I am surprised to be here, but I thought I'd share an idea that may be really stupid. I recently watched a few documentaries on space and time so I'm obviously an instant expert, but if space is expanding, and space is a substance that may or may not be antimatter, and time and space are related, then perhaps time is expanding (moving forward) and the possibility of going back in time is about as possible as contracting a part of the universe. In other words, we can't go back in time unless where in an area where space is contracting. Okay, have fun!
The product m0=xc = ctc cannot be less than or equal to zero or there will be:
a) no ruler (Einstein)
b) no mass (QM)
(c and tc can both be negative for "holes" - i.e., anti-matter with (smaller) actions above and below the action of the Fermi level )*
Trust Me! ..
*there is more to this story.....