# Special & General Relativity Questions and Answers

## Why does the decay of an orbit mean that gravitational radiation is being produced?

It doesn't, at least not for all systems. For the Earth-Moon system for example, the work that each body does on the other to deform it tidally, is partially dissipated by friction within the bodies of the Earth and Moon. Conservation of angular momentum then requires that the separations of the bodies INCREASE to compensate for the loss of energy. For dense bodies such as neutron stars, gravitational tidal effects are important, but so is the fact that they are very close to one another in orbits that are not circular relative to the center of mass of the system. The result is that instead of a weak, and constant gravitational acceleration as experienced by the Moon orbiting the Earth, the neutron stars undergo very strong, and variable accelerations. This causes them to emit gravitational radiation according to the relativistic theory that applies to system in strong gravitational fields in non-circular orbits. The amount of energy loss by gravitational radiation can be calculated in detail once the masses and orbital parameters for the system are known, and these can be determined with high precision for the famous Hulse-Taylor binary pulsar. When tidal dissipation and other effects are eliminated, the system is still 'seen' to be loosing energy at a rate closely predicted by gravitational radiation energy loss.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.