Einstein's Unfinished Symphony.
UNLESS YOU ARE READING THIS IN Braille, you were born with a pair of highly sensitive detectors of electromagnetic radiation. The pictures and articles in Sky & Telescope shamelessly pander to your consequent natural chauvinism, though you may find an occasional mention of the detection of neutrinos from the Sun and Supernova 1987A. Sometime in the next decade, you will probably download from S&T's Web site the sounds of black-hole event horizons forming and zipping together, and some years later, the whole gravitational symphony of all the universe's binary stars and black holes. This book is the story, from the trenches, of the ongoing battle to make this possible: humankind's struggle to detect gravitational waves.
Albert Einstein's general theory of relativity predicts that these ripples in space-time are produced by all the moving things in the universe--especially dense things being jerked about rapidly --and race outward from them at the speed of light, just like the waves from a pebble dropped in a pond. Einstein's theory has been well tested in weak gravitational fields such as those in our solar system. And, as nicely described in the book's chapter titled "Pas de Deux," the orbit of the binary radio pulsar PSR 1913+16 has been shrinking since its 1974 discovery at exactly (within 3 parts in 1,000) the rate needed to balance the energy that relativity predicts must be carried off by gravitational radiation. This indirect finding has convinced virtually all physicists and astrophysicists that Einstein's gravitational waves exist.
Yet despite 40 years of effort, gravitational waves still have not been detected directly. The waves are predicted to interact so weakly with our instruments that we have so far simply had no chance of measuring them.
So why this book now? It is written in a spirit of optimism. For the past decade, scientists around the world have been developing a new array of huge and expensive ($500 million and growing) gravitational-wave sensors incorporating laser interferometers. Marcia Bartusiak describes the science and the scientific and political battles that have led us this far. If the will of the funding agencies continues, sometime between next year and the next decade, as these are deployed, debugged, and improved, they will very likely detect gravitational waves.
The ultimate reward for catching gravitational waves will not be so much in the find itself, challenging though it will be, but in the use of the observed signals to study the weird and wonderful predictions of general relativity in strong gravitational fields. The signals ought to map the space-times around black holes, record the fireworks and subsequent formation of black holes when neutron stars merge, and reveal the zipping together of horizons when pairs of black holes merge. More speculatively, they may even record the sudden changes in the laws of physics that may have occurred in the very earliest moments of the universe. These predictions are more subtle, remarkable, and uncertain than the existence of the gravitational waves themselves.
Bartusiak's writing emphasizes the actions of the scientists, as well as their personalities and environments. These draw the reader rapidly through the book--I found it harder to put down than some mystery novels. The text begins with a conventional, but engagingly written, history of the development of the general theory of relativity, during which some of the theory's principal ideas are planted painlessly in the reader's mind.
Next on the scene is the remarkable Joseph (Yonah) Weber, who was successively naval officer, engineer, pioneer of quantum electronics, pioneer of gravitational-wave physics, and finally generally acknowledged nutty scientist. Not only did he show how one might, in principle, detect gravitational waves; he and his student Robert Forward also built and operated the prototypes of the two modern classes of detector (resonant bars and laser interferometers).
The next two chapters describe the American side of the development and political sale of the large laser interferometer as a gravitational-wave detector, and the construction by Caltech and MIT of the Laser Interferometer Gravitational-Wave Observatory (LIGO), the pair of enormous interferometers in Louisiana and Washington state (S&T: October 2000, page 40). Afterward, we learn about the European and Japanese interferometers, GEO 600, VIRGO, and TAMA 300. The characters involved in these four projects are almost, but not quite, as interestingly unusual as Weber.
The text moves on to discuss the sources of gravitational waves that astrophysicists know about, though the history of the introduction of new detectors in astronomy suggests that the strongest and most interesting sources will be the ones we haven't yet imagined. The final chapter introduces the mother of all gravitational-wave detectors: the Laser Interferometer Space Antenna (LISA). Just this year, NASA and the European Space Agency have agreed to develop LISA for launch around 2010. Its great sensitivity to long-wavelength gravitational waves (inaccessible to terrestrial detectors) means that it can observe, among other things, the binary stars and supermassive black holes of greatest interest to astronomers.
The pace of the book is sometimes breathless, yet it works well and is generally accurate. Still, a number of errors have crept in. The discussion of Lorentz contraction and time dilation in special relativity repeats some of the whoppers found in most popular accounts of relativity, which we teachers must subsequently struggle to remove from students' brains. On page 32 we read, "Look from Earth at a clock on that swiftly receding spaceship. You will see time progressing more slowly than here on Earth." This is mostly a purely Newtonian effect due to the increasing time it takes light to travel the distance. The relativistic effect needs to be measured by an array of synchronized clocks as the spacecraft speeds by them.
"You will also see the spaceship foreshortened in the direction of its motion," the text continues. This is false. You will see the spacecraft rotated but undistorted, as first realized by James Terrell in 1959. A network of scientists on a network of Earths would, in a technical sense, measure the spacecraft to be shortened, but with their eyes they would see it undistorted.
The discussion of the still-to-be-launched Gravity Probe B satellite says, "The axis of a spaceborne gyroscope should move an infinitesimal 0.0007 of a degree each year because of Earth's dragging the framework of space-time around itself." That sought-after effect is actually 0.000012[degrees] per year, correctly given in the next sentence as the width of a hair seen from a quarter mile away. The total precession of the gyroscope will in fact be much larger: 0.002[degrees] per year, due to the better-tested geodetic precession caused by the gyroscope's motion through Earth's gravitational field.
Radio receivers don't focus signals (page 83)--that's the job of the telescope and feed horns. And contrary to the statement on that same page, the rate that gravitational waves carry energy from a binary-star system was worked out by Philip C. Peters and Jonathan Mathews a decade before the twin pulsars were discovered (to a precision still well beyond the sensitivity of the data).
Bartusiak says, "LISA will ultimately gather fewer data than detectors on the ground." Since LISA hasn't been built and the detectors on the ground haven't detected, it may be unwise to argue. But if both work as designed and the sources are as expected, LISA will probably gather more bits of signal data than the ground-based detectors. The latter will of course gather trillions of bits of uninteresting instrumental noise from which the signals must be winnowed.
If you combine an interest in astronomy with one in relativity, you will enjoy reading this fast-paced book. When the new field of gravitational-wave astronomy is born, you will appreciate not only the tremendous gambles in careers, taxpayer money, and engineering that went into its creation, but also the new baby's astonishing potential to change our view of the universe. This book really has no competition, nor does it need any. If reading it inspires a thirst for a deeper view of the development of relativity and relativistic astrophysics, consider Kip Thorne's Black Holes and Time Warps: Einstein's Outrageous Legacy.
Review by E. Sterl Phinney
STERL PHINNEY, professor of theoretical astrophysics at Caltech, has chaired the LISA Mission Definition Team since 1997. Since his office is in the building housing much of the LIGO team, and some members are bigger than he is, it is his entirely unbiased opinion that LISA and LIGO represent the pinnacle of all modern science.
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|Author:||Phinney, E. Sterl|
|Publication:||Sky & Telescope|
|Article Type:||Book Review|
|Date:||Sep 1, 2001|
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