arXiv subjects

Neil Ashby

Publications and source records attributed to Neil Ashby.

14 recordsLinked to original sources

arXiv2025

A comparative study of time on Mars with lunar and terrestrial clocks

As space exploration extends into cislunar space and further towards Mars, understanding the relativistic effects on clocks on Mars, particularly in relation to multibody gravitational influences, becomes increasingly important for accurate clock synchronization. This study estimates clock rates on Mars and compares them to those on the Moon and Earth. We find that, on average, clocks on Mars tick faster than those on the Earth's geoid by 477 microseconds per day, with a variation of 226 microseconds per day over a Martian year. Additionally, there is an amplitude modulation of approximately 40 microseconds per day over seven synodic cycles. We also introduce a formalism for addressing the effects of solar tides on the Earth-Moon system for predicting clock rates on the Moon and Mars more accurately when compared to using only Keplerian orbit approximations. Our analysis quantifies the relativistic proper time offsets among Martian, lunar, and terrestrial clocks, highlighting important implications for mission planning and the implementation of timekeeping systems on Mars.

gr-qc↗

arXiv2024

A Relativistic Framework to Establish Coordinate Time on the Moon and Beyond

As humanity aspires to explore the solar system and investigate distant worlds such as the Moon, Mars, and beyond, there is a growing need to establish and broaden coordinate time references that depend on the rate of standard clocks. According to Einstein's theory of relativity, the rate of a standard clock is influenced by the gravitational potential at the location of the clock and the relative motion of the clock. A coordinate time reference is established by a grid of synchronized clocks traceable to an ideal clock at a predetermined point in space. This allows for the comparison of local time variations of clocks due to gravitational and kinematic effects. We present a relativistic framework to introduce a coordinate time for the Moon. This framework also establishes a relationship between the coordinate times for the Moon and the Earth as determined by standard clocks located on the Earth's geoid and the Moon's equator. A clock near the Moon's equator ticks faster than one near the Earth's equator, accumulating an extra 56.02 microseconds per day over the duration of a lunar orbit. This formalism is then used to compute the clock rates at Earth-Moon Lagrange points. Accurate estimation of the rate differences of coordinate times across celestial bodies and their inter-comparisons using clocks onboard orbiters at relatively stable Lagrange points as time transfer links is crucial for establishing reliable communications infrastructure. This understanding also underpins precise navigation in cislunar space and on celestial bodies' surfaces, thus playing a pivotal role in ensuring the interoperability of various position, navigation, and timing (PNT) systems spanning from Earth to the Moon and to the farthest regions of the inner solar system.

gr-qc↗

arXiv2021

Falling Atoms

Atomic fountain clocks launch laser-cooled balls of atoms upwards to some height $h$ in the local gravitational field where they experience both second-order Doppler shifts and gravitational frequency shifts. It is shown in this paper that the net shift, relative to a reference at the launch point, is $ g h/3 c^2$. We derive the next most significant correction to this expression, and show that the value of $g$ should be corrected for Coriolis and centripetal effects.

physics.atom-ph↗

arXiv2019

Comparison of open and solid falling retroreflector gravimeters

We study whether the optical properties of a solid glass retroreflector influence the value of the acceleration of gravity $g$ determined by dropping both solid and open retroreflectors in an absolute ballistic gravimeter. The retroreflectors have equivalent optical centers and are dropped from the same height, at a fixed location, in the same gravimeter while recording time data corresponding to fixed fringe separation intervals of 400 fringes. The data for both types of retroreflectors are processed with commercial software, as well as with independently developed software based on a relativistic treatment of the phase difference between reference beam and test beams, and a realistic treatment of the effect of frequency modulation, with modulation index $\beta \gg 1$, on the interference signal. After applying corrections for polar motion, barometric admittance, tides, and ocean loading we find agreement between the values of $g$ determined with both types of retroreflectors, whether processed with commercial software or with our independently developed software. We suggest two procedures for computing relativistic corrections; the two methods agree to better than .01 $\mu$Gal.

physics.geo-ph↗

arXiv2018

Reply to comment by S. Svitlov [2] on "Relativistic Theory of the falling cube gravimeter"

In the subject paper [1] of the comment [2], light propagation through an absolute gravimeter was analyzed, including the propagation delay through the falling retroreflector and through the vacuum. The resulting expression for the interference signal applies without any subsequent "speed-of-light" correction. Other corrections appeared for the three fitting parameters $Z_0,\ V_0$ and $g$, which are the initial position and velocity, and the acceleration of gravity at the reference point. The comment assumes the value of $Z_0$ is known apriori, this case was not addressed in [1]. Also, the comment misunderstands statements made in [1] regarding the derivation of the relativistic/nonrelativistic parts of the corrections (Eq. (47) of [1]), and mistakenly claims they apply to the undifferenced interference signal. In this reply we show why, because of inapplicable assumptions, approximations and misunderstandings, the comment does not apply to the results of [1]\,.

physics.geo-ph↗

arXiv2017

Relativistic Theory of the Falling Retroreflector Gravimeter

We develop a relativistic treatment of interference between light reflected from a falling cube retroreflector in the vertical arm of an interferometer, and light in a reference beam in the horizontal arm. Coordinates that are nearly Minkowskian, attached to the falling cube, are used to describe the propagation of light within the cube. Relativistic effects such as the dependence of the coordinate speed of light on gravitational potential, propagation of light along null geodesics, relativity of simultaneity, and Lorentz contraction of the moving cube, are accounted for. The calculation is carried to first order in the gradient of the acceleration of gravity. Analysis of data from a falling cube gravimeter shows that the propagation time of light within the cube itself causes a significant reduction in the value of the acceleration of gravity obtained from measurements, compared to assuming reflection occurs at the face. An expression for the correction to the acceleration of gravity is derived and found to agree with experiment. Depending on the instrument, the correction can be several microgals, comparable to commonly applied corrections such as those due to polar motion and earth tides. The controversial "speed of light" correction is discussed.

physics.geo-ph↗

arXiv2017

A null test of General Relativity: New limits on Local Position Invariance and the variation of fundamental constants

We compare the long-term fractional frequency variation of four hydrogen masers that are part of an ensemble of clocks comprising the National Institute of Standards and Technology,(NIST), Boulder, timescale with the fractional frequencies of primary frequency standards operated by leading metrology laboratories in the United States, France, Germany, Italy and the United Kingdom for a period extending more than 14 years. The measure of the assumed variation of non-gravitational interaction,(LPI parameter, $\beta$)---within the atoms of H and Cs---over time as the earth orbits the sun, has been constrained to $\beta=(2.2 \pm 2.5)\times 10^{-7}$, a factor of two improvement over previous estimates. Using our results together with the previous best estimates of $\beta$ based on Rb vs. Cs, and Rb vs. H comparisons, we impose the most stringent limits to date on the dimensionless coupling constants that relate the variation of fundamental constants such as the fine-structure constant and the scaled quark mass with strong(QCD) interaction to the variation in the local gravitational potential. For any metric theory of gravity $\beta=0$.

gr-qc↗

arXiv2017

Incorporating an Optical Clock into a Time Scale

This paper discusses how to build a time scale with an intermittently-operated optical clock. In particular, it gives suggestions on how long and how often to run an optical clock. It also explores the benefits of having an optical clock in a time scale, by comparing with the current UTC(NIST) performance and the time scale with a continuously-operated Cs fountain.

physics.app-ph↗

arXiv2014

Frequency shifts in NIST Cs Primary Frequency Standards due To Transverse RF Field Gradients

A single-particle Green's function (propagator) is introduced to study the detection of laser-cooled Cesium atoms in an atomic fountain due to RF field gradients in the Ramsey TE011 cavity. The detection results in a state-dependent loss of atoms at apertures in the physics package, resulting in a frequency bias. A model accounting only for motion in one dimension transverse to the symmetry axis of the fountain is discussed in detail and then generalized to two transverse dimensions. Results for fractional frequency shifts due to transverse field gradients are computed for NIST F-1 and F-2 Cesium fountains. The shifts are found to be negligible except in cases of higher RF power applied to the cavities.

physics.atom-ph↗

arXiv2014

Isosynchronous paths on a rotating surface

In special relativity, clock networks may be self-consistently synchronized in an inertial frame by slowly transporting clocks, or by exchanging electromagnetic signals between network nodes. However, clocks at rest in a rotating coordinate system--such as on the surface of the rotating earth--cannot be self-consistently synchronized by such processes, due to the Sagnac effect. Discrepancies that arise are proportional to the area swept out by a vector from the rotation axis to the portable clock or electromagnetic pulse, projected onto a plane normal to the rotation axis. This raises the question whether paths of minimal or extremal length can be found, for which the Sagnac discrepancies are zero. This paper discusses the variational problem of finding such "isosynchronous" paths on rotating discs and rotating spheres. On a disc, the problem resembles the classical isoperimetric problem and the paths turn out to be circular arcs. On a rotating sphere, however, between any two endpoints there are an infinite number of extremal paths, described by elliptic functions.

gr-qc↗

arXiv2013

Why there is no noon-midnight red shift in the GPS

Although the effects of solar (and lunar) gravitational potentials on the frequencies of orbiting Global Positioning System (GPS) clocks are actually no more than a few parts in 10^(15), a naïve calculation appears to show that such effects are much larger, and depend on whether the orbiting clock is between the earth and the sun, or on the side of the earth opposite to the sun. Consequently questions about whether such effects have been properly accounted for in the GPS continue to arise. This issue has been discussed in a misleading way in terms of cancellations arising from a second-order Doppler shift in the literature for almost 50 years. The purpose of this article is to provide a correct argument, based on fundamental relativity principles, so that one may understand in a simple way why the effects of external solar system bodies on orbiting or earth-bound clocks in the GPS are so small. The relativity of simultaneity plays a crucial role in these arguments.

gr-qc↗

arXiv2011

Optical Clock and Drag-Free Requirements for a Shapiro Time-Delay Mission

In the next decade or two, extremely accurate tests of general relativity under extreme conditions are expected from gravitational wave observations of binary black hole mergers with a wide range of mass ratios. In addition, major improvements are planned in both strong and weak equivalence principle tests; clock measurements based on the ACES program on the ISS; more accurate light-bending measurements; and other new types of tests. However, whether these tests are all consistent with general relativity or not, it still appears desirable to proceed with a much improved measurement of the Shapiro time delay. A suggested approach is based on using a high-quality optical clock in a drag-free spacecraft near the sun-earth L1 point and a smaller drag-free transponder spacecraft in a two-year period solar orbit. Laser phase travel-time measurements would be made between the two spacecraft over a period of 10 or 20 days around the time when the line of sight passes through the Sun. The requirements on the optical clock stability and on the drag-free systems will be discussed. The accuracy achievable for the time-delay appears to be better than 1 part in 100 million.

gr-qc↗

arXiv2011

Discrete simulation of power law noise

A method for simulating power law noise in clocks and oscillators is presented based on modification of the spectrum of white phase noise, then Fourier transforming to the time domain. Symmetric real matrices are introduced whose traces--the sums of their eigenvalues--are equal to the Allan variances, in overlapping or non-overlapping forms, as well as for the corresponding forms of the modified Allan variance. Diagonalization of these matrices leads to expressions for the probability distributions for observing a variance at an arbitrary value of the sampling or averaging interval $τ$, and hence for estimating confidence in the measurements. A number of applications are presented for the common power-law noises.

physics.data-an↗

arXiv2009

Accurate light-time correction due to a gravitating mass

This work arose as an aftermath of Cassini's 2002 experiment \cite{bblipt03}, in which the PPN parameter $\gamma$ was measured with an accuracy $\sigma_\gamma = 2.3\times 10^{-5}$ and found consistent with the prediction $\gamma =1$ of general relativity. The Orbit Determination Program (ODP) of NASA's Jet Propulsion Laboratory, which was used in the data analysis, is based on an expression for the gravitational delay which differs from the standard formula; this difference is of second order in powers of $m$ -- the sun's gravitational radius -- but in Cassini's case it was much larger than the expected order of magnitude $m^2/b$, where $b$ is the ray's closest approach distance. Since the ODP does not account for any other second-order terms, it is necessary, also in view of future more accurate experiments, to systematically evaluate higher order corrections and to determine which terms are significant. Light propagation in a static spacetime is equivalent to a problem in ordinary geometrical optics; Fermat's action functional at its minimum is just the light-time between the two end points A and B. A new and powerful formulation is thus obtained. Asymptotic power series are necessary to provide a safe and automatic way of selecting which terms to keep at each order. Higher order approximations to the delay and the deflection are obtained. We also show that in a close superior conjunction, when $b$ is much smaller than the distances of A and B from the Sun, of order $R$, say, the second-order correction has an \emph{enhanced} part of order $m^2R/b^2$, which corresponds just to the second-order terms introduced in the ODP. Gravitational deflection of the image of a far away source, observed from a finite distance from the mass, is obtained to $O(m^2)$.

gr-qc↗