Gravity has been shown in theories of relativity to be the curving of space around massive bodies. Thus, objects in orbits are following a straight line along a curved space. Why massive bodies curve space is not explained. We continue to ask "What is Gravity?" Quantum mechanics unites theories of electro-magnetism (QED), the weak nuclear force (EWT), and the strong nuclear force (QCD) in the standard model of particle physics, or with a grand unified theory (GUT) sought for these three fundamental forces. As yet there is no empirically verified quantum theory of gravity unified with these three fundamental forces. Considering gravity to be the curving of space, it is evident that gravity supervenes from the properties of space itself. In this short paper, we will attempt to define one of these spatial properties. We will not attempt to define the properties of time, though time appears to be a part of a complete model of gravity. At least in this regard, and likely in many others, our model will be incomplete. We will build a case for the massive collapse of probability density waves (PDWs) in surrounding space, due to the interactions of particles in massive bodies. The collapse of these probabilities, of each particle's possible superposition somewhere in the surrounding space, causes the apparent "curving" of space. We will conclude that space is not the absence of things. Space is a thing in itself. Included in the properties of space is the potential to contain/transmit PDWs. This potential is suggested by both the theories of relativity and the experimental observations of quantum mechanics. In the presence of massive bodies, particle superposition and the probability of existence in the surrounding space is, to varying degrees, lost and space appears to curve as a consequence. (C) 2019 Physics Essays Publication.
