Just a note to readers. I had posted this with three vote choices. Apparently James R has amended my vote choices to include "Don't Care". Well, I don't really care either that he has done that but I find it in poor taste for him to have done so without comment or permission. That is not unlike altering the text of my post. There is currently a thread running which advocates a principle of Relativity for which the stated conclusion is deliberately misleading. [thread=41419]Thread[/thread] The issue has to do with the very requirement by Relativity for something called reciprocity in cases of constant relative velocity and/or equal acceleration. That means whatever happens to A happens to B. In the diagrams given in this case the calculations are made for only one direction of a light signal, frankly the light signal is a complexiety which isn't required to properly analyze the problem. After running a one hour test under such conditions at 0.9c relative velocity the following would be true statements: Clock A reads 3,600 seconds but percieves Clock B as reading 1,569.2 seconds. Clock B reads 3,600 seconds but percieves Clock A as reading 1,569.2 seconds. Upon returning the clocks to a common rest frame they will both still read 3,600 seconds. There is no physical shift of time. The illusion is created by only calculating one half of the actual physical situation. This mathematical trick is obvious if you consider this simplified demonstration. If I tell you that X = Y<sup>2</sup>. You can plot that relationship and you would see a curve. You would claim that this is reality and that X and Y have this non-linear relationship. However, if you are forced to apply reciprocity you would have to also claim that Y = X<sup>2</sup>. Substituting: X = (X<sup>2</sup>)<sup>2</sup> = X<sup>4</sup> Y = (Y<sup>2</sup>)<sup>2</sup> = Y<sup>4</sup> X<sup>4</sup> = Y<sup>4</sup> X = Y So a non-linear illusion of reality is created by looking at only half of the problem. The reality is it is linear. Likewise, in Relativity one sees T2 = T1(1 - v<sup>2</sup>/c<sup>2</sup>)<sup>.5</sup> as a non linear time function between clocks and it is claimed that flying around the universe at relavistic speeds will cause you to return younger than your twin you left at home. Well, the truth is that no such thing happens as a consequence of relative velocity. Upon return your watches (and ages) may only differ a small amount due to General Relativity involving acceleration and gravity but no changes due to long term relative velocity. This is made abundantly clear if one actually computes the total reality of the situation and not calculate only one half of the reality.