A first pass

In this section we make a simple outine of the puzzle, and the solutions that have been proposed.

The main point is the following. In a theory of gravity, it appears that we can make a strange kind of object called a 'remnant'. This object has a lot of internal structure, but little or no mass.

Moreover, the number of such remnants appears to be infinite. And if there are an infinite number of remnants, then it is difficult to make a good formulation of quantum mechanics.

So what is the resolution? Should we ask that remnants spontaneously vanish into the vacuum? Should we assume they exist, but try to make a good quantum theory to include them? Should a correct theory of physics exclude remnants? Or should the basic tenets of physics be altered to deal with the situation?

Most solutions which have been proposed fall into one of these categories, and in later sections we will keep returning to this list to see which resolution looks most promising.