As we shall see, locality is in some conflict with Quantum Mechanics. To illustrate the conflict we will only need the fact that we can view phenomena such as light as both a wave and as a particle. The following discussion is at the level of an upper year liberal arts course in modern physics without mathematics that is given at the University of Toronto. The legend for the figure is shown to the left. In the figure, the reflecting surface is drawn as the thick one.
Light leaves the source and travels to the first half-silvered mirror.
What is happening is that the two beams, U and D , constructively interfere at Detector 1 and destructively interfere at Detector 2 . The details of why this is so are sort of complex and not important for our purpose here. This type of interferometer is still in regular use in laboratories around the world. It turns out that the balance of constructive and destructive interference at the detectors is extremely sensitive to any phase changes in the two beams of light. Thus, by inserting, say, a gas sample into the path of one of the beams, the additional phase shift caused by the gas allows the deduction of information on the density, pressure and temperature of the gas by observing the changes in intensity of the signals arriving at the two detectors.