Would we see the past if we look to a mirror 1 light year away?

Question

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

Answer 1

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...!