Gravitational lensing is a powerful probe of cosmology and astrophysics. With the prospect of the first strongly lensed gravitational waves on the horizon, we highlight an opportunity to test fundamental physics. In this work, we assume a nonzero mass for the graviton, which leads to gravitational waves following timelike geodesics instead of null geodesics. We derive standard gravitational lensing equations, such as the scattering angle, the time delay between different images, and the magnification, which normally rely on the assumption of null geodesics. We show that a single strongly lensed multimessenger event is enough to constrain the graviton mass to $m<3\times {10}^{-23}\mathrm{eV}/{\mathrm{c}}^2$. Notably this constraint is independent of the lens model, of the waveform model, and of cosmology. Additionally, we explore magnification of images and find that they offer at least 3 orders of magnitude weaker bounds than the time delay, and have a dependence on the correct modeling of the lens and cosmology.

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• Alternative gravity theories
• General relativity
• Gravitation
• Gravitational lenses
• Gravitational waves
• Gravitons