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Zenith Grant Awardee

Wojciech Zurek

Theiss Research

Project Title

Events, Irreversibility, and the Objective Past of a Quantum Universe

Project Summary

We perceive sequences of events arranged into histories. In a classical world that evolved according to Newton’s dynamics history was a sequence of causally connected events: What happened next was completely determined by what happened before, and, ultimately, by the initial condition. In quantum physics this deterministic progression is no longer the only possibility. Often (as in quantum measurements, or in the famous Schroedinger cat gedankenexperiment) history can split into a superposition of several possibilities (corresponding, e.g., to the measurement outcomes or to the fate of the cat). I will study how such superpositions of possibilities turn into hard facts, so that, in the end, we can expect there is a single objective past of the quantum Universe we inhabit. The main ingredient I shall rely on is the idea that a history is not an abstract entity, but, rather, our reconstruction for what has happened that is based on evidence, on the records of events (that can be arranged into a causally connected sequence, a history). When these records are inscribed, in many copies, into the environment that is also responsible for decoherence, the history becomes indelible. Such histories comprise objective past of our Universe.

Technical Abstract

Quantum Darwinism recognizes role of redundant records in identifying, as candidate events, quantum states with resilience characteristic of objective classical reality. I will explore implications of redundancy for consistent histories and study irreversibility that accompanies imprinting of records. Events are causally connected, so histories matter in defining them: Only some events can follow a certain past event, so in that sense events record one another. Moreover, probabilities of consistent sets of histories must add up in accord with Boolean logic. Quantum Darwinism suggests that events comprising objective past should be recorded in many subsystems of the environment. Such redundantly recorded past becomes independently accessible to many observers, who can reconstruct it and arrive at compatible conclusions. I will investigate whether redundant records suffice to select, from the many alternative sets of consistent histories (most of them flagrantly non-classical), sequences of events with characteristics of the familiar classical past. I will also investigate similarities and differences between quantum and classical information flows, and study their relation to the familiar sources of the second law (e.g., decoherence). The origins of irreversibility in quantum measurements will be explored operationally – through observer’s ability to reverse measurement-like evolution in quantum and classical setting.

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