Originally Posted by mettw The problem with most explanations of Special Relativity (SR) is that they use the old "Classical" mindset to explain it. I'll try to explain it using the modern concepts of symmetry and rotation. Hold a pencil up horizontally. It's vertical length is very small, just a few millimeters. It's horizontal length is much longer though, say 15cm. Now rotate the pencil by 90 degrees. Now the horizontal length is only a few millimeters while the vertical length is now 15cm. That is, vertical and horizontal length are relative quantities that depend on your orientation relative to the pencil. None of this is a problem however since we know that the total length of the pencil (L) is invariant. By Pythagoras theorem: L^2 = x^2 + y^2 or in 3-D L^2 = x^2 +y^2 + z^2 Now, in newtonian physics space and time are two completely different things. This means that you can't rotate anything through time since time is only a single dimension and so therefore the time between two events is invariant and everyone measures the same time. In SR however it was realised that time and space are just different aspects of the same thing: space-time. That is, instead of having the 3 space dimensions and one time dimension being two completely different things they are actually both parts of a single thing - the 4 dimensional space-time continuum. The upshot of this is that we can now do rotations involving time. These rotations are what we call velocity. So if someone is moving relative to me then his space-time is rotated relative to mine. Imagine two astronauts floating in space. If they are rotated relative to one another and one sees a spanner floating horizontally (relative to him) then the other will see the spanner at some angle to the horizontal, so that part of its length is horizontal and part is vertical. We could say then that the first astronaut's horizontal axis is partly vertical relative to the second astronaut. In the same way a space-time rotation (that is, velocity) will make the other person's time dimension partly space relative to me and his space dimension, in the direction of his travel, is partly time relative to me. Now in the pencil example the horizontal distance got smaller while the vertical one gets larger. The same thing happens here, the space distance gets smaller (length contraction) while the time distance gets larger (time dilation). Also as with the pencil example, none of this is a problem since there is an invariant length that everyone measures to be the same value: s^2 = x^2 + y^2 + z^2 - t^2 Note the minus sign on the 't' dimension. So we may disagree on the value of x, y, z or t between two events, but that doesn't matter since everyone measures the interval s between the two events to have the same value. So, basically, you need to stop thinking of time as being a separate thing in itself. Time is just one of four dimensions in the space-time continuum and what seems to you to be time may to me seem to be space and visa versa. Does that make any sense at all? I do understand. And I do agree as far as time being another dimension, as it relates to 3d coordinates. A simply way I look at it is; 10 seconds ago I started this reply. But I started it in a whole other area of space. I don't b elieve in space-time as a dimension that go fast for some, and slower for others; as it is observed. We have absolutely no physical proof of time being variant as it pertains to speed. One MAJOR-MAJOR paradox to me is, the simple state of motion. And the more that relativist say that crap about feeling acceleration, so that's the one that knows there moving - the more infuriated I get. And another ironic point is, of all the math that defines time dilation, length contraction, distance contraction, etc., there is no math that pertains to acceleration. NONE. So to me, it is a paradox of the twins. By Einsteins own words, how do we know who's moving!? Again, I don't buy acceleration. It's a cop-out.