The Cosmic Speed Limit

As an object speeds up toward light speed, its energy and momentum grow without limit — the Lorentz factor blows up. There's no hidden gain in mass; it's the energy that runs away.

This is why only massless particles like photons travel at light speed — they were born at that speed and can never go slower. The Large Hadron Collider pushes protons to 99.999999 percent of light speed, yet that last fraction stays permanently unreachable.