Researchers at the U.S. National Institute of Standards & Technology (NIST) say they have created the most accurate atomic clock to date — one that can measure time down to the 19th decimal place.

BOULDER • Every second in a small laboratory room in Boulder, a green light flashes. Within the webs of yellow wire and shelves of computer systems, this green light represents the passage of time.

Time dilation, a captivating concept embedded in Einstein’s theory of relativity, has been experimentally validated on Earth. With the use of ultra-precise clocks, scientists have been able to ...