Why are we changing the way we keep time?

Timekeeping scientists around the world voted to stop using leap seconds, but what does this mean for us?

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:17Flora Lichtman?

  Hey, it's Flora, and you're listening to Science Friday. Time passes, but measuring it is actually kind of tricky. So the time we see on our phones and our clocks is based on the Earth's rotation. It sounds simple, but the problem is that the Earth's rotation speed can vary. And so if it speeds up or slows down even a tiny itty-bitty little bit, our time measurements get thrown off. To solve this problem, timekeeping scientists have been deploying leap seconds. That is, until last week, when a global committee of timekeepers voted to change the way the world keeps time. After decades of debate, they killed the leap second.

• 1:02Flora Lichtman?

  And in doing so, they made the historic decision that the time we live by will be just a tiny bit out of sync with the spin of the planet. Why is this a big deal? And what does it mean for us? Here to help is Emma Gomez, who reported on this for Scientific American. Thanks for taking the time, Emma.

• 1:20Speaker 02

  Thank you so much for having me on, Flora.

• 1:22Flora Lichtman?

  Tell us, is this a big deal in the timekeeping world?

• 1:26Speaker 02

  Yes, I would say it's a big deal. Like you mentioned, it's been decades of debate. Historically, we have always had time very closely connected to the Earth's movements and the angle of the Earth in comparison to the Sun. And so this will be a decision that means that the world will be keeping time according to the count of the seconds, which is new, as opposed to the movements of the Earth. So it's a divergence from this way that we've always known how to tell time.

• 1:57Flora Lichtman?

  That's wild. I want to unpack this a little bit. So obviously, we used to measure time by looking at the sky. There were sundials. How is time measured now?

• 2:05Speaker 02

  Yes, so time is something of a democratic process, which I find interesting. There are more than 90 labs across the world that all have a suite of atomic clocks that they keep. And then they send the data from those clocks to this lab in France called the BIPM, which is the International Bureau of Weights and Measures. And then they do like a special weighted average of all of that information. And they are the official record of coordinated universal time. And then that's where the leap seconds come in. The International Earth Rotation Service, they kind of keep note of like how the Earth's rotation is doing.

• 2:45Speaker 02

  And if they notice that the atomic clock is going to fall out of sync with time by more than a second, they let the BIPM know, and the BIPM says, OK, guys, like, we all have to hold our clocks. Traditionally, the Earth has been a little slower. So we hold our clocks for one second to bring our time back in alignment with the Earth's rotation. So that has been the traditional system of how we tell time.

• 3:11Flora Lichtman?

  And the atomic clocks that you mentioned, these are just very precise counters of seconds?

• 3:17Speaker 02

  Yes, they use a cloud of cesium atoms to do this. So cesium atoms, when they're bathed in a certain microwave frequency, they enter a quantum state that causes them to oscillate between two energy states. It's called their resonant frequency. So basically, when you blast a microwave frequency at the cesium atoms, when you notice that they have entered that, you know, flipping energy state, state, the quantum state, that's how you know that the microwave frequency aligns with the seconds.

• 3:54Speaker 02

  So it's 9,192,631,770 oscillations per second. So that frequency can be converted into seconds. So they observe the cesium atoms in this state. They say, OK, they're resonating. That's how we know the microwave frequency is correctly measuring the seconds, basically. It's very confusing, but the reason that we do it this way is because we know that those atoms, like, they don't change over time. Like, they're never going to give you an incorrect reading of what that frequency is. So that's why we do it that way.

• 4:36Flora Lichtman?

  Why was the leap second killed?

• 4:38Speaker 02

  The leap second was killed because it's really, really, really annoying for everyone involved. It's annoying for timekeepers because it's, they call it a discontinuity. It's a discontinuity. And so timekeepers have to take their very precious special instruments and their, you know, complicated system for disseminating the time and hold everything for a second, which machines really, really hate. But it's even more annoying for computers, which are sending and receiving messages constantly, and those messages need to be time stamped. So if you get a message from somebody that was sent after you're actually supposed to be receiving the message, it creates a bug, it creates outages. A lot of really important internet services, computer services,

• 5:20Speaker 02

  like CloudFlare, DNS Systems, Mozilla, Reddit, Meta, they've all experienced outages because of the leap second, just because the computer doesn't understand that there's a discontinuity in the time. So Google, Meta, lots of big software companies have tried to get the BIPM, the International Telecommunications Union, to end the leap second because it creates outages. And sometimes it can be dangerous. Sometimes, like, in a plane system, they have to restart it when the leap second has happened. Like, just because time is really, really essential to internet services, phone towers, and even GPS, like navigation. It's all really critical infrastructure

• 6:05Speaker 02

  that really depends on either, you know, being continuous or being able to treat discontinuities very carefully so that it doesn't mess things up.

• 6:16Flora Lichtman?

  But are we just going to get more and more out of sync with Earth's rotation? Like, is there going to be a time when this is actually a problem we might notice?

• 6:26Speaker 02

  It'll be like thousands of years before we would ever have to use a leap hour. So basically, the decision is kicking the can down the road to, you know, a population of people who, we don't know what's going to happen with them. We have no idea what the world is going to be like in 3,000 years. But I'd like to say, like, we already kind of live in a quote-unquote "different solar time" because of time zones. Like, if you move like 100 yards down, technically, it's a slightly different time than it would be 100 yards, you know, east of where you were. I think I'm living because of daylight saving times, like, more than an hour off from the solar time in my area.

• 7:09Flora Lichtman?

  Makes a ton of sense. Emma, thank you so much. This is delightful.

• 7:14Speaker 02

  Thank you, Flora. This was great.

• 7:16Flora Lichtman?

  Emma Gomez from Scientific American, and you can read their piece there. If we already live out of sync with astronomical time because of time zones and daylight savings, then why was there so much debate, and for so long, about changing how time is kept? Clocking in to give us the inside scoop is one of America's top timekeepers, Dr. Judah Levine, a physicist specializing in precision measurements of time and frequency at the University of Colorado Boulder. Judah, welcome to Science Friday.

• 7:50Speaker 03

  Thank you. It was nice, nice to be here.

• 7:53Flora Lichtman?

  I hear that your coworkers call you Father Time.

• 7:56Speaker 03

  Yeah, I think that's unfortunate. There's an implication that I'm the only guy in the business, and that's not accurate. It's a lot of people who do this.

• 8:06Flora Lichtman?

  It's, it sounds like there's many people who do it, based on what we heard from Emma. So it seems like you all came up with a good solution. Why was there so much debate about this? Like, what are the two sides of it?

• 8:20Speaker 03

  Well, I think there are applications that depend on, on what's called UT1, which is the angle of the Earth with respect to the stars. And there was a lot of concern about a divergence between...

• 8:36Speaker 03

  Atomic time and UT1, the leap second business was essentially to try to keep them within one second. And, but there was some concern that if you, you shouldn't abandon the link altogether.

• 8:51Flora Lichtman?

  Was it emotional?

• 8:54Speaker 03

  Well, I, I think, I think time has been linked to astronomy since, since forever, since antiquity. Almost every religion has some linked astronomical times. It was regarded as an important quantity. And, and people have the idea that, that when you go out to eat lunch, the sun ought to be high in the sky.

• 9:15Flora Lichtman?

  That's fair. I mean, so what, what pushed people over the edge to unlink time a little bit from astronomical time?

• 9:25Speaker 03

  Well, I think, I think there are, yeah, there are two things. The first thing is that, um, uh, applications that depend on time interval and frequency became more and more important. And, and leap seconds were not predictable. And that means that they happened when they happened, but you couldn't program them in advance. Uh, it's not like February 29th where you can program it in advance forever. You know when it's going to happen, but leap seconds couldn't be predicted more than a few months in advance. And that was a big pain because it meant that, that they had to be handled administratively. They couldn't be handled, you know, by programming so that, that clocks couldn't be built once for, for all time.

• 10:05Speaker 03

  The second thing is that, uh, the leap second introduced a discontinuity in time interval because there was this extra second, uh, which was added into the time, but, but, uh, real time processes like navigation didn't stop. And so when you wanted to measure the elapsed time, which was how you measure distance when measuring the elapsed time, you got into trouble because of this leap second business. Uh, but the real trigger was the concern that there would be a negative leap second, that the earth was speeding up and, and, um, and that instead of atomic time being ahead of the earth, uh, the earth might be ahead of atomic time and that the negative leap second had never been, never been done before, never been used before.

• 10:48Speaker 03

  And so there were concerns that there would, that that would be a problem. And I think, I think that was really the trigger.

• 10:54Flora Lichtman?

  So let me get this straight. Earth was moving slower and then now earth is, is speeding up or earth is predicted to speed up its rotation?

• 11:04Speaker 03

  No, no, no, no, no. Earth is not predicted. This actually happens. Uh, that is, starting in about 2016 or 2017, uh, the earth began to speed up. Uh, that was not a prediction. That was not an expected thing. Uh, the, the, the concern about a negative leap second was an extrapolation of that, of that increase, that it would continue and eventually grow to, to something approaching a second. Uh, but it, it happened. It was, was, was really very clearly observed.

• 11:34Flora Lichtman?

  So we're losing the leap second. This will go into effect in a couple of months. Does that mean we're just going to get more and more out of sync with astronomical time?

• 11:45Speaker 03

  Sure, but, but you gotta keep, you gotta, you know, pay attention to the scale of things. But the answer is that's true. Uh, but for example, uh, you know, since we started counting leap seconds, we've added 37 leap seconds. So that's about something like a minute per century. Uh, so yeah, we're going to get out of sync, but, but we're going to get out of sync at the level of a minute per century. So, so, uh, it's not a, you know, you don't have to get into panic mode about this.

• 12:11Flora Lichtman?

  And then, okay, so, but you know, centuries later, you know, you might get to an hour, you might notice it. So what do you do?

• 12:19Speaker 03

  Well, that's a good question. I don't know what you do, but, but, but at the rate of a minute per century, it's going to take a long time to get this to, to an hour. Uh, and the answer is-

• 12:28Flora Lichtman?

  This is not your problem, is what I'm hearing.

• 12:30Speaker 03

  Well, I understand. I mean, it's not my problem, uh, but it might be somebody's problem.

• 12:35Flora Lichtman?

  You know, you've spent a lot of your career working in standardization. Why? Why is that important? What's the big picture here?

• 12:43Speaker 03

  Well, I think, I think, uh, standards are very important because you don't have to, you don't have to worry about, for example, when you, when you call your friend in, in, in France from, from, from New York, you don't have to worry about how the call is being carried, uh, because all the carriers have agreed on standards of time and frequency. Uh, when you, when you, when you go to the supermarket, you buy stuff by the pound and you don't have to worry about whose pound it is. There's a whole standards community that is, is making sure that, that, that pounds are pounds and gallons are gallons and so on and so on. And, and, and so time and frequency just fits into that, that same framework of making

• 13:27Speaker 03

  it possible to, to, to, to do things in a modern society without paying attention to what units you're dealing with. They're all the same.

• 13:35Flora Lichtman?

  Dr. Judah Levine is a faculty member in the Department of Physics and a Fellow of Jilla at the University of Colorado Boulder. Thank you so much for joining us today.

• 13:44Speaker 03

  A pleasure. Nice to talk to you.

• 13:46Flora Lichtman?

  Likewise. This episode was produced by Iman Moyn. If you had the time of your life listening, why not leave us a review? It really does help. You can find the reviews on any of the podcast platforms that you listen on. Until next time, I'm Flora Lichtman.