Apparent Superluminality of Lensed Gravitational Waves

Understand

We consider gravitational wave (GW) sources with an associated electromagnetic (EM) counterpart, and analyze the time delay between both signals in the presence of lensing.

• If GWs have wavelengths comparable to the Schwarzschild radius of astrophysical lenses, they must be treated with wave optics, whereas EM waves are typically well within the approximation of geometric optics.
• With concrete examples, we confirm that the GW signal never arrives before its EM counterpart, if both are emitted at the same time.
• However, during the inspiral of a binary, peaks of the GW waveform can arrive before their EM counterpart.