Is time universally consistent? Is say one minute the same for me as for the rest of the known Universe? Why is it so or not so?
No, it isn’t.
See any good book about Einstein’s theory.
Or, do a search on these here very boards:
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=4665&highlight=relativity
In physics up to Relativity, the measure of time was assumed to be consistent everywhere and at all times. Relativity denies this assumption. In any two inertial reference frames if we measure the space and time coordinates of two events (where an event is a point in space at a moment in time), the two frames may disagree as to the distance and the time difference. If s and t are the distance as measured by one observer and s and t are the distance and time as measured by the other, then s[sup]2[/sup] - t[sup]2[/sup] = s[sup]2[/sup] - t[sup]2[/sup].
Oops. Make that: s[sup]2[/sup] - (c t)[sup]2[/sup] = s[sup]2[/sup] - (c t)[sup]2[/sup]
Not only is it not so for the rest of the Universe it is not so right here on earth. A person at the top of a tree has time pass for them more quickly than it does for a person on the ground. I’ll grant the effect isn’t very big but it is there and has actually been measured (using water towers or elevator shafts among other things but probably not trees).
Also, I think the cosmonauts aboard the Mir Space Station who stayed up there for about 6 months were somewhere around 3 seconds younger than the rest of the population on earth upon their return (although they were further from the earth causing time to speed up for them they were moving quite quickly which more than made up the difference and caused time to slow down for them).
As for the why of it Dr. Matrix has a good answer.
Actually, the gravitational effect on the cosmonauts was larger than the speed effect. Or, more accurately, since General Relativity includes Special Relativity, the speed effect is part of the gravitational effect. The most quickly that time can pass for any observer is if they’re not accelerating, and (in a relativistic sense) a person in orbit isn’t accelerating.