In 1801 Thomas Young sent one beam of sunlight through two slits and saw stripes of light and dark — proof that light adds and cancels like a wave.
Two ordinary lamps never make stripes: each atom emits its own tiny burst of light with a random phase, so the pattern redraws itself billions of times a second and the average is a smooth grey. That randomness is what the coherence dial pours in.
When the sources do agree, the bright stripes sit Δy = λL/d apart — pull the sources apart and the comb tightens. Young ran this backwards to measure the wavelength of light itself.
The same table, scaled to 4-kilometre arms, is how LIGO hears gravitational waves: a passing ripple in spacetime shifts the fringes by less than a proton's width.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.