Skip to main content

View Diary: Which book next? (97 comments)

Comment Preferences

  •  Problem is not only "what" but "when?" (3+ / 0-)
    Recommended by:
    Ahianne, Fabian, winterbanyan

    And that's where the metaphor between quantum theory and bookkeeping (or information theory) really comes apart, not on what the information is, but when it is.

    A common example concerns the question of which of two paths was taken by stream of photons.  If you don't detect which path, you'll get an interference pattern suggesting they took both.  If try to detect which path, they collapse to one path.

    The key (and non-intuitive) part is that you get the same results no matter where along the path you put the "which path" detector.  This is Wheeler's delayed choice experiment and it's been confirmed by observations, both in the laboratory and ... more mind-blowingly ... by astronomers.

    The confirmation by astronomers is so mind-blowing because the "which path" detectors near earth will collapse the path information for light that has been traveling across the universe for millions of years.  The beam splitter can be a galaxy hundreds of thousands of light years away.

    Put another way, next Tuesday you decide to run one of these astronomical observations, by turning on "which path" detectors on two precisely positioned satellites in orbit:

    1. Until you turn the detectors on, you get an interference pattern on light from a a star a million light-years from earth, split by the gravitational field of a galaxy a half-million light years from earth.
    1. Next Tuesday when you turn the "which path" detectors on, the interference stops; the light has now taken only one path for its entire million-year journey, based on a change of condition you made next week.
    1. If you turn the "which path" detectors off (or the satellites' orbits take them out of alignment with the beam), back to the interference pattern.  Now light was taking both paths for its entire million-year journey, again based on a change of condition next week.

    So when did those photons take all paths or just one path: (a) a million years ago when they left the distant star; or, (b) next Tuesday when you turned the detectors on and then off?

    On the issue of information of matter and energy, quantum theory predicts (and experiments confirm) both "what" problems and "when" problems.  Quantum superposition plays on matter, energy, and time.

Subscribe or Donate to support Daily Kos.

Click here for the mobile view of the site