section: Sources of gravitational waves
question: Which direction does a dumbbell have to fall in order to radiate gravitational waves?
article: Gravitational wave
candidates: 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.