Quantum physicists have a saying: "If you don't think quantum physics is weird, you don't understand quantum physics." In spite of the fact that quantum physics comes to some conclusions that seem weird by everyday world standards, I choose to believe that its major findings are correct. In particular, I believe a central finding of quantum physics: we bring the everyday world into being by our observations.
Three variations of the famous two-slit experiment (also called double-slit experiment) point to this conclusion. The classic version of the two-slit experiment, done more than 200 years ago, demonstrated that light traveled as a wave. Light rays passing through two parallel slits displayed characteristic wave behavior (think here of water waves) by interfering with each other, creating a pattern of light and dark patches on a photosensitive screen positioned behind the slits. The patches corresponded to the points on the screen where the peaks and troughs of waves diffracting out from the two slits combined with one another. Light patches occurred when the crests of two light waves came together while dark patches occurred when the crest of one wave met the trough of another wave. Thus, in the classic version of the experiment light was demonstrated to travel as a wave.
However, other research findings suggested that under some circumstances light consisted of discrete quantized packets and in the twentieth century, physicists re-performed the classic two-slit experiment with low-intensity light to show that this interference pattern was evident even when particles of light (photons) passed through the apparatus one at a time. Light, therefore, was shown to exhibit both particle-like and wave-like properties. (These experiments can also be done with other subatomic particles with the same results.)
The second experiment was a modest but important variation of the classic two-slit experiment. In this version, detectors were placed at the slits to determine through which slit a particle was passing. It was found that using detectors destroyed the interference pattern on the screen. The behavior of photons thus was changed depending on whether or not an attempt was made to observe them.
The most astonishing of this important triad of experiments, the delayed-choice experiment, was proposed by John Wheeler in 1978.[2] In this experiment the decision whether to turn on or off the detector was delayed (from our perspective) until after a photon had passed the detector. The astonishing part: it was found that the later decision determined what happened at the earlier time. [3]
Wheeler's conclusion: "[W]e, by observing the universe, contribute to the ongoing creation of not just the present and the future but the past as well."[4] Another way to express this same idea: elementary particles (and the real-world items "composed of" elementary particles) are not there waiting for us to observe them but rather are brought into being by our observations. By our thoughts we bring our world into existence.
Notes
1. This Post is adapted from Donald W. Jarrell, At the Edge of Time: Reality, Time, and Meaning in a Virtual Everyday World (North Charleston, South Carolina: CreateSpace Independent Publishing Platform, 2012, rev 2014), 2-5 and 27-29. See At the Edge of Time.
2. John Archibald Wheeler, “The ‘Past’ and the ‘Delayed-Choice’ Double-Slit Experiment,” in Mathematical Foundations of Quantum Theory, A. R. Marlow (ed.) (New York, NY: Academic Press, 1978), 9-48.
3. It may be helpful in understanding the double-choice experiment to read an alternate explanation by Paul Friedlander at AlternateExplanation.
4. Tim Folger. Interview with John Wheeler, “Does the Universe Exist if We’re Not Looking?”, Discover, June 1, 2002. See WheelerInt.
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