text: The answer is simple: we travel through time constantly.  Is it possible to travel backwards?  Science does not currently know the answer. Travelling backwards would entail travel at speeds greater than c (the speed of light).  But it is a fact that to move any body from rest to that speed requires an infinite amount of energy, which does not exist.  So, the only option is to use a particle which ALWAYS travels faster than c.  This particle is called a tachyon, as predicted by M-Theory, but whether or not such a particle exists is currently speculative.  The tachyon has a negative mass, travels at velocities greater than c, and travels in the opposite direction of time as we do.