Lensa

Meet Lensa, a mass so large she bends the light passing by her.

General relativity says mass curves spacetime, and light follows that curvature. A ray passing a mass M at closest distance b is deflected by an angle θ ≈ 4GM/(c²b) when the deflection is small. That is twice what a simple Newtonian argument predicts, so measuring it was a test of Einstein's theory.

For light grazing the edge of the Sun, the formula gives about 1.75 arcseconds. During the total solar eclipse of 29 May 1919, expeditions led by Arthur Eddington and Frank Dyson measured star positions near the darkened Sun and found shifts in line with Einstein's value.

Today gravitational lensing is a working tool. Galaxy clusters act as natural telescopes, magnifying and distorting galaxies behind them into arcs. Lensing maps reveal dark matter, and microlensing by single stars has been used to find exoplanets.

The simple formula is for weak fields far from the mass. Near black holes, or when the source, lens and observer line up, the full theory gives rings and multiple images.