I Twist Space Itself

The Twister does not push light. Light goes straight. The Twister changes what straight means.

Send a ray past the Sun and it bends. Newton's physics can be made to predict this, if you treat light as fast particles that feel gravity: grazing the edge of the Sun, the bend comes to about 0.87 arcseconds. Einstein's general relativity, in its finished form of 1915, predicts twice that, about 1.75 arcseconds. The first formula on this page has a 4 where the Newtonian version, the second line, has a 2.

Where does the extra half come from? Near a mass, gravity in Einstein's theory does two things. It slows clocks, and that part alone reproduces the Newtonian answer. It also curves space itself, and the curvature adds an equal share. A beam of light, which always takes the straightest available path through spacetime, picks up both.

Stars beside the Sun show only during a total eclipse. On 29 May 1919, British expeditions organised by Frank Dyson and Arthur Eddington photographed the stars around the darkened Sun from Sobral in Brazil and the island of Príncipe, and compared them with the same stars at night. The shifts favoured Einstein over Newton. How cleanly they did so has been argued ever since: the plates were difficult, and one Sobral set that leaned toward the smaller value was set aside for a focusing problem. Later radio measurements, timing quasars as the Sun passed in front of them, confirmed general relativity far more precisely.

Today the effect is a tool. Galaxy clusters bend the light of galaxies behind them into arcs, and the arcs weigh the cluster, including mass that gives off no light at all.

The Twister never touches a photon. They are only heavy, and nearby.