Experiment #01 · June 2026 · second postulate

Does light from a fast source move faster?

A 1964 test at CERN's Proton Synchrotron. The instrument below is interactive: set the pion velocity, fire the protons, and watch the ballistic prediction race the measured speed of light.

CERN · Proton Synchrotron1964Alväger · Farley · Kjellman · Wallin

The Source Was Moving
At 0.99975c.
The Light Wasn't.

High-energy protons strike a beryllium target, creating neutral pions (π⁰) that hurtle forward at almost light speed. Each pion decays into two gamma rays. If light's speed depended on its source (the old ballistic theory), those gamma rays would fly at nearly twice c. Run the experiment and watch the race.

TIME-OF-FLIGHT BASELINE · 31 m BALLISTIC PREDICTION ⟶ c(1+β)

Pion velocity β = v/c 0.99975

π⁰ Source

the moving emitter

0.99975 c

kinetic energy≈ 6 GeV

decay π⁰ → 2γ~10⁻¹⁶ s

Ballistic Theory

Ritz · "c + v"

1.99975 c

γ speed5.994 ×10⁸ m/s

arrival over 31 m51.7 ns

Relativity / Measured

2nd postulate · "always c"

1.00000 c

γ speed2.998 ×10⁸ m/s

arrival over 31 m103.4 ns

The Verdict

Despite a source rushing forward at 0.99975 c, the gamma rays travelled at exactly c. Alväger and colleagues expressed the result as a speed of c′ = c(1 + kβ) and measured k = (−3 ± 13) × 10⁻⁵, consistent with zero.

The ballistic prediction of k = 1 (gamma rays at about 2c) was ruled out by a factor of thousands. Light does not care how fast its source moves. Einstein's second postulate held in the GeV regime.

Source: T. Alväger, F. J. M. Farley, J. Kjellman, I. Wallin, "Test of the second postulate of special relativity in the GeV region," Physics Letters 12, 260 (1964). Energies, β, and timing figures are illustrative of the published apparatus; speeds shown are rounded. Animation is time-stretched for visibility.

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