Science Friday - SciFri Extra: About Time

Episode Date: June 25, 2019

The official U.S. time is kept on a cesium fountain clock named NIST-F1, located in Boulder, Colorado. On a recent trip to Boulder, Ira took a trip to see the clock. He spoke with Elizabeth Donley, acting head of the Time and Frequency Division at the National Institute of Standards and Technology, about keeping the official U.S. time on track—and how NIST is using advanced physics to develop ever more precise and stable ways to measure time. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Hi, I'm Iroflato. Last week, we visited Boulder, Colorado as the guests of KUNC, and we saw a ton of great stuff. In fact, too much stuff to fit in just one hour of our program. Ironically, the segment we didn't have enough time for was about time. So here's a special extra for you podcast listeners, a conversation about measuring time and how to do it better. This is Science Friday. I'm Ira Flato coming to you from the, you. Chautauqua Auditorium in Boulder, Colorado. In Alice in Wonderland, when the white rabbit

appears before he startled Alice saying, oh dear, oh dear, I shall be too late, Lewis Carroll is expressing for all of us, I think, are deep-seated feelings about time. It's slipping away. You never have enough. And, oh, just how accurate is my watch? No one is more fixated with time than those who study it. Researchers are obsessed with measuring it, as accurately as possible down to the billionth, the trillionth, or more of a second. And Boulder is a great place to have a conversation about time, because did you know that one of the most accurate clocks in the world, the official clock for the United States,

is secreted behind locked doors right here in Boulder. And spoiler alert, it does not look anything like the wind-up pocket watch carried by the White Rabbit. Dr. Liz Donnelly is the head of the time and frequency division of the National Institute of Standards and Technology, NIST here in Boulder. She's in charge of both maintaining and distributing the U.S. official time and researching ways to do it better.

Welcome to Science Friday. Nice to have you here. We visited, I had a chance to visit NIST yesterday and you showed us some of your clocks, including something called a cesium fountain clock. Yes. What is, can you describe that? In 1967, the definition of the second change from being based on the orbit of the Earth around the Sun

to a microwave transition in cesium atoms that oscillates at about 9.2 gigahertz. And so this change was made because you can measure this transition frequency very, very precisely. And it can also be realized all over the world because every cesium atom is the same, just because of quantum mechanics, you have always the same number of electrons and protons and neutrons in the atom. So they're all identical. They can build their own clock. Yeah, so every cesium atom in the world can be a clock

if you can measure that microwave frequency that excites this transition in the atoms. And so cesium atoms have a split ground state. It's called hyperfine ground state splitting, and it's split by this 9.2 gigahertz frequency, And what that physically comes from is that the nucleus of the atoms, it has its own spin, and then there's one single valence electron, like a hydrogen atom. It's in the first column of the periodic table of the elements.

And so both of those can act like little magnets, and so they can either be aligned or counteraligned. They can't be anything in between. So those two states are what we probe. So we probe transitions between these two states, And what makes the oscillator in the clock is the radiation that we use to excite the atoms to change state. And so the cesium fountain clock at NIST, it's about 20 years old, and it's been upgraded recently, and it's based on laser-cooled cesium because we want to be able to probe the atoms for as long as possible to have a narrow transition band that we're investigating or measuring.

Let me ask you about that. Why do you need to have such an accurate, accurate clock? You were telling me yesterday that the clock's accuracy, some of the clocks you're working on, is one second in 38, 28 billion years? So that's, yeah, so that's a way to put it. So we have these frequencies are very, very, very precisely measured to the point where if you wanted to compare it to how long it takes them to be off, by one second. It's twice the age of the universe. Well, why do you need that? Why do you need that kind of precision?

What do you do with that that's useful? You know? I mean, my watch is pretty good. Why do I need to have one and dissolve times twice the age of the universe? Well, there's, so with the cesium ones, okay, so since we redefine the second in 1967, they've improved by about a factor of 10,000. they're not improving anymore because they've reached the state of the art. It's like really heroic to measure it because you split this little band of frequencies that we measure by about a factor of a million to be able to tell exactly what it is. So you have to go to a different technology that doesn't use.

Well, so now, so there's these atomic clocks are all over the place, and we use output from the GPS satellite clocks all the time. So the way that the GPS system works is you measure these signals and you measure it from multiple different satellites and then you know based on the code that's in the signals how long it took those signals to reach you from different places. So you can figure out what time it is. But they all have atomic clocks on board

that are all synchronized to within about a nanosecond of each other. So it helps us accurately. locate where we are kind of similar, but the one, so the one that you saw yesterday, and it's the primary frequency standard for the United States, that one is much better than the ones that are in the satellites, but it's the one that we use to actually calibrate official at the U.S. time. And so we all, all over the world, we use coordinated universal time, and we have to compare our realization of it to other realizations.

around the world via satellite, to synchronize all the national labs around the world. And it takes about a month, each time you want to make a measurement with that fountain to really reach the limit that it performs at. But why do we need even better clocks? So yesterday you also saw, maybe there's a photo of it there.

You saw one of the other really nice optical clocks. It actually has the current world record for accuracy, And so that one has a part in 10 to the 18. When you say an optical clock, it's not working with the cesium atoms anymore. It's working with lasers. Yes. So we excite the electrons in the atom to go to a higher orbital, basically. If you remember the simple version of like the bore model of an atom,

you have all of these electron orbitals, and you can go to higher ones. So we move the electron around. and that's at much higher frequency. So you can still get about the same line width that you measure, but you get a factor of 10 to the 5, so about almost a factor of 100,000 higher frequency. So that's what sets your ability to measure it with such high precision because you can divide the second down by a factor of 100,000 more.

So that's 100,000 more accurate than the CZM? much more accurate with that thing? Well, it's about a factor of a hundred now or maybe a little bit better. That's pretty good. Yeah, so it's, I mean, so it's enough better that the definition of the second is going to change probably over the next 10 years to be able to realize that higher level of accuracy for more applications. Let's go to the other.

Let's start here. Yeah, come up to the mic. Don't be afraid. Yes, go ahead. Yeah, so this is kind of in response to your question, Ira, on the possibility of recombining molecules as in Star Trek transporters, I would think that the timing would have to be just so perfect. How far away from that type of technology are we? You did a paper on that recently, right?

They'd have to, they would. The two places would have to be really coordinated, wouldn't they? Yeah, so we do do what are called quantum teleportation experiments in the lab as well. What have you moved so far? What have you transferred? See, that's not what he wants to eat. Not what he's talking about, but it's beginning. Okay, question over here. I'm just curious, though, with such a precise clock, do we have a definition for a year

and exactly how long is it right now? The years are actually still determined by astronomy. So all of our precision applications use UTC, which is tied to atomic time. But one of the unfortunate things is that because the Earth's orbits and rotations around the sun are not stable, we have to add leap seconds to UTC, coordinated universal time.

And those cause a lot of disruption because they're always added at midnight UTC, which is different times all over the world. And so a lot of things that are synchronized these days are synchronized of very high precision at the microsecond level. And so you can suddenly have an extra second it can cause a problem.

You know, I hear on WWV, anybody ever listen to the clock? Master clock. Does that come from your office? Yeah. Yeah, she's responsible for that. For synchronizing all those clocks. Next time you come, you can go visit the radio stations. Ooh, I blew it this time.

Yes, over here. Hi, thanks for coming. I was wondering and moving on to a more accurate version from the cesium clock, is that due to experimental inaccuracies or their fluctuations or aberrations in the cesium system that make it difficult to pin down that accuracy? So what it comes down to is what we call the Q factor, the quality factor, and the atom,

and it's just much better in the optical than it is in the microwave region of the electromagnetic spectrum, at least for these transitions that we're measuring. And so it's just a fundamental limit. You just can't do any better with cesium. It's reached its full maturity. And so to be able to have higher precision, time and frequency standards, we have to change the definition. It's old news.

CZM has had it. Called nodes. Yes. So I'm curious, with these extremely accurate clocks, they can be affected by relativity and even the effect of gravity can adjust that. I'm curious if there's any particular reason why the clock here was chosen instead of something, say, at sea level. Well, the NIST- chose to move the time and frequency division out here in the 1950s.

And back then, it really didn't care at the level that you have these relativistic effects. So all the frequencies of these clocks shift by about a part in 10 to the 16 per meter above sea level. And so we have to correct all of them for that. So when you have these clocks that are this precise, you're no longer really probing time or time interval or frequency, or frequency or probing space time. So we have to be very careful with how we do the measurements and how we correct for those effects.

But yeah, it isn't, it's not a very fortunate choice for that reason that our most precise clocks in the United States are here. Not just because of that, but once you get to the level of the part in 10 to the 18, the flat irons probably have an effect, you know? Wait a minute. Gravity, gravitational potential is not spherical.

I mean, it's got perturbations. They approximate it by like spherical harmonics, 256 spherical harmonics, and those have a pretty big size. So you're saying they should shave off the flat iron a little bit and get a better clock out of it? You're not advocating that? No, not at all. Okay, let's go right over here. Hi, when some people are, most people are asked with the greatest invention in human history, as they'll save the wheel. But some scientists are saying that it's the construct of time. Now, while a lot of animals have an intuitive sense of time, none of them have the participation in precision that humans do.

So my question is, beyond humans, does time exist? So that's a good philosophical question, but I bet that if there's another, and I'm sure there are another intelligent, beings in the universe, they probably have their own concept of time, I would imagine. They probably want to make appointments and do more and more things just like we do. Well, as I'm apt to say, our time has run out. I know, I'm sorry.

Dr. Liz Donnelly, thank you for taking time to be with us today. Head of the Time and Frequency Division of the National Institute of Standards and Technology here in Boulder. That was recorded on June 5,000. and the Chautauqua Auditorium in Boulder, Colorado. Catch you on Friday. I'm Ira Flato in New York.