terms become highly significant when performing calculations such as hydrodynamic simulations of core-collapse supernovae. These predictions for the roles of pressure, momentum and stress as sources of spacetime curvature are elegant and play an important role in theory. In regards to pressure, the early universe was radiation dominated, and it is highly unlikely that any of the relevant cosmological data (e.g. nucleosynthesis abundances, etc.) could be reproduced if pressure did not contribute to gravity, or if it did not have the same strength as a source of gravity as mass–energy. Likewise, the mathematical consistency of the Einstein field equations would be broken if the stress terms did not contribute as a source of gravity. Experimental test of the sources of spacetime curvature Definitions: Active, passive, and inertial mass Bondi distinguishes between different possible types of mass: (1) is the mass which acts as the source of a gravitational field; (2) is the mass which reacts to a gravitational field; (3) is the mass which reacts to acceleration. is the same as in the discussion of the equivalence principle. In Newtonian theory, The third law of action and reaction dictates that and must be the same. On the other hand, whether and are equal is an empirical result. In general relativity, The equality of and is dictated by the equivalence principle. There is no "action and reaction" principle dictating any necessary relationship between and . Pressure as a gravitational source The classic experiment to measure the strength of a gravitational source (i.e. its active mass) was first conducted in 1797 by Henry Cavendish (Fig. 5-9a). Two small but dense balls are suspended on a fine wire, making a torsion balance. Bringing two large test masses close to the balls introduces a detectable torque. Given the dimensions of the apparatus and the measurable spring constant of the torsion wire, the gravitational constant G can be determined. To study pressure effects by compressing the test masses is hopeless, because attainable laboratory pressures are insignificant in comparison with the of a metal ball. However, the repulsive electromagnetic pressures resulting from protons being tightly squeezed inside atomic nuclei are typically on the order of 1028 atm ≈ 1033 Pa ≈ 1033 kg·s−2m−1. This amounts to about 1% of the nuclear mass density of approximately 1018kg/m3 (after factoring in c2 ≈ 9×1016m2s−2). If pressure does not act as a gravitational source, then the ratio should be