- In general relativity, the effects of both the stationary field and gravitational radiation are described by the tidal forces they produce on free test masses. In other words, single geodesics alone cannot detect gravity or gravitational radiation; we need at least a pair of geodesics. While the stationary tidal force due to the Newtonian potential of a self-gravitating source at a distance falls off as , the tidal force due to the gravitational wave amplitude that it emits at wavelength decreases as . Therefore, the stationary coulomb gravitational potential is the dominant tidal force close to the gravitating body (in the near zone, where ). However, in the far zone () the tidal effect of the waves is much stronger.
- The stationary part of the tidal field is a DC effect, and simply adds to the stationary tidal forces of all other objects in the universe. It is not possible to discriminate one source from another. Gravitational waves carry time-dependent tidal forces, and so they can be discriminated from the stationary field if one knows what kind of time dependence to look for. Interferometers are ideal detectors in this respect because they sense only changes in the position of an interference fringe, which makes them insensitive to the DC part of the tidal field.

Because gravitational waves couple so weakly to our detectors, those astronomical sources that we can
detect must be extremely luminous in gravitational radiation. Even at the distance of the Virgo
cluster of galaxies, a detectable source could be as luminous as the full Moon, if only for a
millisecond! Indeed, while radio astronomers deal with flux levels of Jy, mJy and even Jy, in the
case of gravitational wave sources we encounter fluxes that are typically 10^{20} Jy or larger.
Gravitational wave astronomy therefore is biased toward looking for highly energetic, even catastrophic,
events.

Extracting useful physical, astrophysical and cosmological information from gravitational wave observations is made possible by measuring a number of gravitational wave attributes that are related to the properties of the source. In the rest of this section we discuss those attributes of gravitational radiation that can be measured via gravitational wave observations. In the process we will review the basic formulas used in computing the gravitational wave amplitude and luminosity of a source. These will then be used in Section 3 to make an order-of-magnitude estimate of the strength of astronomical sources of gravitational waves.

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