<P> The gravity model was expanded by William J. Reilly in 1931 into Reilly's law of retail gravitation to calculate the breaking point between two places where customers will be drawn to one or another of two competing commercial centers . </P> <P> Opponents of the gravity model explain that it cannot be confirmed scientifically, that it's only based on observation . They also state that the gravity model is an unfair method of predicting movement because its biased toward historic ties and toward the largest population centers . Thus, it can be used to perpetuate the status quo . </P>

Who created the gravity model of spatial interaction