Pressure measures momentum exchange. System external pressure puts force per area on system-boundary surfaces. It is due to kinetic energy, which increases with temperature.
internal pressure
System internal pressure {internal pressure}| puts force per area on system particles. It measures system potential energy changes as system expands or contracts while keeping temperature constant. Internal pressure is positive for attractive forces and negative for repulsive forces.
Vacuum has no forces, so its internal pressure is zero. Particles have no internal forces, so their internal pressure is zero. Solids have attractive forces, but particle distances do not change at constant temperature, so internal pressure is zero.
positive internal pressure
Gas particles slightly attract, and system volume can change at constant temperature, so particle distances can change at constant temperature, and gases can have positive internal pressure. Hotter gases push particles farther apart against attractive forces, increasing positive potential energy, so hotter gases have more internal pressure than cooler gases. Photons have radiation pressure that pushes against electromagnetic forces, increasing positive potential energy, so photon "gases" have positive internal pressure.
negative internal pressure
Systems that have internal repulsive (negative) forces have negative potential energy and negative internal pressure. For example, if external force compresses rubber membranes, rubber has repulsive forces that tend to push particles apart. The internal restoring force is negative, so internal potential energy is negative, with negative internal pressure.
gravity
At space-time points, gravity G depends on mass-energy density M and on internal pressure P: G ~ M + 3 * P. Hotter gas has more positive internal pressure than cooler gas and so more positive gravity. Photon "gas" has positive internal pressure that is one-third of energy density, so gravity doubles: M + 3 * (M/3) = 2 * M.
Quantum vacuum has negative (repulsive) force that expands space, increasing negative potential energy (dark energy) by subtracting universe positive kinetic energy, and so cooling the universe. Quantum vacuum has negative internal pressure between one-third and one of mass-energy density, so repulsive antigravity is between zero and negative two times mass-energy density: M + 3 * -(M/3) = 0 and M + 3 * (-M) = -2*M.
Physical Sciences>Physics>Relativity>General Relativity>Gravity
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Date Modified: 2022.0224