The ability to travel back in time, though entirely hypothetical, isn't explicitly forbidden by our current understanding of space and time, embodied in the general theory of relativity. Time travel tends to play havoc with other laws of physics, however, and in the 29 May Physical Review Letters researchers report another example. They show that data encryption systems relying on quantum principles can be broken by allowing the data stream to interact with a quantum state that travels back in time. This scenario doesn't present an immediate threat to information security, the authors assert. Rather, it's an example of the kind of contradiction that any unified theory of quantum mechanics and gravity will have to resolve.
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