Could it be that an observed object moving back in time is indistinguishable from an object moving forward in time? To measure the signed rate of time flow we could use a light clock with 3 mirrors (A, B, C) reflecting the light beam in a triangle, in clockwise direction (A -> B -> C …). If someone would observe this clock moving back in time, he would see the light beam traveling counterclockwise (A -> C -> B …). So it appears that we could determine if such a clock is moving backward or forward in time. But what if the entire clock was mirrored in space, when moving back in time. Then the light beam traveling counterclockwise would still hit the mirrors in the normal order (A -> B -> C …), and we could not distinguish it from a clock moving forward in time. Now you ask - Why should a clock moving back in time be mirrored in space? One reason for this assumption are the formulas for time dilation und length contraction, which are connected by the Lorentz factor. A clock with a negative proper time (going backwards in time) would imply a negative Lorentz factor. Put into the length contraction formula, this would imply a negative scaling along one space dimension (which means mirroring). Aside of this pure algebraic approach there is also a geometrical interpretation of this formulas, presented by L. C. Epstein in his book “Relativity Visualized”. He uses a space-propertime diagram, where time dilation and length contraction are determined by simple projections on the time and space axes respectively. An interactive version can be viewed here: http://www.adamtoons.de/physics/relativity.swf From this, it can be easily seen, that a negative proper time component, would mirror the spatial representation of the object. So maybe traveling back in time is not that exciting at all, and takes place all around us, without being noticed. The key point is that nothing can be observed to change it’s direction in time. So we can never see those objects going back in time, when they are not mirrored. Therefore we assume their mirrored appearance to be normal. OK, enough unverifiable speculations for today.