And directly related to that: How can we be sure of the speed of light? I've been searching for a good answer as to how we measure the SOL and am yet to have someone give me a solid explaination. Today we shine a laser at a mirror on the moon and time how long it takes to get back. What it sounds like to me is: we use the SOL to measure a distance that we use to measure the SOL. I've also heard that we have a device that measures how many times light bounces between 2 points within 1 meter. If im not mistaken thats 3x10^8 times in 1 sec. This to me seems like a pretty important thing to know considering our science is pretty well based on the SOL.