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Knowability and unknowability of the universe
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All ability to detect which path an elementary particle took (in this case a photon of light) must be removed to obtain interference.
[If] one is ignorant of which path the photon took, then an interference pattern is not just possible, it must occur.
As an electron ?jumps? from a higher-level step to a lower-level step, it emits a photon of light.
However, if an electron can jump down by two possible paths when emitting a photon, and one cannot tell which path the electron took, then they too must produce an interference pattern even though there is no double slit or classical wave-interference experimental setup going on at all.
They are not waves made of anything but probabilities, or tendencies to exist.
Yet they can interfere with each other in a wave-phenomena-type way before they are measured, and so ?turn? from probability waves to measured particles. (This ?collapse of the wave function? is also said to take place instantaneously, as we shall discuss more in article four.)
Richard Feynman pointed out that the mathematics really does mean all possibilities.
a kind of infinite-slit experiment ? and then these infinite numbers of paths all cancel in the multi-dimensional mathematics ? called Hilbert space ? so that only one result is finally measured.
Professor John Wheeler of Princeton University
?The Participatory Universe.?
In this approach one can look at the universe as directly participating in each quantum effect in real time. In other words, the concept of a First Cause starting things off (winding up the clock of the universe, one might say) and then leaving the laws of physics to run things, may be what is incorrect in the basic approach of classical physics.
Rather, in this participatory scenario, the Cause of the laws of physics remains an active Participant.
It would appear that what has ?happened? in the distant past in this case may be determined by what is happening right now even though it is supposed to have ?happened? over a billion years ago. The choice of which path, in other words, has somehow been ?delayed.? One might view this as the Universe playing more the part of an active participant in what is happening rather than just in what has happened in the past in this case. Hence the ?Participatory Universe? conceptualization.
This interesting Gedanken experiment points out what may be the main difference between general relativity and quantum physics. In general relativity time is a definite dimension, part of the already unalterable space-time continuum. While in quantum physics, time is, at best, a variable, and is also quantized (i.e. there are particles of time). Thus far from being an absolute, time in quantum physics is not a solid background upon which particles in space change. In quantum physics time is not yet really, in a sense, even there until the ?time particles? are measured.
