Emma Chapman's "The Echoing Universe" is a vivid survey of the hidden parts of the cosmos and those who seek them out.

When he was 16 years old, Albert Einstein imagined chasing after a beam of light and, as the story goes, this feat of imagination helped him develop the now-famous theory of special relativity.

HoloRadar uses radio waves to see around corners, allowing it to detect people at T-shaped intersections like the one pictured here. Penn Engineers have developed a system that lets robots see around ...