Screening mammography is an important tool against breast cancer, but it has its limitations. A "normal" screening mammogram means that a woman doesn't have cancer now. But doctors wonder whether "normal" images contain clues about a woman's risk of developing breast cancer in the future.
This may come as a shock, if you're moving fast enough. The shock being shock waves. Far out in the cosmos, a collapsing star generates shock waves from particles racing near the speed of light as the star goes supernova.