[ETL]

Here's an explanation of time dilation that I wrote myself. Hope it makes sense. The graphic I found on the Web.

"One second is defined as 'the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom'..."
http://en.wikipedia.org/wiki/Second

Now imagine, instead of a vibrating 'caesium 133 atom', we have a beam of light bouncing back and forth between two mirrors within a vertical tube. Now let's say, for the sake of argument, that it takes precisely 'one second' for the light beam to reach the top mirror (tick), reflect off it, reverse and reach the bottom mirror (tock).

Now let's say the light tube, or 'light clock', is resting on a flatbed train car, and on the flatbed is an observer who we will call "Observer A". To Observer A, who is moving along with the train and is therefore 'at rest' with respect to it, the light beam simply travels from the bottom of the tube *vertically* to the top of the tube and then straight back down again. From the relationship, speed equals distance over time, we get time equals distance over speed. So this is then how Observer A defines time (t=distance/speed). Important to note here is that light travels at the SAME SPEED for ALL observers.

Now let's say there is an observer B standing on the embankment alongside the train watching it pass by. From this observer's point of view, or frame of reference, the light beam does NOT simply travel vertically up and down. Rather, it travels on a slanted or diagonal path since the train is in motion, let's say from left to right as Observer B sees it. Now since the light beam travels a diagonal path between tick and tock, again, from OB's stationary point of view, the light beam therefore is traveling a LONGER distance (from OB's perspective). Therefore, since the light beam is traveling a longer distance (from OB's perspective) AND since light travels at the same speed for all observers, the light beam MUST take a longer time to bounce between the two mirrors (tick-tock). Therefore, the two observers (A and B) do NOT agree on what a "second" is.

http://galileo.phys.virginia.edu/classes/252/srelwhat.html

The mathematical relationship between the two paths is based on the Pythagorean Theorem for right-triangles that many of us used in high school.-ETL

[James C. Bennett]

However, when considering velocities, one must also keep the frame of reference fixed.

[rawcatslyentist]

Here's a little spice for the differences in time.

Which observer is correct in his definition of time?

Neither? Both? or both Neither and both?

[The Cajun]

Shouldn't your top reflector be moving at the same speed as your clock in order to be considered part of the clock?

[carenot]

HUH????!!!!!!

[hadit2here]

What happens if the clock is moving vertically, like in an elevator, instead of horizontally?

Going “up” fast enough would shorten the tock part because the bottom mirror would be closer to the position the top mirror was in when the light beam left it on the return journey.

Hey, mabye I’ve discovered how to “warp” time by taking the clock with me and moving fast enough in the vertical direction!

Nahhh.