Einstein's Constant

Einstein's constant or Einstein's gravitational constant, denoted κ (kappa), is the coupling constant appearing in the Einstein field equation which can be written:

where Gαγ is the Einstein tensor and Tαγ is the stress-energy tensor.

This equation relates to the curvature of space and time, telling that stress-energy is what causes the disturbance of spacetime, thus gravitation. Einstein used Newton's law of universal gravitation in his field equations, and the constant of κ is found to have a value of:

N.B.: Writing Einstein's constant depends on how the stress-energy tensor is defined, so the Einstein field equations are always invariant (see details in the section "About the two possible writings" further).