Does light from a fast source move faster?
A ball thrown from a speeding train flies faster: the train's speed adds to the throw. Does light do the same? In 1964 at CERN, physicists made light come from something moving at almost the speed of light, and timed it to find out. Here is how it works and what they saw. Then you can run it.
First, make a fast-moving source of light.
You cannot wave a flashlight anywhere near light speed. So the experiment builds a moving light source from scratch, in three quick steps.
Protons hit a target
A proton beam, about 6 GeV, slams into a small block of beryllium.
A pion flies out, near light speed
The crash makes a neutral pion (π⁰) racing forward at 0.99975 c.
It turns into light
Almost at once the pion decays into two gamma rays, thrown forward. That light now comes from a source moving at 0.99975 c.
Everything, on one clock.
Here is the real chain of events, in order. The pion is born and lives only about 10⁻¹⁶ seconds before it decays. Then the two gamma rays fly the 31 m baseline and reach the detector 103.4 ns later. That flight time is the one number the experiment measures.
Show the timings
The "make the light" steps are practically instant: the pion is created and decays within about 10⁻¹⁶ s, travelling barely a hair's width. The clock for the speed starts at the decay, when the gamma rays are born, and stops 103.4 ns later when they reach the detector 31 m away.
That time means c, not c + v.
103.4 ns over 31 m is exactly the speed of light, c. If the gamma rays had also carried the pion's speed (c + v, nearly 2c), they would have arrived in about 51.7 ns, twice as fast. They did not. Light went at c, and the source's 0.99975 c added nothing.
Where does c + v even come from?
c + v is not the obvious default. It follows from one assumption: that light is a thrown object that carries its source's motion, like the ball from the train. That is the ballistic (emission) theory, Ritz 1908.
Treat light as a wave in a medium instead and its speed is set by the medium, not the source: a sound wave from a fast jet still moves at the speed of sound in air. So the wave theory already predicted no source effect. Relativity goes one further: c for every observer.
Measuring c rules out the ballistic assumption, the only one that predicted c + v. It does not, by itself, separate the wave from relativity, since both expected the source to drop out; that took Michelson and Morley, testing the observer's motion. What this experiment settles is sharp: light does not inherit the speed of whatever emits it.
The exact figures
Alväger and colleagues pinned any source dependence to k = (−3 ± 13) × 10⁻⁵, consistent with zero, ruling out the ballistic c + v by a factor of thousands.
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). Figures are illustrative of the published apparatus and rounded; the animation is time-stretched for visibility.
Run the experiment.
Fire the protons and watch the two predictions race the 31 m to the detector: amber for c + v, green for c. Drag the slider to change the source speed and watch the head start grow or shrink.
Press Step to walk through it one event at a time, or Fire Protons to play it all.
Want to interrogate this result? The graph holds it as a sourced, time-stamped fact.
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