39 research outputs found
Clocks around Sgr A*
The S stars near the Galactic centre and any pulsars that may be on similar
orbits, can be modelled in a unified way as clocks orbiting a black hole, and
hence are potential probes of relativistic effects, including black hole spin.
The high eccentricities of many S stars mean that relativistic effects peak
strongly around pericentre; for example, orbit precession is not a smooth
effect but almost a kick at pericentre. We argue that concentration around
pericentre will be an advantage when analysing redshift or pulse-arrival data
to measure relativistic effects, because cumulative precession will be drowned
out by Newtonian perturbations from other mass in the Galactic-centre region.
Wavelet decomposition may be a way to disentangle relativistic effects from
Newton perturbations. Assuming a plausible model for Newtonian perturbations on
S2, relativity appears to be strongest in a two-year interval around
pericentre, in wavelet modes of timescale approximately 6 months.Comment: Accepted for publication in MNRA
Geometrical vs wave optics under gravitational waves
We present some new derivations of the effect of a plane gravitational wave
on a light ray. A simple interpretation of the results is that a gravitational
wave causes a phase modulation of electromagnetic waves. We arrive at this
picture from two contrasting directions, namely null geodesics and Maxwell's
equations, or, geometric and wave optics. Under geometric optics, we express
the geodesic equations in Hamiltonian form and solve perturbatively for the
effect of gravitational waves. We find that the well-known time-delay formula
for light generalizes trivially to massive particles. We also recover, by way
of a Hamilton-Jacobi equation, the phase modulation obtained under wave optics.
Turning then to wave optics - rather than solving Maxwell's equations directly
for the fields, as in most previous approaches - we derive a perturbed wave
equation (perturbed by the gravitational wave) for the electromagnetic
four-potential. From this wave equation it follows that the four-potential and
the electric and magnetic fields all experience the same phase modulation.
Applying such a phase modulation to a superposition of plane waves
corresponding to a Gaussian wave packet leads to time delays.Comment: Accepted for publication in Physical Review D, matches published
versio
Molecular dynamics simulations of bubble nucleation in dark matter detectors
Bubble chambers and droplet detectors used in dosimetry and dark matter
particle search experiments use a superheated metastable liquid in which
nuclear recoils trigger bubble nucleation. This process is described by the
classical heat spike model of F. Seitz [Phys. Fluids (1958-1988) 1, 2 (1958)],
which uses classical nucleation theory to estimate the amount and the
localization of the deposited energy required for bubble formation. Here we
report on direct molecular dynamics simulations of heat-spike-induced bubble
formation. They allow us to test the nanoscale process described in the
classical heat spike model. 40 simulations were performed, each containing
about 20 million atoms, which interact by a truncated force-shifted
Lennard-Jones potential. We find that the energy per length unit needed for
bubble nucleation agrees quite well with theoretical predictions, but the
allowed spike length and the required total energy are about twice as large as
predicted. This could be explained by the rapid energy diffusion measured in
the simulation: contrary to the assumption in the classical model, we observe
significantly faster heat diffusion than the bubble formation time scale.
Finally we examine {\alpha}-particle tracks, which are much longer than those
of neutrons and potential dark matter particles. Empirically, {\alpha} events
were recently found to result in louder acoustic signals than neutron events.
This distinction is crucial for the background rejection in dark matter
searches. We show that a large number of individual bubbles can form along an
{\alpha} track, which explains the observed larger acoustic amplitudes.Comment: 7 pages, 5 figures, accepted for publication in Phys. Rev. E, matches
published versio
Clocks around Sgr A*
The S stars near the Galactic Centre and any pulsars that may be on similar orbits can be modelled in a unified way as clocks orbiting a black hole, and hence are potential probes of relativistic effects, including black hole spin. The high eccentricities of many S stars mean that relativistic effects peak strongly around pericentre; for example, orbit precession is not a smooth effect but almost a kick at pericentre. We argue that concentration around pericentre will be an advantage when analysing redshift or pulse-arrival data to measure relativistic effects, because cumulative precession will be drowned out by Newtonian perturbations from other mass in the Galactic Centre region. Wavelet decomposition may be a way to disentangle relativistic effects from Newton perturbations. Assuming a plausible model for Newtonian perturbations on S2, relativity appears to be strongest in a two-year interval around pericentre, in wavelet modes of time-scale ≈6 month
Direct Simulations of Homogeneous Bubble Nucleation: Agreement with CNT and no Local Hot Spots
We present results from direct, large-scale molecular dynamics (MD)
simulations of homogeneous bubble (liquid-to-vapor) nucleation. The simulations
contain half a billion Lennard-Jones (LJ) atoms and cover up to 56 million
time-steps. The unprecedented size of the simulated volumes allows us to
resolve the nucleation and growth of many bubbles per run in simple direct
micro-canonical (NVE) simulations while the ambient pressure and temperature
remain almost perfectly constant. We find bubble nucleation rates which are
lower than in most of the previous, smaller simulations. It is widely believed
that classical nucleation theory (CNT) generally underestimates bubble
nucleation rates by very large factors. However, our measured rates are within
two orders of magnitude of CNT predictions - only at very low temperatures does
CNT underestimate the nucleation rate significantly. Introducing a small,
positive Tolman length leads to very good agreement at all temperatures, as
found in our recent vapor-to-liquid nucleation simulations. The critical
bubbles sizes derived with the nucleation theorem agree well with the CNT
predictions at all temperatures. Local hot spots reported in the literature are
not seen: Regions where a bubble nucleation events will occur are not above the
average temperature, and no correlation of temperature fluctuations with
subsequent bubble formation is seen.Comment: 15 pages, 13 figures. Submitted to PRE. Simulation movies available
at http://www.ics.uzh.ch/~diemand/movies
Simple improvements to classical bubble nucleation models
We revisit classical nucleation theory (CNT) for the homogeneous bubble
nucleation rate and improve the classical formula using a new prefactor in the
nucleation rate. Most of the previous theoretical studies have used the
constant prefactor determined by the bubble growth due to the evaporation
process from the bubble surface. However, the growth of bubbles is also
regulated by the thermal conduction, the viscosity, and the inertia of liquid
motion. These effects can decrease the prefactor significantly, especially when
the liquid pressure is much smaller than the equilibrium one. The deviation in
the nucleation rate between the improved formula and the CNT can be as large as
several orders of magnitude. Our improved, accurate prefactor and recent
advances in molecular dynamics simulations and laboratory experiments for argon
bubble nucleation enable us to precisely constrain the free energy barrier for
bubble nucleation. Assuming the correction to the CNT free energy is of the
functional form suggested by Tolman, the precise evaluations of the free energy
barriers suggest the Tolman length is independently of the
temperature for argon bubble nucleation, where is the unit length of
the Lenard-Jones potential. With this Tolman correction and our new prefactor
one gets accurate bubble nucleation rate predictions in the parameter range
probed by current experiments and molecular dynamics simulations.Comment: 10pages, 6figures, Accepted for publication in Physical Review
Prospects for Measuring Planetary Spin and Frame-Dragging in Spacecraft Timing Signals
Satellite tracking involves sending electromagnetic signals to Earth. Both
the orbit of the spacecraft and the electromagnetic signals themselves are
affected by the curvature of spacetime. The arrival time of the pulses is
compared to the ticks of local clocks to reconstruct the orbital path of the
satellite to high accuracy, and to implicitly measure general relativistic
effects. In particular, Schwarzschild space curvature (static) and
frame-dragging (stationary) due to the planet's spin affect the satellite's
orbit. The dominant relativistic effect on the path of the signal photons is
Shapiro delay due to static space curvature. We compute these effects for some
current and proposed space missions, using a Hamiltonian formulation in four
dimensions. For highly eccentric orbits, such as in the Juno mission and in the
Cassini Grand Finale, the relativistic effects have a kick-like nature, which
could be advantageous for detecting them if their signatures are properly
modeled as functions of time. Frame-dragging appears, in principle, measurable
by Juno and Cassini, though not by Galileo 5 and 6. Practical measurement would
require disentangling frame-dragging from the Newtonian 'foreground' such as
the gravitational quadrupole which has an impact on both the spacecraft's orbit
and the signal propagation. The foreground problem remains to be solved.Comment: 10 pages, 6 figures, provisionally accepted for publication in
Frontiers in Astronomy and Space Sciences, section Fundamental Astronom
Towards relativistic orbit fitting of Galactic center stars and pulsars
The S stars orbiting the Galactic center black hole reach speeds of up to a
few percent the speed of light during pericenter passage. This makes, for
example, S2 at pericenter much more relativistic than known binary pulsars, and
opens up new possibilities for testing general relativity. This paper develops
a technique for fitting nearly-Keplerian orbits with perturbations from
Schwarzschild curvature, frame dragging, and spin-induced torque, to redshift
measurements distributed along the orbit but concentrated around pericenter.
Both orbital and light-path effects are taken into account. It turns out that
absolute calibration of rest-frame frequency is not required. Hence, if pulsars
on orbits similar to the S stars are discovered, the technique described here
can be applied without change, allowing the much greater accuracies of pulsar
timing to be taken advantage of. For example, pulse timing of 3 microsec over
one hour amounts to an effective redshift precision of 30 cm/s, enough to
measure frame dragging and the quadrupole moment from an S2-like orbit,
provided problems like the Newtonian "foreground" due to other masses can be
overcome. On the other hand, if stars with orbital periods of order a month are
discovered, the same could be accomplished with stellar spectroscopy from the
E-ELT at the level of 1 km/s.Comment: 22 pages, 9 figures, published in the Ap
Bubble Evolution and Properties in Homogeneous Nucleation Simulations
We analyze the properties of naturally formed nano-bubbles in Lennard-Jones
molecular dynamics simulations of liquid-to-vapor nucleation in the boiling and
the cavitation regimes. The large computational volumes provide a realistic
environment at unchanging average temperature and liquid pressure, which allows
us to accurately measure properties of bubbles from their inception as stable,
critically sized bubbles, to their continued growth into the constant speed
regime. Bubble gas densities are up to 50 lower than the equilibrium vapor
densities at the liquid temperature, yet quite close to the gas equilibrium
density at the lower gas temperatures measured in the simulations: The latent
heat of transformation results in bubble gas temperatures up to 25 below
those of the surrounding bulk liquid. In the case of rapid bubble growth -
typical for the cavitation regime - compression of the liquid outside the
bubble leads to local temperature increases of up to 5, likely significant
enough to alter the surface tension as well as the local viscosity. The
liquid-vapor bubble interface is thinner than expected from planar coexistence
simulations by up to . Bubbles near the critical size are extremely
non-spherical, yet they quickly become spherical as they grow.Comment: 14 pages, 14 figures. Accepted for publication in Physical Review E,
now matches published versio