there is this mind-twisting problem in which a time traveller visits the past and kills his grandfather before his grandmother is even in the picture, meaning the time traveller will never exist and so cannot have travelled into the past to kill his grandfather. but the mathematics of quantum theory says that the quantum state that describes them evolves both forwards and backwards in time (the quantum particles such as photons and electrons are not bound by the arrow of time). this odd state of affairs has led to some researchers claiming that the normal rules of causality don’t apply, so things that happen in a quantum particle’s future will affect its past.
there are two ways to time travel in theory: through a tunnel in space-time, so-called wormhole or quantum teleportation in time using quantum mechanics. the latter has been demonstrated experimentally countless times. the process exploits a curious quantum property called entanglement, by which two particles, such as photons, become so closely linked that they share the same existence. entangled particles are special because a measurement on one immediately influences the other, no matter how far away it is.
now imagine that you want to teleport a third space-travelling particle from A to B. the trick is to make a pair of entangled particles and place one of them at A and one at B, then carry out a set of measurement at both locations, if you do this just right, you can use this “spooky action at distance”, as Einstein called it, to ensure that the second particle ends up in a state that is exactly the same as the “space traveller”. – in fairness, the traveller hasn’t physically moved, but the quantum information that completely describes the traveller has made the trip instead and this allows the second particle at B to take on the traveller’s identity. the curious thing about teleportation is that it occurs instantly. in this process, the quantum information moves from point A to point B, so it is natural to think that the measurement at A set the journey in motion. but because teleportation happens instantaneously, it is just as valid to think that the measurement at point B triggers the journey, even though it takes place moments later. this is post-selection in action and it is a feature that quantum physicists use all the time to do things like quantum computation. it is this ambiguity between cause and effect that Steinberg and Lloyd (of the university of Toronto, Canada) exploit in the time-travel simulator.
Work in Progress 