>>11942633If you built the mirror on Earth, you would need to travel faster than light to get it into position. If there were already someone 10 ly away, you would need to send information faster than light to tell them to build it.
Even if you could make the mirror materialize 10 ly away instantly, the light from yourself 20 years ago would already be 20 ly away from you (i.e. 10 ly behind the mirror) and thus wouldn't be reflected. You wouldn't see the mirror itself for 10 years after it was built, since its light would take that long to reach you. When it finally did, you would see yourself from 10 years before it was built (or 20 years before the present).
If you wanted to see yourself from 20 years ago, you'd need to build the mirror 20 ly away and wait 20 years for the light to get back. But my point at the beginning still stands: the light emitted by you from the past is traveling away from you too fast to catch up with without violating causality.
However, all of this assumes that space is 100% vacuum, which it isn't, so the light is really traveling ever-so-slightly slower than c. If you could create something that is able to travel through space just barely faster than your own light (but still slower than c), it would take thousands of years to catch up (at least) and the same amount of time for the light, once intercepted and reflected, to come back to Earth. By then, any trace of your existence would be wiped off the face of the Earth.
When astronomers looked into the night sky millennia into the future, this massive project forgotten, they would be shocked to find a blue marble just like ours. As they looked closer, though, they would notice something different. Their sun has no Dyson sphere. Their atmospheric CO2 is far higher. The people are all so very confused. Zooming in, they would see someone lost to time, sitting at a screen from an ancient piece of technology, making a 4chan thread about troll physics.
And they would say, "What a faggot."
