Time Travel's 'Grandfather Paradox' Solves Itself at Quantum Level

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A paradox perennially posed to would-be time travelers may resolve itself — but only for a single photon at a time, and only at the quantum level. The "Grandfather Paradox" points out that if time travel were possible and you went back and prevented your grandparents from meeting, you would prevent your own birth and subsequent time travel. It's an insoluble paradox — except, perhaps, at the smallest of scales.

Researchers at the University of Queensland simulated what would happen if a single photon were to be caught in a "closed timelike curve," a theoretical wormhole that returns the photon to an earlier position in space-time. By interacting with itself, it should affect its own future, creating a nano-scale version of the Grandfather Paradox — but the researchers determined that the inherent "fuzziness" of quantum states prevents that from happening. The photon is already in a quantum superposition of combined existence and nonexistence, so unlike the hypothetical grandfather, it doesn't seem to matter whether it follows one path or another. The study, published in Nature Communications, adds to the growing scholarship regarding the poorly understood border between classical and quantum physics.



—Devin Coldewey

Hat tip to Lee Billings at Scientific American.