The physicists' question is: "Why are quantum mechanical effects like wave-particle duality, probability wave theory and tunneling effects not seen in the macroscopic world?" The philosophers' question is: "Where do all the odd socks go?"
The answer is simple and can be found in a domestic appliance: the humble washing machine. It seems that socks are tunnelling from one machine to another.
Philosophers had for a long time suspected that the household washing machine was the answer, but lacked sufficient evidence. Their limited knowledge of physics suggested that the socks could not just disappear without trace, unless some extraordinary mass-to-energy transformation was possible. But since this obviously cannot happen, they reluctantly surmised that washing machines were not the answer.
The washing machine theory may be hard to accept, but there is some compelling evidence to support it:
Socks disappear from inside washing machines. This statement is so blatantly true that it hardly needs proving. However, if you doubt it just check your collection of socks. It is almost certain that some socks do not form pairs, showing conclusively that some are missing. Socks can also appear inside washing machines. This is not so clear to the casual observer, but again a quick check of your sock collection should soon convince you. You are bound to find at least one odd sock that is not yours. You did not buy it and were never given it. The sock just appeared one day. Clearly the spontaneous creation and destruction of socks inside a washing machine would violate all conservation laws, especially sock parity, and the conversion of the mass of the sock into energy would certainly play havoc with a low-temperature wash.
If socks disappear and appear in washing machines without being spontaneously destroyed or created in the machines, there is only one other explanation: they tunnel between machines. The exact mechanism is unclear, but recent observations would suggest that washing machines are connected via some, or all, of the 'left-over' dimensions of string theory.
For the doubters amongst you there are experiments that provide evidence for this phenomenon. These experiments have the added attractions of requiring no more than household items and being easily reproducible.
All that is required is a washing machine, a duvet cover and as many socks as possible. A duvet cover with some sort of fastening at the opening, like poppers of a zip, is preferable but not essential. Close the fastening and roll the cover up so the opening is on the inside of the ball and therefore neither visible nor accesible. Place the socks and the duvet cover in the washing machine and wash.
At the end of the wash empty the machine and check inside the duvet cover for any socks whose probability waves have collapsed inside the cover. Given enough socks and enough washes you will, sooner or later, find a sock inside the cover. Remember that you are looking for a random event that will not occur to order, so the experiment may need repeating. Do not forget to watch for the complete disappearance (or appearance) of any socks while conducting these experiments.
Now the probability of a sock managing to get inside a duvet cover under these conditions is very small, in spite of the churning of the washing machine. If the cover has a zip that is done up before the experiment and which remains so throughout, it should be impossible for a sock to get into the cover. However this is exactly what happens on occasion.
In fact I have even experienced multiple tunnelling, in which the probability waves of several objects collapsed around eachother. This resulted in a sock inside another sock, inside a pillow case, inside a duvet cover. This result is extremely rare, but multiple tunnelling can occur, leading to many socks inside a single duvet cover.
While these experiments provide evidence of macroscopic probability waves and tunnelling, they do not help to prove that socks actually tunnel between washing machines. However it is simple to see how this theory can be proved conclusively. Just put a marked sock in a washing machine and wash it until it disappears. Then, search all the other washing machines in the universe until you find the machine that contains the marked sock.
As socks are macroscopic quantum objects and have a probability wave, the marked sock must be in one of the machines. It is impossible to determine in which machine the probability wave collapses and the sock appears - until you look. However, when the right machine is opened and the sock found, the theory will be proven beyond doubt.
Author's note: This experiment can be attempted by the academic community though I suggest that scientists do not try to get funding from any government agency. Perhaps a chain of launderettes could be persuaded to sponsor the research. In any case, I look forward to hearing of the further results in this fascinating and squeaky clean science.