GRAVITATIONAL WAVES: RIPPLES IN THE FABRIC OF SPACE-TIME
Albert Einstein predicted the existence of gravitational waves in 1916 as part of the theory of general relativity. He described space and time as different aspects of reality in which matter and energy are ultimately the same. Space-time can be thought of as a "fabric" defined by the measuring of distances by rulers and the measuring of time by clocks. The presence of large amounts of mass or energy distorts space-time -- in essence causing the fabric to "warp" -- and we observe this as gravity. Freely falling objects -- whether a soccer ball, a satellite, or a beam of starlight -- simply follow the most direct path in this curved space-time.When large masses move suddenly, some of this space-time curvature ripples outward, spreading in much the way ripples do the surface of an agitated pond. Imagine two neutron stars orbiting each other. A neutron star is the burned-out core often left behind after a star explodes. It is an incredibly dense object that can carry as much mass as a star like our sun, in a sphere only a few miles wide. When two such dense objects orbit each other, space-time is stirred by their motion, and gravitational energy ripples throughout the universe.
In 1974 Joseph Taylor and Russell Hulse found such a pair of neutron stars in our own galaxy. One of the stars is a pulsar, meaning it beams regular pulses of radio waves toward Earth. Taylor and his colleagues were able to use these radio pulses, like the ticks of a very precise clock, to study the orbiting of neutron stars. Over two decades, these scientists watched for and found the tell-tale shift in timing of these pulses, which indicated a loss of energy from the orbiting stars -- energy that had been carried away as gravitational waves. The result was just as Einstein's theory predicted.
LIGO: AN OBSERVATORY FOR THE 21ST CENTURY
LIGO should begin the new millennium by directly detecting gravitational waves for the first time, perhaps recording the final death spiral of two orbiting neutron stars just before they collide and merge into one. Physicists have predicted that such an event will produce a burst of gravitational waves with a characteristic pattern -- its own "fingerprint" -- that LlGO should be able to detect and measure, initially out to distances of 70 million light years. As happens so often when we enter new domain of measurement, totally unexpected discoveries may surprise us. Improved detectors will probe deeper into the universe and hunt for more exotic events.Science of this type is the epitome of basic research. As always with basic work, no one now can predict the full depth of its development, or the long range of benefits it may ultimately entail. For example, 19th Century scientists classified the spectral lines found in sunlight merely because it was interesting, having no idea that a century later their work would lead to the understanding of atomic structure and the development of quantum mechanics. In turn, inventors of the laser built upon the foundation of quantum mechanics, never imagining their invention would be used for delicate eye-saving surgery, at supermarket checkout counters, for printing daily newspapers, or as a light source for LlGO.
Will the discoveries made by LIGO have such an impact? We hope so, but we need to do the experiments first.
HOW LIGO WORKS
LIGO will detect the ripples in space-time by using a device called a laser interferometer, in which the time it takes light to travel between suspended mirrors is measured with high precision using controlled laser light. Two mirrors hang far apart, forming one "arm" of the interferometer, and two more mirrors make a second arm perpendicular to the first. Viewed from above, the two arms form an L shape. Laser light enters the arms through a beam splitter located at the corner of the L, dividing the light between the arms. The light is allowed to bounce between the mirrors repeatedly before it returns to the beam splitter. If the two arms have identical lengths, then interference between the light beams returning to the beam splitter will direct all of the light back toward the laser. But if there is any difference between the lengths of the two arms, some light will travel to where it can be recorded by a photodetector.The space-time ripples cause the distance measured by a light beam to change as the gravitational wave passes by, and the amount of light falling on the photodetector to vary. The photodetector then produces a signal defining how the light falling on it changes over time. The laser interferometer is like a microphone that converts gravitational waves into electrical signals. Three interferometers of this kind were built for LlGO -- two near Richland, Washington, and the other near Baton Rouge. Louisiana. LlGO requires at least two widely separated detectors, operated in unison, to rule out false signals and confirm that a gravitational wave has passed through the earth.
LIGO: A NEW WAY TO EXPLORE THE UNIVERSE
Suppose you watched a concert on television with the volume turned down. The rousing musical score can only be imagined. Could we, in fact, even imagine the music if we had never heard music before? Throughout human history, we have viewed the cosmos in a similar way. First with our unaided eyes, and then with telescopes, we examined the visible light from heavenly objects to discover their secrets. Eventually we learned to view a broader variety of radiation -- including infrared light, x-rays, gamma rays and radio waves -- invisible to our eyes but clearly detectable by electronic devices. Yet still, all these different kinds of radiation, including light, are made purely of electricity and magnetism.Today we know that only about ten percent of all the matter in the universe can be observed in this way. What new tools might we use to gain insight into the majority of matter in the universe? We now have the technology to wield a very different force -- the force of gravity -- to explore this uncharted realm. LIGO, the Laser Interferometer Gravitational-Wave Observatory, is an instrument for sensing the presence of matter, whether shining or dark, in the distant reaches of the cosmos. LIGO does this by detecting the gravitational waves -- ripples in the force of gravity -- created by violent events such as the collisions of stars and the vibrations of black holes.
Imagine now turning up the volume on that televised concert and hearing the stirring sounds of a symphony. What a difference it makes to experience music with this new sense! What new experiences await us when we begin exploring the heavens with LIGO?
PUSHING THE LIMITS OF TECHNOLOGY
LIGO must measure the movements of its mirrors, separated by two and a half miles, with phenomenal precision. To achieve its goal, LlGO must detect movements as small as one thousandth the diameter of a proton, which is the nucleus of a hydrogen atom. Achieving this degree of sensitivity requires a remarkable combination of technological innovations in vacuum technology, precision lasers, and advanced optical and mechanical systems.LIGO's interferometers are the world's largest precision optical instruments. They are housed in one of the world's largest vacuum systems, with a volume of nearly 300,000 cubic feet. The beam tubes and associated chambers must be evacuated to a pressure of only one-trillionth of an atmosphere, so that the laser beams can travel in a clear path with a minimum of scattering due to stray gases. To do this, LIGO scientists and engineers have worked with industry to produce steel with a very low dissolved hydrogen content.
The LIGO laser light comes from high-power, solid-state lasers that must be so well regulated that, over one hundredth of a second, the frequency will vary by less than a few millionths of a cycle. This severe requirement makes the LIGO detectors among the most precise test beds available for laser stabilization and has attracted significant laser development activity worldwide.
The suspended mirrors must be so well shielded from vibration that the random motion of the atoms within the mirrors and suspension fibers can be detected. The high-precision, vibration-isolation systems needed to achieve this are very closely related to equipment used for the masking and etching of circuitry on silicon in semiconductor manufacturing.
More than 30 different control systems are required to hold all the lasers and mirrors in proper alignment and position, to within a tiny fraction of a wavelength over the four-kilometer lengths of both arms of the interferometers. These control systems must be monitored continuously and able to function without human intervention. Sophisicated simulation software and state-of-the-art electronics design are used to perform these tasks.
PARTICIPATION IN LIGO
LIGO is a scientific collaboration of the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT). Funded by the National Science Foundation, LlGO will function as a national resource for both physics and astrophysics. There will be significant involvement with many other universities and institutions, both within the United States and abroad.The LIGO Scientific Collaboration (LSC) is the organization that fosters such participation. It offers a mechanism for national and international collaboration in the scientific uses of LIGO, for guidance in design decisions today, as well as the science program requirements of the future.
LIGO will strongly support science instruction and other educational activities in the states and communities where the observatories are located. The resident staff at the Washington State and the Louisiana observatories, as well as the steady stream of top scientists visiting from all over the globe, will contribute to the intellectual and cultural life of the local communities.
LIGO Home Page on the Web: http://www.ligo.caltech.edu
LIGO Scientific Collaboration (LSC) Home Page on the Web: http://www.ligo.org
LIGO Laboratory (Caltech): 626.395.2129