Pedagogy: the Bubble Analogy and the Difference between Gravitational Forces and Rocket Thrust in Spatial Flow Theories of Gravity

We present a physical analogy which can be used to understand the issues involved in the Principle of Equivalence in so-called spatial flow theories of gravity, and we discuss the essential kinematic properties of the flow which distinguish its gravitational, non-inertial, and inertial modes. We also point out that the acceleration experienced by a body moving in the flow does not always coincide with the comoving derivative of the flow itself. In spatial flow (aether flow) theories of gravity [1], the gravitational force on a body does not arise from the same underlying physical process as does the force which appears when that body is temporally accelerated, even though the passive gravitational mass and the inertial mass remain proportional. We are going to discuss this proportionality of inertial and passive gravitational mass and explain how the physical differences between the phenomena of inertia (inertial mass) and weight (passive gravitational mass) can be helpful in achieving a deeper understanding of spatial flow models of gravity. What we are going to describe here is already implied (albeit extremely succinctly) in our paper General Relativity and Spatial Flows: I. Absolute Relativistic Dynamics [1]. Since that paper was written in a fairly concise form, and since it was couched in rather formal mathematical language, we think it might be helpful if we explain the concepts of inertia and gravitational mass in the spatial flow picture in a non-mathematical fashion and by using a somewhat primitive physical analogy. For pedagogical reasons, we will freely * http://www.gravityresearch.org/pdf/GRI-020928.pdf