Skip to main content

Currently Skimming:

6 What Are the Limits of Physical Law?
Pages 105-131

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 105...
... There are two quite separate reasons for carrying out this program. The first is to check the basic assumptions made when analyzing exotic cosmic objects like white dwarfs, neutron stars, and black holes.
From page 106...
... By contrast, cosmic ray protons are created in distant, astronomical sources with energies some 300 million times greater than those produced by the largest particle accelerators on Earth (see Box 6.11. These collide with atoms in the upper atmosphere, and the products of these collisions are observed on the ground as sprays of particles called air showers.
From page 107...
... Yet this density pales in comparison with that of neutron stars, formed by the spectacular supernova explosions of more massive stars, that have density beyond that of nuclear matter, 1,000 trillion times that of normal matter and initial temperatures of over 100 billion degrees (see Box 6.21. Neutron stars themselves have a maximum mass (less than three solar masses)
From page 108...
... (c) One of the Cerenkov shower detectors in the Pierre Auger Observatory array in Argentina; image courtesy of the Pierre Auger Observatory.
From page 109...
... Not all neutron stars have the same mass; many do have well-determined masses of around 1.4 solar masses. A neutron star of mass greater than about 3 solar masses cannot support itself against gravity, and collapse to a black hole is inevitable.
From page 110...
... These black holes are the engines for the hyperactive galactic nuclei called quasars, which for a time in the early universe outshone whole galaxies by up to a thousandfold. Quasars, too, are powered by accretion disks, fueled by gas supplied by their host galaxies.
From page 111...
... The wavelengths of these spectral lines are altered by several effects: a Doppler shift due to each atom's orbital velocity around the hole (the shift can be positive or negative, depending on whether the motion is toward or away from the observer) ; an overall shift to longer wavelengths that occurs for all radiation struggling to escape from black holes; and another longward shift due to the time-dilating effects of high-speed motion near the hole.
From page 112...
... However, they have not yet been detected directly. The most promising sources of gravitational waves, which involve the collisions and coalescences of large black holes, are even more luminous than gamma-ray bursts.
From page 113...
... There are several likely strong sources of gravitational radiation: inspiraling binary neutron stars, supernova explosions, and merging supermassive black holes in galactic nuclei. In all cases, the goal is to measure the gravitational wave profile as the mass falls together and compare it with nonlinear predictions using general relativity.
From page 114...
... To detect the gravitational radiation from the formation of more massive black holes, a larger detector that is sensitive to lower frequencies is required. Because of natural size limitations as well as seismic noise, such a detector would have to be deployed in space.
From page 115...
... to gravitational waves of different frequencies. Different sources of gravity waves are expected to produce waves of different kinds; the LIGO and LISA project are expected to complement each other to cover a broader range of the gravity wave spectrum.
From page 116...
... It turns out that gas can remain close to the black hole and produce such a strongly broadened and redshifted line only if the hole spins nearly as fast as possible. On this basis, at least some active galactic nucleus black holes are already thought to rotate very rapidly.
From page 117...
... Although the quantum mechanical principles necessary to calculate these effects are understood, the atoms are so complex in practice that it is necessary to mount a focused program in experimental laboratory astrophysics to make the most of existing observations of accretion disks. A second approach to measuring a black hole's spin comes from monitoring the rapid quasi-periodic oscillations of the x-ray intensity from selected galactic binary sources.
From page 118...
... Nearly as difficult as building these observatories, however, is the task of computing the gravitational waveforms that are expected when two black holes merge. This is a major challenge in computational general relativity and one that will stretch computational hardware and software to the limits.
From page 119...
... In the former case, the dense nuclear matter is extremely hot, about 1 o42 K, whereas neutron stars usually have temperatures of less than 109 K, which from a nuclear standpoint is cold. Scientists are still quite unsure about the properties of cold matter at densities well above nuclear matter density.
From page 120...
... An even more direct approach to learning the composition of highly compressed nuclear matter involves neutron star cooling. Neutron stars are born hot inside supernovae, which also create shells of expanding debris known as supernova remnants (see Figures 6.2 and 6.31.
From page 121...
... It may therefore be possible to observe QED at work in magnetars by observing x-ray polarization and mapping out the neutron star magnetic field. Measuring x-ray polarization is difficult, but, encouragingly, it has recently become possible to measure the circular polarization of x rays from laboratory synchrotrons.
From page 122...
... , (b) G11.2-0.3 (image courtesy of NASA/McGill, V
From page 124...
... The two means of classification do not necessarily coincide, and we lack a detailed theoretical understanding of how to make the correspondence. Nevertheless, Type la supernovae are observed to have very similar intrinsic luminosities and have provided convincing evidence that the expansion of the universe is speeding up (see Chapter 51.
From page 125...
... In fact, it is strongly suspected that supernovae, once again, must be the place where the remaining elements up to uranium are built, but there is no detailed understanding of how the process occurs. Resolving this problem requires observational data from supernova remnants, experimental data from both nuclear physics and neutrino physics, and the ability to make detailed, fully three-dimensional, theoretical calculations of supernova explosions.
From page 126...
... Cosmic rays with energies up to at least 1014 eV are probably accelerated at the shock fronts associated with supernova explosions, and radio emissions and x rays give direct evidence that electrons are accelerated there to nearly the speed of light. However, the evidence that high-energy cosmic-ray protons and nuclei have a supernova origin is only circumstantial and needs confirma
From page 127...
... As is the case for the high-energy cosmic rays, the sources of such energetic photons must all be relatively local on a cosmological scale, since photons of this energy also tend to be destroyed in traveling through space by combining with background infrared photons from starlight to create electron-positron pairs. Many of the highest-energy gamma rays are probably emitted as a byproduct of the acceleration of the mysterious ultrahigh-energy cosmic rays.
From page 128...
... O , and gamma-ray bursts are thought to involve highly relativistic bulk motion, ultimately powered by accretion onto massive black holes. Scientists only have quite speculative theories to offer at this stage, but future observations, in particular of high-energy radiation, can provide important constraints.
From page 129...
... The needed observations include x-ray line Doppler shifts and linewidths from black holes, quasiperiodic fluctuations of x-ray intensity from oscillating accretion disks, and gravitational radiation from mergers of compact objects. What Are the New States of Matter at Exceedingly High Density and Temperature?
From page 130...
... The opportunities include (1) measuring neutron-star radii from x-ray line gravitational redshifts and from absolute distance and x-ray intensity measurements, neutron-star rotation speeds from x-ray linewidths, x-ray timing measurements from quasi-perioidic oscillations, and the cooling rate of neutron stars in expanding supernova remnants and (2)
From page 131...
... Only by observing many more of these particles, or perhaps the associated gamma rays, neutrinos, and gravitational waves, will scientists be able to distinguish these possibilities. To realize this opportunity, large cosmic-ray air shower detector arrays and observations of high-energy gamma rays and neutrinos will be needed, as described in Box 6.1.


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.