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The Nature of Compact Objects and Probes of Relativity
Astronomical observations have verified that general relativity provides an accurate description
of gravity on solar-system scales, but an unanswered question, and the most challenging test of general
relativity, is whether it works in the strong gravity fields around black holes. Current studies using X-ray
spectroscopy of gas disks around black holes are consistent with the predictions of general relativity and
yield preliminary estimates of the black hole spin. Over the next decade the precision of these tests can
be dramatically improved.
Also feasible within the decade is the detection of gravitational waves from mergers of million
solar mass black holes or low mass objects captured by more massive ones. Such events produce clean
signals that can be used to map spacetime with tremendous precision in regions where gravity is very
strong. An important theoretical and computational breakthrough in this decade was the ability to
compute the merger of two black holes, yielding highly accurate predictions of the gravitational wave
emission patterns. Combined with detections of these waves, such computations provide stringent tests of
the theory of relativity in regimes not accessible by any other means. Deviations from Einstein’s
predictions would cause us to rethink one of the foundational pillars of all of physical science.
Gravitational wave detection would not only test general relativity, but also measure the spins and
masses of the merging black holes. Furthermore, the discovery and understanding of such merging
systems would uniquely probe the conditions at the centers of galaxies and the cosmological history of
galaxy formation and growth. Black holes are common in the centers of galaxies and our estimates of
their abundance, masses, and merger rate are poised for steady improvement in precision through a spacebased
interferometer that can reach back in time to “hear” the spacetime echoes of mergers of
supermassive black holes.
Observations with X-ray telescopes provide a complementary probe of the nature of spacetime
near the event horizon at the edge of a black hole. Such observations allow us to track the motions of
material as it swirls “down the drain”, and thereby to measure the spin of the black hole. This is
currently only possible for a handful of nearby black holes, but more powerful facilities in the future
would enable us to extend these measurements to large samples. Since any black hole can be fully
characterized by its mass and spin, this is fundamental information about how black holes work and how
they were formed.
Yet another probe of black holes is the jets that are frequently created by massive spinning black
holes in active galactic nuclei. Radio telescopes have shown that the emitting gas travels with speeds
close to that of light. X-ray and now gamma-ray telescopes are able to trace the emission down to quite
close to the black hole itself. Plasma and magneto-hydrodynamic physics, which we understand best from
solar and solar system studies, play important roles in many astrophysical contexts. It is proposed to
combine the results from many types of telescopes operating simultaneously to understand how jets are
made and how they shine. This will then lead to a better understanding of how gravity operates around a
black hole. Black holes⎯either spinning massive holes in active galactic nuclei or newly formed stellar
ones in gamma ray bursts⎯are also suspected to be the source of the Ultra High Energy Cosmic Rays
which are detected when they hit the Earth’s atmosphere. These can have energies as large as that of a
well-hit baseball and despite great advances in understanding their properties that have come from the
Auger-South facility in Argentina, we still do not know for sure what they are, how they interact with
matter and how they are made.
Only slightly less remarkable than black holes are the neutron stars. It is for them that the
investments over the last decade in ground-based gravitational wave detectors are likely to pay off first, as
frequent detections of merging neutron stars in other galaxies are expected from Advanced LIGO.
Formed as the catastrophic collapse of the core of a dying massive star, these amazing objects contain a
mass larger than the Sun's, squeezed into a region the size of a city. The centers of neutron stars contain
the densest matter in the universe, even more tightly compressed than the matter inside the nucleus of a
single atom. Some neutron stars also have the largest inferred magnetic field strengths in the universe,
trillions of times that of the Earth.
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