|A material medium (such as a spring) is not necessary
to transmit forces between matter. Indeed if one removes the connecting spring
between the lumps of matter in the example above the latter will still
accelerate towards each other when released from rest. In this case it is the
attractive force called gravitation that is solely responsible.
According to Newton this is a universal attraction between every element of
matter in the Universe and was assumed negligible compared with the elastic
forces due to the spring in the example above. It differs from the elastic
forces by being always attractive no matter what the separation is between the
elements of matter. For two steel ball bearings 1cm in diameter and separated
by 1cm the gravitational acceleration of each on the other is very small. By
comparison it is 3 millionths of 1 per cent of the acceleration of one of the
ball bearings released from rest just above the surface of the earth. The
latter acceleration is due to the gravitational attraction between the earth
and the ball bearing and is so much larger because it varies directly with the
mass of the source of attraction (i.e. the earth).
If either of two isolated masses in space is given an initial sideways disturbance (rather than being released from rest) then the system can execute a steady orbital motion in which each mass dances around the other in a periodic manner under their mutual gravitational attraction. Similar orbital patterns often emerge when several bodies are locked together under the tentacles of the gravitational force. Gravitation is the dominant force that keeps the planets in their orbit around the sun and the stars in their motion about the centre of the galaxy. Not all motions under the force of gravitation correspond to bound orbits. Sometimes a wandering comet visits from beyond the solar system swings around the sun and disappears never to return. If it passes close to a planet it may induce a wobble in the planet's orbit. Such orbital oscillations are mercifully rare since they can potentially upset the orbits of neighbouring planets and lead to planetary instabilities that would be very difficult to control. More common are visits from rogue asteroids that stray from their orbits between Mars and Jupiter. Clearly it is important to understand the orbital vibrations induced by the forces of gravitation!
One of the most striking features of Einstein's description of gravitation is the prediction of gravitational waves: ripples of tidal acceleration that propagate through space at the speed of light. Such vibrations are thought to be produced by astrophysical phenomena ranging from the coalescence of orbiting binaries to violent events in the early Universe. Their detection would herald a new window for the observation of natural phenomena. Great ingenuity is being exercised in attempts to detect such waves in the vicinity of the earth using either laser interferometry or various resonanting devices following Joseph Weber's pioneering efforts with aluminium cylinders some decades ago. Due to the masking effects of competing influences and the weakness of gravitation compared with the electromagnetic interactions the threshold for the detection of expected gravitationally induced signals remains tantalisingly close to the limits set by currently technology. In order to achieve the signal to noise ratios needed for the unambiguous detection of gravitational waves in the vicinity of the earth a number of experimental strategies are currently under consideration.
Further information on the "Lasso" project for detecting gravitational waves. (Adobe pdf file - 110kb)