Quantum Physics Can Change the Past, Physicists Show

Quick Overview

Physicists have demonstrated that changing the past in quantum mechanics is conceptually possible through specific thought experiments, such as Wigner's Friend paradox variants, but the results of these experiments, when interpreted under standard assumptions, lead to contradictions, suggesting that the assumption that measurements are irreversible must be false, or that reality is not objectively real, as shown by recent papers like Mukherjee & Hance's "Limits of Absoluteness of Observed Events in Timelike Scenarios: A No-Go Theorem."

Key Points: A recent paper by Mukherjee and Hance (arXiv:2510.26562) presents a 'No-Go Theorem' challenging the assumption that events are absolute in quantum mechanics. The video discusses thought experiments involving observers (Alice, Charlie, Bob, Debbie) and an entangled system, showing that if observers and their friends agree on measurement outcomes, contradictions arise. If Alice's measurement result is 'Left' (00:32) and Wigner 2 (outside the lab) concludes the particle was definitely in one state, while Wigner 1 concludes it was in the other, this implies the past can be changed. The core conflict arises when assuming both the validity of quantum mechanics and the irreversibility of measurements (the second observer cannot discard the first measurement result). The video concludes that either the assumption that measurements are irreversible is false (implying the past can be undone/changed) or that objective reality itself does not exist, referencing articles suggesting this impossibility. The discussion uses analogies like playing cards (Queen vs. Ace) to illustrate how different observers can experience conflicting, yet internally consistent, realities based on when they make their measurements.

Context: This science news segment, hosted by Sabine Hossenfelder, explores profound implications arising from recent theoretical work in quantum physics concerning causality, time, and the nature of reality. The discussion centers on paradoxes like Wigner's Friend, which challenge the classical notion that reality is observer-independent and that past events are fixed. The video references new theoretical papers that formalize these paradoxes into 'No-Go Theorems,' suggesting that one of the core assumptions underlying our understanding of physics—either the absolute nature of past events or the irreversibility of measurement—must be incorrect.