Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ucsd!rutgers!rochester!pt.cs.cmu.edu!andrew.cmu.edu!ap1i+ From: ap1i+@andrew.cmu.edu (Andrew C. Plotkin) Newsgroups: comp.ai Subject: Re: Robots & free will (was Re: The limitations of logic) Message-ID: Date: 4 Feb 89 18:48:11 GMT References: , <280@tahoma.UUCP> Organization: Class of '92, Carnegie Mellon, Pittsburgh, PA Lines: 38 In-Reply-To: <280@tahoma.UUCP> /> The damn state function collapses if a /> measurement is made, even if the measurement is made by a mindless instrument /> and recorded on tape that's locked in a closet for ten years. In fact, it'll /> even collapse if a measurement is *going* to be made; and if you change your /> mind and turn off the instrument *after* the event but *before* the /> measurement, you'll find that the function outguessed you and didn't collapse. / / What does all this mean? I'm not a physicist but I would guess that a / mindless instrument collapses the state function of a quantum event *for / itself only* when it makes a measurement and records the result on paper / tape. Until you look at it, the paper tape record is also represented / by a superposition of quantum states. Just because something is big / dosn't mean that quantum physics don't apply . . . That makes perfect sense, and it's a pity that it doesn't work that way... (This discussion is out of place on this board by now, but what the heck.) The experiment in question has a point source of light shining on two slits. If you put a measuring device on each slit, then as each photon goes past, it will show up either at slit A and slit B, then go through and hit approximately beyond that slit (with a possibility of bending left or right, because of the nature of waves going through slits. The result is two overlapping fuzzy bright spots on the wall. Quantum weirdness 1: If you do *not* put measuring devices on the slits, the photon will go 50 / 50 through *both* slits, have a chance of bending left or right from each one, and the two "halves" will interfere, producing two overlapping spots streaked with light and dark bands. Now change the wall to a movable screen. Add some lenses so that the waves coming from the two slits are bent inwards somewhat. If the screen is in, you have the second experiment; if the screen is out, the photons go past it and diverge somewhat, so you *can* tell which slit each photon came from -- you're back to the first experiment. According to QM, the photon does the appropriate thing depending on where the screen is. Even if you pull it out *after* the photon has passed the slits, it still gets it right. Accckkk. --Z (The book I got this from is _Beyond Schroedinger's Cat_, by John Gribbin.)