Gravitational Radiation and Waves
The general theory of relativity predicts the existence of gravitational radiation or gravitational waves. Just as an accelerated charge produces electromagnetic radiation in the form of electric and magnetic fields, an accelerated mass produces gravitational radiation in the form of vibrations in the curvature of space-time analogous to the ripples in a pond.
The classical problem, which Albert Einstein solved in 1916, was to consider the radiation pattern from a thin rod rotating about an axis perpendicular to its center. The corresponding situation in electromagnetism produces a radiation pattern whose leading term is given by the electric-dipole moment. The mass-dipole moment is the time rate of change of the momentum of the spinning rod. Because this quantity vanishes by momentum conservation, the power of the radiation scales as the quadrupole moment, and its details are considerably more subtle.
The power of the radiation distribution makes the detection of gravitational waves very difficult. Consider a rod of length L and mass M. The moment of inertia through an axis perpendicular to the center is I= 4ML2 /3. If the rod is spinning with angular momentum , the power is given by the relation: P = 32GI2 6 /5 c5 = 1.73 x 10-52 I2 6 where G is the gravitational constant and c is the speed of light. Assuming that the rod has a mass of 105 kg, a length of 10 m, and an angular velocity of 10 rad/s, the power dissipated in gravitational radiation is roughly 10-32 Watts. This is too small to be detected in the laboratory.
Joseph Weber devised an experiment in which a large cylinder of aluminum is supercooled under a vacuum. When gravitational waves strike the bar, it vibrates at its fundamental frequency. By considering a pair of such bars separated by hundreds of miles, Earth-based vibrations are subtracted out, and the apparatus records gravity waves incident from space. Weber claimed to have detected gravity waves in 1969, but independent measurements have not corroborated this result.
The existence of gravity waves has been inferred by examining the steady loss of orbital energy in binary pulsar systems such as PSR 1913+16. This work contributed to the 1993 Nobel Prize in physics, which was awarded to Russell Hulse and Joseph Taylor. The Laser Interferometer Gravitational Observatory (LIGO) is a present-day attempt to detect gravitational radiation originating in such energetic events as supernovae or black-hole collisions.
Just as there is a quantum theory of electromagnetism (called quantum electrodynamics or QED), there are hopes for a quantum theory of gravity. Though the technical issue of renormalizability complicates the issue, some generic comments can be made about the graviton, the gravitational analogue of the photon.
The fundamental forces of nature other than gravity (electromagnetism, the strong force, and the weak force) are gauge interactions that fit into a theory particle physicists call the standard model. Particle interactions within the standard model arise through the exchange of vector bosons. The photon is responsible for electromagnetism; the gluon, which carries color, is responsible for the strong force; and the intermediate vector bosons W+ , W- , and Z are responsible for the weak interaction. Since these gauge particles each carry spin-1, the potential experienced between two particles that carry the same charge is repulsive. For example, in the case of electromagnetism, a pair of electrons repel, while oppositely charged particles like the electron and positron attract. This is the content of Coulomb's law.
Unlike the three gauge interactions, gravity is always attractive. Newton's law of universal gravitation states that any pair of objects experience a force of attraction proportional to the product of their masses and inversely proportional to the square of their separation. The general theory of relativity says precisely the same thing except that it argues that the force originates from the curvature of space in the presence of matter or energy. The universally attractive character of the potential means that gravity cannot be mediated by a vector (spin-1) boson. Rather, the particle must have spin-2. This tensor, the quantum of the gravitational field, which encodes the curvature of space-time, is called the graviton. According to string theory, the graviton is the lowest energy vibrational mode of the closed string.
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