Physics
Curved spacetime, gravitational waves and precession.
orbiting masses shake spacetime at twice their orbit rate
A compact binary radiates gravitational waves at twice its orbital frequency; chirp mass and distance set a strain of around 10⁻²¹ or smaller.
more mass, deeper well, tighter orbits
Mass curves spacetime: picture a gravity well whose depth follows −GM/r, with the Schwarzschild radius 2GM/c² marking where light can no longer escape.
relativity keeps the ellipse from quite closing
General relativity adds a small 1/r⁴ pull that makes an orbit's closest point creep forward each lap: about 43 arcseconds per century for Mercury.