Sources of gravitational waves
Which direction does a dumbbell have to fall in order to radiate gravitational waves?
Gravitational wave
Gravitational wave Gravitational-wave detector Quadrupole Radiate Gravitational-wave detector
Gravitational_wave-Sources_of_gravitational_waves Gravitational-wave_detector-Abstract Quadrupole-Gravitational_quadrupole Radiate-Abstract Gravitational-wave_detector-Pulsar_timing_arrays
In general terms, gravitational waves are radiated by objects whose motion involves acceleration, provided that the motion is not perfectly spherically symmetric (like an expanding or contracting sphere) or cylindrically symmetric (like a spinning disk or sphere). A simple example of this principle is provided by the spinning dumbbell. If the dumbbell spins like wheels on an axle, it will not radiate gravitational waves; if it tumbles end over end like two planets orbiting each other, it will radiate gravitational waves. The heavier the dumbbell, and the faster it tumbles, the greater is the gravitational radiation it will give off. If we imagine an extreme case in which the two weights of the dumbbell are massive stars like neutron stars or black holes, orbiting each other quickly, then significant amounts of gravitational radiation would be given off. Some more detailed examples: Two objects orbiting each other in a quasi-Keplerian planar orbit (basically, as a planet would orbit the Sun) will radiate.  A spinning non-axisymmetric planetoid — say with a large bump or dimple on the equator — will radiate.  A supernova will radiate except in the unlikely event that the explosion is perfectly symmetric.  An isolated non-spinning solid object moving at a constant velocity will not radiate. This can be regarded as a consequence of the principle of conservation of linear momentum.  A spinning disk will not radiate. This can be regarded as a consequence of the principle of conservation of angular momentum. However, it "will" show gravitomagnetic effects.  A spherically pulsating spherical star (non-zero monopole moment or mass, but zero quadrupole moment) will not radiate, in agreement with Birkhoff's theorem. More technically, the third time derivative of the quadrupole moment (or the "l"-th time derivative of the "l"-th multipole moment) of an isolated system's stress–energy tensor must be nonzero in order for it to emit gravitational radiation. This is analogous to the changing dipole moment of charge or current necessary for electromagnetic radiation.
A gravitational-wave detector is any device designed to measure gravitational waves, tiny distortions of spacetime that are predicted by Einstein's theory of general relativity. The existence of gravitational radiation is a prediction of general relativity. Gravitational waves are perturbations in the curvature of spacetime caused by accelerated masses. Since the 1960s gravitational-wave detectors have been built and constantly improved. The present-day generation of resonant mass antennas and laser interferometers has reached the necessary sensitivity to detect gravitational waves from sources in the Milky Way. As of early 2014, no direct detection of gravitational waves has yet been accomplished, although a number of experiments have now added to the evidence that gravitational waves are more than mathematical anomalies of relativity calculations (ref. the 1993 Nobel Prize in Physics). On 17 March 2014, astronomers at the Harvard-Smithsonian Center for Astrophysics announced the apparent detection of gravitational waves, which, if confirmed, may provide strong evidence for inflation and the Big Bang. 
The mass quadrupole is very analogous to the electric charge quadrupole, where the charge density is simply replaced by the mass density. The gravitational potential is then expressed as: formula_19 For example, because the Earth is rotating, it is oblate (flattened at the poles). This gives it a nonzero quadrupole moment. While the contribution to the Earth's gravitational field from this quadrupole is extremely important for artificial satellites close to Earth, it is less important for the Moon, because the formula_20 term falls quickly. The mass quadrupole moment is also important in general relativity because, if it changes in time, it can produce gravitational radiation, similar to the electromagnetic radiation produced by oscillating electric or magnetic dipoles and higher multipoles. However, only quadrupole and higher moments can radiate gravitationally. The mass monopole represents the total mass-energy in a system, which is conserved—thus it gives off no radiation. Similarly, the mass dipole corresponds to the center of mass of a system and its first derivative represents momentum which is also a conserved quantity so the mass dipole also emits no radiation. The mass quadrupole, however, can change in time, and is the lowest-order contribution to gravitational radiation. The simplest and most important example of a radiating system is a pair of mass points with equal masses orbiting each other on a circular orbit (an approximation to e.g. special case of binary black holes). Since the dipole moment is constant, we can for convenience place the coordinate origin right between the two points. Then the dipole moment will be zero, and if we also scale the coordinates so that the points are at unit distance from the center, in opposite direction, the system's quadrupole moment will then simply be formula_21 where M is the mass of each point, and formula_22 are components of the (unit) position vector of one of the points. As they orbit, this x-vector will rotate, which means that it will have a nonzero first, and also the second time derivative (this is of course true regardless the choice of the coordinate system). Therefore the system will radiate gravitational waves. Energy lost in this way was first inferred in the changing period of the Hulse–Taylor binary pulsar, a pulsar in orbit with another neutron star of similar mass. Just as electric charge and current multipoles contribute to the electromagnetic field, mass and mass-current multipoles contribute to the gravitational field in general relativity, causing the so-called "gravitomagnetic" effects. Changing mass-current multipoles can also give off gravitational radiation. However, contributions from the current multipoles will typically be much smaller than that of the mass quadrupole. 
Radiate may refer to:  Radiation, a process by which energetic particles or energetic waves travel  Radiate (coin), an ancient Roman bronze coin  Radiate crown, headgear symbolizing the sun  
A different approach to detecting gravitational waves is used by pulsar timing arrays, such as the European Pulsar Timing Array, the North American Nanohertz Observatory for Gravitational Waves, and the Parkes Pulsar Timing Array. These projects propose to detect gravitational waves by looking at the effect these waves have on the incoming signals from an array of 20–50 well-known millisecond pulsars. As a gravitational wave passing through the Earth contracts space in one direction and expands space in another, the times of arrival of pulsar signals from those directions are shifted correspondingly. By studying a fixed set of pulsars across the sky, these arrays should be able to detect gravitational waves in the nanohertz range. Such signals are expected to be emitted by pairs of merging supermassive black holes. 