# When are gravitational waves emitted during a merger of black holes?

When black holes are merging, and emitting gravitational waves, are we seeing those waves start when the event horizons are merging, or when the singularities are merging?

• Gravitational waves are emitted by both regardless. Do you mean the "chirp" that LIGO detected in its signal? – Sir Cumference Sep 8 '16 at 14:09
• I was thinking specifically of the chirp. So gravitational waves are being emitted by any two orbiting objects? – joseph.hainline Sep 8 '16 at 14:21

The gravitational waves are actually emitted as long as you have a quadrupole moment with a second time derivative that is non-zero. The quadrupole is defined as: $$Q_{ij}^{tt}(x) =\int\rho \left(x^i x^j - \frac{1}{3}\delta_{ij}r^2\right)\mathrm{d}^3x.$$ To simplify this, here is a rule of thumb: if you have a system that is not spherically symmetric and that is changing with time (like two black holes), you can expect gravitational waves to be emitted. For example, the system made of the Earth and the Sun is emitting ridiculously small gravitational waves (I think it is emitting a few Joules per year), but it is.