you didnt have a mirror in the box.
you just had a box with a black interior.
and plasticine at one end.
a mirror would slow down the light slightly as it travels
through the glass medium to the reflective coating and then
out of the mirror.
gold would not absorb the light like the mirror would
it would only reflect it.
using gold would reduce the p=mv to the inside of the box.
and more of the energy of the EM source would be converted
into heat by the plasticine because less light is being absorbed
by the other 5 sides of the box.
you would have a reactionless propulsion device using tiny
amounts of gold leaf and some modeling clay and a box.
but it has no value until you can prove it.
and the people that you would need to prove it to are paid
to not believe it.
so why bother with it.
just buy more gasoline for your spaceship.
