Quantum Mechanics demystified, a try
From 17th century onwards light was believed to be formed of particles as per widely adopted Sir Isaac Newton’s understanding.
At the very beginning of 19th century, Thomas Young proved light must be of wave nature by his double slit experiment.
Which ones is it in the end? See Being a particle for that.
Later on double slit experiment has been refined in various ways. E.g. it has been shown to work for electrons, atoms and even molecules of moderate sizes. See some more details on atom’s case in article Atom’s reality
One of the landmarks was repeating the test with shooting photons one by one, which advanced laser techniques made possible in 1970’s.
When single photons were fired one at a time, but in sequence one after another, an interference pattern appears. A proof for being of wave nature, isn’t it? At least by common understanding. As if consecutive photons were aware of each other and ending up interacting with each other.
Nothing like that. Instead, pretty unconventionally, a single photon is partially passing through both slits and interfering with itself. As the slits are close enough to have the particle’s wavefunction cover each slit with some amount of probability. See Tunneling, in which similar thing happens when e.g. protons cross the barrier of electromagnetic repulsion in specific conditions, causing them to get combined due to strong nuclear force. Fusion that is!
Not all of the single photons fired will make through the slits anyway, some will interact with the slit walls, see photon bouncing. This has been seen on the tests.
The ones making through, they either travel fully through one slit or partially tunneling through both. In either case only one dot is created on the observing back-screen. Dot being formed at the spot where intact photon’s wavefunction or partial photons’ combined (interfered) wavefunctions give the highest probability. For the next photon another dot is again created. But not necessarily at the same spot as the previous one. Probabilities may be different this time. So after some thousands of single photons have been fired, a nice interference pattern starts to accumulate on the detecting screen.
But it gets even more amazing, when a detector is assembled on the slit(s) to find out through which slit the photon traveled. Guess what, interference pattern vanished! I.e. observing the photon just prior or after the slit stripped of the interference phenomenon. This is referred to as the ‘Measurement Problem’. Making an observation on a particle in superposition makes it to loose it’s superposition and to adopt a classical state instead. This is also referred as collapse of the wavefunction.
How could this be explained? Why making an observation will destroy the interference? I will give it a try in Measurement problem explained.