The Unruh effect (1976) predicts that an observer undergoing constant proper acceleration a through the quantum vacuum perceives a thermal bath at temperature T = ℏa/(2πck_B). To an inertial observer the field is in the vacuum state; to the accelerating observer it contains real thermal particles. This reveals that "particle" is an observer-dependent concept — the vacuum of inertial observers is a thermal state for Rindler observers. To reach T = 1 K requires a ≈ 2.5×10²⁰ m/s², making direct detection extremely challenging. The Unruh effect shares mathematics with Hawking radiation via the Bogoliubov transformation between Minkowski and Rindler modes.