Tao level 0: the meta-mystery which cannot go away

In 2002 Physics World published a list of experiments considered the finest in the history of physics, the winner was the Yound experiment or the double-slit experiment and applied to light or electrons, one of the foundations that have generated experimental development of quantum mechanics.
The original experiment performed by Thomas Young the November 24, 1803 focused on determining if light is represented by waves or particles (as suggested by Newton).
Young's experiment was simple.

A ray of sunlight was passed through a hole in a card, then reached a second screen with two holes. The light that passed through the two holes of the second screen finally ended on a screen, where it created a set of lights and shadows that Young explained as a consequence of the fact that light spreads through the two holes in the form of waves. These waves give rise, in places where you add, clear bands (constructive interference), while in places where no sisommano banded dark (destructive interference).
Young's experiment was accepted as evidence that the light shines through the waves. In fact, if light were composed of particles, would not have seen this alternation of light and shadow, but it would be observed only two light bands, one for the forum. The experiment confirmed the predictions of Maxwell classical electromagnetism, where there are solutions in form of  electromagnetic wave propagation.
In 1900 Max Planck, Nobel Prize in Physics 1918, suggested by other experimental data such the photoelectric effect explained by Einstein in a seminal article in 1905 - for which he received the Nobel Prize for Physics in 1921 - that light was composed of particles or quantum of energy.
The experiment was repeated with both light and electron, with techniques that can launch a  single photon or  electron at a time:


The result is that increasing (from left to right) the intensity of photons or electrons one passies from a wave to a particle behavior. Furthermore, at  high intensity, if the slits are open you get the two wave-like behavior, it is open only one it gives the particle behavior. Electrons fired into a double-slit experiment produce the interference pattern on the screen detector (in this case a screen similar to a television set) and must move as a wave. However, on arrival, they generate a single spot of light, thus behaving as particles. It has therefore led to conclude that electrons travel as waves but  become as particles arrive arrival.If the electron was a particle we deduce that each particle passes through either of two holes in the experiment, but the interference pattern generated on the screen shows that these are waves which pass through the two holes at the same time.
Another experimental configuration requires not only a detector for the two slits but two individual detectors on each of the slits. If the detector is only one it is obtained the wave result,if they are two one get the particles behaviour, and from which detector is activeted one can deduced from which slit the particle has passed.The quantum entities therefore prove capable of passing through two slits at the same time, not only have a sort of awareness of past and future, so that each can choose to make its contribution to the interference pattern at the correct point, what contributes to the creation of the pattern, rather than its destruction.
The result of these experiments was conclusive to formulate the wave-particle duality or the Complementarity Principle, formulated by Niels Bohr, revealeing that the observed behavior depends on the experimental configuration used to measure it, and then, ultimately, by the observer.
In the words of Richard Feynman in the double-slit experiment and in the wave/particle duality is enclosed "the quantum mystery which cannot go away” (Feynman, 1977), or the "central mystery" of quantum mechanics. This is a phenomenon which is impossible to find a classical analogous and explanation, and well represents the core of quantum mechanics. "

From this basic experiment had two dual formulations of quantum mechanics: the wave mechanics developed by Erwin Schrödinger, Nobel Prize in Physics 1933, and the particle one made by Werner Heisenber, Nobel Prize in Physics 1932.Both theories lead to two eigenvalue equations; one in the form of wave equation, the other ont he form of operator equation for the function probability amplitude, which describes in statistical / probabilistic properties of quantum entities.

The dual wave/particle behavior was always subject of intense discussions, of which the most historically significant occured between 1930 and 1980, called the the  Copenhagen interpretation, according to which conscience, through the exercise of observation, at least in part determines reality.
The Copenhagen interpretation, formulated by Niels Bohr and Werner Heisenberg during their collaboration in Copenhagen in 1927, explains the double-slit experiment as follows:

• the electron leaves the electron gun like particle;
• immediately dissolves into a series of superimposed probability waves, or a superposition of states;
• waves pass through both slits and interfere with each other to create a new superposition of states;
• the detector screen, making a measurement of the quantum system, make a collapse of  the wave function into a particle, in a well defined point of the screen;
• immediately after the measurement, the electron starts to dissolve into a new superposition of waves.

According to the Copenhagen interpretation the objective existence of an electron in a certain point in space, for example in one of the two slits, independently of any actual observation, makes no sense. The electron seems to show a real existence only when it is observerved. The reality is created, at least in part, by the observer.

In 1978 John A. Wheeler proposed an ingenious mental version of the experiment, called the delayed- choice double-slit experiment, starting from the experiments that show that when you place a detector on the slits and is analyzed by which slit the photon passes, the interference pattern disappears.

In the delayed-choice experiment of the detector is placed at a point between the two slits and the detector end, to see which path is taken by each photon after passage between the two slits, but first to get to the final detector. Quantum theory says that if you turn off the detector and intermediate analyzing the trajectories of photons, these will form an interference pattern. If however, we observe the photons to determine which slit passed, even if the observation is made after that they crossed, there will be no interference pattern. The "delayed choice" comes into play just because you can choose to analyze the photon (decision made randomly by a computer) after that the photon has passed through the slit(s). The decision, according to quantum theory, appear to affect the way in which the photon behaves when passing through the slit(s), actually just an infinitesimal fraction of time before the observation.

In the words of Wheeler, " Thus one decides the photon shall have come by one route or by both routes after it has already done its travel".
The enormous importance of the Wheeler conceptual experiment has stimulated a series of experiments to achieve results, the most conclusive of  which was produced by the Quantum Optics Group of CNRS lead by Alain Aspect with single-photons lasers using interferometry.





The result is against any classic commonsense even in the wave/particle duality, the dtected behavior depends not only by the configuration of the screen but also from that determined after the particle has passed the screen, with an effect of back-causality connection, working backwards in time.In Wheeler's words: "we have a strange inversion of the normal order of time. We, now, by moving the mirror in or out have an unavoidable effect on what we have a right to say about the already past history of that photon"

Among various theories proposed to try to explain such behavior the most fascinating is the MWI (Many Worlds Interpretation) proposed by Everett in the early 50's and supported by Wheeler.
This theory has the idea that every time the world faces a choice at the quantum level (for example if an electron can choose in which slit to pass on the double-slit experiment), the universe splits into two (or in many parts as there are possible choices), so that all options are implemented (in the experiment described above, in a world the electron passes through slit A, the other through slit B).