# David Tsang's research while affiliated with University of Maryland, College Park and other places

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## Publications (11)

There are many spacetime geometries in general relativity which contain
closed timelike curves. A layperson might say that retrograde time travel is
possible in such spacetimes. To date no one has discovered a spacetime geometry
which emulates what a layperson would describe as a time machine.
The purpose of this paper is to propose such a space-ti...

Symplectic integrators are widely used for long-term integration of
conservative astrophysical problems due to their ability to preserve the
constants of motion; however, they cannot in general be applied in the presence
of nonconservative interactions. In this Letter, we develop the "slimplectic"
integrator, a new type of numerical integrator that...

We further develop a recently introduced variational principle of stationary
action for problems in nonconservative classical mechanics and extend it to
classical field theories. The variational calculus used is consistent with an
initial value formulation of physical problems and allows for time-irreversible
processes, such as dissipation, to be i...

I derive a fully analytic expression for the linear corotation torque to
first order in eccentricity for planets in non-barotropic protoplanetary disks,
taking into account the effect of disk entropy gradients. This torque formula
is applicable to both the co-orbital corotation torque and the non co-orbital
corotation torques -- for planets in orbi...

We show that the first order (non co-orbital) corotation torques are
significantly modified by entropy gradients in a non-barotropic protoplanetary
disk. Such non-barotropic torques can dramatically alter the balance that, for
barotropic cases, results in the net eccentricity damping for giant
gap-clearing planets embedded in the disk. We demonstra...

This white paper is an explanation of Ben and Dave's TARDIS time machine,
written for laypeople who are interested in time travel, but have no technical
knowledge of Einstein's Theory of General Relativity.
The first part of this paper is an introduction to the pertinent ideas from
Einstein's theory of curved spacetime, followed by a review of othe...

Using a fast semi-analytic raytracing code, we study the variability of
relativistically broadened iron lines due to discoseismic oscillations
concentrated in the inner-most regions of accretion discs around black holes.
The corrugation mode, or c-mode, is of particular interest as its natural
frequency corresponds well to the ~0.1-15Hz range obser...

We demonstrate that resonant shattering flares can occur during close
passages of neutron stars in eccentric or hyperbolic encounters. We provide
updated estimates for the rate of close encounters of compact objects in dense
stellar environments, which we find are substantially lower than given in
previous works. While such occurrences are rare, we...

Magnetars are neutron stars with X-ray and soft γ-ray outbursts thought to be powered by intense internal magnetic fields. Like conventional neutron stars in the form of radio pulsars, magnetars exhibit 'glitches' during which angular momentum is believed to be transferred between the solid outer crust and the superfluid component of the inner crus...

We examine timing noise in both magnetars and regular pulsars, and find that
there exists a component of the timing noise with strong magnetic field
dependence above B ~ 10^{12.5} G. The dependence of the timing noise floor on
the magnetic field is also reflected in the smallest observable glitch size. We
find that magnetospheric torque variation c...

Using a fast semi-analytic raytracing code, we study the variability of
iron lines due to discoseismic oscillations concentrated in the
inner-most regions of accretion discs around black holes. The dependence
of the relativistically broadened line profile on the oscillation-phase
is studied for discoseismic corrugation modes. The corrugation mode,...

## Citations

... Searching for the extremum of this functional numerically using the parameters μ = 0.5, κ = 1, ξ = 0.00071 (cf. ref. [37]) with initial conditions x i = 1ẋ i = 0 on discretized paths with N t = 32 steps and regularized SBP21 operator leads to the results for x 1 (red circles) and x 2 (blue crosses) shown in Fig. 12. ...

... [17][18][19] by implementing the classical limit of the "in-in" approach [20,21] (see also the formalism developed for nonconservative classical systems in Refs. [22,23]). At 3.5PN order, the radiation-reaction force was first calculated in Refs. ...

... [108,249,106]). Visualisations of such oscillations can be found in Fig 2 of Tsang and Butsky [228]. The study of c modes is part of the wider field of discoseismology, which encapsulates a number of different modes of oscillation (e.g. ...

... As a result, mechanisms for exciting eccentricity are needed. Planet-disc interactions are capable of exciting eccentricities, but are limited to modest values 0.2 (Goldreich & Sari 2003;Tsang, Turner & Cumming 2014;Duffell & Chiang 2015;Ragusa et al. 2018). Recently, Petrovich, Wu & Ali-Dib (2019) studied a mechanism for transferring eccentricity from an outer planet to a WJ during the dispersal of a massive protoplanetary disc ...

... An important result of this analysis is the derivation of a "master equation" describing the global behavior of nonaxisymmetric perturbations driven by an external gravitational potential in a two-dimensional (2D) disk. Such an equation was first presented by Goldreich & Tremaine (1979), for the case of adiabatic perturbations in disks with uniform entropy, and was later generalized to disks with general entropy profiles (Baruteau & Masset 2008;Tsang 2014). We present an even more general version of the equation for adiabatic perturbations in disks with thermal relaxation, which reduces to the locally isothermal and adiabatic regimes in the appropriate limits (short and long cooling timescales, respectively). ...

... General relativity challenges this view. The Einstein equations, describing the relationship between spacetime geometry and mass-energy [1], have counterintuitive solutions containing closed time-like curves (CTCs) [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. An event on such a curve would be both in the future and in the past of itself, preventing an ordinary formulation of dynamics according to an "initial condition" problem. ...

... Within the context of standard general relativity, there has now been over 33 years of serious theoretical work on the possibility of "traversable wormholes" [1][2][3][4][5], 29 years of recent work on "time machines" [6][7][8][9][10][11][12], and over 27 years of work on the theoretical possibility of "warp drives" [13][14][15][16][17][18]. These analyses, and their subsequent refinements, are based on "reverse engineering" the space-time metric to encapsulate some potentially interesting physics, and then using the Einstein equations to deduce what the stress-energy tensor must be to support these space-times [1,5,19,20]. ...

... Ho we ver, this would not explain the QPO being much stronger in the Comptonized spectrum than in the disc spectrum, and the line profile variations produced by c modes are quite subtle compared with what we observe (see e.g. fig. 5 in Tsang & Butsky 2013 ), due to the c -mode oscillation only occurring in a narrow range of disc radii. The spiral waves of the AEI model are also consistent with the asymmetric illumination profile we measure here (Varniere, Rodriguez & Tagger 2002 ). ...

... Other electromagnetic counterparts to neutron star mergers have been proposed, with no confirmed observations so far. This include bursts of radiation before merger (Tsang 2013), continuous emission from magnetosphere interactions (Palenzuela et al. 2013), coherent emission from magnetosphere interactions (Most and Philippov 2022), and months to decades-long synchrotron radio emission from the mildly relativistic ejecta as it interacts with the interstellar medium (Hotokezaka et al. 2016). Neutrinos have no impact on the first three, however, and only a minor impact on the third (as neutrino-matter interactions may slightly change the mass/velocity of the outflows). ...

... Strong temporal variability in the detected radio flux, pulse-profile shape, and polarization fraction are often observed (Levin et al. 2012;Camilo et al. 2018;Dai et al. 2019;Lower et al. 2021b). Moreover, rotational variations in the form of rapid increases in spin-down and sudden spin-up (glitches) or, less commonly, spin-down events (antiglitches) are also detected following outbursts (Archibald et al. 2013;Scholz et al. 2017;Hu et al. 2020;Younes et al. 2020;Caleb et al. 2022). These observational properties are likely a result of magnetic stresses from the decaying internal magnetic fields precipitating sudden crustal motion or fracturing, leading to an impulsive release of energy in the form of electromagnetic radiation and occasionally an alteration of the external magnetic field topology (Duncan & Thompson 1992;Harding et al. 1999;Thompson et al. 2002;Younes et al. 2022b) With a spin period of ∼2.1 s, 1E 1547.0−5408 ...