Clocks on GPS satellites drift against clocks on the ground every single day, and without correcting for that drift positions would go wrong fast. Travel into the future is therefore measured fact, differing only in degree. Travel into the past sits firmly on the other side of the line: solutions containing closed timelike curves have existed on paper since Gödel in 1949, but holding a wormhole open calls for negative-energy matter nobody knows can exist in quantity. Novikov self-consistency, many-worlds branching and Hawking's chronology protection conjecture are ideas, none of them theorems.
Clocks on GPS satellites drift against clocks on the ground every single day, and without correcting for that drift positions would go wrong fast. Travel into the future is therefore measured fact, differing only in degree. Travel into the past sits firmly on the other side of the line: solutions containing closed timelike curves have existed on paper since Gödel in 1949, but holding a wormhole open calls for negative-energy matter nobody knows can exist in quantity. Novikov self-consistency, many-worlds branching and Hawking's chronology protection conjecture are ideas, none of them theorems.
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