String theory
When the Standard Model is set to general relativity in order to include mass, probability densities of infinity appear.  This is presumed incorrect, as probability ordinarily ranges from 0 to 1—0% to 100% probability.  Some theoretical physicists suspect that the problem is in the Standard Model, which represents each particle by a zero-dimensional point that in principle can be infinitely small. Yet in quantum physics, the Planck's constant is the minimum energy unit that a field can be divided into, perhaps a clue to the smallest size a particle can be.  So there is a quest to quantize gravity—to develop a theory of quantum gravity.