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Gravitation The law of gravitation, first formulated by Isaac Newton in 1684, states that the gravitational attraction between two bodies is directly proportional to the product of the masses of the two bodies and inversely proportional to the square of the distance between them. In algebraic form the law is stated F=G m1m2/r2 where F is the gravitational force, m1 and m2 the masses of the two bodies, d the distance between the bodies, and G the gravitational constant. The value of this constant was first measured by the British physicist Henry Cavendish in 1798 by means of the torsion balance. The best modern value for this constant is 6.67 × 10-11 N m2 kg-2. The force of gravitation between two spherical bodies, each with a mass of 1 kilogram and with a distance of 1 metre between their centres, is therefore 6.67 × 10-11 newtons. This is a very small force; it is equal to the weight (at the Earth's surface) of an object with a mass of about 0.007 micrograms (a microgram is one millionth of a gram). Effect of
Rotation Acceleration Gravitational
Forces Gravitational
Lens Gravitational
Waves The existence of gravitational waves is predicted by Albert Einstein's general theory of relativity, but they have not yet been detected directly. Indirect evidence for their existence has been found, however. Gravitational waves would transport energy away from the system in which the waves originate. Energy losses of the expected order of magnitude have been observed in the "decay" of the orbits of the two components of PSR 1913+16; this object consists of two neutron stars that circle each other. Indeed, the 1993 Nobel Prize for Physics was awarded to Russell A. Hulse and Joseph H. Taylor of Princeton University for this work and for the discovery of binary pulsars. More direct observation of gravitational waves would give further and more detailed confirmation of relativity and also lead to a new field of astronomy in which new information may be gained about how stars collapse, how black holes interact and how fast the universe is expanding. But such detection is one of the most challenging problems in present-day experimental physics. It depends on sensing strains in space-extremely small changes in the distance between distinct pieces of matter, caused by the waves. In most situations the movements are larger if the overall distance is greater. Theoretical analysis suggests that detectors need to be sensitive to strains of the order of 10-21 to 10-22. This ratio is equivalent to a change of approximately the diameter of an atom, or less, in the distance of the Earth from the Sun. This change can occur over timescales that range from milliseconds to hours, depending on the type of source to be detected. The design of any detector depends on the timescale of the signals it is to search for. A number of experiments for the detection of gravitational waves are being developed. Some are based on sensing the oscillations induced by gravitational waves in aluminium bars weighing several tonnes and cooled to temperatures below 1 kelvin. This technique was pioneered by Joseph Weber of the University of Maryland in the late 1960s. However, the most promising ways to make very sensitive detectors are to hang masses of a few tens of kilograms on pendulums several kilometres apart on Earth and use laser interferometry to sense their movements, or to make measurements between drag-free artificial satellites separated by several million kilometres, placed in orbits sufficiently high to be free of the effects of the Earth's atmosphere. In these cases the two reference objects are positioned to form two paths at an angle to each other and changes in the relative lengths of the two arms are sensed. In the United States two detector systems, each with arms 4 km (2.5 mi) long, are being built as part of the LIGO project; the first is in Washington state, the second in Louisiana. In Europe the Italian/French VIRGO interferometer, with arms 3 km (1.9 mi) long, is being built near Pisa in northern Italy. GEO 600, a shorter interferometer with arms of 600 m (about 2,000 ft) and of advanced technological design, is being built near Hanover in north-western Germany by German and British research groups in collaboration. A detector of somewhat shorter arm length is being constructed in Japan. It is expected that these detectors will be operational towards the end of the century and that collaborative observations will allow the first direct detection of gravitational waves to be made soon thereafter. Plans are also being developed for long-baseline detectors in Australia and Japan. Because of fluctuating local gravitational effects, Earth-based gravitational wave detectors will be restricted to looking for signals above a few hertz (1 hertz, or Hz, is one cycle per second). This allows searches for stellar collapses, the coalescence of neutron stars in binary systems, and signals from pulsars, either isolated or in binary systems. However, there is a wealth of sources at much lower frequency; these range from compact binary star systems to massive black holes, interacting with each other or with ordinary matter, and it is very important that the low-frequency regime should be explored. In order to make this possible LISA, a space system using satellites and a laser interferometer detector with arm lengths of 5 million km (3 million mi), is being considered by the European Space Agency for launch around 2015. |
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