(Moderator's Note: this thread was split from https://www.physicsforums.com/threads/no-such-thing-as-instantaneous-speed.800941/) Does a particle reach its terminal speed instantaneously or not? Specifically a massless photon (restmass). Without mass there is no inertia. Without inertia, how can there be acceleration? What then prevents a massless particle (such as a photon) from starting at SOL and forces it to consume time in order to accelerate until it reaches SOL? IMO, length of travel, trajectory, time required are irrelevant to the question.