How quantum physics can rewrite history | Avshalom Elitzur

Episode Overview

• Physicist and philosopher Avshalom Elitzur explores the peculiar nature of time as described by quantum mechanics, arguing it is far stranger than even Einstein's theories suggest.
• The Schrödinger's cat thought experiment is re-examined not as a paradox of existence, but as a paradox of time, questioning what the cat's history was before the box was opened.
• Elitzur introduces the concept of retrocausality, where a quantum measurement in the present can determine the entire history of a particle, extending backward in time.
• The talk delves into the Two-State Vector Formalism, a theory suggesting that the overlap of unrealized "fake" pasts and "fake" futures can temporarily create real, observable particles.

Key Concepts

• Quantum Temporal Peculiarities: The central theme is that quantum mechanics reveals aspects of time, such as bi-directional causality, that are absent from classical physics and even relativity.
• Retrocausality in Measurement: The idea that the act of observing a quantum system doesn't just determine its present state but also solidifies its entire past history. The measurement's effect propagates both forward and backward in time.
• Schrödinger's Cat and History: The paradox is used to illustrate retrocausality. If a cat is left in the box for two days, opening it reveals not just a dead or alive cat, but one that has a history consistent with that state (e.g., a decomposed dead cat or a hungry, messy live one), implying the history was determined upon observation.
• Two-State Vector Formalism (TSVF): A framework developed by Yakir Aharonov that describes a quantum system's state at any given moment by considering both its starting conditions (pre-selection) and its final, measured outcome (post-selection).
• "Fake" Futures and "Fake" Pasts: In TSVF, the paths a particle could have taken but didn't are referred to as "fake" histories. Elitzur explains that when a "fake future" and a "fake past" overlap, a real physical particle can temporarily manifest, a phenomenon that has been experimentally observed.

Quotes

• At 00:08 - "Leave the cat there for say, two days... You may find a dead cat, but also decomposed. So here is a proof that it has not been in a superposition. It has been alive all along and dead all along." - Context: The speaker recounts a student's clever argument for resolving the Schrödinger's cat paradox by making the experiment long enough to leave historical evidence of the cat's state.
• At 00:37 - "when I open the box, I determine not only the state of the cat, but its entire history, backwards." - Context: This is the core conclusion drawn from the modified Schrödinger's cat experiment, introducing the idea that quantum measurement affects the past.
• At 12:14 - "In a history where 'fake future' and 'fake past' overlap, a real particle will temporarily appear." - Context: Explaining the surprising and experimentally verified prediction of the Two-State Vector Formalism, where something tangible emerges from the intersection of two unrealized possibilities.

Takeaways

• Our classical intuition that cause must always precede effect is challenged by quantum mechanics. The act of measurement can be seen as an event that defines its own history backward in time.
• The "story" of a quantum particle is not fixed until it is complete. Both its beginning and its end point are necessary to describe its properties at any intermediate moment.
• Physical reality may be stranger than we assume, potentially allowing for the temporary existence of particles in places they "never were," born from the overlap of unfulfilled past and future possibilities.