In special relativity, the problem arises because while all observers agree about the time ordering of events linked by a subluminal signal, for a superluminal signal different observers disagree on whether the signal was received after or before it was emitted. To understand these new developments, we first need to question the origin of the received wisdom that superluminal motion necessarily leads to unacceptable causal paradoxes. Quantum effects such as vacuum polarization in gravitational fields appear to permit “superluminal” photon propagation and give a fascinating new perspective on our understanding of time and causality in the microworld. Difficulties with causality only arise if a return signal B´ to C is possible, where C is in the past light-cone of A. A superluminal signal A to B that is forward in time in one inertial frame may be backward in time A to B´ in another inertial frame related to the first by a Lorentz transformation. Graham Shore describes recent research that sheds new light on these old questions. Is it possible to travel faster than light? Can we travel back in time, or send signals into the past? These questions have intrigued physicists since the discovery of special relativity nearly a century ago highlighted the fundamental nature of the speed of light and revolutionized our concept of time.
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