Gravitational Physics Exploring the Structure of Space and Time (1999) / Chapter Skim
Currently Skimming:
3 Achievements and Opportunities in Gravitational Physics
3 Achievements and Opportunities in Gravitational Physics
Pages 32-98
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 32... ...
Remarkably, one idea yet to be fully checked is the feature most closely related to the principle of relativity: gravitational waves. Just as interestingly, the eventual detection of gravitational waves will probably provide the best possible way to verify the other most spectacular of the unverified predictions of general relativity: the existence of black holes.
|
From page 33... ...
Still, the reception and study of gravitational waves can help answer many key questions in basic physics and astrophysics: · Do waves such as those predicted by Einstein propagate away from dynamic massive objects, and do they interact with test bodies in the way described by general relativity?
|
From page 34... ...
The two neutron stars, by virtue of their motion about one another, execute precisely the sort of motion that generates gravitational waves. Those waves carry away energy.
|
From page 35... ...
That effect is strongest in highly relativistic systems such as the binary pulsar PSR1913+16. One measure of this decrease in orbital period is the steady shift over time of the time of the pulsar's closest approach (periastron)
|
From page 36... ...
in 1991. This run determined the strongest upper limit yet on the flux of gravitational waves.
|
From page 37... ...
Since 1991 Allegro has functioned as the most sensitive continuously operating gravitational wave detector in the world. (Courtesy of Bill Hamilton, Louisiana State University Physics and Astronomy.)
|
From page 38... ...
carry out numerical simulations of gravitational collapse in three spatial dimensions (i.e., without restrictive symmetries) , together with improvements in integrating realistic microphysics into the description of the collapsing stellar matter, gave results that demonstrated a remarkable sensitivity of the gravitational wave output from a supernova to the details of neutrino physics, hydrodynamics, and thermal physics.
|
From page 39... ...
Another was the development of a nearly complete description of the gravitational wave emission from a small mass orbiting a massive black hole and of the "ringing" modes of distorted black holes. Finally, over the last 5 years a new method was developed to study the properties of gravitational waves emitted in the very final stages of black hole mergers.
|
From page 40... ...
(Courtesy of the Jet Propulsion Laboratory, California Institute of Technology.) The work of Taylor and his collaborators tracking the orbit of the binary pulsar PSR1913+16 established dramatically that gravitational waves were being emitted by the binary neutron star system, with a rate of energy loss in agreement with the predictions of general relativity.
|
From page 41... ...
· What is the nature of gravity in the strong-field regime where general relativity makes its most dramatic predictions? Gravitational waves are emitted most strongly when large masses move at relativistic speeds in close proximity to one another, especially as those masses approach the degree of compactness of black holes (as in neutron stars or black holes themselves)
|
From page 42... ...
The "cleanest" test of the existence of black holes (and of the predictions of theory regarding strong-field gravity in general) would be the measurement of the gravitational waves emitted when a black hole is disturbed, or when it forms in a gravitational collapse or merger.
|
From page 43... ...
Other popular models of gamma-ray bursts involve collisions of black holes or core collapse events inside massive stars; either of these classes of models also would generate coincident gravitational wave signals. Whether or not such models turn out to be correct, gravitational wave observations will make an important contribution to the understanding of the enigmatic gamma-ray bursts.
|
From page 44... ...
Gravitational waves have been detected indirectly through the analysis of the orbit of the Hulse-Taylor binary pulsar, in excellent agreement with the predictions of general relativity. Rather, there is uncertainty concerning the frequency of violent events in the universe which could produce gravitational radiation strong enough to be detected on Earth.
|
From page 45... ...
(For more on this, see "Supermassive Black Hole Mergers and Space-based Detectors" in the addendum to this section.) If a binary of two such black holes were to coalesce anywhere within the visible universe, the resulting gravitational wave signal should be detectable at very high signal-tonoise ratio by the planned space-based interferometers.
|
From page 46... ...
Just as our ability to probe the interior of a supernova electromagnetically is frustrated by the opacity of the outer layers of the star, so too do our electromagnetic probes of the universe run into the opaque "surface of last scattering," when hydrogen was last ionized about 300,000 years after the big bang. Gravitational waves emitted before this epoch could be bathing Earth.
|
From page 47... ...
By the same token, ongoing observations of gamma-ray bursts may allow a more sensitive search for the gravitational wave signals, by establishing the arrival times of the signals at Earth. Another kind of violent astrophysical event that may create strong gravitational waves is the stellar core collapse that initiates a Type II supernova.
|
From page 48... ...
Even upper limits in any range of wavelengths can provide important information on early universe physics. For example, if the radiation is less than predicted from the merger of bubbles arising in first-order phase transitions or from the evaporation of cosmic strings, that would yield a much clearer picture of the nature of inflation and the origin of fluctuations.
|
From page 52... ...
Gravitational collapse of very massive stars is expected to produce black holes with masses of a few or a few tens of solar masses. Several candidate black holes have been discovered.
|
From page 53... ...
The discovery of real examples of black holes has been a central goal of gravitational physics for many years. Several excellent black hole candidates have been identified by astrophysicists, and we speak loosely of black holes having been discovered.
|
From page 54... ...
. The detection of gravitational waves from the final merger of a pair of black holes in a binary will probably provide even firmer proof.
|
From page 55... ...
These observations confirm that the objects are very compact. The discovery in the 1960s of luminous quasars prompted astrophysicists to consider the possibility that supermassive black holes may have existed at the centers of galaxies when the universe was young.
|
From page 56... ...
56 to g
|
From page 57... ...
57 to to to to / I j j j j I' I , , 7, , , j j I ~I I .......
|
From page 58... ...
For these reasons, they are considered strong black hole candidates. While there is no doubt that both these classes of objects described above are compact enough to be black holes and are too massive to be neutron stars, scientists cannot yet claim victory.
|
From page 59... ...
This is the simplest problem that includes the full panoply of general relativistic difficulties gravitational waves, black holes and their associated singularities, and dynamics in three spatial dimensions with no symmetry. It also happens to
|
From page 60... ...
The Binary Black Hole Alliance has taken computational general relativity from simulations in two spatial dimensions to full three-dimensional simulations, and its results suggest that the ultimate goal of simulating something as complicated as two black holes in binary orbit is within reach. Neutron Stars.
|
From page 61... ...
ACHIEVEMENTS AND OPPORTUNITIES IN GRAVITATIONAL PHYSICS 6 FIGURE 3.6 Head-on collision of two black holes to form a single black hole that becomes spherical at late times. The location of the black hole horizons in the numerically generated spacetime is found by tracing the path of light rays (yellow)
|
From page 62... ...
Recently, an explanation has been found in quantum gravity. Quantum states of some black holes can be counted, and the result is in complete agreement with the thermodynamic prediction.
|
From page 63... ...
This is an example of how the combination of analytic methods and numerical simulation can uncover new fundamental qualitative features of the theory. Opportunities Detection and Study of Gravitational Waves from Merging Black Holes The discovery of signals from the merger of a binary black hole system, and the demonstration that the signals are indeed from black holes, would be a spectacular achievement.
|
From page 64... ...
Computational General Relativity Computer simulations of Einstein's equations are necessary to calculate the detailed shape (the waveform) of the gravitational waves emitted by the inspiral and merger of binary black holes or neutron stars in the strong-field regime.
|
From page 65... ...
Prime examples involve nonspherical supernova calculations and simulations of neutron star mergers, but it can also be expected that the modeling of phenomena involving quasars, x-ray binaries, and gamma-ray bursts will begin to incorporate general relativity. Detection and Study of Black Holes by Astrophysical Means As described in the previous parts of this section, astrophysicists have demonstrated beyond reasonable doubt that there exist objects in the universe that are extremely compact and that cannot be neutron stars.
|
From page 66... ...
Increasing the population of known black hole systems will also lead to a better understanding of many statistical questions. In particular, the question of event rates for gravitational wave detectors requires more observational input for better estimates.
|
From page 67... ...
Is Einstein's theory right on the largest scales? Is the big bang model the correct description of our universe?
|
From page 68... ...
The observed abundances of the light elements agree well with these predictions, providing one of the most compelling pieces of evidence for the big bang. Quantum gravity ~ = , ?
|
From page 69... ...
Today CMB Fluctuations The universe at the big bang was almost perfectly smooth, but it must have contained the seeds that condensed by gravity to evolve into today's galaxies. These seeds are seen in tiny 30-millionths-of-a-degree fluctuations in the temperature of the cosmic microwave background shown here as different colored regions in a map of the sky.
|
From page 70... ...
The strongest curvatures in the universe occur in the initial big bang. Some of the places where gravitational physics and cosmology intersect most strongly are described below.
|
From page 71... ...
ACHIEVEMENTS AND OPPORTUNITIES IN GRAVITATIONAL PHYSICS 7 pies to theory, we can test different ideas for the origin of density fluctuations and for the composition and evolution of the universe. Gravitational Lensing.
|
From page 72... ...
Gravitational Waves from the Early Universe. The early universe is a source of gravitational radiation.
|
From page 73... ...
Is it connected with the fundamental dynamical theory? These are the kinds of questions quantum cosmology seeks to answer by working at the intersection of the disciplines of cosmology, quantum mechanics, and quantum gravity.
|
From page 74... ...
These observations directly probe the distribution of matter only 300,000 years after the big bang, the moment when electrons and protons first combined to form hydrogen, and the universe became transparent to most electromagnetic radiation. Ground- and balloon-based microwave background experiments have confirmed the COBE result and detected additional fluctuations on smaller angular scales.
|
From page 75... ...
Origin of Elements and the Number of Light Neutrinos The big bang model successfully explains the origin of primordial deuterium, helium, and lithium. These light elements were produced in the first minutes of the universe.
|
From page 76... ...
In studying inflation, cosmologists tie together and test ideas from grand unified theories and from quantum gravity. Over the past 20 years, theorists have developed the conceptual and numerical tools needed to make detailed quantitative comparisons of the variants of the inflationary model with the data, primarily the distribution of galaxies.
|
From page 77... ...
, which is scheduled for launch in late 2000, and the European Space Agency's Planck Surveyor, which is scheduled for launch in 2007, will map the microwave background radiation across the whole sky with angular resolution 30 to 60 times better than that obtained with the COBE satellite. These microwave background observations will measure the level of inhomogeneities in the universe 300,000 years after the big bang.
|
From page 78... ...
Lensing observations measure the statistical properties of dark matter. Microwave background observations measure the distribution of matter and radiation in the early universe.
|
From page 79... ...
However, if the universe did begin with violent physics, then the gravitational waves will bear the imprint of these first moments. That makes their detection interesting even if it is not clear whether this is an opportunity for the next decade or future ones.
|
From page 80... ...
The possibility of new physics is not idle speculation; it is motivated by today's searches for theories of the fundamental interactions. For example, string theory suggests that general relativity will fail at some level, although the level is not yet predictable.
|
From page 81... ...
None was found down to the level of about 2 parts in 1000. typical of unified theories (such as string theory)
|
From page 82... ...
The pendulum is suspended in a vacuum and the entire instrument is rotated continuously at about one revolution per hour. A violation of the equivalence principle would show up as a pendulum twist that varied at this rotation frequency.
|
From page 83... ...
Another test of the equivalence principle was made possible by techniques developed within atomic physics, such as atom and ion traps, and laser cooling, which were used to put exquisitely stringent constraints on any anisotropy, or preferred direction, in local physics that might be generated by new cosmic interactions. Pulsar Tests of Relativistic Gravity The Hulse-Taylor binary pulsar provided a definitive test of the existence of gravitational waves, in agreement with the prediction of general relativity to a third of a percent.
|
From page 84... ...
84 GRAVITATIONS PHYSICS: E~LOHNG THE STRUCTURE OF SPACE^D TIME BOX 3.4 Gravity Probe B Gravity Probe B (GP-B) is a space project to measure the tiny precession of gyroscopes relative to distant stars.
|
From page 85... ...
At present, the spacecraft is built, construction and installation of the flight experimental hardware is complete, and the project is proceeding toward a proposed launch in 2000. For the purpose of this report, Gravity Probe B is treated as part of the committed, ongoing program in gravitational physics.
|
From page 86... ...
Careful study of these images can provide a map of the mass distribution in the tensing system, and it plays a role in the search for dark matter and the measurement of cosmological parameters. An analogous phenomenon is microlensing, in which a foreground mass passes in front of a distant star, augmenting its intensity momentarily through gravitational focusing.
|
From page 87... ...
In this regime, the gravitational forces are necessarily weak compared to electromagnetic interactions between neutral objects, so that new techniques are required. It is useful to point out that many of the experimental improvements needed to perform these delicate measurements, such as vibration isolation and sensitive gravity "radiometers, may have payoffs in other areas, such as gravitational wave detection and geophysics.
|
From page 88... ...
For example, the fortuitous discovery of binary pulsars of the right characteristics, such as systems containing both a pulsar and a black hole, could result in a 10-fold improvement in accuracy in the test of gravitational radiation damping, provide a high-precision measurement of a companion black hole mass, detect the precession of the spin of a neutron star, contribute to a determination of the distribution of neutron star masses, and help sharpen the event rate of inspiraling and coalescing neutron star binaries. The Newtonian Gravitational Constant The Newtonian constant G
|
From page 89... ...
In certain situations, effects of quantum gravity can be important even when the gravitational fields are significantly weaker than the above estimate. In particular this is the case around black holes.
|
From page 90... ...
. · What are the quantum properties of black holes?
|
From page 91... ...
In particular, it became physically meaningful to assign thermodynamic parameters such as temperature and entropy to black holes in terms of their geometric properties. This posed a concrete challenge to any candidate quantum theory of gravity: Explain the origin of these thermodynamic properties in terms of microscopic degrees of freedom as is done for ideal gases and other everyday systems.
|
From page 92... ...
However, a few years later it was shown that when quantum effects are included, this analogy becomes exact. Because of an analog of the tunneling process in elementary quantum mechanics called the Hawking effect, particles can escape from black holes.
|
From page 93... ...
This is not sufficient to answer the important questions about the big bang or what happens deep inside a black hole. These issues appear to require a complete non-perturbative formulation of string theory which is not yet available.
|
From page 94... ...
This procedure is motivated by two considerations. The first comes from general relativity in which some of the simplest and yet most interesting physical systems black holes and gravitational waves consist of "pure geometry." The second comes from quantum theory where the occurrence of infinities
|
From page 95... ...
Since a black hole in general relativity is "pure geometry," it is natural to use quantum geometry to unravel its microscopic degrees of freedom. Recently, this task was carried out for nonrotating black holes, possibly with charges.
|
From page 96... ...
While this is not believed to be the case in nature, this formulation can still be used as a model to study quantum gravitational processes such as the evaporation of black holes. This proposal incorporates a novel "holographic" view of space and time, in which our usual notions of locality and causality hold only approximately.
|
From page 97... ...
Much progress would occur if there were more interaction between the two communities. It is clear that recent results on the quantum properties of black holes will be extended using both string theory and quantum geometry.
|
From page 98... ...
Indeed it could be argued that while most effects of quantum gravity are not observable, the effects of the quantum fluctuations in geometry and matter near the big bang are all around us. We see them in anisotropies in the cosmic background radiation and in the large-scale distributions of galaxies.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.