A metric on the space of projections admitting nice isometries

Tom 191 / 2009

Lajos Molnár, Werner Timmermann Studia Mathematica 191 (2009), 271-281 MSC: Primary 47B49; Secondary 54E40. DOI: 10.4064/sm191-3-8


Motivated by the concept of separation between propositions in quantum logic, we introduce the so-called separation metric or Santos metric on the space of all projections in a Hilbert space. We show that the resulting metric space has only “nice” surjective isometries. On the nontrivial projections they are all unitarily or antiunitarily equivalent to the identity or to taking the orthogonal complement. We relate this result to Wigner's classical theorem on the form of quantum mechanical symmetry transformations.


  • Lajos MolnárInstitute of Mathematics
    University of Debrecen
    P.O. Box 12
    H-4010 Debrecen, Hungary
  • Werner TimmermannInstitut für Analysis
    Technische Universität Dresden
    D-01062 Dresden, Germany

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