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What would we see if we place a mirror 1 light year away? Would we see the present or the past?

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    $\begingroup$ Light would take one year to reach the mirror and one year to come back. You'd see two years in the past. $\endgroup$ – QuantumBrick Apr 17 '15 at 20:35
  • $\begingroup$ possible duplicate of Will we ever be able to view the past? $\endgroup$ – Mike Apr 17 '15 at 21:11
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    $\begingroup$ Amateur radio operators call this a moonbounce, though the distance is only a couple of light-seconds and it's quite challenging to pull off. $\endgroup$ – rob Apr 17 '15 at 21:38
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The light from the observation point that hit the mirror and returned would be two years old by the time it returned to the observation point, but there is a very big problem with this set up. The mirror would have to be huge and curved to reflect enough light from the observation back again so that it could be seen. (Imagine tyring to brush your hair in a mirror 100 m away...).

More seriously, often in 'pump probe' laser experiments the positions of mirrors are moved to change the delay between two pulses of light that allows experiments to be performed with variable delays between two laser pulses. These experiments use exactly the principle that is relevant to your question.

  • So the further away you stand from the mirror - the younger you will look...!
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