# Why can’t we accelerate photons, and is it possible for a photon to have mass? [duplicate]

• Is not true that you cannot accelerate a photon, at least nominally. The speed of photons in a refractive medium is given by $\frac{c}{n}$, with n the refractive index of the medium. If you build a refractive material with a carefully engineered gradient of the refractive index, you can make light to accelerate or decelerate within it – lurscher Sep 20 '18 at 14:45