# Quantum Physics Can Change the Past, Physicists Show Source: https://www.youtube.com/watch?v=Xb9GpYM9Xa4 Recap page: https://rapidrecap.app/video/Xb9GpYM9Xa4 Generated: 2026-01-18T16:36:46.062+00:00 --- ## 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. ![Screenshot at 01:22: An animated diagram illustrating the 'Wigner's Friend' thought experiment setup, showing Alice inside a sealed lab measuring a particle while her friend Charlie observes, and an external observer \(Wigner\) exists outside the lab, setting up the scenario where conflicting realities can emerge.](https://ss.rapidrecap.app/screens/Xb9GpYM9Xa4/00-01-22.jpg) **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. ## Detailed Analysis The video begins by posing the question of whether the past can be changed in quantum physics, referencing a recent paper by Mukherjee and Hance titled "Limits of Absoluteness of Observed Events in Timelike Scenarios: A No-Go Theorem." Sabine Hossenfelder explains that quantum mechanics accurately predicts probabilities for measurement results, but conceptually, the wave function ($\psi$) is not tangible. The discussion moves to the Wigner's Friend thought experiment, which involves two nested observers: Alice (inside a lab) and Charlie (her friend, who performs the measurement), observed by Wigner (outside the lab). If Alice measures a particle and gets 'Left' (00:32), Wigner, using quantum mechanics, can predict that Alice and her entire lab exist in a superposition of states (e.g., one where the particle was 'Left' and one where it was 'Right') even after Alice measures it. The paradox deepens when introducing a second friend, Wigner 2, who performs a measurement on Alice's entire system. If Wigner 2 measures the system and discards the first measurement, they effectively 'undo' Alice's past, leading to contradictions if all observers must agree on the final outcome. The video shows that if both Wigner 1 and Wigner 2's results are considered valid, there is no combination of measurement results that satisfies all assumptions, implying that either quantum mechanics cannot consistently describe itself (as suggested by the Frauchiger & Renner paper title shown at 2:11), or the fundamental assumption that measurements are irreversible must be false. Hossenfelder concludes that if measurements are irreversible (as they appear to be in our everyday experience, represented by the playing card analogy where the Queen cannot turn into an Ace), then the quantum mechanical description of reality is flawed, leading to the conclusion that objective reality may not exist outside of observation. ### Introduction to Changing the Past - Physicists show the past can be changed in quantum physics - The wave function ($\psi$) describes probabilities, not tangible reality - Introduction to the Wigner's Friend Paradox scenario (00:50-01:10) ### The Wigner's Friend Setup - Alice measures a particle inside a sealed lab, while Wigner waits outside - If Alice measures 'Left', Wigner sees Alice in a superposition of states (01:22-01:33) - If Wigner 2 measures the whole system later, they can potentially discard Alice's past measurement (02:57-03:07) ### The Contradiction and Conclusion - No combination of measurements allows all observers to agree if quantum mechanics is applied consistently (02:33-02:40) - This suggests reality is not objectively real, or measurements are not irreversible (02:40-02:45) - The implication is that the past is not fixed unless we abandon quantum theory's predictive power (03:33-03:50) ### Sponsor Message - NordVPN advertised as a solution for accessing geo-blocked content and protecting privacy online (05:46-06:40) ![Screenshot at 00:04: Text overlay asking "Changing the past?" next to a clock graphic, symbolizing the video's central topic regarding time manipulation in physics.](https://ss.rapidrecap.app/screens/Xb9GpYM9Xa4/00-00-04.jpg) ![Screenshot at 00:12: Display of the academic paper title: "Limits of Absoluteness of Observed Events in Timelike Scenarios: A No-Go Theorem," establishing the theoretical basis for the discussion.](https://ss.rapidrecap.app/screens/Xb9GpYM9Xa4/00-00-12.jpg) ![Screenshot at 01:10: Animation showing two overlapping blue spheres that separate into two distinct spheres, illustrating a quantum state before and after measurement/separation.](https://ss.rapidrecap.app/screens/Xb9GpYM9Xa4/00-01-10.jpg) ![Screenshot at 01:47: Diagram from the thought experiment where Wigner 2 outside sees Alice report 'It's left!' while Alice 2 reports 'It's right!', illustrating conflicting realities.](https://ss.rapidrecap.app/screens/Xb9GpYM9Xa4/00-01-47.jpg) ![Screenshot at 04:58: A graphic displaying a gauge with the needle pointing far into the red zone labeled "BULLSHIT," indicating Hossenfelder's low rating for the concept of changing the past.](https://ss.rapidrecap.app/screens/Xb9GpYM9Xa4/00-04-58.jpg)