Does light climbing away from a mass lose frequency — and if a gravitational field is 'just' a force, how could it shift a light wave at all? Einstein said gravity is indistinguishable from acceleration; does that alone force the redshift Δν/ν = gh/c² that Pound & Rebka measured on a Harvard tower?
▶ Launch the interactive simulationThe equivalence principle turns the gravitational field into a rocket accelerating at g in flat space: a light crest emitted from the floor toward the ceiling (height h) flies for ≈ h/c while the ceiling accelerates away, so the received light is Doppler-shifted. The lab TRACES successive wave crests to the receding ceiling (the crest-meets-ceiling quadratic, evaluated cancellation-free) and reads the received/emitted period ratio — with no redshift formula coded. The dimensionless coefficient K = z/(gh/c²) is Richardson-extrapolated to the weak field, the g/h/c dependence is swept, and a Monte-Carlo 'Mössbauer' measurement at 1% noise recovers the tower shift.
light REDDENS as it climbs, by exactly Δν/ν = gh/c² — the coefficient comes back K = 1.0000 (Einstein's z = ΔΦ/c²), recovered from the traced crest ratio with no formula fed, and applied to Earth's 22.5 m tower it gives 2.455×10⁻¹⁵, matching Pound & Rebka's 1959 measurement (Pound–Snider 1965 refined it to 0.9990 ± 0.0076 of the prediction). The shift scales z ∝ g·h/c² (log–log slopes +1, +1, −2), which is why it needed a tall tower and a part-per-quadrillion nuclear line to see. The decisive rival is the pre-1960 view that a static gravitational field, being merely a force, cannot shift a light wave's frequency (z = 0): setting the frame's acceleration to zero gives exactly z = 0, and the measured 2.455×10⁻¹⁵ sits thousands of standard errors above it — rejected. This is the g_tt (time-dilation) piece of the metric, the sibling of ?world=lightbending's g_rr light bending and ?world=schwarzschild's perihelion advance. The on-screen colour shift is exaggerated (~10¹⁴×) so the reddening is visible; the printed K = 1.0000 and 2.455×10⁻¹⁵ are the true traced values.