Cloudscape

To travel back in time is to travel back in space. What appears to be the most esoteric of technologies, then, actually happens daily. Step backward in the selfsame way you stepped forward, and your body has traveled through time.
The same applies to a molecular, atomic or subatomic level: if two classical particles collide on a straight line, both will travel back in time as they rebound. If every quantum in the universe could move in exactly the opposite direction, it would regress to what it was earlier. Doing so in any closed system would cause the same for that system.
However, doing so would require to surmount the obstacle of Heisenberg uncertainty - at least, it likely would. If the movement of every particle in an isolated system would be reversed at the same moment, however, it’s possible that they’d move in exactly the same way, but in the other direction. This could be so if Heisenberg uncertainty is caused by the system itself. If it is not, Heisenberg uncertainty would violate causality, as it would mean that it is purely stochastic, independent of the state of the system.
It’s possible that one wouldn’t need to know the position and momentum of every the quanta in order to revert their motion, however: doing so would, in principle, require only one very simple alteration: changing the sign of their mass.
As kinetic energy is proportional to mass, if the mass will change sign, so will the kinetic energy. Therefore, its vector will change sign, which means that it will move in the opposite direction. If the mass of every particle in an object would change sign, then, the object would regress to its former state. This makes the manipulation of mass extremely important, as it may allow any lost information to be retrieved. Most notably, this technology could be used to revive people upon information-theoretic death. If this method would be applied on a large enough area, it could save anyone or anything from the worst cataclysm long after it has happened.
If we could also control the absolute value of mass, we could also control time in any way at all: that is, we could really manipulate time like we do in motion pictures, pausing, rewinding, going forward - even though we’d not need to know the past or future to do so: we could; we could slow it down or speed it up, raise it to infinity or stop it. And this all would in no way entail the absurdly high energies of ultrarelativistic speeds.
Another way of manipulating time without needing to reach the speed of light, however, is manipulating the speed of light itself, as lowering the speed of light could make it easier to travel through time: right now, we’d have to speed up to a speed of three hundred million meters per second, so that it would take a mass of two thousand tons an energy of 10^22 joules to slow down time thousandfold. But suppose that we’d have to speed to only three hundred millionths of a meter per second, then this would take only 10^–10 joules, 10^–32 times less.
Aside from time travel, changing mass would have many more applications. For instance, as E=mc^2, in accordance with the law of conservation of energy the speed of light should change if the mass of an object would change, which could make the applications in time travel said above possible.
Also, changing the mass of an object would automatically change its speed. In this way, though we cannot undo energy, we can undo its effects: for mass is what impedes energy it in its expression in movement. And if forces are mediated through exchange of momentum, then controlling mass, which will change momentum, could allow us to control the mediation of forces. If we could control mass, then, we could control the universe. The Higgs boson, supposed to be responsible for mass, fully justifies its apellation “God particle.”