The Snapshot Paradox: On the Ontological Impossibility of Past Time Travel

AbstractThis paper proposes the Snapshot Paradox, a theoretical argument that backward time travel to an identical past state of the universe is ontologically impossible. A past moment corresponds to a complete quantum microstate of the universe. Reinstating that state would require restoring all particles and fields to their exact configuration, which is forbidden by quantum uncertainty, entropy increase, and the no-cloning theorem. Furthermore, the introduction of any observer or apparatus would necessarily alter the microstate, producing a distinct Cauchy surface and therefore a different spacetime event. Consequently, any reconstructed past would be a new state rather than the original, rendering true time re-entry logically incoherent rather than merely technologically infeasible.