# Time it takes for a single unit of movement [duplicate]

This question already has an answer here:

I've just been wondering, what is the time that passes between one moment to another.

Lets take an example that we have a single light source, so small that it emits only a single, constant beam of light in only one direction. This beam hits a plane at a 90° angle. We are observing the dot that forms on this plane (like a laser dot). Now the light source starts moving in a single line with a set speed. Lets say to the left.

We are now observing the dot which moves. Since the light from the light source is pointing in only one direction and it is created at the light source at every moment, when does the light change from position 0, to position 0 + 1Left?

As far as I see it even if this goes down to infinite there must still be a moment there somewhere where the dot was at 0 and then suddenly at 0 + 1Left.

Does this make any sense?

## marked as duplicate by Martin, ACuriousMind♦, WetSavannaAnimal, Waffle's Crazy Peanut, Ryan UngerSep 13 '15 at 14:32

• "when in time": If the source starts moving at time $t=0$, the dot starts moving at time $t=d/c$. The position of the dot isn't quantized so there's no jumpiness to this process. The dot isn't an object; it's a wave phenomenon capable of moving faster than light. In my shut-off/start-up scenario, it's possible for one observer to momentarily see two dots while another observer sees none. If the dot sweeps faster than light, it might be seen to move in opposite directions from different perspectives. But none of this deals with the granularity of time itself. – Blackbody Blacklight Sep 11 '15 at 11:30