I found this extraordinarily helpful: http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf Th

I found this extraordinarily helpful: http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf The Lorentz transform allows you to calculate exactly how time/distance differ between two reference frames. The above link derives the transform from first principles - assuming only spatial symmetry and the invariance of the speed of light in different reference frames. There are other derivations, but I found this one the most straightforward.