Why are we changing the way we keep time?

The end of the leap second

0:00Hey, 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

0:32deploying 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. 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.

1:04Thanks for taking the time, Emma. Thank you so much for having me on, Flora. Tell us, is this a big deal in the timekeeping world? 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

1:36Earth. So it's a divergence from this way that we've always known how to tell time. 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? 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

2:10Weights 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. 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, okay, guys, like, we all

2:40have 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. And the atomic clocks that you mentioned, these are just sort of very precise counters of seconds? Yes, they use a cloud of cesium atoms to do this. So cesium atoms, when they're bathed in a certain

3:11microwave 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 these 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. So it's 9,192,631,770 oscillations per second. So that frequency can be converted into seconds.

3:55So they observe the cesium atoms in this state. They say, okay, 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.

Why computers hate leap seconds

4:21Why was the leap second killed? The leap second was killed because it's really, really, really annoying for everyone involved. It's annoying for timekeepers because 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 that which are sending and receiving messages

4:52constantly. And those messages need to be timestamped. 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 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

5:28Telecommunications Union, to end the leap second because it creates outages. And sometimes it can be dangerous. Sometimes like in plain system, they have to restart it when the leap second has happened. Just because time is really, really essential to internet services, phone towers, and even GPS, like navigation, it's all really critical infrastructure that really depends on either, you know, being continuous or being able to treat discontinuities very carefully so that it doesn't

6:00mess things up. 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? 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

6:37because 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. It makes a ton of sense. Emma, thank you so much. This is delightful. Thank you, Flora. This was great. Emma Gomez from Scientific American, and you can read their piece there.

Inside the history of timekeeping

7:06If 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.

7:36Thank you. It was nice to be here. I hear that your co-workers call you Father Time. 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. 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? What are the two sides of it? Well, I think there are applications that depend on what's called UT1,

8:11which is the angle of the Earth with respect to the stars. And there was a lot of concern about a divergence between atomic time and UT1. The leap second business was essentially to try to keep them within one second. But there was some concern that you shouldn't abandon the link altogether. Was it emotional? Well, I mean, I think time has been linked to astronomy since forever, since antiquity.

8:50Almost every religion has some link to astronomical time. So it was regarded as an important quantity. And people have the idea that when you go out to eat lunch, the sun ought to be high in the sky. Anyway, that's fair. I mean, so what pushed people over the edge to unlink time a little bit from astronomical time? Well, I think there are two things. The first thing is that applications that depend on time interval and frequency became more and more important.

9:21And leap seconds were not predictable. And that means that they happened when they happened, but you couldn't program them in advance. 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 they had to be handled administratively. They couldn't be handled, you know, by programming so that clocks couldn't be built once for all time. The second thing is that the leap second introduced a discontinuity in time interval because there was this extra second which was added into the time.

10:02But real-time processes like navigation didn't stop. And so when you wanted to measure the elapsed time, which was how you measured distance when measuring the elapsed time, you got into trouble because of this leap second business. But the real trigger was the concern that there would be a negative leap second, that the Earth was speeding up, and that instead of atomic time being ahead of the Earth, the Earth might be ahead of atomic time, and that the negative leap second had never been done before, never been used before.

10:32And so there were concerns that that that would be a problem. And I think that was really the trigger. So let me get this straight. Earth was moving slower, and then now Earth is speeding up, or Earth is predicted to speed up its rotation? No, no, no, no, no. Earth is not predicted. This actually happens. That is, starting in about 2016 or 2017, the Earth began to speed up. That was not a prediction. That was not an expected thing.

11:04The concern about a negative leap second was an extrapolation of that increase that it would continue and eventually grow to something approaching a second. But it happened. It was really very clearly observed. So we're losing the leap second. This will go into effect in a couple months. Does that mean we're just going to get more and more out of sync with astronomical time? Sure, but you've got to keep, you've got to pay attention to the scale of things.

11:37The answer is that's true. But, for example, since we started counting leap seconds, we've added 37 leap seconds. So that's about something like a minute per century. So, yeah, we're going to get out of sync, but we're going to get out of sync at the level of a minute per century. So it's not a, you know, you don't have to get into panic mode about this. 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? Well, that's a good question. I don't know what you do, but at the rate of a minute per century, it's going to take a long time to get this to an hour.

12:13And the answer is... This is not your problem, is what I'm hearing. Well, I understand. I mean, it's not my problem, but it might be somebody's problem. You know, you've spent a lot of your career working in standardization.

12:26Why? Why is that important? What's the big picture here? Well, I think standards are very important because you don't have to worry about, for example, when you call your friend in France from New York, you don't have to worry about how the call is being carried because all the carriers have agreed on standards of time and frequency. When you go to the supermarket, you buy stuff by the pound, and you don't have to worry about whose pound it is.

12:57There's a whole standards community that is making sure that pounds are pounds and gallons are gallons and so on and so on. And so time and frequency just fits into that same framework of making it possible to do things in a modern society without paying attention to what units you're dealing with. They're all the same. Dr. Judah Levine is a faculty member in the Department of Physics and a fellow of JILLA at the University of Colorado, Boulder.

13:28Thank you so much for joining us today. A pleasure. Nice to talk to you. Likewise. This episode was produced by Iman Moyne. 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. I'm Flora Lichtman. I'm Flora Lichtman. I'm Flora Lichtman. I'm Flora Lichtman.

13:59I'm Flora Lichtman. I'm Flora Lichtman. I'm Flora Lichtman. I'm Flora Lichtman. I'm Flora Lichtman.