The days after the end of Daylight Saving Time can be a bit confusing. Your cell phone probably adjusted automatically, but maybe the clock in your car did not leading to brief panic. Am I late for work? In the first of an occasional exploration of time that we're calling Tick-Tock, NPR's Geoff Brumfiel reports that sort of momentary confusion is nothing compared to the confusion facing researchers who study time.

　　GEOFF BRUMFIEL, BYLINE: Here's the first question - what is time?

　　O'BRIAN: My own personal opinion is that time is a human construct.

　　BRUMFIEL: Tom O'Brian's opinion matters. He keeps time at the National Institute of Standards and Technology.

　　新时钟可能颠覆现有时间概念

　　O'BRIAN: It's a way that we as humans have developed to try to understand and put some order in this very fascinating and complex universe around us.

　　BRUMFIEL: We bring order using clocks. And O'Brian oversees America's master clock, one of the most accurate clocks on the planet.

　　O'BRIAN: And it is accurate to the equivalent of about 0.000000000000001. I think I said it right, but I might've miscounted (laughter).

　　BRUMFIEL: Put another way, this clock could tick for 300 million years and stay accurate to within a second, but here's the crazy thing...

　　O'BRIAN: We can measure time much better than the weight of something or an electrical current, but what time really is a question that I can't answer for you, and I'm embarrassed to say that as chief of the Time and Frequency Division. But I feel I'm in good company because even people like Albert Einstein would readily admit that he didn't really understand what time was.

　　BRUMFIEL: Maybe it's because we don't understand it that we keep trying to measure it more accurately, and why at the nearby University of Colorado they're developing a clock to end all clocks.

　　So I'm standing in the basement of the University of Colorado Boulder, and it's just chaos - wires hanging from the ceiling, wires on all the tables, wires held up with binder clips. I mean, I have never seen such a mess. And this is the home of the most accurate clock on the planet.

　　(SOUNDBITE OF CLOCK TICKING)

　　BRUMFIEL: Actually this tangle of wires and lasers is the clock. It's spread out on a giant table, parts of it wrapped in shiny foil.

　　There's tinfoil, man. How can the most accurate clock on the planet depend on tinfoil?

　　TRAVIS NICHOLSON: Well, first of all that's research-grade tinfoil, so...

　　BRUMFIEL: Graduate students Travis Nicholson and Sara Campbell keep the clock running day-to-day.

　　Do you guys really know where all these wires go?

　　SARA CAMPBELL: I think half of them go nowhere.

　　NICHOLSON: Yeah. We can trace them out if we need to.

　　BRUMFIEL: At its heart, this new clock is actually just atoms of an element called strontium. Strontium vibrates at an incredibly fast frequency. It's a natural, atomic metronome ticking out teeny, teeny fractions of a second. Jun Ye is the scientist who built this clock. He says it could keep perfect time for 5 billion years.

　　JUN YE: It's about the whole entire age of the Earth.

　　BRUMFIEL: So if this clock were running since the start of the age of the Earth, it would have the time down to within a second?

　　YE: Right. And our aim is that we'll have a clock that would - during the entire age of the universe, would not have lost a second.

　　BRUMFIEL: But this perfect clock has run into a big problem because this thing we call time doesn't tick at the same rate everywhere in the universe or even on our planet. On top of Mount Everest, time is passing just a little bit faster than it does in Death Valley. That's because the speed at which to time passes depends on the strength of gravity. You don't need to take this to extremes either. If you take a clock off the floor and hang on the wall...

　　YE: The time will speed up by about one part in 10 to the 16.

　　BRUMFIEL: One part in 10 to the 16 is a sliver of a second, but this isn't some effect of gravity on the clock's machinery. Time itself is flowing more quickly on the wall than on the floor. These differences didn't really matter till now, but this clock is so sensitive little changes in height throw it way off.

　　YE: So we're talking about lifting up a couple centimeters and you will start to see that difference.

　　BRUMFIEL: This new clock can sense the pace of time speeding up as it moves inch by inch away from the earth's core, and that's a problem because to actually use time, you need different clocks to agree on the time. Think about it - if I say let's meet at 3:30, we use our watches, but imagine a world in which your watch starts to tick faster because you're working on the floor above me. Your 3:30 happens earlier than mine. This clock works like that. Tiny shifts in the Earth's crust can throw it off even when it's sitting still. And two of them will never agree.

　　YE: At this level, maintaining absolute timescale on Earth is in fact turning into nightmare.

　　BRUMFIEL: This nightmare clock they've built doesn't just look chaotic; it's turning our sense of time into chaos. But Chief Timekeeper Tom O'Brian isn't worried about all this because as confusing as these clocks are, they're going to be really useful.

　　O'BRIAN: Scientists can make these clocks into exquisite devices for sensing a whole bunch of different things.

　　BRUMFIEL: They're getting so sensitive to gravity. Maybe they can map the interior of the Earth or detect gravitational waves from black holes and exploding stars. They could change our view of the universe. They just might not be able to tells what time it is. Geoff Brumfiel, NPR News.