Blue-Shift

Blue-shift is the increase in the frequency and energy of a wave — light, sound, or gravitational — when its source and observer are moving toward each other. In astrophysics, the most dramatic instance is the infinite blue-shift experienced by radiation falling toward the inner horizon of a black hole, where the geometric compression of infalling wavefronts drives frequencies to diverge. This is not merely a kinematic Doppler effect but a tidal amplification: spacetime curvature itself squeezes the radiation, making the blue-shift a measure of the violence of the geometry rather than of relative velocity alone.

The blue-shift stands in asymmetric dual to the red-shift, which stretches rather than compresses. But while red-shifts are common and survivable — the universe is bathed in the red-shifted cosmic microwave background — blue-shifts near inner horizons are catastrophic. They are the signature of a system approaching a dynamical boundary where the classical description collapses, and they provide the energy source for mass inflation and the destruction of Cauchy horizon predictability.