Quantum compass might help birds 'see' magnetic fields

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week, homing in on migratory birds’ magnetic compass…

Host: Nick Petrić Howe

And would alien astronomers be able to see Earth? I’m Nick Petrić Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson.

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Host: Nick Petrić Howe

First up on the show this week, reporter Adam Levy has been focusing in on a magnetic mystery in sensory biology.

Interviewer: Adam Levy

For decades researchers have tried to answer a simple question with a remarkably complex answer: how do birds find their way?

Interviewee: Eric Warrant

We have eyes for vision, we have ears for hearing, but we have no organ that we know of that’s involved in magneto reception. It’s a complete mystery.

Interviewer: Adam Levy

This is Eric Warrant, a zoologist from Lund University in Sweden.

Interviewee: Eric Warrant

One of the greatest mysteries in sensory biology is how animals are able to sense the Earth’s magnetic field and to use it to navigate.

Interviewer: Adam Levy

And the solution to this magnetic mystery could come from a surprising discipline – quantum physics.

Interviewee: Henrik Mouritsen

I had never thought that I would get to a place where we would start understanding the quantum mechanical mechanism that goes on inside the bird.

Interviewer: Adam Levy

This is Henrik Mouritsen, a biologist from the University of Oldenburg in Germany. Researchers have shown that some animals can indeed sense Earth’s magnetic field, using it to find their way. But how birds like the European robin detect such a weak field has long baffled biologists.

Interviewee: Eric Warrant

It’s the last sense we effectively know nothing about, and the solution of this problem, I would say, is the greatest holy grail in sensory biology.

Interviewer: Adam Levy

And although the answer has remained elusive, researchers have put forward theories that they hope could explain this mystery.

Interviewee: Eric Warrant

Over 40 years ago, in the late 1970s, a physicist first dreamt up the idea that a light sensitive molecule might be used by animals to sense the Earth’s magnetic field.

Interviewer: Adam Levy

This theory proposed that animals could sense magnetic fields using a quantum mechanical phenomenon called radical pairs. Here’s Henrik again.

Interviewee: Henrik Mouritsen

The radical pair hypothesis suggests that you have a light-sensitive molecule, it absorbs light, and in the end you have a radical pair, so two unpaired electrons.

Interviewer: Adam Levy

The magnetic spins of this radical pair can take on two states, both pointing in the same direction, a triplet state, or both pointing in opposite directions, a singlet state. The pair swings between these two states but a magnetic field tips the balance, making one or the other more likely.

Interviewee: Eric Warrant

And it’s this change in the balance of the lifetimes of the triplet state relative to the singlet state which is thought to be the basis of magneto reception.

Interviewee: Henrik Mouritsen

And there was only one class of molecules that people knew originally could use light to generate radical pairs in plants, and they were called cryptochromes.

Interviewer: Adam Levy

Henrik and his collaborators hunted for these cryptochrome molecules in birds and since cryptochromes need light to work, they searched specifically in bird eyes.

Interviewee: Henrik Mouritsen

We found cryptochromes in the eyes of birds and so did others, and then the big challenge was now, well, how do you now test whether quantum-mechanics-electron-spin-based mechanisms goes on inside the eyes of a bird.

Interviewer: Adam Levy

This week in Nature, Henrik and collaborators from biology, physics and quantum chemistry have worked to shed light on the cryptochrome hypothesis.

Interviewee: Henrik Mouritsen

We needed to have the molecule in isolation to actually measure things on it because it’s very, very difficult to do it inside the eye of a bird. We started trying to make cryptochromes in 2004, and we now have 2021. Not only could we show that the electrons jump inside the molecule exactly as predicted by the quantum chemists in theory, we could also show that the photochemistry of that radical was actually magnetically sensitive. Now, it’s not a hypothesis that this molecule is magnetically sensitive. We can see that it’s magnetically sensitive.

Interviewer: Adam Levy

The team were also curious how cryptochrome proteins compared between birds that migrate and birds that don’t.

Interviewee: Henrik Mouritsen

We then also made cryptochromes from an extreme non-migratory bird, basically the chicken, and it looks like the cryptochrome 4 from the migratory birds are significantly more magnetically sensitive than the same molecule from a chicken.

Interviewer: Adam Levy

Previous work has shown that birds process magnetic field information in the visual region of their brains, so Henrik and team members believe birds may actually see the Earth’s magnetic field.

Interviewee: Henrik Mouritsen

So, maybe it’s kind of a shadow on top of whatever else you would be seeing as a bird, but what exactly the bird is seeing we do not know because we cannot ask the bird.

Interviewer: Adam Levy

But the puzzle isn’t solved. The team have shown that cryptochromes are indeed magnetically sensitive but they haven’t yet proven that these proteins are the critical ingredient in magneto sensing in birds. Here’s Eric again.

Interviewee: Eric Warrant

I think having studied quantum mechanics myself years and years ago, I would never had realised then or even believed, I think, that it would be possible that quantum mechanical effects could be actually housed within the eye of a bird. That is a huge surprise, and it’s nothing we learned in quantum mechanics when I was young. The advances that the authors have made in this study have brought us a long way to understanding how magneto reception works. There’s no question that the study was a spectacular piece of science. But because this is a laboratory study – it’s not a study which is being performed on living animals – it’s still no actual proof that cryptochromes in general are actually used in magneto sensing.

Interviewer: Adam Levy

Henrik, though, is optimistic that the team will one day overcome these challenges and finally reveal how some birds are able to sense the Earth’s magnetic field and find their way.

Interviewee: Henrik Mouritsen

We are of course going to try to get closer to the natural situation inside the eye of the bird. Things like that would have been utopia before you could actually make the molecule in isolation, and we are very excited about the possibilities there.

Host: Nick Petrić Howe

That was Henrik Mouritsen from the University of Oldenburg in Germany. You also heard from Eric Warrant from Lund University in Sweden. You can find a link to Henrik’s paper over in the show notes.

Host: Benjamin Thompson

Coming up in the podcast, if alien astronomers exist, would they be able to see the Earth? Right now, though, it’s time for the Research Highlights, read by Shamini Bundell.

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Host: Shamini Bundell

Certain amphibious fish benefit from splitting their time between water and air, as recent experiments show that fish dwelling in both become smarter than fish that only reside in water. A team of researchers placed mangrove killifish, a species that thrives both in and out of water, in small containers that exposed some of the fish to periodic drops in water levels. Other killifish were placed in a terrarium every few days and encouraged to jump about on land for three minutes, while control fish were left to swim about undisturbed. The trained jumpers and those exposed to air subsequently navigated a maze and found the meal at the end more quickly and in a shorter distance than the control fish. They also had more cell proliferation in an area of the brain linked to spatial learning. The authors say the work is a step towards showing how ancient fish evolved and adapted while making the move from water onto land. Think that research is a breath of fresh air? Find out more in the Proceedings of the Royal Society B.

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Host: Shamini Bundell

With the weather heating up here in the UK and elsewhere, many of us will be suffering from our usual seasonal allergies. But while constant sneezing can be unpleasant, you can console yourself with the news that a team of scientists have successfully pinpointed the precise population of neurons that are causing you to go ‘Achoo!’ Researchers made mice sneeze by getting them to inhale droplets of sneeze-inducing compounds. The team then screened the signalling molecules released by the sensory neurons of the nose and zeroed in on one that was essential for sneezing, called neuromedin B. The team found that certain neurons in a well-known sneeze-evoking region in the brain stem responded to neuromedin B and caused sneezing. But they also found some key neurons in another part of the brain, one which controls exhalation. When they injected neuromedin B into this new area, the mice started sneezing thus revealing the full nose-to-brain pathway. Sniff out that research in full in Cell.

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Interviewer: Benjamin Thompson

Back in 1992, Nature published a landmark astronomy paper which confirmed for the first time the existence of planets outside our Solar System orbiting another star. Since then, researchers have continued to point their telescopes to the skies and have discovered thousands more of these so-called exoplanets, often by looking for the faint dimming of a star as a planet passes in front of it, known as the transit method. This week in Nature, scientists have turned the telescope back on us and, using a wealth of data on the movement of stars in our galactic neighbourhood, ask who might be looking back at Earth. Could alien astronomers somewhere be recording the blink of the Sun as the Earth orbits? And if so, what might they be seeing? One of the authors is Lisa Kaltenegger from Cornell University in the US. I gave her a call to find out more and she told me about the dataset used to make the calculations.

Interviewee: Lisa Kaltenegger

So, there’s this amazing mission up right now. It’s called the Gaia mission from the European Space Agency, and there’s a slew of information that it provides us because it basically maps all the stars around us. And because it measures over a couple of years, it tells you where the stars are going and where they came from. And so that led to the idea where we were thinking, ‘Well, you know if this vantage point is special in fleeting to see the Earth as a transiting planet, so if we go with civilisations on the Earth, right, so roughly 5,000 years ago, civilisations started to bloom on our planet. Who could have seen us since then?’

Interviewer: Benjamin Thompson

Yeah, so you constrained the data then to which stars, if you’re looking from that vantage point, could see the Earth from 5,000 years ago to 5,000 years in the future.

Interviewee: Lisa Kaltenegger

Right, and the thing is it’s kind of, if you think about it, just geometry. So, you can see how big the Sun is and you know where the Earth is, and so if you go out from that view, you can figure out what part of the sky can see the Earth go in front of the stellar disc. And so, we found about 2,000 stars that can see, in this 10,000-year period, the planet go in front of the Sun.

Interviewer: Benjamin Thompson

In terms of all the stars in the sky that’s quite a small number.

Interviewee: Lisa Kaltenegger

Absolutely. It’s a tiny number in terms of how many stars are in the sky, of course. But what we wanted was we wanted the closest stars because light needs time to travel, so the closer those stars are that see us as a transiting planet, the more recently they see us. So, we concentrated on the closest stars, everything within about 100 parsecs, and that’s the astronomer scale and that translates into about 326 lightyears. And among these stars, there are 117 that are within 100 lightyears. And when we’re talking about how light needs time to travel and that about 100 years we actually started to transmit radio and so that is the reach where basically the radio waves have washed over again.

Interviewer: Benjamin Thompson

And of course, Lisa, you and I are talking about stars here, but for someone to have a look at us down their hypothetical telescope, they’ll need to be on a planet, and I’m imagining that the number of potential planets circling these 2,000 stars is a lot smaller. What were some of the stars’ hosting planets that stood out to you as having the highest chance of seeing the Earth moving in front of the Sun?

Interviewee: Lisa Kaltenegger

In our sample of 2,000 stars, we know of 7 known exoplanet hosts. But of course, now that we have actually described this sample, there’s a lot more people looking specifically at this sample, trying to find planets around them. But even among the ones that we know, there are a couple of systems that have planets in the so-called habitable or ‘Goldilocks’ zone, and there it’s fascinating to me that one of them, for example, Ross128 b, is only about 12 lightyears away from us, so kind of nothing in cosmological scales. That one saw us starting 3,000 years ago transiting the Sun, and 900 years ago it lost this vantage point,and so it saw us for more than 1,000 years. And we have a couple of other stars, Teegarden’s Star that’s also about 12 lightyears away. That will start to see us pretty soon but it hasn’t seen us yet. And Trappist-1, this really, really famous system with 7 Earth-sized planets at about 40 lightyears away. That one has 4 planets within this ‘Goldilocks’ habitable zone and that one will start to see us in about 1,000 years. And so, I just think it’s pretty exciting to think about we see some of those planets that are in the right distance in the Goldilocks zone but they don’t see us yet, and some other ones have seen us already. So, it’s not this whole thing that I see you and you see me. There’s actually a really interesting interplay that brings the dynamic of the cosmos into this question.

Interviewer: Benjamin Thompson

So, one of the ways you talked about to look for signs of potential life is to look in the atmosphere of exoplanets and look for disequilibrium between levels of gas, for instance, which could be caused by life of some sort. And so, in this kind of window, this sort of 5,000 years in the past to 5,000 years in the future, it really seems to me that most of what humanity has done, let’s be honest, in terms of changing the atmosphere, really began with the Industrial Revolution. But otherwise really you’re looking millions of years ago, the sort of Cambrian explosion when oxygen flooded the atmosphere, so could it be that if you weren’t looking all that time ago, there’s really a really narrow band where you could see these kind of changes to see that intelligent life at the very least is about?

Interviewee: Lisa Kaltenegger

Well, that was one of the really interesting questions that we also got to. So, we know that you could find life 2 billion years ago because of the oxygen, the Cambrian explosion, and all of these things, but what about the Anthropocene, right? What about when we started to change the climate? And there’s an argument when you have enough chemicals that are man-made or technology-made to actually be able to spot it and I agree in 100 years maybe. One question, once you think about these timescales, was what is it that you could see about an intelligent civilisation? And then the next question that comes onto this, a technological civilisation that changes the atmosphere. Once it becomes hopefully a little bit more intelligent, it actually will stop doing that, right. It will actually start to safeguard the planet and not keep in a snowballing effect changing the atmosphere until this planet is not habitable anymore. But for this paper we were very conservative. We basically said we are looking about how these stars move, who could see us and it is really a question of how big their telescopes would be if alien astronomers were looking. And so, we restricted ourselves to the technology we have right now because you can envision anything, right? You could say they had like this huge, I don’t know, 1,000-metre telescope in space and they can see a flamingo dancing on the Earth. Then it’s not a problem to find intelligent life, right? But if you have telescopes like we have, who could find us and who could thus also see signs of life in the atmosphere?

Interviewer: Benjamin Thompson

So, it seems to me that in your work, you’ve looked at this kind of Venn diagram of stars that could see us, that are likely to have exoplanets, maybe that aren’t too far away so they could detect radio waves but also see the effects that humans are having on the atmosphere, or maybe in the future that are done to kind of mitigate the effects of what humans have done to the atmosphere. It does appear that this is, to an extent, a thought experiment. Given that there is this aspect, is there anything else that you can gain from this work or is it really more of a cataloguing exercise right now?

Interviewee: Lisa Kaltenegger

Well, the idea here was to give you the best targets because if you’re saying another civilisation evolved, and hopefully they get further than we do, which are the stars that would actually watch and so where should we focus our attention? And a lot of those stars that we identified have actually not been scrutinised at all because they’re in a part of the galaxy where there’s a lot of other stars behind them, and so it’s very, very hard to see small planets. But with this, it basically gives you a motivation for why you should put in the extra work, and that’s basically where this thought experiment gets into the practice of what we’re doing right now as astronomers all over the world, where we are looking for planets and where we hope somebody might be looking at us.

Interviewer: Benjamin Thompson

That was Lisa Kaltenegger from Cornell University. Head over to the show notes where you can find a link to her paper.

Host: Nick Petrić Howe

Now, it’s time for the Briefing chat, where we talk about a couple of our favourite stories from the Nature Briefing. Ben, seeing as you’ve already brought the topic up, I’ll go first this week as I have another space story, although this one’s from a little bit closer to home. It’s coming to us from Mars.

Host: Benjamin Thompson

Well, Nick, there’s a lot of science going on on Mars and I guess around Mars as well. We took a look at It a few weeks back. What’s the latest?

Host: Nick Petrić Howe

So, this story is from Nature and it’s focusing on NASA’s mission to Mars, which is Perseverance the rover and Ingenuity the helicopter, and it’s actually all about Ingenuity. So, I don’t know if you’ve seen, you might have done, there have been a lot of pictures and videos floating around of the helicopter flying around the Martian surface, and one thing that a lot of people picked up on is there’s a heck of a lot of dust.

Host: Benjamin Thompson

Right, well this is a step forward, right, because last time we checked in on this helicopter it was just going up and then coming down again, right? So, now it’s zooming around, you say. Why is the presence of dust exciting? I mean, I can’t say I’m surprised that there is dust on Mars and a helicopter kicking it up.

Host: Nick Petrić Howe

No, I mean it’s not surprising at all that there’s dust on Mars and, in fact, they’ve built the helicopter to deal with that, to deal with a lot of dust being on Mars. But normally, and if you think of helicopters on Earth landing in the desert for instance, you get a lot of dust kicked up when the helicopter is landing and taking off. However, what seems to be happening is the helicopter is picking up the dust and is travelling along the Martian surface with it as it flies about.

Host: Benjamin Thompson

Right, so it’s kind of enveloped then in this sort of sphere of particulate matter.

Host: Nick Petrić Howe

Yeah, exactly, and the reason that’s really interesting is it might give us an indication of how these dust whirlwinds – they’re known as ‘dust devils’, I think, in the US – form on the Martian surface because that’s long been a mystery because the thing with Mars is it’s atmosphere is really, really thin, so it’s hard to work out how the wind gets enough energy to actually pick up a lot of dust. But if the helicopter itself is able to do it, we might be able to work out how that’s happening on the Martian surface.

Host: Benjamin Thompson

Right, and to what end then, Nick? Other than being able to know a bit more of the weather on Mars, what are researchers sort of saying about all of this?

Host: Nick Petrić Howe

Well, the thing about Ingenuity is it was only really a proof of principle. It was only there to say, ‘Hey’, we can fly on other planets,’ so anything else is sort of like, one of the researchers interviewed in the article described it as the icing on the cake. So, it’s just really interesting to get any extra science that we can from this mission, and this can be a nice little insight into how the atmosphere works on Mars and how like these dust storms form and things like that. But it’s really just like a nice little bonus that we didn’t expect to get from this mission.

Host: Benjamin Thompson

Awesome, well, it’s a very intrepid little helicopter then. What’s next for this mission because we’ve proved it can fly, we’ve proved it can zoom about, and it’s giving some information on the meteorology of the planet. What’s next for the little helicopter that could?

Host: Nick Petrić Howe

Well, for the helicopter itself, it’s just going to be doing a flight every couple of weeks and we may get other unexpected things like this. Perseverance itself – the rover – will be going around the Jezero crater looking for signs of life, and that’s the sort of main aim of the mission but because of the success of this helicopter it could be that we see more flying vehicles going to Mars because it is actually a really good way to get around. You don’t get your wheels stuck. So, that might be something we see more of in the future. But, Ben, what have you found for this week’s Briefing chat?

Host: Benjamin Thompson

Well, Nick, it was only a matter of time before we had to talk about this subject. The strange world of non-fungible tokens (NFTs) has kind of collided with the world of science, and this is a story that was reported in Nature.

Host: Nick Petrić Howe

Well, Ben, we might have to go a few steps back just for my benefit because my knowledge of what these NFTs are is only about as deep as the memes I’ve seen of them, so beyond that I don’t know a lot about them so can you tell me to start with what an NFT is?

Host: Benjamin Thompson

Well, I will do my very, very best. So, the concept of NFTs was born in the early 2010s but it’s become super popular this year. And what it is, it’s kind of a digital certificate of ownership, and let me try and run you through an example. Imagine I drew a picture on my computer of a house. I’m quite old school so I’d probably use MS Paint. So, I draw a nice picture of a house, right. I stick it online. You can look at it. Anyone else can look at it as well. But what I can do is use some sort of clever computer software to generate an NFT, generate this token, which says that I own the original version of that picture, and this ownership token gets put on the blockchain on this kind of database of ownership and now anyone can see this image, they can copy it, they can share it with their friends, but there is a record of me owning it and I can sell this ownership to you or to anybody else, much like I might sell a piece of traditional kind of painted artwork.

Host: Nick Petrić Howe

Okay, that makes sense. So, how is this NFT thing coming into the world of science?

Host: Benjamin Thompson

So, so far, this has been used to sell JPEGs, songs, animated GIFS, but yes, the sort of world of science is now getting involved too and there are a few examples of what has been going on. One is that very, very soon, Tim Berners-Lee, who as we know invented the World Wide Web, is auctioning an NFT featuring the source code of the original web browser, along with a silent video of the code being typed out. And of course, we can watch that video but the token which proves ownership is being sold. But there’s other examples too. So, at the University of California, Berkeley, what they’re doing is they’re trying to use NFTs to raise funds for the university, and one of the ways they’re doing that is they auctioned off an NFT based on the documents related to the work of Nobel-prizewinning cancer researcher James Allison. And what happened here is a team of designers from the university scanned legal papers filed with the university along with some handwritten notes and some faxes and all the rest of it, and they used that to make an artwork called the The Fourth Pillar. And again, that’s available for anyone to see online, but they minted this token of ownership and they auctioned it and it raised about US$54,000, and that’s due to be split between the NFT auction site, a Berkeley research fun and carbon offsetting.

Host: Nick Petrić Howe

Right, okay, so this is a way for maybe research and universities to make a bit more money, to raise money for research, that sort of thing?

Host: Benjamin Thompson

Yes, that’s right, Nick. That’s one possible way that this could be used. There’s also talk of using these auctions to showcase science to the public, and maybe as a way, when it comes to genomics, to allow people to maybe profit when a pharma company buys access to their genomic data to use in one of their studies, for instance.

Host: Nick Petrić Howe

Well, that all sounds quite positive, right?

Host: Benjamin Thompson

Well, this is such an interesting one because this technology, as I say, really exploded onto the scene this year. The figures have been astonishing – US$325 billion in sales in May. I think the question is really is this a bubble? There’s a lot of talk about the amount of energy it requires to make these tokens and to write the blockchain. Of course, when we’re thinking about climate change and what have you, is that really the best way to be going ahead. And in our article, there’s one person who’s saying maybe in terms of the papers, it would be better to just auction the papers off rather than a JPEG image of the papers that then has to go through all this kind of scenario. And when it comes to genomes as well, certainly an interesting idea but there are a lot of ethical issues of course about ownership of genomes and how much a family member’s is similar to yours so how much of it can you sell, and all the rest of it. So, whether this is a flash in the pan that kind of disappears next week or whether this is something that is around for a long time, it’s really, really hard to know at the moment.

Host: Nick Petrić Howe

Well, speaking of disappearing, Ben, I think that’s all we’ve got time for on the Briefing this week, but thanks so much for chatting to me. And for everyone listening, if you’re interested in more stories like this but instead as an email then make sure you check out the Nature Briefing. We’ll put a link in the show notes where you can sign up.

Host: Benjamin Thompson

And that’s all for this week’s podcast. If you’ve got time to leave us a review, that would be amazing and would really help other people find the show. And of course, you can drop us a line anytime on email – podcast@nature.com – or on Twitter – we’re @NaturePodcast. I’m Benjamin Thompson.

Host: Nick Petrić Howe

And I’m Nick Petrić Howe. Thanks for listening.