3 ms·

One key difference is that time has a direction from past to future, while space doesn't.

• echelon 8y ago
  At a cosmic level, space doesn't either. We're all affected by the big bang and can't go back. At small scales there are time-reversible (or symmetric) systems called time crystals. (I am not a physicist, so feel free to refute me.)
• gus_massa 8y ago
  That is true, but it's not related to this problem. One important point is that you can't replace the "something" in the equation with whatever you like, for example something2 must be always smaller than something1 (if you add a few c here and there to have the correct units). If you select an event (i.e. a particular point of space, in a particular time) there is a cone of future events and a cone of past events and a region with the remainder events. The transformations don't change the future and past cones, but they mix space and time, so they must be mixable.