[LibWhacker]

Yippee, I see the mid-point thingy! Hey, that's progress for me, slow as it may be. And the observer can stay there as long as he wants. :-)

[Physicist]

I forgot to mention: the interval is invariant for all observers.

[spunkets]

Sorry, maybe the fs and Es weren't to clear. f₁ is a high clock rate, time passes fast. As the relative speed of separation increases towards the speed of light f₂ appears to slow down to someone on Earth.