Monday, April 27, 2009

Quantum mechanics is best understood when you're blind

Occasionally I go through some threads of newsgroup sci.physics.foundations. This moderated newsgroup is dedicated to my main subject of interest and one is bothered neither by flames or spam as in sci.physics nor with biased moderation. So for everyone with an interest in physics foundations, this is a good place to be.

Recently there have been discussions on the difference between classical and quantum. I appreciate the description given by Charles Francis, which I quote hereafter:

Classical mechanics is the mechanics of bodies whose position can be continuously known up to the limit of experimental accuracy.

Quantum mechanics is the mechanics of particles whose position cannot be known between observations even in principle.

I like the fact that this definition goes beyond the historical bias that ordinary objects behave classically and submicroscopic particles behave quantum-mechanically. Charles Francis's definition focuses on the way we describe the behavior of objects, not on the way the objects behave. In other words, this definition doesn't assume a boundary between classical vs. quantum bodies. It merely assumes that we may choose between two descriptions. It assumes we have bodies and particles and we can choose to describe them either classically or quantum-mechanically. This point is very important: it empowers us to go beyond the historical bias. We just have to set up a suitable experimental procedure where:
  • positions cannot be known between observations even in principle --> use quantum mechanics.
  • positions can continuously be known --> use classical mechanics.

So a body is neither classical nor quantum, it just is. Depending on our purpose, we choose to describe it classically or quantum-mechanically.

As an illustration, we may refer to the perception of blind people. A blind man relies on his blind stick to analyze the behavior of objects surrounding him. He cannot know the position of objects between two "blind stick observations" even in principle. His blind stick "scans" the objects quantum-mechanically, with intrinsic uncertainties, intrinsic probabilities. At each hit of his stick against an object, the "state" of that object collapses at the location of interaction. It had some probability to be detected at any other place depending on the environment, on the boundary conditions and on the respective angles (phases) of the stick and the geometry of the detected object, but the fact of detecting the object collapsed the body at a specific location. It instantly pops up at that location.

I guess a blind man will intuitively choose a quantum-mechanical description with states whose phase varies with time and location, with probabilities, with uncertainties, with interference patterns, etc. Teaching him to describe the world classically would be counter-intuitive because he cannot continuously know the position of the objects. He has to infer it with square state (blind stick projected on detected object) probabilities.

Paradoxically, we could say that quantum mechanics is best understood when you're blind...


  1. Thanks for such a great analogy. Found this site because someone referred to it on Twitter. Look forward to spending more time reading.

    I didn't realize people actually believed there were actually different kinds of bodies. Thanks for the insight.

  2. Hello Christine,

    Thanks for commenting on this "blind stick" analogy. Actually, most physicists think that there are "quantum particles" on the nanoscopic side, and "classical objects" at the macroscopic side and that there is a "decoherence" frontier between both domains. I show with analogies that quantum particles may be simulated with macroscopic objects which are neither quantum nor classical. Particles or objects just "are" and depending on the environment and the chosen detection process, we may choose the appropriate description (quantum or classical). A classical description seems 'weird' when you reason quantum-mechanically. A quantum description seems 'weird' when you reason classically.

  3. Two possible interpretations of the quantum mechanics .
    The first, the photon able of timetravel.
    Goes somewhere, afterwards goes back in the time-dimension .
    This way the photon can travel on all possible path.

    The second possible interpretation, the vacuum is a grid.
    The grid always filled with inactive photons.
    Doesn’t matter matter , how many photon we see.
    Probability density is real density of particles , always.

  4. @Anonymous
    Interesting ideas. Can you develop those ideas?