Hi, Mr. Red. About your first post: when I was a college freshman, I read a book about relativity by Einstein and Infeld. Most of us couldn't follow it, but that was all right because most of the teachers couldn't either. After some years, I decided to lick this thing, and I read more books on relativity. I still haven't licked it.

But one thing I do know is that when we want to calculate the speed of two systems passing each other, we think we should just add or subtract. Like this (I think)

Train A is heading east at 70 mph.
Train B is heading west at 50 miles per hour.
How fast do they seem to be going past if one is a passenger? answer 120 mph, 70+50.

But Einstein and others figured out that there is another term added to the equation, and it's a fraction with some element of the system (I forget which element) divided either by the speed of light or the speed of light squared. Obviously this fraction with a huge denominator will have a tiny, tiny value. Detecting the difference it made was definitely beyond Newton and other early scientists.

In fact, earthlings ignore it entirely, unless they are working with GPS's.