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# 9: Gravitational Waves

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• 9.1: The Speed of Gravity
In Newtonian gravity, gravitational effects are assumed to propagate at infinite speed, so that for example the lunar tides correspond at any time to the position of the moon at the same instant. This clearly can’t be true in relativity, since simultaneity isn’t something that different observers even agree on. Not only should the “speed of gravity” be finite, but it seems implausible that it would be greater than c.
• 9.2: Gravitational Radiation (Part 1)
The Hulse-Taylor system  contains two neutron stars orbiting around their common center of mass, and the period of the orbit is observed to be decreasing gradually over time. This is interpreted as evidence that the stars are losing energy to radiation of gravitational waves. As we’ll see later, the rate of energy loss is in excellent agreement with the predictions of general relativity.
• 9.3: Gravitational Radiation (Part 2)
In this section we study several examples of exact solutions to the field equations. Each of these can readily be shown not to be a mere coordinate wave, since in each case the Riemann tensor has nonzero elements.
• 9.E: Gravitational Waves (Exercises)

Thumbnail: Two-dimensional representation of gravitational waves generated by two neutron stars orbiting each other. Image used with permission (Public Domain; NASA).