A moving clock ticks slow. dt = γ·dτ — the coordinate time between two ticks of a clock whose own proper time is dτ. Because γ>1 for any motion, the muon born high in the atmosphere lives long enough, in ground time, to reach the ground. Rendered, not quoted.
Two events on one worldline — tick, then tock — are separated by proper time dτ in the clock's own rest frame (same place, so only time differs). In a frame where that clock moves at speed v, the two events are at different places, and the coordinate time between them is
Since γ>1 whenever v>0, the moving clock's dτ is squeezed into a larger ground interval dt: fewer ticks per second of ground time — it runs slow. At v=0, γ=1 and dt=dτ.
Moving clocks run slow — Einstein 1905, §4. dt = γ dτ is the time component of the-lorentz-transformation, reciprocal with the-length-contraction (L = L₀/γ), and the reason the-twin-paradox is real once one twin accelerates.
Neighbour sphere: the-lorentz-transformation. This window is that transform read on the time axis alone.
Re-runs the pure engine now, in your browser, including the planted tamper below. Green = every relativistic identity holds; it flips red the instant window 6 corrupts the law.
witness idle
A clock's proper time dτ and its speed v (as β=v/c). Drag the slider to set β.
Left: a light clock — a photon bouncing between mirrors. Boosted, its path lengthens to a zig-zag, so each tick takes longer: the γ stretch, drawn live. Right: γ(β) with your β marked. Nothing here is typed in; every number is computed from γ=1/√(1−β²).
Coordinate time per tick, and the muon reach. Proven at boot:
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Muon, γ=10, proper lifetime τ: ground lifetime = 10τ; distance travelled ≈ 9.9499 τc — far enough to reach sea level, which at rest it never could.
"If it's all relative, the OTHER frame sees YOUR clock slow too — so who is really slow? The whole thing is a contradiction."
Answer: both are right, and there is no contradiction. Dilation is symmetric because it depends on relative velocity only. The apparent paradox is resolved only by adding a real, frame-independent event — a reunion, which requires acceleration (the-twin-paradox). Symmetry of the effect ≠ symmetry of the histories.
Planted void, disclosed. This swaps the law to dt = dτ/γ — a moving clock that runs fast and ends up ahead. The Witness (7) catches it live.
law intact