We may advance through time with rate c, however, once rate c is fully used up by motion through space (tipping of the light cone by 45o), and passage of relativistic time decreases to rate 0, then we no longer have any left over supply to "increase beyond c" through space (and travel back in time). Besides... doing so would require an infinite impulse (as relativistic mass tends to infinite) which is of course impossible.

Alex, I don't believe it's possible to tip our light cone upside down (and travel into different frame's past). Even relativity with all its "funny buisness" doesn't allow for that. You may go back and forth along x-axis but only one direction (although at dilated rates) through t-axis (must be relative to another frame). Of course... proper-time remains invarient (you can't travel into your own future of course), so we're speaking with regard to comparison of somebody elses frame. With no other frame to refer to (no other matter), relativity no longer makes sense (Mach's principle says that there can be no motion with respect to "background" space).

• Is not Mach principle dead yet? - alexander - November 6, 2001 - 02:12 UTC