Podcast: Atomic clock accuracy and wind farm worries

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week, we’ll be hearing how clocks can be used to measure gravity.

Host: Shamini Bundell

And woes for wind farms. I’m Shamini Bundell.

Host: Benjamin Thompson

And I’m Benjamin Thompson.

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Interviewer: Benjamin Thompson

Listeners, if you were to ask my friends what my timekeeping is like, they would happily tell you that it is terrible. The old phrase, ‘He’d be late for his own funeral,’ pretty much sums up my ability to meet someone when I say I will. But apparently, I’m not the only one who needs to improve their timekeeping. Being able to accurately tell the time is really important for loads of technologies. Here’s Andrew Ludlow from the National Institute of Standards and Technology in the US with a few examples.

Interviewee: Andrew Ludlow

I think some of the more ubiquitous examples like navigational systems and communication systems often require very stringent timing requirements. You know, Global Navigation Satellite Systems basically exploit atomic clocks, on-board satellites and distributing those timing signals in order to deduce relative position. The ability to kind of keep time very well enables other types of technologies as well.

Interviewer: Benjamin Thompson

In this week’s Nature, Andrew and his colleagues describe a pair of atomic clocks that keep time with astonishing accuracy. They even suggest that these clocks could, in the future, not just be used to measure time, but to measure the strength of gravity on different parts of the Earth. But more on that in a bit. First, let’s talk about atomic clocks. Rather than relying on the swinging of a pendulum to count the passing of time, atomic clocks use something else.

Interviewee: Andrew Ludlow

Here, the time base really is the atom. Mostly it’s electronic oscillations in an atom, and it turns out that, you know, nature has given us some atoms in particular who give us extremely stable oscillations where they don’t change for many reasons. They’re quite fixed in time and therefore they can be very good at making an atomic clock.

Interviewer: Benjamin Thompson

Andrew’s clocks are optical lattice clocks, considered to be the next generation of atomic clocks and in this case use atoms of the element ytterbium. By using a finely-tuned laser, it’s possible to excite electrons within these ytterbium atoms into a different energy state. By measuring the oscillations in the electrons as they move between these states, it’s possible to measure the passage of time. And these transitions happen very, very quickly.

Interviewee: Andrew Ludlow

It’s at about 518 terahertz, and so that’s 5 x 10^-14 times per second, and I should say that that’s a big number. And that’s one of the reasons why it’s useful for a clock is each oscillation is dividing time up into extremely fine intervals, and so that gives us a lot of precision in trying to make measurements of time.

Interviewer: Benjamin Thompson

Andrew and his colleagues have been working on their clocks for a few years now, trying to hone the mechanisms and get them to work as accurately as possible. This is what they’ve detailed in their new paper.

Interviewee: Andrew Ludlow

We showed that we were able to make big advances in the three most important figures of merit for atomic clocks, so these are called systematic uncertainty, instability and reproducibility. And basically, each one of these three are important details that determine how good these clocks are, how useful they are. At the end of the day, all of them together contribute to what you might consider as accuracy – how accurate is the clock. And so, we were able to show advances in each one of these areas, ultimately showing that these systems are capable of making measurements at the level of, you know, one part in 10⁻¹⁸

or even better than that.

Interviewer: Benjamin Thompson

So, when it comes to measurements, the team are working to 18 digits of precision. But what can these clocks be used for?

Interviewee: Andrew Ludlow

In the article, we highlighted especially one application that there’s been a fair bit of anticipation for these clocks being useful for, and this is what’s kind of been known as relativistic geodesy.

Interviewer: Benjamin Thompson

Relativistic geodesy is the idea that you can use a pair of atomic clocks to measure the gravitational strength of a particular location. As you move further away from the centre of the Earth, gravitational strength decreases. So, the strength of gravity on top of a mountain is less than it is at sea level, for example. And this change in the strength of gravity does some peculiar things to time, put forward by Einstein in his theory of general relativity. To put it very simply, as gravity strength decreases time moves faster, although the effects seen here on Earth are very subtle. However, if you were to have, say, a pair of super-accurate optical lattice clocks, you could in principle send one up a mountain and leave one at sea level. By measuring the tiny differences in the ticking rate of the clocks, you could get a super-accurate measurement of the gravitational strength at the different locations. As well as being able to tell you things like precisely how high a mountain is above sea level, Andrew thinks there could be other benefits to these new measurements.

Interviewee: Andrew Ludlow

In its first go, it will improve geodetic models that, you know, the models that say what the gravitational shape of the Earth is. And then ultimately, those models could have impact in a lot of different areas, including surveying, including water and ice flow – that could be related to climate studies. There’s really a handful of reasons why these models are used quite extensively now.

Interviewer: Benjamin Thompson

While this all might seem a little hypothetical at the moment, efforts to send optical clocks out into the field have been made. You might remember, for instance, us talking here on the podcast about a group who sent an optical lattice clock into the Alps here in Europe. Andrew’s clocks aren’t ready to go anywhere just yet. While that might prevent them being used to measure gravitational strength at different locations, he says there are plenty of things they can be used for in the meantime.

Interviewee: Andrew Ludlow

We’re using these clocks right now in the lab to try to look for new physics, to better understand our Universe. And the idea is pretty straightforward – if you have a device that’s able to measure some quantity up to 18 digits, it’s sensitive to very subtle effects. And basically, as these clocks keep getting better and better, they become more sensitive probes for exploring these new physics and kind of constraining the possibility of deviations from the existing physical theories that we have.

Host: Benjamin Thompson

That was Andrew Ludlow. You can read his paper over at nature.com/nature.

Host: Shamini Bundell

Later in the show, Flora Graham will be coming by to tell us about the latest lander to touch down on Mars – that’s coming up in the News Chat. Now though, it’s time for the Research Highlights, brought to you this week by Ali Jennings.

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Interviewer: Ali Jennings

If you’d been wandering the Siberian steppes late in the last Ice Age, you might have seen a unicorn. Though on closer inspection, you would have discovered that it was in fact a three-and-a-half-ton, long-horned, rhinoceros. It was originally thought that this so-called ‘Siberian unicorn’ died out about 200,000 years ago, but now a team from the Natural History Museum in London have updated that extinction estimate. They used radiocarbon dating on the remains of 23 specimens to show that the youngest had died only 36,000 years ago – around about the time that a certain Homo sapiens arrived in the area. But humans probably weren’t responsible for the unicorns’ disappearance – the authors note that the massive mammal was a specialised grazer, so its demise was more likely down to climatic shifts that affected its habitat. You can track that study down in Nature Ecology and Evolution.

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Interviewer: Ali Jennings

For people with spinal cord injuries, breathing problems are the leading cause of disability and death. This is because the nerves controlling breathing can be severed after injury. If treatment is applied soon after the trauma, the nerves can be helped to regrow. But restoring nerve function after long-term injury has so far proven elusive because scar tissue grows across the severed nerve endings, stopping their regeneration. Now, a study has worked out a way round this problem in rats with long-term spinal injury. They injected an enzyme into the rats’ spines that broke down the built-up scar tissue. The nerves started sprouting and over time the rats regained almost complete control of their breathing, control that still lasted six months after treatment. Find that reinvigorating research over at Nature Communications.

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Host: Shamini Bundell

Next up, Noah Baker has been getting to grips with wind wakes.

Interviewer: Noah Baker

As a wind turbine spins, huge rotors extract energy from the passing breeze.

Interviewee: Julie Lundquist

So, you can imagine a stick with a big circle spinning on top, and that circle is inscribed by the wind turbine blades. As the wind enters that disc, then energy from the wind or momentum from the wind is extracted.

Interviewer: Noah Baker

That’s Julie Lundquist from the University of Colorado, Boulder. The wind energy is converted into electricity and pumped to hungry, green-conscious customers, but that’s not the whole story. You see, the wind which drove the turbine doesn’t just stop when it hits the blades. It continues on, but with less energy and riddled with turbulence.

Interviewee: Julie Lundquist

And if you want to think about a metaphor for this, you can think about water in a stream flowing past a rock. The streamflow is faster upstream of the rock and when you look downstream from the rock, it tends to be slower and it also tends to be more turbulent, and the same thing happens in the wind turbine wake as well.

Interviewer: Noah Baker

This wake can have all kinds of impacts. The turbulent air can cause strain and damage to other turbines in a farm, and the reduced wind speed can lead to a reduction in energy extraction. As a result, the wakes of wind turbines have been widely studied to try to mitigate these impacts. Julie too is interested in wind wakes, but she’s been looking on a broader scale.

Interviewee: Julie Lundquist

So, when you have a lot of wind turbines built together into a wind farm, you can imagine that those individual wakes from those individual wind turbines all merge together into one larger, more complicated wind farm wake.

Interviewer: Noah Baker

Julie turned her attention to publically available data from two nearby wind farms in Texas, on the hunt for wind farm wake effects.

Interviewee: Julie Lundquist

First, we needed two wind farms that were in close proximity to each other so that they could interact, so we needed an upstream farm and a downstream farm. We highlighted a Texas one in our paper because Texas is the state in the United States that has the highest production of electricity from wind.

Interviewer: Noah Baker

Now, Julie is an atmospheric scientist in an interdisciplinary team. While she and one of her students were simulating the atmospheric dynamics of the wind farm wakes, others started looking at the numbers, specifically the cold, hard variety – cash.

Interviewee: Julie Lundquist

Dan Kaffine, the economist on the project, we were hoping that he would be able to find evidence of the economic impacts of the wakes. So, he did a very clever analysis that showed that the presence of the upwind wind farm had a discernible and statistically significant economic impact on the downwind wind farm.

Interviewer: Noah Baker

The losses are not insignificant. In fact, the economic model suggests that over four years, the downstream wind farm had lost revenue of about US$3.7 billion, with an error of about US$2.4 billion. In other words…

Interviewee: Julie Lundquist

Best case scenario that’s about a US$1.3 billion loss, and worst case scenario that’s about a US$6.1 billion loss. So, remember that this is a predictive model and these are estimates, but that is pretty significant.

Interviewer: Noah Baker

It’s perhaps unsurprising then, with numbers like this, that Julie and her team wanted to delve into the laws and regulations surrounding wind farm wakes. After all, businesses have bottom lines and they tend to like to protect them. But the team was surprised by just how few rights people have with respect to wind wakes. Here’s K. K. DuVivier from the University of Denver Sturm College of Law.

Interviewee: K. K. DuVivier

There is a real question whether you have any ownership right in the energy in the wind.

Interviewer: Noah Baker

K. K. thinks that wind has been overlooked by policymakers quite literally because it’s invisible. But wind is also a difficult thing to litigate. For example, is wind energy a public or private resource?

Interviewee: K. K. DuVivier

In some countries, the wind is definitely a public resource. In the United States, it’s not quite so clear. You know, when the wind is passing over your land, presumably you have the right to get whatever you can of what is passing over your land, but there is a question in the law about whether you own anything before it hits your boundary line. Some states like Texas which is our number one wind state, that’s a state where you can own the wind separately from the land surface and you, an individual, can own it, so it’s not really considered public property.

Interviewer: Noah Baker

Things get even trickier when you take into account the distances that Julia’s calculated wakes could have an impact over.

Interviewee: Julie Lundquist

So, you know we have worst case scenarios where these wakes can extend 50 kilometres downwind.

Interviewer: Noah Baker

Now, these worst-case scenario conditions only occur something like 4% of the time, but in those situations 50 kilometres could start taking you across state lines where there are different laws. And given that in the USA the vast majority of wind farms are within 40 kilometres of another wind farm, these small likelihoods could actually have very real impacts. K. K. and Julie both hope that some form of policy may come into play to help solve the problems of waking, but K. K. doesn’t think that the way to do that is through protecting individual farms’ rights. Instead, she took inspiration from another industry – wind’s dirty cousin – oil.

Interviewee: K. K. DuVivier

They created something called pooling or unitisation so that the spacing of the oil wells had to depend on what was best for getting the most resource out of a particular pool rather than what one property or another might benefit more from. So, I would recommend something like that for wind – to try to look at the whole area, figure out the best spacing to maximise the recovery from the whole area, and then, you know, have that be controlling rather than each one trying to maximise what their own farm could produce at the cost of another wind farm.

Interviewer: Noah Baker

For any such law to come into play though, much more work will need to be done on the impacts of wakes in different landscapes or with different turbine designs and atmospheric conditions. So for now, it’s unclear if wind wakes will make their way into law at any point soon. Julie, for one, was unwilling to make any predictions.

Interviewee: Julie Lundquist

I think you’re asking me to make a forecast and, you know, doing a weather forecast is hard enough as it is, I don’t think I would even attempt to make a political forecast.

Host: Shamini Bundell

That was Julie Lundquist from the University of Colorado, Boulder. Before her, you heard K. K. DuVuvier from the University of Denver. You can read K. K. and Julie’s paper in Nature Energy. Head over to nature.com/nenergy to get your mitts on it.

Interviewer: Benjamin Thompson

Listeners, it’s the final part of the show which is of course the News Chat. Joining me in the studio today is Flora Graham, Senior Editor of the Nature Briefing. Hi Flora.

Interviewee: Flora Graham

Hi, thanks for having me.

Interviewer: Benjamin Thompson

For our first story today Flora, let’s talk about, well, quite a late-breaking story actually and one that’s caused a lot of discussion across the world. What’s been going on?

Interviewee: Flora Graham

Yeah, well a researcher in China claims that two healthy babies, twin girls, have been born after being gene edited as a single-celled embryo. So, previously embryos have been edited but only for research – they’ve never, ever been implanted as a pregnancy and actually become healthy, living babies.

Interviewer: Benjamin Thompson

Lots of ethical chat to unpick there, but before we do, maybe, I mean this is obviously a big science story. Where has it been published?

Interviewee: Flora Graham

That’s the thing – it hasn’t been published anywhere. It hasn’t been peer-reviewed. This is one fellow’s claims. He’s put up a lot of YouTube videos about it but really this is just kind of promotional stuff, so we really don’t know if this genuinely happened or not.

Interviewer: Benjamin Thompson

Well Flora, what’s he claiming to have done?

Interviewee: Flora Graham

Well, this researcher, He Jiankui, says that just after this embryo was fertilised, he used CRISPR gene editing to change a gene that’s involved in HIV infection. It’s called the CCR5 gene and the idea is that he’s given these children a mutation that does occur naturally in some of the population which makes them less likely to be infected with HIV.

Interviewer: Benjamin Thompson

And what are scientists saying?

Interviewee: Flora Graham

Well, there are a lot of questions swirling around this. The first is, did this actually happen? So, even if the editing took place as this researcher describes, there’s a question about whether the mutation did take hold in the children’s DNA as they developed and after they were born and that now they have this uniform set of cells in their body. We do have some scientists who’ve looked at some documents that were associated with the research and they say that the data does seem to be consistent with the fact that the gene editing did actually take place.

Interviewer: Benjamin Thompson

But I guess we need independent review to make sure?

Interviewee: Flora Graham

Absolutely. We haven’t seen a paper, we haven’t seen the in-depth data. These are documents that were filed with the administrative bodies in China and were dug up by journalists, as far as I understand.

Interviewer: Benjamin Thompson

And what about the ethics of the thing then, right, because this does seem like it’s a heck of a topic.

Interviewee: Flora Graham

There’s a lot to unpick here. I think the first and foremost is gene editing an embryo. This is something that’s not even legal in a lot of countries, the reason being is because once you’ve edited a person’s genome, that can then be passed on to any potential offspring. So, they call that editing the germ line – this is something that so far, we absolutely have not agreed internationally about whether this is even a good idea. There absolutely is gene editing but it’s normally for people who are adults or they’re already older and this is not something that would then be passed on to their offspring.

Interviewer: Benjamin Thompson

Well, one of the ethical problems that seems to be going on here is that this isn’t a treatment; it’s a prevention.

Interviewee: Flora Graham

That’s right. A lot of ethicists and scientists are questioning why edit a child’s DNA for a preventive reason. I mean there’s no guarantee that these children are going to be at risk of HIV in future. There’s a lot stronger argument amongst people who are saying this could save the life of a child who’s at imminent risk of severe suffering or death from a genetic disorder, but in this case there’s no question that they wouldn’t have necessarily been born healthy without any gene editing. Now, what kind of adds interest to this story is in this case the girls’ father is HIV positive. Now, there’s no huge risk that they would become infected from the father. A lot of scientists told us there are safe and effective ways to prevent transmission between the parent and the child. It’s more that the family feels emotionally that they are very concerned that their kids could in future be infected with HIV, and subject to a lot of the intense discrimination, particularly in China, against people who are HIV positive.

Interviewer: Benjamin Thompson

So, what happens next then Flora? I mean where do we go from here?

Interviewee: Flora Graham

Well, the timing of this is interesting because we’re just about to open an international meeting in Hong Kong on genome editing, where scientists and ethicists are coming together to talk about this very type of thing. The fact that this researcher has chosen to announce his claim in such an unconventional way, just on the eve of this conference, I think raises almost more questions than it answers.

Interviewer: Benjamin Thompson

Yes, and listeners, this is sort of a developing story so head over to nature.com/news to see any more updates as and when they occur. But for now, let’s move over to our second story for today then Flora, and we’re going to take a visit to the red planet.

Interviewee: Flora Graham

Yes, we were all glued to or live feeds and Twitter streams on Monday when NASA’s InSight lander successfully made its touchdown on the red planet.

Interviewer: Benjamin Thompson

Well, another lander which is obviously very exciting. What’s this one going to do?

Interviewee: Flora Graham

Well, this one is going to be peering inside Mars. It’s going to be trying to detect tiny tremors – tiny ‘marsquakes’ we’re calling them – and hopefully those seismic readings will reveal what’s inside the core of the planet.

Interviewer: Benjamin Thompson

Wow. I mean why would we want to know necessarily what’s inside the core of the planet then?

Interviewee: Flora Graham

This is going to tell us about how planets in the solar system developed, how they came together and no doubt it’s good to reveal things that will help us understand Earth better as well.

Interviewer: Benjamin Thompson

Well as I understand it, the last lander that we sent to Mars didn’t necessarily go as planned. How did this one sort of successfully make it down to the planet’s surface?

Interviewee: Flora Graham

Rights, well landing on Mars is notoriously difficult. In this case, NASA chose a pretty well-understood landing method, so no sky cranes or giant bubbles or anything weird this time. The lander dropped off the spacecraft, it had a heat shield, it dropped through Mars’ very thin atmosphere, then the parachute deployed and slowed it down quite a bit, parachute let’s go, the lander then uses radar to detect the surface and kind of gently drops upon the Earth and a big cheer goes up all round.

Interviewer: Benjamin Thompson

Easy.

Interviewee: Flora Graham

Easy peasy.

Interviewer: Benjamin Thompson

One thing that I’ll say Flora, and you said there that you managed to be sort of be glued to the coverage as it was happening. How did you manage to do that? Often I think there’s kind of a delay and there’s kind of that window where everything goes dark and then it’s either success or failure.

Interviewee: Flora Graham

Normally one might have to wait until, let’s say, another orbiting spacecraft was able to observe the successful landing, or you have to wait until the lander itself has kind of deployed its communications equipment and is able to transmit back to Earth. But in this case, two tiny satellites called cubesats – these are the size of tiny shoe boxes – were actually launched at the same time as InSight. They made their own way to Mars and they actually observed the InSight landing, and they were able to communicate back to Earth almost in real time, and this is the first time that a cubesat has gone so far from the Earth.

Interviewer: Benjamin Thompson

So, a lander successfully on the ground now then. How exactly is it going to be looking for these marsquakes?

Interviewee: Flora Graham

Well, it’s got several instruments. Some extremely delicate that are being held inside vacuum chambers that have to be delicately placed onto the surface, using the landers kind of appendages. There’s an amazing instrument called the heat-flow probe which is actually going to burrow 5 metres down into the surface of Mars – far deeper than we ever dug during the moon missions – and that’s going to detect things even closer to the core.

Interviewer: Benjamin Thompson

Well final question then Flora, how long is this intrepid lander’s mission going to last?

Interviewee: Flora Graham

Well it’s planned to work for a little more than one Martian year, which is almost two Earth years. And the researchers say it should measure maybe up to a hundred marsquakes during that time.

Interviewer: Benjamin Thompson

Brilliant. Well, we’ll keep an eye out to see what it sends back. Listeners, don’t forget you can sign up for the Nature Briefing at nature.com/briefing, where you’ll get even more science news like this directly to your inbox.

Host: Shamini Bundell

And that’s it for this week’s show, but as always if you’d like to get in touch you can tweet us @NaturePodcast, or send us an email: podcast@nature.com. I’m Shamini Bundell.

Host: Benjamin Thompson

And I’m Benjamin Thompson. Thanks for listening.

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