A perfect turkey sandwich is a scientific impossibility.
Much like perpetual motion, the perfect turkey sandwich can not exist within our universe. The average turkey sandwich is composed of Turkey, Lettuce, Tomato, Mayonnaise, and Bread. What one must realize is that each of these is a chaotic element. Even while refrigerated, Turkey has the ability to spontaneously become dry and/or spoiled. Lettuce is subject to similar spontaneous changes, as is tomato. While mayonnaise is a much more stable substance, it exists with a tremendous amount of "potential foulness." (This is a similar concept to potential energy. Just as a body of mass can store the potential to fall by being lifted up, many edible substances store potential microbial life by being fresh.) This is the case for bread as well.
Now, it is fully possible to acquire and store fresh smoked turkey, juicy, vine-ripe tomatoes, crisp lettuce, light mayonnaise and thinly sliced rye bread. The flaw is that, each being a chaotic element, they will inherently refuse to ever exist in the same universal space-time coordinates. Attempting to combine such high levels of potential foulness will inevitably cause one of the five elements to spontaneously spoil. Adding more ingredients only serves to increase the sum potential foulness.
Recently, culinary scientists have begun collaborating with physicists to determine once and for all if the Turkey Sandwich can actually exist. While none of these studies have been finalized, the standing theory is much like the current observations on the existence of the Top Quark. The theory is that at the precise moment of combination under ideal conditions, the Perfect Turkey Sandwich (pTS) is known to exist for a remarkably brief duration, before instantly decaying into what is known as an "Average Turkey Sandwich," or aTS. This aTS then continues to spend its potential foulness, until reaching its final form, known as a "Shitty Turkey Sandwich," or sTS.
Despite the compelling evidence, most humans continue to be unconvinced. Many amateurs can be found fiddling in their kitchens at lunchtime in hopes of at last discovering and patenting the pTS. Perhaps someday man will finally break through the confines of science by reaching absolute zero or traveling faster then the speed of royalty, (royalty travels faster then the speed of light. When a king dies, the next of kin instantly becomes the new royal figure, as theorized by Terry Pratchett.) but as for the pTS, I fear it is a quest mankind shall never accomplish.