). They arise, most likely, when black holes form. Their expected progenitors: Collapsars and in particular neutron star binary mergers are the classical candidates for sources of Gravitational Radiation. Moreover, the acceleration process of their jets also leads to gravitational radiation emission that might be detectable. Finally, the high energy photons and possibly neutrinos emitted in GRBs provide the best tools to explore and set limits on Lorentz Invariance Violations.

We study the dynamics of a mini-collapse induced by a phase transition in the core of an isolated rotating neutron star by means of performing, with the use of general relativistic hydrodynamical code CoCoNuT, a series of numerical simulations. The immediate cause of the collapse is the back bending instability that can be reached via the angular momentum loss. We study a resonant nonlinear coupling of axisymmetric modes of pulsation, their damping and resonant effects, as well as the emission of gravitational waves for configurations composed of a microphysical equation of state with kaon-condensed core and for an analytic equation of state, approximating the mixed-phase transition to quark matter.

We investigate the possibility that stellar mass black holes, with masses in the range of 3:8M_⊙ and 6M_⊙, respectively, could be in fact quark stars in the Color-Flavor-Locked (CFL) phase. Depending on the value of the gap parameter, rapidly rotating CFL quark stars can achieve much higher masses than standard neutron stars, thus making them possible stellar mass black hole candidates. Moreover, quark stars have a very low luminosity and a completely absorbing surface - the infalling matter on the surface of the quark star is converted into quark matter. A possibility of distinguishing CFL quark stars from stellar mass black holes could be through the study of thin accretion disks around rapidly rotating quark stars and Kerr type black holes, respectively. Strange stars exhibit a low luminosity, but high temperature bremsstrahlung spectrum, which, in combination with the emission properties of the accretion disk, may be the key signature to differentiate massive strange stars from black hole.

Matter falling onto a black hole is trans-relativistic, transonic, and close to the horizon it is sub-Keplerian. Such a flow shows the existence of multiple critical point, shocks etc. Employing relativistic equation of state and realistic composition, it has been shown that the solution strongly depend on the composition of the uid. Electron-positron fluid is the least relativistic, to the extent that multiple critical points, shocks etc do not form. The most relativistic fluid is the one whose ratio of proton to electron number density is ~ 0.2. Since the solution strongly depend on composition, the emitted radiation should strongly depend on the composition too.

Gaia will estimate the astrometric and physical data of approximately one billion objects. The core of this process, the global sphere reconstruction, is represented by the reduction of a subset of these objects, which will constitute the largest and most precise catalog of absolute astrometry in the history of Astronomy. General Relativity will be tested by estimating the PPN parameter γ to unprecedented accuracy. As the Hipparcos mission showed, and as it is natural for all kind of absolute measurements, possible errors in the data reduction can hardly be identified at the end of the processing, and can lead to systematic errors in subsequent analyses. In order to avoid such type of problems, a Verification Unit was established by the Gaia Data Processing and Analysis Consortium (DPAC). One of its jobs is to implement and perform an independent global sphere reconstruction, parallel to the baseline one, to compare the two results, and to report any significant difference.

Numerical models of spherically symmetric relativistic stellar clusters with anisotropic distribution function are developed by generalizing isotropic solutions with an energy cutoff.

Why, despite all efforts to the contrary, have attempts at unification based on the supposedly more fundamental quantum theory failed miserably? The truth is that the essential idea or concept of the quantum itself has never been fully understood. What is the quantum, or rather, what is its ultimate nature? Science may be able to work adequately with the quantum; in a sense science is quite articulate in the language of the quantum, i.e., its mathematical interpretation of the quantum mechanics, but science has no idea of the true physical nature of the quantum. Scientists and philosophers have wasted energy and efforts on irrelevant issues such as the debate over determinism and indeterminism instead of carefully analyzing the physical source of the quantum. Only with a true understanding of the physical nature of the quantum will the unification of the quantum and relativity ever become a reality.

Based on pre-Einstein classical mechanics, a theoretical model is constructed that describes the behavior of objects in a liquid environment that conduct themselves in accordance with the formal laws of the special theory of relativity. This model reproduces Lorentz contraction, time dilation, the relativity of simultaneity, the Doppler effect in its symmetrical relativistic form, the twin paradox effects, Bell effect, the relativistic addition of velocities. The model makes it possible to obtain Lorentz transforms and to simulate Minkowski four-dimensional space-time.

Minkowski spacetime predates quantum mechanics and is frequently regarded as an extension of the classical paradigm of Newtonian physics, rather than a harbinger of quantum mechanics. By inspecting how discrete clocks operate in a relativistic world we show that this view is misleading. Discrete relativistic clocks implicate classical spacetime provided a continuum limit is taken in such a way that successive ticks of the clock yield a smooth worldline. The classical picture emerges but does so by confining unitary propagation into spacetime regions between ticks that have zero area in the continuum limit. Clocks allowed a continuum limit that does not force inter-event intervals to zero, satisfy the Dirac equation. This strongly suggests that the origin of quantum propagation is to be found in the shift from Newton’s absolute time to Minkowski’s frame dependent time and is ultimately relativistic in origin.

Physicists’ understanding of relativity and the way it is handled is at present dominated by the interpretation of Albert Einstein, who related relativity to specific properties of space and time. The principal alternative to Einstein’s interpretation is based on a concept proposed by Hendrik A. Lorentz, which uses knowledge of classical physics to explain relativistic phenomena. In this paper, we will show that on the one hand the Lorentz-based interpretation provides a simpler mathematical way of arriving at the known results for both Special and General Relativity. On the other hand, it is able to solve problems which have remained open to this day. Furthermore, a particle model will be presented, based on Lorentzian relativity, which explains the origin of mass without the use of the Higgs mechanism, based on the finiteness of the speed of light, and which provides the classical results for particle properties that are currently only accessible through quantum mechanics.