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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?

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

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The delay between movement of the laser and the movement of the dot is equal to the distance between them divided by the speed of light. Your thought experiment seems to have implied that the distance is constant, so relativity doesn't even really come into play.

Simply put: the light emanates from the laser, travels the distance at constant speed, then hits the surface and scatters so you can see it.

Of course, the laser can't jump from 0 to 1Left instantaneously, but if it did (or, say, you redirected the beam or shut off a laser at 0 and started another laser at 1Left), it would still just be a constant time delay. Even shut-off isn't really instantaneous, but the question of how quickly EM waves can be attenuated, or how short a laser pulse can be, is a different one.

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  • $\begingroup$ Yes, there is a time delay between the movement of the laser and the dot beacuse of the distance between, but that is not what I meant. We are observing the dot because that may be easier to imagine and in theory the dot moves just the same as the laser but with a little delay time. What I'm wondering is when in time is that dot no longer at its current position but at its next. $\endgroup$ – Fort Ash Sep 11 '15 at 8:33
  • $\begingroup$ "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. $\endgroup$ – Blackbody Blacklight Sep 11 '15 at 11:30

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