What is gravitational pressure?

Answer 1

To gain an heuristic grasp of gravitational pressure, let us begin by making three obvious assertions: 1. Pressure has units of force per unit area: In MKS units, we write p = N/m2. 2. Any quantity multiplied by unity remains unchanged: S x (1) = S. 3. The rules of algebra apply to the units of denominate numbers just as they apply to the numbers themselves: e.g., (m/s) X s = m. Let us now write unity in denominate terms of meters divided by meters: 1 = m/m, and multiply pressure by this term. We obtain a term identical to pressure but with units energy per unit volume or energy density: (N/m2)(m/m) = (Nm/m3) = J/m3. In electrodynamics, by way of comparison,we study the energy density of the electromagnetic field in terms of the electromagnetic constants of free space and the squares of the field values. For the electric field, the energy density (as a scalar) has the form (1/2)e0E2 where e0 is the dielectric constant of free space and E is the local electric field. The gravitational equivalent of e0 has the form 1/4(Pi)G where G is Newton's constant of gravitation (N.B., for comparison, e0 = i/4(Pi)k where k is Coulomb's constant). We may write, therefore, that gravitational energy density (pressure) must have the [classical or Newtonian] form ag2/8(pi)G where a is a constant to be determined. If the analogy with the electric field case strictly holds, then a = 1. Since the gravitational field is an energy field, it is appropriate to speak of its energy per unit volume. This concept emerges in general relativity. Anecdotally, Maxwell spoke of the "pressures" of the electric and magnetic fields, using terminology for energy density that is now regarded as out-of date. Joseph C. Kolecki Retired NASA planetary physicist