where does the idea of travel back in time come from?

[Tim_Lou]

where does it suggest that it is possible to travel back time?

when something's velocity is greater than speed of light, gamma becomes imaginary...
but imaginary number has nothing to do with travel back time...
time dilation becomes imaginary, but nothing to do with "negative" time.

where does the idea come from then?

[Bo]

you need to ask yourself if you look at these formulas: what does time reversal mean in the language of such a formula? it is simply a transformation from t -> -t. If you look at this new set of equations, you'll see that they have exactly the same properties you described above, so time reversal and v>c are mathematicly equivalent.
(notice that if you do both transformations, you get real numbers again, as you would expect for a physical quantity)

Bo

[pgrmdave]

How do we know that time is not independant of the speed of information. I understand that perception of time is dependant, but how could motion backwards in time occur, theoretically?

[Bo]

time is dependent on the speed of information (which is the speed of light); alternating the speed of light will alter the speed of time (however if the change is global, and everywhere the same, then no one will notice, i guess)

how could motion backwards in time occur, theoretically?

the notion of time is very much linked to the notion of causality. time travels (for our conception) in a certain direction, since we see that A causes B, so A is before B.
Since, to pass from (speeds < c) to (speeds > c) you have to cross an infinite barrier (disregarding quantum effects) it is tempting to think that these 2 cases are not causally connected. and since physics in this regime works the same, only with time walking in the opposite direction, B will cause A there, and thus B happens before A in that regime.

Bo