9,941 research outputs found
AI Feynman: a Physics-Inspired Method for Symbolic Regression
A core challenge for both physics and artificial intellicence (AI) is
symbolic regression: finding a symbolic expression that matches data from an
unknown function. Although this problem is likely to be NP-hard in principle,
functions of practical interest often exhibit symmetries, separability,
compositionality and other simplifying properties. In this spirit, we develop a
recursive multidimensional symbolic regression algorithm that combines neural
network fitting with a suite of physics-inspired techniques. We apply it to 100
equations from the Feynman Lectures on Physics, and it discovers all of them,
while previous publicly available software cracks only 71; for a more difficult
test set, we improve the state of the art success rate from 15% to 90%.Comment: 15 pages, 2 figs. Our code is available at
https://github.com/SJ001/AI-Feynman and our Feynman Symbolic Regression
Database for benchmarking can be downloaded at
https://space.mit.edu/home/tegmark/aifeynman.htm
Dynamical inference from a kinematic snapshot: The force law in the Solar System
If a dynamical system is long-lived and non-resonant (that is, if there is a
set of tracers that have evolved independently through many orbital times), and
if the system is observed at any non-special time, it is possible to infer the
dynamical properties of the system (such as the gravitational force or
acceleration law) from a snapshot of the positions and velocities of the tracer
population at a single moment in time. In this paper we describe a general
inference technique that solves this problem while allowing (1) the unknown
distribution function of the tracer population to be simultaneously inferred
and marginalized over, and (2) prior information about the gravitational field
and distribution function to be taken into account. As an example, we consider
the simplest problem of this kind: We infer the force law in the Solar System
using only an instantaneous kinematic snapshot (valid at 2009 April 1.0) for
the eight major planets. We consider purely radial acceleration laws of the
form a_r = -A [r/r_0]^{-\alpha}, where r is the distance from the Sun. Using a
probabilistic inference technique, we infer 1.989 < \alpha < 2.052 (95 percent
interval), largely independent of any assumptions about the distribution of
energies and eccentricities in the system beyond the assumption that the system
is phase-mixed. Generalizations of the methods used here will permit, among
other things, inference of Milky Way dynamics from Gaia-like observations
Large-scale structure perturbation theory without losing stream crossing
We suggest an approach to perturbative calculations of large-scale clustering
in the Universe that includes from the start the stream crossing (multiple
velocities for mass elements at a single position) that is lost in traditional
calculations. Starting from a functional integral over displacement, the
perturbative series expansion is in deviations from (truncated) Zel'dovich
evolution, with terms that can be computed exactly even for stream-crossed
displacements. We evaluate the one-loop formulas for displacement and density
power spectra numerically in 1D, finding dramatic improvement in agreement with
N-body simulations compared to the Zel'dovich power spectrum (which is exact in
1D up to stream crossing). Beyond 1D, our approach could represent an
improvement over previous expansions even aside from the inclusion of stream
crossing, but we have not investigated this numerically. In the process we show
how to achieve effective-theory-like regulation of small-scale fluctuations
without free parameters.Comment: added pedagogical explanation of key math trick in appendi
Tidal Tails Test the Equivalence Principle in the Dark Sector
Satellite galaxies currently undergoing tidal disruption offer a unique
opportunity to constrain an effective violation of the equivalence principle in
the dark sector. Theories in which cold dark matter (CDM) couples to a light
scalar field naturally lead to a long-range force between dark matter
particles. An inverse-square-law force of this kind would manifest itself as a
violation of the equivalence principle in the dynamics of CDM compared to
baryons in the form of gas or stars. In a previous paper, we showed that an
attractive force would displace stars outwards from the bottom of the
satellite's gravitational potential well, leading to a higher fraction of stars
being disrupted from the tidal bulge further from the Galactic center. Since
stars disrupted from the far (near) side of the satellite go on to form the
trailing (leading) tidal stream, an attractive dark-matter force will produce a
relative enhancement of the trailing stream compared to the leading stream.
This distinctive signature of a dark-matter force might be detected through
detailed observations of the tidal tails of a disrupting satellite, such as
those recently performed by the Two-Micron All-Sky Survey (2MASS) and Sloan
Digital Sky Survey (SDSS) on the Sagittarius (Sgr) dwarf galaxy. Here we show
that this signature is robust to changes in our models for both the satellite
and Milky Way, suggesting that we might hope to search for a dark-matter force
in the tidal features of other recently discovered satellite galaxies in
addition to the Sgr dwarf.Comment: 29 pages, 13 figures, final version published in PR
The general relativistic two body problem
The two-body problem in General Relativity has been the subject of many
analytical investigations. After reviewing some of the methods used to tackle
this problem (and, more generally, the N-body problem), we focus on a new,
recently introduced approach to the motion and radiation of (comparable mass)
binary systems: the Effective One Body (EOB) formalism. We review the basic
elements of this formalism, and discuss some of its recent developments.
Several recent comparisons between EOB predictions and Numerical Relativity
(NR) simulations have shown the aptitude of the EOB formalism to provide
accurate descriptions of the dynamics and radiation of various binary systems
(comprising black holes or neutron stars) in regimes that are inaccessible to
other analytical approaches (such as the last orbits and the merger of
comparable mass black holes). In synergy with NR simulations, post-Newtonian
(PN) theory and Gravitational Self-Force (GSF) computations, the EOB formalism
is likely to provide an efficient way of computing the very many accurate
template waveforms that are needed for Gravitational Wave (GW) data analysis
purposes.Comment: 43 pages, 4 figures, to appear in the Brumberg Festschrift, edited by
S. M. Kopeikein, and to be published by de Gruyter, Berlin, 2014. arXiv admin
note: substantial text overlap with arXiv:1212.316
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