As an example of how these ripples are produced, think of man whose mass is 80 kg. All matter is a source of gravity and, according to general relativity; gravity curves space and slows down time. So the 80 kg man is surrounded by a slight warping of space and time commensurate with his mass.

Now suppose that this man begins waving his arms. Although his total mass does not change, how his mass is distributed does change. The geometry of space and time must adapt to these changes, because the gravitational field of the man with his hands over his head is slightly different from that of the man when he has his hands at his sides. These minor readjustments appear as tiny ripples in the overall geometry of space and time surrounding the man. In the same way, a bouncing ball, the Moon going around the Earth, or binary stars all produces gravitational waves. From the equations of general relativity, it is possible to prove that gravitational radiation moves outward from its source at the speed of light.

Gravitational waves are difficult to detect, because they carry very little energy. To appreciate how weak gravitational waves are, imagine two electron separated by a short distance. Because they each possess mass and charge, these electrons exert both gravitational and electric forces on each other. The gravitational force is about 10^42 time weaker than the electric force. If these two electrons are made to wiggle back and forth, they will radiate both gravitational and electromagnetism waves. Because gravity is so much weaker than electromagnetism, the resulting gravitational waves are subdued by a factor of 10^-42 compared to the electromagnetic waves.

Processes involving dramatic changes in intense gravitational fields produce the strongest bursts of gravitational radiation. For example, the collapse of massive star's core during a supernova explosion emits substantial gravitational radiation. Of course, we cannot observe the actual outer layers of light. However, gravitational waves from the collapsing core carry detailed information about how this dense matter is being rearranged. With a gravitational wave antenna, we should be able to observe directly the creation of a neutron star or black hole.

Although an actual burst of gravitational waves has not yet been conclusively detected, many astronomers believe that the effects of gravitational radiation have been observed. In 1974 Joseph Taylor and his colleagues at the University of Massachusetts discovered a pulsar in a binary system. The system apparently consists of two neutron stars separated by only 2.8 solar radii. One of the two stars emits radio pulses every 0.059 second, and the orbital period of the two stars about each other is only 7.75 hours. The average orbital velocity of these stars is thus enormous about  0.1% of the speed of light.

Because these two stars have strong gravitational field and are moving so rapidly, this entire binary system should be a substantial source of gravitational waves. As gravitational radiation carries energy away from the system, the two stars should gradually spiral in closer and closer to each other, causing the orbital period of the two stars to decrease. Because one of the stars is a pulsar, radio astronomers have been able to measure its orbital period with extreme accuracy. These observations prove that the two stars are indeed spiraling in toward each other at exactly the rate required by the emission of gravitational waves.  

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