in explaining the interference pattern that happens in double slit, single particle interference - some of the interpretations, for example De Broglie–Bohm interpretation: assumes that (some sort of) "waves" travel through both slits simultaneously and interfere with each other the question is that do we really need waves to explain the interference? for example let's discard waves and look at the below explanation: when the photon starts its journey towards the slits - the photon/electron can be anywhere within the small narrow say 99% cloud....also it can only be in certain orbitals (separated by some multiple of planks length) now some would be at one edge and others at other edge...of the 99% probability distribution now some photons will strike edge of slit A, others will strike edge of slit B, while others will pass right in middle of slit A, while others will pass right in middle of slit B, while others will be slightly to the left of the middle of slit A, while others will be slightly to the right of the middle of slit A, while others will be slightly to the left of the middle of slit B, while others will be slightly to the right of the middle of slit B, etc...... isn't this fact alone sufficient to cause an interference pattern? the one striking the edges of the slits would form a dark band further away from the middle of the screen ......and so on...... thus the single particle never interferes with itself.....it simply hits the slits at slightly different angles....and gets deflected.... when we cover one of the slits, we actually constraint the photon paths/behavior....the photons that were to cause the interference pattern never go through because the other slit was closed....there was only one slit available..... and thus the interference pattern disappears side note:there will still be slight bit of interference......i.e. single particle, single slit interference....(with the edges of the slit or related)..... the mach zehnder (which i think is simply a more elaborate form of the double slit experiment) can be explained similarly [/PLAIN] [Broken] http://en.wikipedia.org/wiki/Mach%E2%80%93Zehnder_interferometer[/URL] [Broken] so even if we took tennis balls (one by one) and shot them through a double slit we would get an interference pattern.....assuming that the tennis ball starting position is similar to the probability distribution of the photon/electron (...i.e. the ball can only be in certain positions which correspond to the energy levels of the electron/photon.....the planks length) or mathematically if we were to substitute the probability distribution of the photon into the Schrodinger wave equation we would get the patterns.....without having to consider two physical waves (one wave that split into two at the slits) and their interference note: the wave and amplitude are simply talking about probabilities or their derivatives and are not actual physical waves.....we don't need actual physical waves to explain the interference pattern..