Why Quantum Information is Never Destroyed

The laws of physics are equations of motion. They are mathematical rules that dictate how systems evolve in time. Newton’s equations for classical mechanics, Maxwell’s equations for electromagnetism, and the Schrodinger equation for quantum mechanics. These laws can be used to predict how the universe will evolve into the future. They are deterministic; perfect knowledge of a system in the present perfectly predicts how the system will change in the next instant, and the instant after that, ad infinitum. But determinism in the forward-time direction does NOT guarantee that the same laws can perfectly predict the past. And yet this sort of deterministic symmetry – time-reversal symmetry – is essential for information itself to be conserved. Today we learn why conservation of information is such a fundamental requirement of quantum mechanics. In a future episode we’ll see how this law might be broken by black holes.

Here’s that fun proof that independent quantum states can’t evolve into the same quantum state while preserving unitarity! Apologies for the unavoidable technical notation, but a quick college course in introductory quantum mechanics will get you up to speed in no time 😛

Hosted by Matt O’Dowd
Written by Graeme Gossel and Matt O’Dowd
Graphics by Grayson Blackmon
Assistant Editing and Sound Design by Mike Petrow and Linda Huang
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