Q&A: Diets, Duct Tape & Dark Matter

Does counting calories really work? Could the universe ever implode? And what makes duct tape so sticky?
29 June 2021
Presented by Chris Smith
Production by Phil Sansom.


A woman licking ice cream


It’s Q and A time! Does counting calories really work? Could the universe ever implode? And what makes duct tape so sticky? We’ve assembled a panel of powerful people to answer your science questions: brain and bodyweight expert Giles Yeo, astrophysicist Katie Mack, chemist Kate Biberdorf, and microbiome geneticist Rob Finn...

In this episode

A tray with a burger, chips, and soft drink.

04:36 - What's the problem with counting calories?

Brain and bodyweight expert Giles Yeo says 'calories don't count'. Why?

What's the problem with counting calories?

Brain and bodyweight expert Giles Yeo says 'calories don't count'. Chris Smith asked why...

Giles - What is a calorie? A calorie is a unit of energy. It's the amount of energy it takes to raise a litre of water one degree Celsius at sea level - a food calorie at any rate. So they are all equal once they're in you as a little poof of energy. But because calories are tied up in food, and we actually eat food, not the calories themselves, our body has to work at different rates - put in different amounts of energy - in order to extract the calorie from the food. So it does matter whether or not a calorie comes from a carrot, a doughnut, or a steak.

Chris - The amount of chewing that you expend to extract the calories?

Giles - No, the energy it takes to metabolise the individual units of food. So glucose, fatty acids, amino acids, the energy it takes to actually pull apart the chemical components, is not taken into account on the side of the packets that you actually see everywhere.

Chris - And not just the half pint of cream or custard that inevitably goes with the stewed fruit or whatever.

Giles - Okay, clearly if you have 200 calories of cream it's going to be twice the amount of a hundred calories of cream. But a hundred calories of fruit is very, very different from a hundred calories of cream. So they count - calories count clearly if you're looking within a food type of scenario, butthere are gazillion diets out there which say, "you can only have 400 calories," for example, "for lunch." Well then it's going to make a difference whether or not you're eating 400 calories of sugar or 400 calories of celery. I think we worship the calorie today. I mean, this is the point. We count it, we intermittently eat it, we bat people over the head with them. Whereas I think we need to have a more nuanced view. I'll give you one example: for every 100 calories of protein that we actually eat, 30 calories - 30% - is used up in terms of just trying to take apart the protein and turn it into energy. It doesn't matter whether or not this protein comes from a bean or from a steak. So all protein calories on the side of every single pack are 30% wrong. And I just feel that people need to know that.

Chris - Is there therefore an ideal diet that we should subscribe to that actually takes this into account, or are people just wrong when they just look at calories? They take calories at face value, and basically these diets are prescriptively wrong?

Giles - The explanations for a lot of diets out there are probably wrong, but actually diets do work. The fad diets that we actually see everywhere - most of them actually work for some people at least some of the time, and any diet that results in a reduction in food intake is a diet that works. And so if we look at this caloric availability thing, my purpose is not for people to count calories more accurately, although that is obviously part of the thing. But the two elements of food that influence caloric availability to most are protein and fibre. And if you actually think about the protein content of your food and the fibre content of your food, it is actually quite a good marker of the quality of your diet. And so I guess the message is for people to not be counting the calories per se, but to think about the quality of the diet that you're actually eating. And I think that's far more important than the number, that calorie - which is very easy to see, the number is right there - than just to actually count the calorie in its absolute form.

Rob - So quality often gets conflated with cost, and that's always a balancing act when people are trying to make choices over the food they eat. Do you think it's possible to get a high quality diet for a low cost?

Giles - Yes, it is. Now I think there is a nuance here: clearly we can have a diet of lentils, we can eat dhal, and that is very cheap, but yet very, very nutritious. So undoubtedly you can do that. The problem there is it takes time to make a dhal, and you need to know how to make a dhal. So I think it is cheap, and this is the argument that a lot of middle class people - and we are all by definition here middle class people - just often throw about. "Look, people just need to learn how to cook better and they'll get good food," but you gotta have the time, you got to have sometimes to have the money, but you certainly have to have the knowledge in order to do that.

Oranges and lemons

09:17 - Why does citrus juice cause sunburn?

Lemon and lime juice - if it gets on your skin - can cause a nasty rash in the sun. Why?

Why does citrus juice cause sunburn?

Charlotte had this burning question for chemist Kate Biberdorf...

Katie - It does. It really does. And it can be horrible. One of my very good girlfriends - she just got married a couple of weeks ago - had a horrible, horrible burn because of this. So she made us margaritas like from scratch, she was squeezing out limes, and then we went out to the pool and we hung out all day, and she ended up getting wicked burns on the palms of her hands that lasted for over a month. And it was horrible. It's a rash that forms, and the rash is called phytophotodermatitis. And essentially what is happening from a chemical perspective is there's a molecule that exists in your lemons, your limes, and your citrus, and it is photosensitive. And so when it receives sunlight or any high energy radiation from the sun - so UV visible light, IR, but usually it's that UV light - when the UV light comes in and hits your hands and hits those molecules, it goes through a chemical reaction and it causes this irritation on your skin, and you can get this horrible rash. So it's called lime disease, but not like Lyme disease, not the bacteria infection. Quite different. So it's like the joking 'lime disease' and it's a horrible rash. So be very careful. So if you're using lemons, limes, eating oranges outside, especially if you live where I live in Texas where we get a lot of heat from that sun, wash your hands and just be careful.

Chris - Is this chemistry similar to the reason that people put lemon juice on their hair to lighten it in the sunshine? Is it capturing energy and sunlight and basically feeding it into things that then make chemical reactions happen? And if it's on your skin, it hurts your hands. If it's on your hair, it makes it bleach.

Katie - Yep. Exactly. It's the exact same type of science. You could think about it in the opposite way for sunscreen. So sunscreen has certain molecules that absorb the sunlight, absorb that UV radiation, and then they break down protecting your skin from the sun. And unfortunately this is the kind of the opposite reaction. So the lemon juice like reacts with the sun and then your skin has a horrible reaction to the molecule that forms.

Chris - So wash your hands, but do enjoy your margarita. I thought you were going to say the error was that she put lemon in your margarita. Because I mean, that's just a cardinal sin, isn't it, doing that?

Katie - It would be a cardinal sin. I'm glad we agree on that. But she did use limes, but it's the same molecule that exists in all these citrus. So you've gotta be really careful with that, but just a little soap and water, 15 seconds, 20 seconds wash your hands. Just like we've been doing all year constantly. So it's no big deal.

this is a picture of some micro-organisms down a microscope

12:01 - Where did our microbiome come from?

The trillions of organisms in the human body - how do they get there?

Where did our microbiome come from?

Chris Smith asked geneticist Rob Finn about the origins of our microbiome - the trillions of bugs that live on or inside us...

Rob - They start coming into us as we're being born. So it's been shown that as you are born... those children who are born by caesarean section, they don't actually have as mature microbiome as when they come out. So that's a very important part. And then throughout your life, so as you start feeding, breast milk is not pasteurised like our milk is typically pasteurised, so there are bacteria in that milk. You start ingesting those bacteria. And then as you start moving onto solid food, you start getting a more interesting microbiome. It becomes more diverse just because you're exposed to more things. So really it's about the food we eat, and the environment we're in, and also what happens to your microbiome over time. You don't get it and then it stays fixed. Obviously as you may have courses of antibiotics that may change your microbiome. So it's really one of those things that - you are what you eat and what you get exposed to.

Chris - It's often said that you never dine alone, even if you are on your own, because there are 37 trillion bacteria there to help you digest your dinner.

Rob - Indeed. I mean, they're really, really important in that digestion process. And they are really important to your health. And this is why there are lots of people now advocating that looking after your microbiome is a really important part to human health.

Katie - Is it... so children get their microbiome from putting everything in their mouths and playing in the mud and all of that. Is that something that should be encouraged for little kids to go out and just like devour everything, or is it like... what are the limits to that?

Chris - Social services, you can get in touch with Katie Mack via her email address or find her on Twitter.

Katie - Because I've heard about this, and I'm just wondering what the actual advice is.

Rob - Yeah. You get exposed to so much. So if you take someone who has a very clean sort of lifestyle and never puts anything in, or everything's really disinfected, you're going to get sources from all different shapes and form. There is this notion of, you know, having pets, and - that slightly less sanitised environment is actually better for you. There is a little bit of evidence of it, but there are so many factors to the construction of your microbiome that actually I wouldn't advocate going and licking sort of toys that have fallen on the floor to try and improve your microbiome. You're far better off having a varied and balanced diet.

Artist's impression of the Big Bang.

16:25 - Could the universe contract to a single point?

Given the Big Bang was such a quick expansion, are we at risk of equally quickly shrinking down again?

Could the universe contract to a single point?

Theoretical astrophysicist Katie Mack answered this question from listener Matt...

Katie - We're pretty sure the universe is not going to contract again. So right now we see the expansion of the universe, and the way we see the expansion of the universe is that we see distant galaxies moving farther apart from each other and moving farther away from us. And so we see that there's this uniform expansion throughout space. And for a long time, it wasn't clear if that would continue forever, or if the expansion would slow down and stop and turn around because the expansion was set off by the Big Bang, but all of the gravity of everything in the universe is kind of pulling together and that sort of slows down the expansion and puts on the brakes. And so for a long time, we were measuring the expansion and trying to figure out how quickly it was slowing down to see if it was slowing down enough that it would stop and re-collapse, or if it would just keep going but never actually stop.

When those measurements were completed in the 1990s, it turned out that it was not slowing down. It was actually speeding up, and the expansion of the universe is now accelerating. And we don't know exactly why that is. We attribute it to something we call dark energy. We don't know what dark energy is, but it looks like it might just be a property of space - that there's a certain expansion, a sort of stretchiness in space itself, that kind of pushes out. And if that's the case, if that's what the expansion is accelerating because of, then it's not going to stop and turn around, it's going to keep going and keep expanding forever and the universe will just get bigger and bigger and bigger and emptier and emptier and emptier as the space expands. But there's some caveats there. One of them is that we don't know what the dark energy is. So we don't know for sure that it is just a property of space that'll stay the same forever. It might be something that changes over time. It might be something that will get more powerful. It might turn around and make the universe start coming together again, we don't really know.

A glass of Coke and ice

Which are worse: sugary drinks or diet drinks?

Geneticist and metabolism expert Giles Yeo answered this question...

Giles - This is a hot topic at the moment. Let's just deal with one thing out there: a lot of people says that artificial sweeteners, aspartame, Stevia, whatever... I mean, the first thing is, look, people say that sweeteners are cancerous. They're cancerous if you stuff a hundred litres of a diet soda into a rat. That will cause cancer. But at the levels and at the doses that we actually drink it, like in a can or anything like that, they're not cancerous. So that's the first thing that we need to worry about. But - and this is the big but here - is there something that happens where if you drink a lot of diet soda, you actually break the communication between the taste and the amount of nutrients you're actually absorbing? Because ultimately we evolve through that, "ooh, a red berry, ooh, sweets, ooh, calories". So this is sort of the way that our primitive brains work.

But if you actually break that connection - so in other words you have the sweet flavour, you drink it, but you then absorb no nutrients - the longer term question is, will it then end up affecting your metabolism in some way, subtly or otherwise, and therefore increasing your risk of disease? Now here, I can't give you a straight answer. I think there is some evidence that this does happen, that this disconnect between the flavour and the number of calories, the amount of nutrients you actually eventually absorb, will actually begin in the long term to start messing about with your metabolism. Now, does this mean it is worse for you than the full sugar drink? Now it depends who you are. Now clearly, if you're a diabetic, then it is not. Please don't be drinking the full sugar drink. If you suffer from obesity, maybe you should be considering not drinking the full sugar drink. And if you are a kid, a child, particularly a young child, I think we need to have children drinking less sugar. So my answer is going to be that a diet beverage, a diet drink is going to be not as bad as a full sugar drink for many people, but ultimately water is the best drink that you can drink.

Duct tape

21:16 - How is duct tape so sticky?

What force makes duct tape stick to a wall?

How is duct tape so sticky?

Chemist Kate Biberdorf got stuck into this question...

Kate - It's very sticky. And it's funny - I get this question from kids all the time. I don't know why kids want to know about this, but I am ready to answer this question. So for duct tape, it's really neat. There's basically two main forces that happen here. There's a bunch of molecules in your tape, and some have really strong adhesive forces and some have strong cohesive forces. So 'adhesive' is when something sticks to something else. And so that's the part of the tape that's going to allow your tape itself to tape to the poster, and then also tape to the wall. The problem is you need the tape to stick to itself as well. So that's the really cool part. That's the cohesive part. So when a molecule wants to touch just itself, stick to itself and not touch the wall at all, that's going to be its cohesive properties.

Chris - What actually chemically is going on to give the molecules on the white side of the tape - that are really sticky and pull your fingers apart when you try and get them off your fingers - that actually gives them those adhesive properties?

Kate - So they're called intermolecular forces. And so it's when a molecule wants to interact with another molecule physically, without going through a full-on chemical change. We're not having an explosion here or a bomb, we're just having some intermolecular forces. So usually it's like the negative side of one molecule is attracted to the positive side of another molecule. It's kind of like when you see a hot guy across the bar and you're like, "ooh, I like that". And you just walk across the bar, you just go straight over there. It's the same type of attraction. And so you're coming together, maybe you hold hands for a moment. Maybe you kiss for a second, but you don't chemically change, your body doesn't change. So you're just going to hang out and you're holding hands. But then when you want to rip your poster off the wall, you can just let go of their hand and move on and go on with your day.

Chris - You're a fast mover, Kate. They do things differently in Texas. We know we're a bit more sedate here in the UK. We take our time with these dates.

Kate - We go after what we want here.

Chris - But you can probably anticipate where I'm going to go with the next question, which is, you mentioned about cohesive forces. You said the lovely line, "you need the glue to stick to the tape too". This is going to cue the question in everyone's mind; if Teflon is so slippery, how do they make a stick to the pan?

Kate - It has different sides of the molecule itself. So you can't always assume that one molecule is a hundred percent uniform. Just like we have like long hair on our head, we don't necessarily have hair on our feet. Molecules are kind of a little bit different as they're asymmetrical. So maybe like the head of the molecule is able to attract to the pan itself and that's what's clinging onto the pan for dear life. And then the feet itself gives like the repellent property. And so that's what allows for your food not to actually stick on the pan because it's repelled from that side of the molecule. So it's all about asymmetry.

A young boy kicking a football.

26:13 - Quiz: eye science & football physics!

Test yourself against our scientific head-scratchers! How do you compare to our panel?

Quiz: eye science & football physics!
Katie Mack, North Carolina State University; Rob Finn, European Bioinformatics Institute; Kate Biberdorf, University of Texas; Giles Yeo, University of Cambridge

It's time for a science quiz - and Chris Smith had a range of head-scratchers for the panel, pairing astrophysicist Katie Mack with microbiome geneticist Rob Finn, and chemist Kate Biberdorf with neuroscientist Giles Yeo...

Chris - The first round is called The Eyes Have It. And Question 1, for Katie and Rob: the cornea is the only part of the body with no blood supply - it gets its oxygen directly from the air. Science fact, or science fiction? What do you think?

Rob - Katie, I think that it doesn't have a blood supply, but I don't think it gets its oxygen entirely from the air, because I believe that you have a very thin membrane over the surface of the eye. I'm not sure you think.

Katie - Yeah, I mean it's not fully exposed to the open air I don't think. Are there not other parts of the body that don't have a blood supply? Do teeth... I guess the outer parts of teeth don't, but the inner parts do, I guess? But I'm surprised if it's really the only piece.

Rob - So I guess the only part to add in is: where is the oxygen coming from if it's not coming directly from the air? And I guess the fluid of the eye would be the obvious place where the diffusion of the oxygen comes. But...

Katie - Yeah, that sounds... I mean, there's that vitreous humour, the stuff inside the eye. Maybe that's oxygen dissolved into it, I don't know.

Chris - You're going to have to reach a consensus now. Science fact or science fiction - the cornea is the only part of the body with no blood supply?

Katie - The blood supply part is not the... there were two parts of this, right? No blood supply, and getting oxygen from the air.

Chris - Yeah, in other words, it gets its oxygen directly from the air. True or false?

Katie - I would go with false here, but I don't know.

Rob - Yeah, I think I would go there. False.

Chris - No, I'm really sorry. This is actually true. The cornea is avascular - it has no blood vessels normally, in health, in humans. Some animals do have vascularised corneas, and in humans who wear contact lenses, especially not very gas-permeable, thick, non rigid contact lenses, you can get a phenomenon called neovascularisation when blood vessels will grow into the cornea to deliver oxygen, because the cornea is dependent on close contact between itself and the air that hits the front of the eye to extract oxygen, to nourish that very thin layer of avascular tissue. Let's try Question 2 for Team 2! Kate and Giles: the fear of eyes - and this is eyes, as in things we've just been talking about, not letter I's as opposed like P's and Q's and so on - this is known as Callum phobia. The fear of eyes is known as kalampokiphobia. Is that true or false?

Kate - I'm going with false, but I'm basing that on nothing. That just sounds like a false word.

Giles - ...and I have nothing intelligent to say about this. Kalam... the only bit of this is the suffix of phobia, which I understand. Kalam... say it again? Kalam-hooby-hooby-whatsit?

Chris - Kalampokiphobia.

Giles - There is nothing to confer. I'll go with Kate's gut feeling.

Kate - They made up a fun word to try to trick us into it. That's what I'm going with.

Giles - Yeah, okay. I'll go with Kate. False.

Chris - It is indeed false. It's actually ommetaphobia which is the fear of eyes. But you pair, who accused us of fabrication - very unfairly - kalampokiphobia is actually fear of corn. Although why you'd be frightened of that I don't know. Perhaps if you have some kind of pathological allergy or something. But one point so far to Team 2. Right, Round 2: Bigger is Better. So Katie and Rob, Team 1. Which is the longest: the Nile - that's the river - the Great Wall of China, or the Andes range of mountains. Which is the longest.

Katie - I have no idea.

Rob - They're all very long. I'll give you that.

Chris - Rob's frantically googling...

Rob - I'll put my hands up...

Chris - I'll just list them again. Nile River, Great Wall of China, or the Andes mountain range. The longest please.

Rob - Great Wall of China because you can see it from space.

Katie - You can see anything from space if you have a good enough telescope! I would flip a three-sided coin and get Andes, I guess, but I don't know.

Rob - Let's go Andes then.

Chris - Going for the Andes, and... hooray! Yes, it is the Andes. The Andes range of mountains, 7,600 kilometres in length; the Nile, a close 6,700; the Great Wall of China, only 3,400 kilometres. So still pretty big all the same. Right, level pegging. Let's see if you can retain your lead, Kate and Giles, with your Question 2 of Round 2. What is bigger: lion, tiger, or jaguar?

Giles - Okay. I'm going to put the jaguar out of this, only because I watch too many David Attenborough films. And I'm going to go with tiger. I know 'the king of the beasts', 'Lion King', yada yada, yada... I just get the feeling it's going to be a Bengal tiger.

Kate - I think so, right? Have you seen the video of the tigers where they run through the field and then jump up over the big trucks? I mean, those tigers are huge. I've never seen a lion do that. I'm going with the tiger.

Giles - Yeah, we're going to go with a tiger.

Chris - So Kate's basing this firmly on Discovery Channel, and Giles is ruling out David Attenborough. So on that basis, you're going with tiger, and you get... yep, you're absolutely right, a point for that one. The tiger is the correct answer - the Siberian and Amur tiger. The males have a total length of 2.7-3.3 metres from nose to tail tip. They also weigh up to 306 kilos, almost a third of a tonne, one single cat. So 2 points to Team 2, 1 point to Team 1, and we're into Round 3. This could be the decider. And it is the football at the moment, so we've got to have a round on the Euros, haven't we? I hope you're all sports fans. No, I'm only kidding - it is a round on the Euros, but we're not going to be basing it just on football. Katie and Rob: the impact force of heading a football - and for you Katie, just to translate, that means soccer - that is enough to break a femur. True or false?

Rob - Having mis-headed several balls in my time, I would say it probably is true, because it doesn't half hurt.

Katie - I mean, it takes a lot to break a femur. But also the forehead part of your skull, if you get the angle right, is quite strong. So I guess the question is: if you headed a femur, which would break first...

Chris - You can tell she's a physicist, can't you! So what do you reckon then? We're going to have to press you for an answer. Heading the football: break a femur or not?

Katie - Rob, I'll go with your instinct here.

Rob - True.

Chris - I'm afraid... this is false. If you consider a professional regulation size 5 soccer ball with an inflation pressure of 16 pounds per square inch...

Rob - Ahh, you didn't give me those details!

Chris - ...this yields an average impact force of 3,606 Newtons, but it takes about 4,000 Newtons to break a human femur. So you probably can head with some impunity, but you might get Alzheimer's disease. Unfortunately, nil points there. Let's see if you can retain your crown, Kate and Giles. A strong enough kick with enough spin on a football could, theoretically, make it complete a full circle and return to you. Science fact, or science fiction?

Kate - Oh yeah. Have a ball flip around and come back to you? I'm going with yeah.

Giles - So this is if you're strong enough - this is the if, right? So if you're Ronaldo or someone, you kick the ball and it curves - whether or not you're strong enough to actually maintain the spin. Or are you assuming 'strong'. Lack of gravity, anything... just on earth?

Chris - What do you think?

Giles - I have no idea. When I kick a ball, I try and will it to bend; it normally heads in the opposite direction I'm trying to hit it. So I'm the wrong person to say anything intelligent about this. I'm going to go with Kate again and say, yes, it is possible. Boomerang balls.

Kate - I think so. I'm convinced.

Chris - No, it's false I'm afraid! As the ball moves through the air, frictional losses, the air rubbing on the outside of the ball, does slow it down. And that means the radius of the turn does become smaller and smaller, because the ball will become susceptible to various other forces once it drops down and becomes slow enough for the air to stick to the surface of the ball. So that means the ball theoretically can go in smaller and smaller circles, but in order to do that, it would have to be spinning incredibly fast. And that in fact is much faster than a human could possibly kick it. So it's a no. Anyway, congratulations Team 2, you are the Naked Scientists Big Brain of the Week award winners. Don't tell all your friends.

Twin babies in bunny outfits.

35:19 - Could identical twins have the same microbiome?

If they ate the same things, would their gut bugs be the same?

Could identical twins have the same microbiome?

Microbiome geneticist Rob Finn, and bodyweight expert Giles Yeo stepped in to answer this question...

Rob - It's really hard to answer that because it depends how long they've been eating exactly the same diet for. Your microbiome changes over time. It's a bit like when you sort of plough a field, and you see the succession of plants that come in, and then it was left long enough, you'd eventually get trees. And so your microbiome is constantly changing. We also have to remember, we are carrying around two kilograms of microbes in us. So when we take two individuals, their microbiomes can really differ by 80%. So there's some really interesting sort of things at play there. However, I think that if they had been in a very confined environment, eating exactly the same diet, I think there's a good chance that their microbiomes would be close to identical as well. The only place I've ever seen this sort of thing happen, is people in hospitals when they had these very regimented shake diets, where they had three shakes every day for about a month and a half. And that is the only time when I've seen uniform microbiomes where it was really hard to tell them apart from two individuals.

Chris - Giles, you're nodding - we haven't really brought up the interface between where you're coming from with calories in food and how much energy we extract from our food, and where Rob's coming from with the microbiome that sees our dinner before we do.

Giles - So we know that the heritability - the percentage of variance of a given trait that's down to your genes versus the environment - the heritability of the microbiome is around 40%, I think roughly. So I think in the wild, twins would normally have around 40% of similar microbiome. That's my understanding. Now the role of the microbiome in terms of metabolising our food... I mean, huge. It's clearly going to be the first line. That's where our food gets to first. And there are a number of things that the microbiome in particular plays an important role in metabolising. And they're going to be soluble fibres. So normal fibre, the insoluble stuff, we eat it, it keeps the bugs happy and it comes out the other side, but the soluble fibre, so this is like pectin, the stuff that you make jam, or jelly for the Americans, and the bugs are able to ferment that into short chain fatty acids. The other thing which the bugs also play a role in terms of metabolising are going to be sugars. So I'm not talking here about sucrose, but polysaccharide - so long chains of sugars that are there. And some bugs will be able to ferment that and give off gas. So sometimes some sugars can make us get a bit gassy, and that's going to be down to the bugs that we've got.

An image of a spiral galaxy

Could dark matter just be hidden planets?

Astrophysicist Katie Mack had this answer for us...

Katie - So dark matter is a kind of matter that we can't see. When it was first discovered to be something that's out there in the universe, it was discovered through the way that it affects the rotation of galaxies. So we live in the Milky Way galaxy, it's a sort of disc shape, with a lot more stars and stuff at the centre, and then it sort of flattens out. And the stars orbit around the centre of the galaxy; it takes millions and millions of years. And you can tell by looking at a lot of different galaxies that the stars go around them a lot faster than you would think they should. If you look at the galaxy and you count up all the light in the center of the galaxy and all the light from the stars and the gas and the dust, and even assume that there's a supermassive black hole in the centre, the stars are still going more quickly around the outer edges than they should. You would expect that they would just kind of fling off into space because there's just not enough gravity, from what you can see, to hold them in. And so based on that and a whole huge number of other things, we conclude that the galaxies are actually mostly made of something invisible, something that we can't see, some kind of matter that's providing the extra gravity to hold everything together. And for a long time, we didn't have any ideas for what that could be. It was just something that wasn't putting out light. But we've determined that it really can't be anything like planets or dust or atoms, regular stuff, because it doesn't absorb light. So if it were something like rogue planets, then it wouldn't be invisible, it would be opaque. And we don't see that. And we also don't see the dark matter sort of collapsing together into a disc the way that regular matter does. So in a similar way as how, if you had a lump of pizza dough and you spin it, everything in space, when it comes together with gravity, there's some overall spin and then it all averages out to that kind of discy shape, and dark matter doesn't seem to do that. We can see evidence that dark matter stays kind of puffy and rounded in a way that regular matter doesn't. So we have a number of reasons to suspect that it's not regular matter. It's not something like planets. It does seem to be something different, something that can bump into other dark matter and just pass right through and not collide and get stuck in the middle. And we have a few pieces of evidence for that as well, where we've seen entire clusters of galaxies collide with each other. And a lot of the regular matter gets stuck in the middle, but the bulk of the gravity, the dark matter, passes right through the collision. And we see it show up on the other side after these collisions. So we're pretty sure that there's something weirder than that, we think maybe it's some kind of new particle that we didn't know about before, that just doesn't interact in the ways that most other particles do. Specifically, doesn't do electromagnetism, which is light and magnetism and electrostatic repulsion and all of that. You know, like a neutrino, which is another kind of particle that doesn't seem to do that force. It doesn't do electromagnetism. It's probably some kind of new particle.

A supermarket aisle empty of people.

42:44 - How bad are ultra-processed foods?

What are ultra-processed foods, and what are they doing to us?

How bad are ultra-processed foods?

Geneticist Giles Yeo had the details on ultra-processed food...

Giles - Okay. So first of all, people think that processed food is bad, but cooking is a process. Fermenting is a process. So yoghurt is a processed food. Bread is a processed food. Processed foods have been around forever. Ultra-processed foods, however, are foods that undergo industrial processing that we are not able to replicate in our kitchens or in normal restaurants. So these include extrusion,  and washing the carcass of an animal with a high pressure washer and then heating the stuff up into pink slime, that kind of stuff. That is ultra-processed food - but is it the worst food we can eat?

Chris - I mean, you could argue that cyanide would probably be the worst food you could eat. That probably would be arguably worse for you.

Giles - But is it a food though?

Chris - If it goes in and you eat it intending to feed yourself on it, then I suppose you could regard it as food. You've eaten it. The outcome wouldn't be good...

Giles - So I guess ultra-processed foods... okay. I was going to say, they're not toxic to us. They clearly are toxic to us at a certain dose, but most things are, but ultra-processed foods are not poisonous. So in other words, if you had a little bit of ultra-processed food, it's not going to kill you, it's because we're having too much of it. So the question is why. I think the primary reason is because when you ultra-process a food, a number of things tend to happen. You tend to strip out protein and/or fibre, depending on the type of food you're eating. And you tend to remove flavour, which means you have to add flavour back in. Flavour comes from sugar, fat, salt. These are the holy trinity; yhis is where all flavour comes from. And so ultra-processed  foods tend to be lower in protein and fibre. And so therefore are very calorically available, which means that we're able to extract far more calories from it than unprocessed foods, and tend to be high in fat, sugar, and salt. So that is what an ultra-processed food is. And therefore they tend to be bad for you. They tell you you don't want to eat too much ultra-processed foods. Is it the worst food you could have? It depends how much ultra-processed food you're eating. I mean, if you ate an entire stick of butter, I don't think that's going to be great for you, and butter is not an ultra-processed  food. And if you ate too many carrots, then you can actually end up turning orange and have this carrot poisoning. And that's not a good thing either. So I think the dose makes the poison. It just so happens in the UK, in North America, we get 50% of our calories at the moment from ultra-processed  foods. The problem with ultra-processed foods is the dose that we are actually getting them at.

Pieces of coal

What is activated charcoal?

We put the question to University of Texas chemist Kate Biberdorf, who let Chris Smith in on the answer...

Kate - So activated charcoal is essentially a chunk of carbon, which is just what charcoal is, that has been ground up to a really fine powder. And so when we throw the word activated on it, it just means that it has a lot of surface area. So for example, 1 gram of activated carbon has a surface area of around 3000 metres squared, or for Americans 32,000 square feet. So it's a lot, it's a huge surface area. And so when you use activated carbon like a filter, which is what we do when we try to purify water, it acts exactly like a face mask does - it lets the small molecules go through, like oxygen when we're breathing or in this case water, but the big, bad molecules or the bad pollutants can't go through the face mask and they can't go through the filter. And so your clean little water molecules go through the bottom, but the bigger molecules or those pollutants - things we're trying to filter out - get trapped in the pores of the carbon and they get kind of stuck there, and so it's a really beautiful way to purify water and I'm a big fan.

Chris - So it's not a chemical trick, it's not doing any additional novel chemistry, it's just a structural trick. It's like a fine grain filter with a big surface area.

Kate - Yeah, that is exactly what it is. Now, there are going to be people who are moth chemists or PCM chemists who are going to argue with that because there can be some adhesive properties that happen. And depending on the molecules you can spike it with, you can take this in a whole different direction. But pure, plain, activated carbon is just carbon that's going to trap bigger molecules, that's it.

The image shows a pair of lungs.

47:20 - Does the lung microbiome affect COVID severity?

Could the microbiome of the lungs be the difference between asymptomatic and symptomatic COVID-19?

Does the lung microbiome affect COVID severity?

Listener Joanne had this question, which Chris Smith put to microbiome expert Rob Finn...

Rob - To be honest with you we don't know at the moment, it's very difficult. So we do see some changes between individuals who are asymptomatic and symptomatic, or don't even have coronavirus, but usually people who do actually display symptoms, they're receiving other treatments. And it's really hard to tell -  are those treatments changing the microbiome? Or is it the fact that you've got an altered microbiome that has actually caused you to be symptomatic? So I would say that this is a really interesting research area. The lung microbiome is much harder to tackle than the human gut where stool is used. And so I think that's certainly an area for ongoing research in the future.

Chris - Is there actually a very big assemblage of organisms right deep down in your lungs? Or is the air and the airways right down there relatively clean and most of the bugs are in the back of your throat?

Rob - There is definitely a lung microbiome but it's nowhere near as complex, so the assemblage is much simpler and there are just fewer in terms of number. I mean, we know that your lung microbiome plays a key role in other diseases, so there are things like inflammatory diseases like COPD - the chronic obstructive pulmonary disease - is really a key area where lung microbiome has been implemented. So it definitely plays a role in some inflammatory disease, but with the COVID data it's just not sufficient for us to really draw concrete conclusions yet.

Someone shining a light up from a mountain at the stars and Milky Way.

49:19 - Are we putting up too many satellites?

Astrophysicist Katie Mack weighs in on the state of the sky...

Are we putting up too many satellites?

Chris Smith put the question to North Carolina State astrophysicist Katie Mack, and chemist Kate Biberdorf has a question about space junk too...

Katie - This is a very kind of heated discussion happening among astronomers and space technology people and those who are actually putting these satellites up there. From the astronomer side, one of the things that we worry about with all of these satellites like the Starlink satellites where there's, I think, more than 1000 of them up there now, and there's going to be possibly as many as 10,000 or more, the problem is that those get in the way of our observations. And when we are trying to study deep sky objects and we have to stare at something for a very long time with a telescope when lots and lots of satellites are crossing through the image, you get these stripes that are really hard to deal with for certain kinds of observations. And so the fact that they're somewhat problematic for certain kinds of observations, both visual observations and also potentially radio, because they're also beaming back information to earth and that can give us radio interference when we're doing radio observations with space that can be an issue for astronomers. And then there's also a growing concern that it's just changing the way the night sky looks, and so it's a kind of, for people who have a beautiful dark sky and can watch the stars, it's going to look different when there are lots of little tiny points of light that are moving across the sky all the time. And that's a question of, you know, what's our responsibility to the world to maintain the pristine nature of the night sky, this global resource. So I think it's a complicated question, and there are just lots of aspects of it, and there are really big discussions happening all the time about can these things be controlled? How should we control them? Who should have authority over what gets put into orbit? And it's something that's going to get more and more important over time as more of these things are going up.

Chris - Kate?

Kate - Didn't the ISS just get damaged from space junk? I mean, aren't we worried about that too?

Katie - I mean, we do worry about space junk. Space junk can be anything - it could be a screw that fell out when somebody was doing a repair on something, and there's a lot that's being tracked all the time. And these little satellites, as there are more and more of those, we need more and more systems to mitigate the possibility of collisions. And collisions are still pretty rare, like there's still not so much up there that you have to worry about this stuff too much, but we do need to have kind of a more robust system for if there is going to be a near encounter. Because at the moment each satellite owner could say, "I don't really want to move my satellite. Why don't you move your satellite?" And there have been situations where there's been a conflict over who has to fire their attitude adjustments to move the thing. Or sometimes they can't, sometimes these things don't have the ability to adjust themselves. And then you have to worry about the possibility of an actual collision which can cause really major damage even if it's just between two robotic satellites that don't even come near people, those can damage other satellites and damage the space station and so on. And so that's also something that people are worried about.


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