Measurement, in classical physics, means evaluating a pre-existing definite state for an object. However, in quantum theory, objects do not possess a predetermined and definite state; rather, they are in superposition of every probable state simultaneously. In quantum mechanics the act of measurement by an experimenter/observer reduces the super position to one definite state. The observer has an important role in modern physics. According to Einstein’s special relativity theory, time and space adjust themselves according to the framework of the observer (length elongation and time contraction). We can say that space and time exist in the eye of the beholder (Greene 2005). Here is an example how knowledge of an experimenter changes the wave nature of photons to a particle character.


A simple Mach-Zehender interferometer is sketched in the diagram below. The light source in the interferometer can be regulated so it emits a single photon at a time. The photon then hits a beam splitter (a half-silvered mirror). Classically there is the probability that a single photon will travel along each arm, but never both at the same time. Each arm also has a mirror to reflect the photon, and then, at the juncture of the two paths, there is another beam splitter. At the end of paths there is a detector. In a system with two beams splitters, when the photon hits the detector it is not possible to know which path the photon travelled. In classical terms we do not know which path the photon has taken. In quantum terms, since each pathway is a probability, the single photon passes through both pathways simultaneously (superposition). The photon also has a dualistic particle and wave nature. A phase shifter is placed in one of the lines. In actual experiment, as the phase shifter is varied, the detector at the end of the lines records an interference pattern (meaning that two photon-waves have interfered with each other). This confirms the photon travels both paths concurrently, which is in line with the quantum mechanical prediction.
























                            Diagram A                                                            Diagram B





However if we place a second detector in one of the lines to figure out which path the photon took, we can get the information we were looking for (with our classical logic). Either we find the photon in upper arm or lower arm, never both. As soon as we know the path the photon has taken, the interference in the detector disappears and the photon appears as a classical particle. What does observer, or act of measurement, have to do with the outcome of experiment? Has our awareness changed the actual reality, or has our classical perception taken over and guided us to a logical conclusion?


John Wheeler’s delayed-choice version of the above experiment substantiates the presence of superposition regardless of our observation. If we make the arms long enough and insert the second detector long after photon has passed through the first beam splitter, we still observe the disappearance of the interference pattern. (The quantum eraser experiment of Marlan Scully and Kai Druhl is another version of experimenter’s impedance with the history of the particle (Greene 2005, 186). How are we to make sense of this? Does this paradox direct us to explore the possibility that the analytical left hemisphere interferes by the act of concept formation? Is it possible that the data filters through a complex neuronal circuitry, and at the end there emerges a familiar particle—the logical scenario according to our classical means of perception?


In Alain Aspect’s experiment of entanglement, pair of photons were sent in two opposite directions in space. The spin of each photon was perfectly correlated and was supposed to be in superposition of states. If we measure the spin of one of them and find out that it is clockwise, the spin of the other one abruptly comes out of the superposition and follows suit. We assume that interfering by an experimenter/observer reduces the superposition to one state. Does this mean that wave function really collapses? The measurement/observer problem is at the heart of quantum mechanics paradoxes. Quantum mechanics, and the Schrodinger equation that describes the superposition of states, has no explanation for how measurement by an experimenter/observer can reduce the system to just one classical state. In fact, according to the Schrodinger equation, state reduction never happens; measurement (for example, sending a photon to detect an atom’s energy level) only makes the superposition more complex, because the incoming photon is in superposition itself (infinite regress) (Greenstein and Zajong 1997, 190). Wave-function collapse (coming out of superposition) was introduced, in an attempt to correlate the theory with the observation (or perceived observation). One wonders if the superposition actually disappears or if our logical/classical focus is on just one photon and one path and ignores the other path as irrelevant.


E. T. Jaynes shows that if an atom is raised to an exited state and subsequently decays by releasing a photon, the quantum estate of the emitted photon depends on the measurement we elect to make even long after the atom decays (Greenstein and  Zajong 1997, 195–198). It is incorrect, therefore, to say that a given decay produces a photon with such-and-such properties. Rather, we must accept the fact that the found properties depend on the sort of measurement we elect to make later on.


How can our measurement determine a definite property for a multi-states photon? The solution to this problem may lie in assessing the act of perception itself. In the context of present article, one may presume the superposition as actual multi-state of an object. The subsequent collapse to one state upon measurement may be assumed as the result of the observer’s hunt for a relevant or biased state.

  • Measurement/ Observer Problem

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Wave-Particle Duality