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## Main Question or Discussion Point

Hi.

In the derivation of the relativistic formula for adding velocities, the Lorentz factor drops out. Mathematically, the formula works for inertial frames with relative velocity c and even gives an answer to Einstein's famous question about what happens if you drive at the speed of light and turn your headlights on.

However, is there a physical meaning to this? Massive light sources can't reach c because the required energy would be infinite. But is there some framework (QFT maybe?) where a photon decays into multiple photons such that one of them can be regarded as "headlights" and another as "light from the headlights"?

In the derivation of the relativistic formula for adding velocities, the Lorentz factor drops out. Mathematically, the formula works for inertial frames with relative velocity c and even gives an answer to Einstein's famous question about what happens if you drive at the speed of light and turn your headlights on.

However, is there a physical meaning to this? Massive light sources can't reach c because the required energy would be infinite. But is there some framework (QFT maybe?) where a photon decays into multiple photons such that one of them can be regarded as "headlights" and another as "light from the headlights"?