1. The problem statement, all variables and given/known data From "http://physicsweb.org/articles/world/15/9/1": In 1909 Geoffrey Ingram (G I) Taylor conducted an experiment in which he showed that even the feeblest light source - equivalent to "a candle burning at a distance slightly exceeding a mile" - could lead to interference fringes. This led to Dirac's famous statement that "each photon then interferes only with itself". To be sure that just one photon is producing interference you need to calculate how long the photon last ( t ). Since s = c . t ( space, velocity of light and time ), this will give us a length of the photon. "length is this experiment" an operative definition. How can be calculated the power of ligth that must reach the double slit to be shure that there is just a photon at a time ? 2. Relevant equations Energy of a photon E = h. nu Energy of an electromagnetic wave E = 1/2 E . H ( per volume ) Impedance of the empty space E / H = a given number 3. The attempt at a solution From the tree equations you can derive the volume of a photon, but: volume = surface . length and we dont know the surface of the photon. Besides this seems that is a nosense concept ( volume, length, surface ) when applied to a photon. So my question is "How was calculated the power of ligth that must reach the double slit to be shure that there is just a photon at a time ? " I dont understand QM, so I just want an answer to my question. Thanks in advance.