If you had two identical video cameras and started recording synchronized atomic clocks recording at the same speed and you put one on the spaceship and sent it out traveling at 99% of the speed of light for a while and left the other one on earth both recording the whole time, when the spaceship returned and you played back the recordings of the two clocks on monitors next to each other, would you see the recordings of the two clocks going at different speeds with one clock on earth appearing to move faster?

It seems that you wouldn't see the recordings being different or one recording longer than the other if the experience of the observer on the ship experienced reality at the regular speed the world usually unfolds to the observer, not slow motion, but if you looked at the two clocks they would have different times.

How does that work when you're watching the recordings of the same length and frame rate? The cameras are recording the whole thing yet the clocks are different.

I was thinking about time perception. Does the rate the brain is processing information speed up or slow down relative to an outside observer moving at different speeds, but the subjective experience of time passing for both remain the same?

Suppose there was a streaming video from the spaceship. Would they appear to be moving in slow motion in the video stream?

Would the length of the video be shorter for the recording on the ship?

Would the clocks appear to be moving at the same speed?

What would you see when you played back the videos next to each other?

There is something confusing about this.

  • $\begingroup$ Please see John Rennie's physics.stackexchange.com/q/242043 and the earlier questions he links there. $\endgroup$
    – PM 2Ring
    Mar 10, 2019 at 16:43
  • $\begingroup$ thanks, but a simple answer about differences in the videos was not clear from the links $\endgroup$ Mar 10, 2019 at 17:12
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    $\begingroup$ Regarding your questions about streaming video, see my answer here: physics.stackexchange.com/a/388549/4993 (My answer talks about what happens if you watch the other clock through a telescope, but exactly the same calculations apply to streaming video.) When you play the videos side by side,they of course look identical. Why wouldn't they? $\endgroup$
    – WillO
    Mar 10, 2019 at 17:17
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    $\begingroup$ True, but it's definitely worthwhile to invest the time in reading those answers by John Rennie, IMHO. There are a lot of questions on this site connected to the Twin Paradox, with plenty of good answers, and plenty of not so good answers. ;) But John's articles are pretty hard to beat. And although they don't directly answer your questions they do contain all the info you need to figure out the answers to your questions. $\endgroup$
    – PM 2Ring
    Mar 10, 2019 at 17:54
  • $\begingroup$ if the videos of the two clocks are identical, but the clocks are different when they return? playing back the videos they are the same length but more time has passed. what does that look like if the videos are both recording their own local time. It seems confusing to me. $\endgroup$ Mar 10, 2019 at 18:26

2 Answers 2


The video playbacks would each proceed ticking away at the same rate, looking identical. The traveling video would end a lot sooner than the at-home video.

  • $\begingroup$ How they can be identical if the traveller video ends earlier? $\endgroup$
    – Alchimista
    Mar 11, 2019 at 9:45
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    $\begingroup$ @Alchimista He means that the individual ticks look identical. $\endgroup$
    – Conifold
    Mar 11, 2019 at 21:49
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    $\begingroup$ @Alchimsta l think it's a subjunctive mood continuous future conditional, until it's not? Maybe ask grammar stackexchange. $\endgroup$
    – JEB
    Mar 12, 2019 at 5:28
  • $\begingroup$ Yes it is probably grammar. I read that the subjects of the sentence are looking identical, not the thicks. Sorry if I don't know English very well. Somehow the answer is "Thicks in the two videos are identical. The video recorded by the traveller is shorter than that recorded on earth.". Got it, but I was suspecting it already. $\endgroup$
    – Alchimista
    Mar 12, 2019 at 8:09
  • $\begingroup$ @Conifold yes sure. But the sentence means something else. $\endgroup$
    – Alchimista
    Mar 12, 2019 at 14:08

Each recorder shares proper time with its corresponding clock, so both sets record and show the same amount of time during playback.

The clocks themselves though, after luminal travel, would show different times, because of Time Dilation. Time slows for a moving object relative to an external observer. The Observer would see the moving clock in slow motion. This is a consequence of light traveling at the same speed in all frames of reference. To maintain this constant velocity for all Observers time itself changes between them. The closer you get to light speed relative to another Observer, the more noticeable the Time Dilation.

  • $\begingroup$ Just to be clear: the recordings have the same duration as the time shown on their respective clocks. They do not have the same duration as each other. Everything in the travelling spaceship -- clocks, recording devices, people, etc. -- experiences the same shorter elapsed time. $\endgroup$
    – Eric Smith
    May 11, 2022 at 2:13
  • $\begingroup$ Agreed, thank you for that clarification. Everything in the moving clock's frame of reference would experience the time dilation. So each recorder and clock pair shows the same time, but both pairs show different times. Dont let me plot your next jump into hyperspace! $\endgroup$ May 11, 2022 at 11:41

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