What Hawking Radiation Looks Like as You Fall into a Black Hole

Abstract: We study the measurements of a freely-falling Unruh-DeWitt particle detector near the horizon of a semi-classical Schwarzschild black hole. Our results show that the detector's response increases smoothly as it approaches and crosses the horizon in both the Hartle-Hawking and Unruh vacua. However, these measurements are dominated by the effects of switching the detector on and off, rather than by the detection of Hawking radiation particles. We demonstrate that a freely-falling Unruh-DeWitt detector cannot directly measure Hawking radiation near the horizon because the time required for thermalization is longer than the time spent near the horizon. We propose an operational definition of the effective temperature along an infalling trajectory based on measurements by a particle detector. Using this method, we find that the effective temperature measured by a freely-falling observer in the Hartle-Hawking vacuum increases smoothly from the Hawking temperature far from the horizon to twice the Hawking temperature at the horizon, and continues to rise into the interior of the black hole. This effective temperature closely matches an analytical prediction derived by embedding Schwarzschild spacetime into a higher-dimensional Minkowski space, suggesting that further exploration of higher-dimensional embeddings could provide new insights into the near-horizon behavior of black holes.