Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Observing particles

  1. Nov 30, 2014 #1
    Hi Guys, I just want to put this out there. If a particle changes it's behaviour based on wether it's been observed or not what is the dividing factor? What is actually happening? Is it simply the fact that a conscious mind is watching it and because of that the particle changes its behaviour to suit the reality of the observer. Perhaps. If that is the truth then it might be a good way to gage if something has a conscious mind. How could we play with this phenomena? A novel way to prove or disprove artificial intelligence?
     
  2. jcsd
  3. Nov 30, 2014 #2
    I think when scientists use the term "observe" in quantum mechanics (such as Schrodingers Cat) they mean interfere.
     
  4. Nov 30, 2014 #3
    Thanks j7, what do you mean by interfere?
     
  5. Nov 30, 2014 #4

    Nugatory

    User Avatar

    Staff: Mentor

    Observation by a conscious mind has nothing whatsoever to do with a particle's behavior; that notion was rejected long ago. Unfortunately, by then it had taken hold in the public imagination, and it's proven amazingly hard to uproot it. There are a bunch of threads on this topic already in the QM forum.

    It doesn't help any that for historical reasons scientists still use the word "observation" when "interaction" (meaning that the particle interacts irreversibly with something) would be more appropriate.
     
  6. Nov 30, 2014 #5

    bhobba

    Staff: Mentor

    Its none of those things.

    The formalism of QM is simply a variation of standard probability theory that allows for continuous transformations between so called pure states:
    http://www.scottaaronson.com/democritus/lec9.html
    http://arxiv.org/pdf/quantph/0101012.pdf
    http://arxiv.org/abs/1402.6562

    Alternatively, and very interestingly, it is the most reasonable theory that allows that strange phenomena of entanglement:
    http://arxiv.org/pdf/0911.0695v1.pdf

    That is the formalism. Its like using probabilities to describe throwing a dice - its silent about what happens when the dice is flying up in the air and exactly what causes a particular face to appear. QM is silent about what's going on when you are not observing it and exactly what causes a particular outcome or even why we get outcomes at all (the last is known as the problem of outcomes and, with our modern understanding of decoherence basically replaces the collapse postulate). Different interpretations such as Bohmian Mechanics, Copenhagen, and the Ensemble interpretation have different takes.

    Nugatory's excellent response beat me do it.

    Just to delve into the history a bit. It dates back to an analysis the very great mathematician Von-Neumann did in his influential mathematical Foundations of Quantum Mechanics. Observations occur in an assumed common-sense classical world but the boundary for that world the theory was silent about. Von Neumann showed it could be placed pretty much anywhere - such is called the Von-Neumann cut. What he did was follow it back to the only place that was different - human conciousness and that's we he placed it. Its a very weird view, especially in today's computer age where you can have computers doing the observing leading to all sorts of problems (ironically Von-Neumann made big strides in ushering in that computer age), but did catch on with one very great and influential mathematical physicist - Wigner. Von-Neumann died young but Wigner lived to see some of the early results of research into decoherence by Zurek. Decoherence shows there is a place that is different - just after decoherence, so the reason for its introduction no longer applied and Wigner abandoned it.

    Don't know why (actually I have strong suspicions - but thats a whole new thread) but, despite it being very backwater these days, it still hangs around in popularisations.

    Thanks
    Bill
     
    Last edited: Dec 1, 2014
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook