Significant new imaging potential could be the outcome of research into the relativistic motion of free electrons in the electromagnet fields of light, say scientists at the University of Michigan, Ann Arbor. Their studies confirm several predictions, based on Einstein's theory of relativity, about how electrons behave in extremely high-strength electromagnetic fields, such as those produced by powerful lasers or supernova explosions. In such strong fields, a quantity commonly used as a fundamental physical constant in physics theories -- known as the Thomson cross section -- has now been measured not to be constant. They say the discovery marks the dawn of a new field of study -- "relativistic nonlinear optics" -- that may result in such new technologies as X-rays that could take snapshots on an atomic scale of ultra-fast chemical, physical, and biological processes such as photosynthesis, or make holograms of living cells. They say the most promising technologies may arise as coherent harmonics in the X-ray region of the light spectrum are explored. For example, X-rays generated in this way should have extremely short pulse durations (less than a trillionth of a second) and might be used for time-lapse radiography, crystallography, microscopy, and lithography with atomic-scale resolution. The research team was led by Donald Umstadter, associate professor in the College of Engineering, who coordinates the High-Field Science program of the Center for Ultrafast Optical Science.