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> If, as clock A nears the speed of light it appears that stationary clock B is slowing down, doesn't that mean that at the speed of light is no time? If clock A could reach the speed of light (which it can't), then, yes, clock B would appear to be 'frozen in time'. > Does a photon in a vacuum arrive instantly at its destination? From the perspective of any possible observer, no, it always appears to be traveling at the speed of light. If an observer could travel at the speed of light (which it can't), then, yes, it would appear as if it arrived instantly in the sense that clocks at both its source and destination would appear to be not running at all. > If a photon looks at another photon, does the other photon appear not to be moving at all, or... Ouch my brain. Yeah, this is tricky. Photons can't look or see – what could that mean as looking/seeing involves detecting photons (or, in the sense that something like echolocation is 'seeing', detecting a pattern of matter, e.g. sound)? In a sense, not moving at all or both moving at the same maximum speed don't seem to be much different, from the 'perspective' of a photon. Somewhat related, this video describes how, if one could travel on a vehicle that could accelerate at a constant rate continuously for decades, one would eventually see the cosmic microwave background radiation as a rainbow ring because traveling closer and closer to the speed of light would shift the apparent frequency of the radiation first into the visible spectrum (and then beyond it). |