# Understanding of relativity

mxur
http://en.wikipedia.org/wiki/General_relativity
if you go to this link, and find "Physical description of the theory" not far from the top. the 5th paragraph starts 'More specifically, let us ask how..'
i understand Earth moves in spiral, and light moves in spiral too but with lower angle against horizontal, is this because of time decreasing due to lorentz factor?
also it brings coordinates ct and says its increased by one light year. why introduce the light? also, it shows the angle Earth takes on the spiral is C divided by orbital velocity. why C?? and why 2 speeds show an angle?
sorry if it isn't explained very well.

## Answers and Replies

Staff Emeritus
Science Advisor
If you want to compare time and space, it's very natural to use geometric units where c=1.

Measuring time in years and distance in light years is one choice of geometric units. These are geometric units because the speed of light is exactly

1 (light year) / year

Conveniently enough, 1 g is approximately (1 light year) / year^2 (it's not exact, but close).