Detecting Cherenkov Light
The gamma rays that CTA will detect don’t make it all the way to the earth’s surface. When gamma rays reach the earth’s atmosphere they interact with it, producing cascades of subatomic particles. These cascades are also known as air or particle showers. Nothing can travel faster than the speed of light in a vacuum, but light travels 0.03 percent slower in air. Thus, these ultra-high energy particles can travel faster than light in air, creating a blue flash of “Cherenkov light” (discovered by Russian physicist Pavel Cherenkov in 1934) similar to the sonic boom created by an aircraft exceeding the speed of sound. Although the light is spread over a large area (250 m in diameter), the cascade only lasts a few billionths of a second. It is too faint to be detected by the human eye but not too faint for CTA. CTA’s large mirrors and high-speed cameras will detect the flash of light and image the cascade generated by the gamma rays for further study of their cosmic sources. Learn more about gamma rays and their cosmic sources.