— Schrédinger’s Cat easurement is a big puzzle. What causes the collapse of the Mees function so that the photon stops considering many optional paths and makes a hard and fast decision. When light passes through a series of glass surfaces, it is reflected or transmitted by each. We can stack up as many pieces of glass as we want, but none of the surfaces will cause a measurement — a collapse of the wave function. It is not until the photon reaches a detector that a measurement is made and all the potential reflections and transmissions that might have happened ‘collapse’ into the one choice that actually happened. You might doubt this but there are ingenious experiments that can be performed to prove it. One is quite simple to do and can be set up on your kitchen table with a handheld laser and $100 worth of optical components. You need three ordinary mirrors and a beam splitter. Beam splitters are often made from half-silvered mirrors. They are similar to your bedroom mirror except the silver coating is more thinly applied, allowing only half the light to reflect while the rest passes straight through. Arrange the mirrors and beam splitter on a table at the four corners of an imaginary box, as in the diagram. If you point your laser at the half-silvered mirror, half the light will go straight through and half will be reflected upwards towards the first mirror. It is sent on around the square until it meets the half-silvered mirror again, and the beams meet up. You might expect that half the light reaches the detector but this is not what happens. Depending on the way the mirrors are positioned, either all the light reaches the detector, or none does. (The light does not disappear it just gets sent back to the light source, energy is conserved.) If you turn down the brightness of your laser, this does not change. Even HOUSE_OVERSIGHT_016015