arXiv subjects

Ashok K. Singal

Publications and source records attributed to Ashok K. Singal.

At least 37 records · Page 2Linked to original sources

arXiv2020

GMRT observations of IC 711 -- The longest head-tail radio galaxy known

We present low-frequency, GMRT observations at 240, 610 and 1300 MHz of IC~711, a narrow angle tail (NAT) radio galaxy. The total angular extent of the radio emission, $\sim 22$ arcmin, corresponds to a projected linear size of $\sim 900$ kpc, making it the longest among the known head-tail radio galaxies. The objectives of the GMRT observations were to investigate the radio morphology, especially of the long tail structure, at low frequencies. The radio structure, especially initial $\sim 10$ arcmin of tail being a long straight feature, does not seem to be consistent with a simple circular motion around the cluster centre, as previously suggested in the literature. Two sharp bends after the straight section of the tail cast doubt on the prevailing idea in the literature that the long narrow tails represent trails left behind by the fast moving parent optical galaxy with respect to the cluster medium, as the optical galaxy could not have undergone such sharp bends in its path, under any conceivable gravitational influence of some individual galaxy or of the overall cluster gravitational potential. In fact the tail does not seem to have been influenced by the gravitational field of any of the cluster-member galaxies. The radio spectrum of the head, coinciding with the optical galaxy, is flat ($α\stackrel{<}{_{\sim}}0.4$ for $S \propto ν^{-α}$), but steadily steepens along the radio tail, with the end part of the tail showing the steepest spectrum ($α\, {\sim}$ 4 -- 5) ever seen in any diffuse radio emission region.

astro-ph.GA↗

arXiv2019

Is Interstellar travel to an exoplanet Possible?

It is shown that space travel, even in the most distant future, will remain confined to our own planetary system, and a similar conclusion will hold forth for any other civilization, no matter how advanced it might be, unless those extra-terrestrial species have life spans order of magnitude longer than ours. Even in such a case it is unlikely that they will travel much farther than their immediate stellar neighbourhood, as each such excursion will exhaust the resources of their home planet so much that those will dwindle rather fast and there might not be much left for the further scientific and technological advancements. So the science-fiction fancy of a "Galactic Empire" may ever remain in our fantasies only. And as for the mythical UFOs, whose quiet appearances do get reported in the press once in a while, recent explorations have shown no evidence that any such thing could have an origination within our own solar system itself. And a "quiet trip" back and forth from a distant star is almost impossible as the exhaust in any such trip will dazzle the sky like another sun or perhaps more like a gamma ray burst occurring but not in a distant part of the universe instead going off right in our own solar backyard.

physics.pop-ph↗

arXiv2019

The fallacy of Schott energy-momentum

The incompatibility between Larmor's formula for radiation losses (at a rate proportional to square of the acceleration of the electric charge) and the radiation reaction (the rate of loss of momentum of the accelerated charge proportional to its rate of change of acceleration) was recently shown to arise because a proper distinction is not kept between radiation losses calculated in terms of a retarded time and those expressed in terms of a "real time". However, the occurrence of this disparity between two formulations is usually reconciled in literature by proposing an acceleration-dependent Schott energy lying somewhere in the nearby electromagnetic fields of an accelerated charge. But nobody has yet unambiguously demonstrated where the Schott energy actually lies in the fields. By scrutinizing electromagnetic fields of a uniformly accelerated charge, a mathematically tractable case, we show that contrary to the ideas prevalent in the literature, there is no evidence of any acceleration-dependent Schott energy-momentum in the electromagnetic fields, anywhere in the near vicinity of the charge or elsewhere. Accordingly, we expose the fallacy of the Schott energy-momentum term, which should henceforth be abandoned, in the electromagnetic radiation formulation.

physics.gen-ph↗

arXiv2019

Peculiar motion of the solar system derived from a dipole anisotropy in the redshift distribution of distant quasars

An observer stationary with respect to comoving coordinates of the expanding universe should find the redshift distribution to be isotropic. However, a peculiar motion of the observer would introduce a dipole anisotropy in the observed redshift distribution. Conversely, a dipole anisotropy in observed redshift distribution could be exploited to infer our peculiar motion, or rather of our solar system. We determine here our peculiar velocity by studying the dipole anisotropy in the redshift distribution of a large sample of quasars. The magnitude of the peculiar velocity thus determined turns out to be $2350\pm280$ km s$^{-1}$, not only much larger than 370 km s$^{-1}$ determined from the dipole anisotropy in the Cosmic Microwave Background Radiation (CMBR), but also nearly in an opposite direction. Such large values for peculiar velocity have been found in a couple of radio surveys too, but with a direction along the CMBR dipole. Large genuine differences in the inferred motion, whether in magnitude or direction, are rather disconcerting since a solar peculiar velocity should not depend upon the method of its determination. Such discordant dipoles imply perhaps an anisotropic universe, violating the cosmological principle, a cornerstone of the modern cosmology.

astro-ph.CO↗

arXiv2019

A large disparity in cosmic reference frames determined from the sky distributions of radio sources and the microwave background radiation

The angular distribution of the Cosmic Microwave Background Radiation (CMBR) in sky shows a dipole asymmetry, ascribed to the observer's motion (peculiar velocity of the solar system!), relative to the local comoving coordinates. The peculiar velocity thus determined turns out to be $370$ km s$^{-1}$ in the direction RA$=168^{\circ}$, Dec$=-7^{\circ}$. On the other hand, a dipole asymmetry in the sky distribution of radio sources in the NRAO VLA Sky Survey (NVSS) catalog, comprising 1.8 million sources, yielded a value for the observer's velocity to be $\sim 4$ times larger than the CMBR value, though the direction turned out to be in agreement with that of the CMBR dipole. This large difference in observer's speeds with respect to the reference frames of NVSS radio sources and of CMBR, confirmed since by many independent groups, is rather disconcerting as the observer's motion with respect to local comoving coordinates should be independent of the technique used to determine it. A genuine difference in relative speeds of two cosmic reference frames could jeopardize the cosmological principle, thence it is crucial to confirm such discrepancies using independent samples of radio sources. We here investigate the dipole in the sky distribution of radio sources in the recent TIFR GMRT Sky Survey (TGSS) dataset, comprising 0.62 million sources, to determine observer's motion. We find a significant disparity in observer's speeds relative to all three reference frames, determined from the radio source datasets and the CMBR, which does not fit with the cosmological principle, a starting point for the standard modern cosmology.

physics.gen-ph↗

arXiv2019

Why no radiation occurs in the case of a uniformly accelerated charge

We show that in the case of a uniformly accelerated charge, in its instantaneous rest frame, there is only a radial electric field as the acceleration fields strangely get cancelled at all distances by a transverse term of the velocity fields. Consequently, no electromagnetic radiation will be detected by any observer from a uniformly accelerated charge, even in the far-off zone. This is in contradiction with Larmor's formula, according to which a uniformly accelerated charge would radiate power at a constant rate, which is proportional to the square of the acceleration. On the other hand, the absence of radiation from such a charge is in concurrence with the strong principle of equivalence, where a uniformly accelerated charge is equivalent to a charge permanently stationary in a gravitational field, and such a completely time-static system could not be radiating power at all.

physics.gen-ph↗

arXiv2018

A heuristic derivation of radiative power loss and radiation reaction from the kinetic power of electric inertial mass of a charge

It is shown that formulas for the radiative power loss and radiation reaction from a charge can be derived in a heuristic manner from the kinetic power (rate of change of the kinetic energy) of its electric inertial mass. The derivation assumes a non-relativistic but otherwise an arbitrary motion of the charge. We exploit the fact that as the charge velocity changes because of a constant acceleration, there are accompanying modifications in its electromagnetic fields which can remain concurrent with the charge motion because the velocity as well as acceleration information enters into the field expression. However, if the acceleration of the charge is varying, information about that being not present in the field expressions, the electromagnetic fields get `out of step' with the actual charge motion. Accordingly we arrive at a radiation reaction formula for an arbitrarily moving charge, obtained hitherto in literature from the self-force, derived in a rather cumbersome way from the detailed mutual interaction between various constituents of a small charged sphere. This way we demonstrate that a power loss from a charge occurs only when there is a change in its acceleration and the derived instantaneous power loss is directly proportional to the scalar product of the velocity and the rate of change of the acceleration of the charge.

physics.class-ph↗

arXiv2018

Locating Planets in Sky Using Manual Calculations

In this article, we describe a very simple technique to locate naked-eye planets in the sky, to an accuracy of $\sim 1^{\circ}$. The procedure, comprising just three steps, involves very simple manual calculations for planetary orbits around the Sun; all one needs are the initial specifications of planetary positions for some standard epoch and the time periods of their revolutions. After applying a small correction for the orbital ellipticity, appearance of a planet relative to Sun's position in sky, as seen by an observer from Earth, is found using a scale and a protractor (found inside a school geometry box).

physics.pop-ph↗

arXiv2018

Disparities of Larmor's/Liénard's formulations with special relativity and energy-momentum conservation

It is shown that the familiar Larmor's formula or its relativistic generalization, Liénard's formula, widely believed to represent the instantaneous radiative losses from an accelerated charge, are not compatible with the special theory of relativity (STR). We demonstrate this in the case of a charge emitting synchrotron radiation where the agency responsible for acceleration (the magnetic field) does no work and thus all the energy-momentum carried by the radiation has to be balanced unambiguously against the kinetic energy-momentum of the moving charge. The application of the radiation reaction inferred from Larmor's/Liénard's formulas in two different inertial frames, yields results that are not in conformity with the relativistic transformations. Further, we show that energy-momentum conservation is violated in all inertial frames when we try to balance the energy-momentum carried by the electromagnetic radiation against the loss of kinetic energy and momentum of the radiating charge in accordance with Larmor's formula. We point out a mathematical subtlety in the applicability of Poynting's theorem, that seems to have been overlooked in the text-book derivation of Larmor's formula, where a proper distinction between "real" and "retarded" times was not kept, and which has therefore given rise to such anomalies, including the mysterious Schott term, which happens to be nothing but the difference in rate of change of energy in self-fields of the charge between retarded and present times.

physics.gen-ph↗

arXiv2018

Motion of a rod pushed at one point in a weightless environment in space

We analyze the motion of a rod floating in a weightless environment in space when a force is applied at some point on the rod in a direction perpendicular to its length. If the force applied is at the centre of mass, then the rod gets a linear motion perpendicular to its length. However, if the same force is applied at a point other than the centre of mass, say, near one end of the rod, thereby giving rise to a torque, then there will also be a rotation of the rod about its centre of mass, in addition to the motion of the centre of mass itself. If the force applied is for a very short duration, but imparting nevertheless a finite impulse, like in a sudden (quick) hit at one end of the rod, then the centre of mass will move with a constant linear speed and superimposed on it will be a rotation of the rod with constant angular speed about the centre of mass. However, if force is applied continuously, say by strapping a tiny rocket at one end of the rod, then the rod will spin faster and faster about the centre of mass, with angular speed increasing linearly with time. As the direction of the applied force, as seen by an external (inertial) observer, will be changing continuously with the rotation of the rod, the acceleration of the centre of mass would also be not in one fixed direction. However, it turns out that the locus of the velocity vector of the centre of mass will describe a Cornu spiral, with the velocity vector reaching a final constant value with time. The mean motion of the centre of mass will be in a straight line, with superposed initial oscillations that soon die down.

physics.class-ph↗

arXiv2017

Reply to comment on `Poynting flux in the neighbourhood of a point charge in arbitrary motion and the radiative power losses'

Doubts have been expressed in a comment (Eur. J. Phys., 39, 018001, 2018), about the tenability of the formulation for radiative losses in our recent published work (Eur. J. Phys., 37, 045210, 2016). We provide our reply to the comment. In particular, it is pointed out that one need to clearly distinguish between the rate of the energy-momentum being carried by the electromagnetic radiation to far-off space, and that of the mechanical energy-momentum losses being incurred by the radiating charge. It is also demonstrated that while the Poynting flux is always positive through a spherical surface centred on the retarded position of the charge, it could surprisingly be negative through a surface centred on the "present" position of the charge. It is further shown that the mysterious Schott term, hitherto thought in literature to arise from some acceleration-dependent energy in fields, is actually nothing but the difference in rate of change of energy in self-fields of the charge between the retarded and present times.

physics.gen-ph↗

arXiv2017

Radio VLBI and the quantum interference paradox

We address here the question of interference of radio signals from astronomical sources like distant quasars, in a very long baseline interferometer (VLBI), where two (or more) distantly located radio telescopes (apertures), receive simultaneous signal from the sky. In an equivalent optical two-slit experiment, it is generally argued that for the photons involved in the interference pattern on the screen, it is not possible, even in principle, to know which of the two slits a particular photon went through and that any procedure to ascertain this destroys the interference pattern. But in the case of the modern radio VLBI, it is a routine matter to record the phase and amplitude of the voltage outputs from the two radio antennas on a recording media separately and then do the correlation between the two recorded signals later in an offline manner. Does this not violate the quantum interference principle? We provide a resolution of this problem here.

physics.gen-ph↗

arXiv2017

The genesis of the internal resistance of a battery -- a physical perspective

The standard exposition of the internal resistance of a battery, that a battery comprises a source of emf in series with an internal resistance, as given in engineering and physics text-books, is lacking in proper explanation. It is treated merely as an experimental fact, and not something that should follow from logic. The battery has a tendency to maintain electric potential difference across its terminals equal to its chemical potential, and in an open circuit, when no current flows, these two do match. However in a closed circuit, a drop in electric potential across the battery terminals is inevitable for a steady flow of electric current throughout the circuit, because the chemical reactions driving the electric current within the battery can proceed only if the electric potential at its terminals differs from the chemical potential. It is shown that for small voltage changes, the current passing through the battery is linearly proportional to the change in potential from the open-circuit value (i.e., its chemical potential), giving rise to a semblance of an internal resistance in series with the external resistance. It follows that a battery {\em has to have} an internal resistance in order to function as a power source. It is also shown that Thevenin's theorem does not make our results superfluous, in fact our results are presupposed in its derivation.

physics.class-ph↗

arXiv2016

Radiation reaction from electromagnetic fields in the neighbourhood of a point charge

From the expression for the electromagnetic field in the neighborhood of a point charge we determine the rate of electromagnetic momentum flow, calculated using the Maxwell stress tensor, across a surface surrounding the charge. From that we derive for a `point charge' the radiation reaction formula, which turns out to be proportional to the first time-derivative of the acceleration of the charge, identical to the expression for the self-force, hitherto obtained in the literature from the detailed mutual interaction between constituents of a small charged sphere. We then use relativistic transformations to arrive at a generalized formula for radiation reaction for a point charge undergoing a relativistic motion.

physics.class-ph↗

arXiv2016

Relativistic Doppler beaming and misalignments in AGN jets

Radio maps of AGNs often show linear features, called jets, both on pc as well as kpc scales. These jets supposedly possess relativistic motion and are oriented close to the line of sight of the observer and accordingly the relativistic Doppler beaming makes them look much brighter than they really are in their respective rest-frames. The flux boosting due to the relativistic beaming is a very sensitive factor of the jet orientation angle, as seen by the observer. Quite often large bends are seen in these jets, with misalignments being $90^\circ$ or more and might imply a change in the orientation angle that could cause a large change in the relativistic beaming factor. Such large bends should show high contrasts in the brightness of the jets, before and after the misalignments, if relativistic beaming does play an important role in these jets. It needs to be kept in mind that sometimes a small intrinsic change in the jet angle might appear as a much larger misalignment due to the geometrical projection effects, especially when seen close to the line of sight. Of course what really matters is the final orientation angle of the jet with respect to the observer's line of sight. Taking the geometrical projection effects properly into account, we calculate the consequences of the presumed relativistic beaming and demonstrate that there ought to be large brightness ratios in jets before and after the observed misalignments.

astro-ph.GA↗

arXiv2016

Poynting flux in the neighbourhood of a point charge in arbitrary motion and the radiative power losses

We examine the electromagnetic fields in the neighbourhood of a "point charge" in arbitrary motion and thereby determine the Poynting flux across a spherical surface of vanishingly small radius surrounding the charge. We show that the radiative power losses from a point charge turn out to be proportional to the scalar product of the instantaneous velocity and the first time-derivative of the acceleration of the charge. This may seem to be in discordance with the familiar Larmor's formula where the instantaneous power radiated from a charge is proportional to the square of acceleration. However, it seems that the root cause of the discrepancy actually lies in the Larmor's formula which is derived using the acceleration fields but without a due consideration for the Poynting flux associated with the velocity-dependent self-fields "co-moving" with the charge. Further, while deriving Larmor's formula one equates the Poynting flux through a surface at some later time to the radiation loss by the enclosed charge at the retarded time. Poynting's theorem, on the other hand, relates the outgoing radiation flux from a closed surface to the rate of energy decrease within the enclosed volume, all calculated {\em for the same given instant} only. Here we explicitly show the absence of any Poynting flux in the neighbourhood of an instantly stationary point charge, implying no radiative losses from such a charge, which is in complete conformity with energy conservation. We further show how Larmor's formula is still able to serve our purpose in vast majority of cases. It is further shown that Larmor's formula in general violates momentum conservation and in the case of synchrotron radiation leads to a potentially wrong conclusion about the pitch angle changes of the radiating charges, and that only the radiation reaction formula yields a correct result, consistent with the special relativity.

physics.class-ph↗

arXiv2016

Do the standard expressions for the electromagnetic field-momentum need any modifications?

We investigate here the question raised in literature about the correct expression for the electromagnetic field-momentum, especially when static fields are involved. For this we examine a couple of simple but intriguing cases. First we consider a system configuration in which electromagnetic field momentum is present even though the system is static. We trace the electromagnetic momentum to be present in the form of a continuous transport of electromagnetic energy from one part of the system to another, without causing any net change in the energy of the system. In a second case we show that the electromagnetic momentum is nil irrespective of whether the charged system is stationary or in motion, even though the electromagnetic energy is present throughout. We demonstrate that the conventional formulation of electromagnetic field-momentum describes the systems consistently without any real contradictions. Here we also make exposition of a curiosity where electromagnetic energy decreases when the charged system gains velocity. Then we discuss the more general question that has been raised -- Are the conventional formulas for energy-momentum of electromagnetic fields valid for all cases? Or more specifically, in the case of so-called "bound fields," do we need to change over to some modified definitions which have made their appearance even in standard text books? We show that in all cases it is only the conventional formulas which lead to results consistent with the rest of physics, including the special theory of relativity, and that any proposed modifications are thus superfluous.

physics.class-ph↗

arXiv2016

The pitch angle paradox and radiative life times in a synchrotron source

In synchrotron radiation there is a paradox whether or not the pitch angle of a radiating charge varies. The conventional wisdom is that the pitch angle does not change during the radiation process. The argument is based on Larmor's radiation formula, where in a synchrotron case the radiation power is along the instantaneous direction of motion of the charge. Then the momentum loss will also be parallel to that direction and therefore the pitch angle of the charge would remain unaffected. The accordingly derived formulas for energy losses of synchrotron electrons in radio galaxies are the standard text-book material for the last 50 years. However, if we use the momentum transformation laws from special relativity, then we find that the pitch angle of a radiating charge varies. While the velocity component parallel to the magnetic field remains unaffected, the perpendicular component does reduce in magnitude due to radiative losses, implying a change in the pitch angle. This apparent paradox is resolved when effects on the charge motion are calculated not from Larmor's formula but from Lorentz's radiation reaction formula. We derive the exact formulation by taking into account the change of the pitch angle due to radiative losses. From this we first time derive the characteristic decay time of synchrotron electrons over which they turn from highly relativistic into mildly relativistic ones.

physics.plasm-ph↗