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Observed (top) vs. not observed (bottom)
experiment what he called a “delayed choice” determination. It is a little more complicated but see if you can follow this. They did not understand the relation between the observer and what happened in the experiment (and still do not) but the question Wheeler came up with was “what if we don't observe until after the electron (or photon) has already gone through the slits.”
He was trying to more clearly define the relationship. If the electron has already gone through the two slits as a wave propagation, interfered with itself, what will happen if it is observed at that point? After all, observing it before the slits would make it act as a particle. Interesting question, what? This was a “thought experiment” but later could be performed at the University of Maryland and University of Munich (with the same results in both institutions). Here is what happened.
If the electron was not observed the resultant output pattern was that of a wave. If it was observed, the pattern was of single particles going through. But this makes no sense. If the electron was not observed and therefore went through both slits as a wave pattern how could observation after that event have it look like it went through as particles? It led Wheeler to ask “how did the electron know we were going to look at it?”
“These results indicate that the wave-like or particle-like status of a photon at one point in time can be changed later in time by choosing to measure or observe one of these aspects in spite of the fact that the photon is traveling at light speed.”
“The results of these and other experiments not only show that the observer and the observed system cannot be separate and distinct in space. They also reveal that this distinction does not exist in time. It is as if we caused something to happen "after" it has already occurred. These experiments, like those testing Bell's theorem, unambiguously disclose yet another of the strange aspects of the quantum world —the past is inexorably mixed with the present and even the phenomenon of time is tied to specific experimental choices.”
“For the nonphysicist, it is not immediately obvious what experiments using electrons or photons can possibly say about the vast complexity of the universe in which we live. The simple answer is that what is disclosed in these experiments are general properties of all quanta, and, therefore, fundamental aspects of everything in physical reality.”6
Based on this outcome and other findings (what is called “entanglement) Wheeler, in 1940, proposed that all electrons in the universe could be parts of one single line like a huge tangled knot, traced out by the one electron (the one-electron universe postulate). Any given moment in time is represented by a slice across space-time, and would meet the knotted line a great many times. Each such meeting point represents a real electron at that moment.
Could it get more strange? Further research has confirmed that it is not only electrons or photons that have this nature, but apparently, all physical phenomena including atomsandmolecules.
Quotes from Albert einstein:
“Reality is merely an illusion, albeit a very persistent one.” “Once you can accept the universe as matter expanding into nothing that is something, wearing stripes with plaid comes easy.”
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