[Excerpted from The Way of the Human Being by Calvin Luther Martin]
Visualize a simple physics exercise: imagine an opaque screen with two holes in it, one above the other. Electrons are fired at the screen from an electron source. Some of the electrons pass through the top hole and some through the bottom hole, and on passing through the holes their pattern is registered on an electron detector, similar to a television screen, set up behind the screen.
"Now , when the top hole in the shade [screen] is covered," continues MIT professor Alan Lightman, "a certain pattern of light is seen on the television screen, and when the bottom hole is covered another pattern is seen. We may interpret the first pattern of light as that produced by electrons traveling through the bottom hole, and the second pattern as that produced by electrons traveling through the top." Simple enough. "When we uncover both holes, however, the pattern of light is not the sum of the first two patterns, but a completely different pattern. In fact, the new pattern is what we would expect if each electron could divide itself into many pieces and simultaneously pass through both holes."
The metaphysical bomb begins to tick.
We now install an electron counter behind each hole, a device that merely clicks when an electron passes through the hole but does not obstruct the passage of the electron on its journey. We are now able to see where electrons land on the television screen and can tell which hole they pass through on the way there. The result is unnerving. "What we in fact find is that the two new detectors [counters] never click at the same time. One clicks, then the other clicks, but never two clicks together with the same electron, None of the electrons passes through both holes at once." What's more, "when the two new detectors are in place, the pattern of light on the television screen changes to the sum of two one-hole patterns, as we would expect from a situation in which some electrons travel through the top hole and some through the bottom."
"To sum up, when we don't measure which hole each electron goes through, each electron behaves as if it subdivides on the way to the shade and passes through both holes at once; when we do measure, each electron behaves as if it stayed whole and passed through only one opening."
There are variations on this experiment, which has been repeated many times in laboratories all over the world. In fact, pick up just about any introductory physics textbook and it's there. Sometimes it is described as the double-slit experiment, as here; at others, the beam is split using mirrors. But the results are essentially the same. The point is that what we choose to measure somehow determines the performance of the electron. "Somehow," says Lightman, "the properties of the electron depend on the mind asking the questions." At extreme conditions of reality and matter -- the microcosmic (the subatomic world) and the macrocosmic (the universe itself) -- objectivity seems to vanish and the observer becomes, literally, a part of the experiment, determining the outcome. "The observer," in the words of Princeton physicist John Archibald Wheeler, "is inescapably promoted to participator. In some strange sense, this is a participatory universe."
Mankind, it seems, has an unavoidable role in creating the reality of the universe. There was a time, continued Wheeler, when scientists imagined the universe as something outside them, amenable to inspection "without personal involvement," as though one were looking at it through a glass window. "The truth, quantum theory tells us, is quite different. Even when we want to observe, not a galaxy, not a star, but something so miniscule as an electron, we have, in effect, to smash the glass, to reach in, and install measuring equipment." And when we do we inevitably change the reality of what we are observing."
Hence the chief stumbling block of quantum mechanics is measurement. For whenever one measures the performance, or the nature, of elementary particles, one inevitably changes their behavior in generally unpredictable ways. This poses a serious philosophical challenges for physics, whose whole thrust has been to predict with certainty.
[end excerpt]
