Feynman's Rest of the Universe

When we solve a quantum-mechanical problem, what we really do is divide the universe into two parts - the system in which we are interested and the rest of the universe. We then usually act as if the system in which we are interested comprised the entire universe. To motivate the use of density matrices, let us see what happens when we include the part of the universe outside the system.

It appears that the best way to understand what Feynman said above is to use two coupled harmonic oscillators. One of them is in the world in which we do physics, and the other is in the rest of the universe. It also seems necessary to write and publish an article about this issue. Let us see one of the articles which I published in the American Journal of Physics. Click here.

Time separation variable. We can use the concept of Feynman's rest of the universe to study the variable not measured in the present form of quantum mechanics. The Bohr radius occupies a very important place in quantum mechanics. When the hydrogen atom is watched by an observer on a bicycle, the time-separation variable between the proton and electron becomes important, according to Einstein. But this variable is not mentioned in quantum mechanics. This variable belongs to the rest of the universe.

We can approach this problem using Feynman's concpet of the rest of the universe and mathematical techniques of coupled harmonic osicllators. You may look at this paper.