The LurkerGeneral Relativity (Einstein) 1915

The Lurker never touches you. It changes how fast your watch runs.

Einstein completed general relativity in November 1915. The first equation on this page is the result, ten equations packed into one line. On the left, the curvature of spacetime; on the right, the energy and momentum of matter; between them 8πG/c⁴, a coupling so small that it takes a planet to bend spacetime noticeably. The Λ term is the cosmological constant. Einstein added it in 1917, later dropped it, and it returned when the expansion of the universe turned out to be accelerating.

The second equation is the part you can check with a phone. A clock at distance r from a mass M ticks more slowly than one far away. GPS satellites orbit about 20,000 kilometres up, where gravity is weaker, so their clocks gain about 45 microseconds a day on clocks at the ground. Their orbital speed costs them about 7 microseconds a day, the special-relativity effect. Net: about 38 microseconds a day fast. The system is built to correct for it. Left alone, the error would grow by about 11 kilometres of light-travel distance every day.

The theory's first success was older. Mercury's orbit slowly turns. After every known pull from the other planets is accounted for, about 43 arcseconds per century were left unexplained. In November 1915 Einstein computed the extra turning from his new equations and got 43.

He received the Nobel Prize for 1921, but not for any of this. The citation is for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect.

The Lurker does not chase. It sits still, and lets time run slower near it.