If I understand correctly, the Planck length, or maybe a tenth of it, is considered the point at which quantum effects mess up any attempt to apply our present physical laws, and so one could not even in theory make a dependable measurement of something smaller. However, I was wondering what would be wrong with the following simpler argument; it must be wrong somewhere because my answer is too big. Suppose we had some slit of width d . To measure it would require a photon with a wavelength of d/2 or smaller, that is, an Energy of at least 2hc/d, the equivalent rest mass of 2h/(cd). If we try to get the photon into the slit of width d, d will have to be bigger than the Schwarzchild radius r = 2GM/c2 = 4Gh/(c3d) . That is, d>4Gh/(c3d), or d>2√(Gh/c3) , but that is a factor of 2√(2π) too big, as the Planck length is √(Għ/c3). What is wrong? Thanks.