Since when does a constant exponential growth imply constant growth rate? Exponential growth has an exponential growth rate. Constant growth rate implies linear growth, which is far from exponential. ?The most fundamental difficulties [in trying to describe oil prices] lie in the operational definition of a ?bubble,?? Sornette told PhysOrg.com. ?There is no consensus. One standard definition is ?exponential growth of price.? But exponential growth of price is normal in economics, because it just corresponds to a constant growth rate. Our definition is ?faster-than-exponential' growth of the price, which is necessarily unsustainable.?