"My own personal opinion is that time is a human construct," says Tom O'Brian. O'Brian has thought a lot about this over the years. He is America's official timekeeper at the National Institute of Standards and Technology in Boulder, Colorado.
To him, days, hours, minutes and seconds are a way for humanity to "put some order in this very fascinating and complex universe around us."
We bring that order using clocks, and O'Brian oversees America's master clock. It's one of the most accurate clocks on the planet: an atomic clock that uses oscillations in the element cesium to count out 0.0000000000000001 second at a time. If the clock had been started 300 million years ago, before the age of dinosaurs began, it would still be keeping time — down to the second. But the crazy thing is, despite knowing the time better than almost anyone on Earth, O'Brian can't explain time.
"We can measure time much better than the weight of something or an electrical current," he says, "but what time really is, is a question that I can't answer for you."
