丹尼斯·懷特：核聚變與能源的未來

Lex Fridman Podcast #353 – Dennis Whyte: Nuclear Fusion and the Future of Energy Machine transcription by Qwen3-ASR 0.6B (MLX, local, Apache-2.0). May contain recognition errors. The following is a conversation with Dennis White, nuclear physicist at MIT and the director of the MIT Plasma Science and Fusion Center. And now a quick few second mention of each sponsor. Check them out in the description. It's the best way to support this podcast. We got Rocket Money for helping you cancel unwanted subscriptions, MasterClass for online education and Inside Tracker for bio monitoring. She was wise, and my friends. And now onto the full ad reads. As always, no ads in the middle. I try to make this interesting, but if you skip them, please still check out our sponsors. I enjoy their stuff, maybe you will do. This show is brought to you by Rocket Money, a personal finance app that finds and cancels unwanted subscriptions, monitors your spending and helps you lower your bills all in one place. 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And then also if you can get data from the brain, the electrical, the mechanical, the chemical signals from the brain, high resolution, regular collection of that data all inside an app and make predictions based on that on what you should do with your life. Because otherwise, just like I am right now, you would be deeply lost, deeply lost in the turmoil of the human condition. Services like Inside Tracker can at least give you a little bit of hope. Get special savings for limited time when you go to insidetracker dot com slash lex. This is Alex Friedman podcast to support it. Please check out our sponsors in the description. And now, dear friends, here is Dennis White. Let's start with a big question: What is nuclear fusion? It's the underlying process that powers the universe. So, as the name implies, it fuses together or brings together two different elements—technically nuclei—that come together and if you can push them together close enough that you can trigger essentially a reaction. What happens is that the the element typically changes. So this means that you change from one element to not the chemical element to another. Underlying what this means is that you change the nuclear structure. This rearrangement through equals mc squared releases large amounts of energy. So fusion is the fusing together of lighter elements into heavier elements. And when you go through it, you say, "Oh, look, so here were the initial elements—typically hydrogen—they had a particular mass rest mass, which means just the mass when they're with no kinetic energy. And when you look at the product afterwards, it has less rest mass. And so you go, well, how is that possible? Because you have to keep mass, but mass and energy are the same thing, which which is what equals mc squared means. And the the conversion of this comes into kinetic energy, namely energy that you can use in some way. And that's what happens in the center of stars. So fusion is literally the reason life is is viable in the universe. So fusion is happening in our sun. And what are the elements? The elements are hydrogen that are coming together. It goes through a process which is probably gets a little bit too detailed, but there is it's a somewhat complex catalyzed process that happens in the center of stars. But in the end, stars are big balls of hydrogen, which is the lightest, the simplest element, the lightest element, the most abundant element. Most of the universe is hydrogen. And it's essentially a sequence through which these processes occur that you end up with helium. So those are the primary things, and the reason for this is because helium has features as a nucleus like the interior part of the atom that is extremely stable. And the reason for this is helium has two protons and two neutrons. These are the things that make up nuclei that make up all of us, along with electrons. And because it has two pairs, it's extremely stable. And for this reason, it when you convert the hydrogen into helium, it just wants to stay helium and it wants to release kinetic energy. So stars are basically conversion engines of hydrogen into helium. And I mean, this also tells you why you love fusion. I mean, because our our sun will last you know ten billion years approximately. That that's how long the fuel will last. But to do that kind of conversion, you have to have extremely high temperatures. It is one of the criteria for doing this, but it's the easiest one to understand. Why is this? It's because effectively what this requires is that these hydrogen ions or which is really the bare nucleus. So they have a positive charge. Everything has a positive charge of those ones is that to get them to to trigger this reaction, they must approach within distances which are like the size of the nucleus itself because the nature. In fact, what it's really using is something called the strong nuclear force. There is four fundamental forces in the universe. This is the strongest one, but it has a strange property is that it while it's the strongest force by far, it only has impact over distances which are the size of a nucleus. So to get put that into what does that mean? It's a millionth of a billionth of a meter, okay, incredibly small distances. But because the distances are small and the particles have charge, they want to push strongly apart. Namely, they have repulsion that wants to push them apart. So it turns when you go through the math of this, the average velocity or energy of the particles must be very high to have any significant probability of the reactions happening. And so the center of our sun is at about twenty million degrees Celsius. And on Earth, this means it's one of the first things we teach you know entering graduate students you can do a quick you can do a quick basically power balance and you can you can determine that on Earth it it requires a minimum temperature of about fifty million degrees Celsius on Earth. To perform fusion, to get enough fusion that you would be able to make get energy gain out of it, so you can trigger fusion reactions at lower energy but they become almost vanishingly small at lower temperatures than that. First of all, let me just linger on some crazy ideas. So one, the strong force just stepping out and looking at all of physics. Is it weird to you that there is these forces and they are very particular like it operates at a very small distance and then gravity operates at a very large distance and and they are all very specific and the standard model describes three of those forces extremely well and there is. And this is one of them. Yeah, this is one of them, and it's just all kind of works out. There is a big part of you that's you know an engineer that you step back and almost look at the philosophy of physics. So it's interesting because as a scientist, I see the universe through that lens of essentially the interesting things that we do are through the forces that are get used around those and everything works because of that. Richard Feynman had I don't know if you ever had Richard Feynman it's a little bit of a tangent but he's never been on the he's never been on the podcast he was unfortunately passed away but one like a like a hero to almost all all physicists and a part of it was because of what you said he kind of looked through a different lens at these but typically look like very dry like equations and relationships and he kind of I think he brought out the wonder of it in some sense right for for those. He posited what would be if you could write down a single, not even really a sentence but a a single concept that was the most important thing scientifically that we that we knew about that in other words you had only one thing that you could transmit like a future or past generation. It was very interesting. It was so it's not what you think it wasn't like oh strong nuclear force or fusion or something like this. And it's very profound, which was he was that the reason that matter operates the way that it does is because all matter is made up of individual particles that interact each other through forces. That was it. So just that atomic theory basically, yeah, which is like wow that's like so simple but it's not so simple. It's because like who thinks about atoms that they're made out of like I this is a good this is a good question I give to my students how many atoms are in your body like almost no students can answer this but to me that's like a fundamental thing. By the way, it's about ten to the twenty-eight. Ten to twenty-eight. So that's you know trillion you know million trillion trillion or something like that yes. So one thing is to think about the number and the other is to start to really ponder the fact that it all holds together. Yeah, it all holds together and you are actually that you are more that than you are anything else. Yes, exactly. Yeah, no, I mean there there are people who do study such things of the fact that if you look at the for example the ratios between those fundamental forces, people have figured out oh if this ratio was different by some factor like a factor of two or something. Oh, it's like oh this would all like not work. And I look you look at the sun right it's like so it turns out that there are key reactions that if they had slightly lower probability, no star would ever ignite, and then life wouldn't be possible. It does seem like the the universe set things up for us that it's possible to do some cool things, but it's challenging. So that that keeps it fun for us. Yeah, yeah, that's the way I look at it. I mean, the you know the multiverse model is an interesting one because there are you know quantum scientists who look and figure I was like oh it's like oh yeah, like quantum science perhaps tells us that there are almost an infinite you know variety of other universes. But the way that it works probably is it's almost like a form of natural selection. It's like well the universes that didn't have the correct or interesting relationships between these forces nothing happens in them. So almost by definition, the fact that we're having this conversation means that we're in one of the interesting ones by default. Yeah, one of the somewhat interesting, but there is probably super interesting ones where we I tend to think of humans as incredible creatures. Our brain is is an incredible computing device, but I think we're also extremely cognitively limited. I can imagine alien civilizations that are much much much much more intelligent in ways we can't even comprehend in terms of their ability to to construct models of the world to do physics to do physics and mathematics. I would see it in a slightly different way. It's actually it's because we have, um, we have we have creatures that live with us on the Earth that have cognition right that understand and move through their environment, but they they actually see things in a way or they sense things in a way which is so fundamentally different. It's really hard. Like the it's it's the problem is the translation not necessarily intelligence so it's the perception of the world so I have a dog and when when I go out and I see my dog like smelling things, there is a realization that I have that he sees or senses the world in a way that I can never like I can't understand it because I can't translate my way to this. We get little glimpses of this as humans though by the way because there are some parts of it for example optical information which comes from light. Is that now because we've developed the technology we can actually see things you know I've had I get this you know as a one of my areas of research is spectroscopy so this means the study of light you know and I and I get this quote unquote see things or representations of them from you know the far infrared all the way to like hard hard X rays which is several orders of magnitude of the of the light intensity but our own human eyes like see a teeny teeny little sliver of this. So that even like bees for example see a different place than we do. So I don't I think if you think of there is already other intelligences like around us in a way in a limited way because of the way they can communicate but it's like those are already baffling. In many ways, yeah. If we just focus in on the senses, there is already a lot of diversity, but there is probably things we're not even considering as possibilities. For example, whatever the heck consciousness is, could actually be a door into understanding some physical phenomena we're not haven't even begun understanding. So just like you said, spectroscopy, there could be a similar kind of spectrum for consciousness that we're just like we're like these dumb descendants of apes like walking around. It sure feels like something to experience the color red, but like we don't have. It's the same as in the ancient times you experience physics, you experience light. It's like oh it's bright and you know, yeah, and you construct kind of some. We might actually experience this faster than we thought because we might be building another another kind of intelligence. Yeah, and that that intelligence will explain to us how silly we are. There was an email thread going around the professors in my department already of so what is it going to look like to figure out if students have actually written their term papers or it's chat the chat GPT chat GPT. Um, it was so as usual as we tend to be empiricists in my field, so of course they were in there like trying to figure out if if it could answer like questions for a qualifying exam to get into the PhD program at MIT, which was they didn't do that well at that point. But of course, this is just the beginning of it. So we have some interesting ones to go eventually. Both the students and the professors will be replaced by chat GPT and we'll sit on the beach. I really recommend you know the I don't know if you've ever seen them. It's called The Day the Universe Changed. This is a movie. James Burke, he's a science historian based in the UK. He had had a fairly famous series on on public television called Connections. I think it was, but the one that I really enjoyed was The Day the Universe Changed. And the the reason for the title of it was that, um, he says the universe is what we know and perceive of it. So when there is a fundamental insight as to something new, the universe for us changes. Of course, the universe from an objective point of view is the same as it was before, but for us it has changed. So he walks through these these moments of perception in in in in the history of humanity that like changed what we were right. And so as I was thinking about coming to discuss this, you know people see fusion. Oh, it's still far away, or we've been it's been slow progress. It's like when my when my godmother was born, like people had no idea how stars worked. So you talk about like that day, that insight—the universe changed—is like, oh, this is the I mean, and they still didn't understand all the parts of it, but you know they basically got it. It was like, oh, because of the because of the understanding of these processes, it's like we unveiled the reason that there can be life in the universe. That's probably one of those days the universe changed, right? Yeah, and that was around the nineteen thirties. It seems like technology developing faster and faster and faster. I tend to think just like with chat GPT, I think this year might be extremely interesting, just with how rapid. And how profitable the efforts in artificial intelligence are—that just stuff will happen where our whole world is transformed like this, and there is a shock, and then next day you kind of go on and you adjust immediately. You probably won't have a similar kind of thing with nuclear fusion with energy because there is there is probably going to be an opening ceremony and stuff, and an announcement that take months, but with with digital technology you can just have a immediate transformation of society, and then it'll be this gasp and then you kind of adjust like we always do, and then you don't don't you remember just like with the internet and so on how the days were before. And how the world before right. I mean fusion will be because it's energy it's it's nature is that it will be and anything that has to do with energy use tends to be a slower transition, but they are the most I would argue some of the most profound transitions that we make. I mean the reason that we can live like this and sit in this building and have this podcast and people around the world is is at its heart is energy use and its intense energy use that came from the evolution of starting to use intense energies at the beginning of the industrial revolution up to now. It's that it's like it's a bedrock actually of all of these, but it doesn't tend to come overnight. Yeah, and some of the most important some of the most amazing technologies one we don't notice because we take it for granted because it enables this whole thing. Yeah, which is energy which is amazing for how fundamental it is to our society and way of life is a very poorly understood concept actually just even energy itself people confuse energy sources with energy storage with energy transmission these are different physical phenomena which are very important. So for example, you know you buy an electric car and you go oh good I have an emission free car, and ah but it's like so so why do you say that well it's because if I draw the circle around the car I have electricity and it doesn't emit any anything okay but you plug that into a grid where you follow that wire back there could be a coal power plant or a gas power plant at the end of that oh really. I mean so this isn't like carbon free oh, and it's not their fault it's just you know they don't like the car isn't a source of energy the underlying source of energy was the combustion of the fuel back somewhere. Plus there is also a story of how the raw materials are mined in which parts of the world with sort of basic respect or or deep disrespect of human rights that happens in that mining so the whole supply chain there is a story there that's deeper than just the particular electric car with a circle around it and the physics or the science of it too is the energy use that it takes to do that digging up which is also important and all that. Yeah anyway so. Yeah we wandered away from fusion but yes it's a beautiful but but it's very important actually in the in the context of this just because, you know those of us who work in in fusion and these other kinds of sort of disruptive energy technologies it's it's interesting I do think about like what would it what isn't going to mean to society to have an energy source that is like this, that that would be like fusion you know which has which has such completely different characteristics. For example you know free unlimited access to the fuel but it has technology implications so what does this mean geopolitically what does it mean for how we how we distribute wealth within our society it's. It's very difficult to know, but probably profound. Yeah we're going to have to find another reason to start wars. Instead of resources we're going to we've done a pretty good job of that over the course of our histories yeah. So we talked about the the forces of physics and again sticking to the philosophical before we get to the specific technical stuff. E equals mc squared you mentioned how amazing is that to you that energy and mass are the same. And what does that have to do with nuclear fusion? So it has to do with everything we do it's the fact that energy and mass are equivalent to each other they're just the way we usually comment to it is that they're just energy just in different forms. Can you intuitively understand that? Yes, but it takes a long time. I, um having for all but usually often I I teach the the introductory class for incoming nuclear engineers and and so we put this up as an equation and we go through many iterations of using this to how you derive it how you use it and so forth. And then usually in the final exam I would give I would basically take all the equations that I've used before and I flip it around I basically instead of thinking about energy is equal to mass is sort of mass is equal to energy and I ask the question a different way and usually about half the students don't get it it takes a while. It's to get that intuition yeah so so in the end it's interesting is that this is is actually the source of all free energy because that energy that we're talking about is kinetic energy if it can be transformed from mass so it turns out even even though we we used equals mc squared this is burning coal and and burning gas are and burning wood is actually still equals mc squared the problem is that you would never know this because the relative change in the mass is incredibly small by the way which comes back to fusion which is that equals mc squared okay so what does this mean it tells you that the the amount of energy that is liberated in a particular reaction when you change mass has to because c squared is that the speed of light squared it's a large number it's a very large number and it's totally constant everywhere in the universe which is which is another weird thing which is another weird thing and in all rest frames and actually the relativity stuff gets more difficult conceptually until you get through anyway so you go you go to that and and it's and what that tells you is that it's the relative it's the relative change in the mass will tell you about the relative amount of energy that's liberated and this is what makes fusion and you asked about fusion as well too this is what makes them extraordinary it's because the relative change in the mass is very large as compared to what you get like in a chemical reaction in fact it's about it's about ten million times larger. And that is at the heart of why you use something like fusion it's because that is a fundamental of nature like you can't beat that so of whatever you do if you are thinking about and why do I care about this well because mass is like the fuel right so this means gathering the resources that it takes to gather a fuel to hold it together to deal with it the environmental impact it would have and fusion will always have twenty million times the amount of energy release per reaction that you could of those so this is why you know we consider it the ultimate like environmentally friendly energy source is because of that. So is it is it correct to think of mass broadly as a kind of storage of energy? Yes, you mentioned it's environmentally friendly so nuclear fusion is a source of energy it's cheap clean safe so easy access to fuel and virtually unlimited supply no production of greenhouse gases little radioactive waste produced allegedly. Can can you sort of elaborate why it's cheap clean and safe? I'll start with the easiest one cheap it is not cheap yet because it hasn't been made at a commercial scale right flies when you are having fun yes yeah yeah but yes not yet we'll talk about actually we'll we'll come back to that because it this is cheap or or a more technically correct term that it's economic that it's economically interesting is is really the primary challenge actually of fusion at this point. But I think we can get back to that so what were the other ones you said so so cheap we're actually when we're talking about cheap we're thinking like asymptotically like if you take it forward several hundred years that's sort of because of how much availability there is of resources to use of the fuel yeah of the fuel we should separate those two the fuel will the fuel is already cheap it's basically free right. What do you mean by basically free so if if we were to be using fusion fuel sources to power your and and it's like that's all we had was fusion power plants around and we were doing it the fuel cost per person or something like ten cents a year it's like it's free okay. This is why it's hard in some ways I think it's hard to understand fusion because people see this and go oh if the fuel is free this means the energy source is free because we're used to energy sources like this so we you know we spend resources and drill to get gas or oil or we chop wood or we make coal we find coal or these things. Right so fusion this is what makes fusion and it's also it's not an intermittent renewable energy source like wind and solar it's like but this is this makes it hard to understand so as you are saying the fuel is free why isn't the like why isn't the energy source free and it's because of the necessary technologies which must be applied to basically recreate the conditions which are in stars. In the center of stars in fact so there is only one natural place in the universe that fusion fusion energy occurs that's in the center of stars so that's going to bring a price to it depending on the the cost and sorry the size and complexity of of the technology that's needed to recreate those things. We'll talk about the details of the technologies and which parts might be expensive today and which parts might be expensive in two hundred years exactly. It will have a revolution I am certain of it. So about clean so clean is at its heart what it does is convert it basically converts hydrogen into it's it's it's heavier forms of hydrogen the one the most predominant one that we use on Earth. And converts it into helium and some other products but primarily helium is the product that's left behind so helium safe inert gas you know in fact that's actually what our sun is doing is eventually going to extinguish itself because it'll just make so much helium it doesn't it doesn't do that so in that sense clean because there is no there is no emissions of of carbon or pollutants that come directly from the combustion of the fuel itself. And safe safe yeah we're talking about very high temperatures yeah yeah so this is also the counter intuitive thing so I told you temperatures which that like fifty million degrees or it actually tends to be more like about one hundred million degrees is really what we aim for so how can one hundred million degrees be safe and it's safe because it is. This is so much hotter than anything on Earth where everything on Earth is at around three hundred Kelvin you know it's around a few tens of degrees Celsius and what this means is that in order to get a medium to those temperatures you have to completely isolate it from anything to do with terrestrial environment. It can have no contact like with anything on Earth basically so this means what we this is the technology that I just described is it fundamentally what it does is it takes this fuel and it isolates it from any terrestrial conditions so that it has no idea it's on Earth it's not touching any object that that's at room temperature including the walls of the containment even the including the walls of the containment building or containment device or even air or anything like this so. So it's that part that makes it safe in this and that's actually another aspect to it but that that fundamental part makes it so safe and in the main line approach to fusion is also that it's very hot but there is very very few particles in at any time in in the thing that we view the power plant. Actually the more correct way to do it is you say there is very few particles per unit volume so in a cubic centimeter and cubic meters and so we can do this so right now we're although we don't think of air really as a there is atoms floating around us and there is a density because if I wave my hand I can feel the air pushing against my face that means we're in a fluid or a gas which is around us that has a particular number of atoms per cubic meter right so it's what this actually turns out to be ten to the twenty fifth so this is one with twenty five zeros behind it per cubic meter so we can figure out like cubic like cubic meters about like this the volume of this table like the whole volume of this table. Okay very good so like fusion or a few of those so fusion like the mainstream one of fusion like what we're working on at MIT will have a hundred thousand times less particles per unit volume than that so this is very interesting because it's extraordinarily hot one hundred million degrees but it's very tenuous. And what matters from the engineering and safety point of view is the amount of energy which is stored per unit volume because this tells you about the the scenarios and that's what you worry about because when those kinds of energies are released suddenly it's like what would be the consequences right so the consequences of this are essentially zero. Because that's less energy content than boiling water. Because of the low density, because of the low density. So if you take water is at about a hundred million to a billion times more dense than this, so even though it's at much lower temperature it's actually still it has more energy content. So for this reason, you know one of the ways that I explain this is that if you imagine a power plant that's like powering Cambridge Massachusetts, like if you were to which you you wouldn't do this directly but if you went like this on it it actually extinguishes the fusion because it gets too cold immediately. Yeah so that's the other one and the other part is that it does not in because it works by staying hot rather than a chain reaction it can't run out of control that's the other part of it so by the way this is what very much distinguishes it from fusion it's not a process that can run away from you because it's it's basically thermally stable. What is thermally stable in here? That means is that you want to run it at the optimization in temperature such that if it deviates away from that temperature the reactivity gets lower and and the reason for this is because it's hard to keep the reactivity going like it's a very hard fire to keep going basically. Also it doesn't it doesn't run away from you it can't run away so how difficult is the control there to keep it at that? It varies from from concept to concept but in general it it's fairly it's fairly easy to do that and the easiest thing it can't it can't physically run away from you because the other part of it is that there is just at any given time there is a very very small amount of fuel available to fuse anyway so this means that that's always intrinsically limited to this so if it even if the power consumption of the device goes up it just kind of burns itself out immediately. Yeah. So you are the just to take a another tangent on a tangent you are the director of MIT's plasma science and fusion center. We'll talk about maybe you can mention some interesting aspects of the history of the center in the broader history of MIT and maybe broader history of science and engineering in the history of human civilization but also just the link on the safety aspect you know. How do you prevent you know some of the amazing reactors that you are designing? How do you prevent from destroying all of human civilization in the process? What's the safety protocols? Fusion is interesting because it's not really directly weaponizable because what what I mean by that is that you have you have to work very hard to make these conditions at which you can get energy gain from from fusion. And this means that the the when we design these devices with respect to application in the energy field is that they you know while while while they will because they're producing large amounts of power and they will have hot things inside of them this means that they have like a level of industrial hazard which is very similar to that you would have like a chemical processing plant or anything like that any kind of energy plant actually has these as well too. But the underlying under underneath it core technology like can't be directly used in an nefarious way because of the power that's being emitted it just basically well if you try to do those things typically it just stops working. So the safety concerns have to do with just regular things that like equipment malfunctioning, melting of equipment like all all this kind of stuff that that has nothing to do with fusion necessarily. I mean usually what we worry about is the viability because in the end we build pretty complex objects to realize these requirements and so what we try really hard to do is like not damage those components which but those are things which are internal to the to the fusion device and and it's this is not something that you would consider about like it would as you say destroy human civilization because that release of energy is just inherently limited because of the fusion process. So it doesn't say that there is zero so you asked about the other feature that it's safe so it is the process itself is intrinsically safe but because it's a complex technology you still have to take into account consideration aspects of the safety so it produces ionizing radiation instantaneously so you have to take care of this which means that you shield it. Think of like your dental X rays or or treatments for cancer and things like this we we always shield ourselves from this so we get the beneficial effects but we minimize the harmful effects of those so there there are all those aspects of it as well too. So we'll return to MIT's plasma science and fusion center but let us linger on the destruction of human civilization which brings us to the topic of nuclear fusion what is that? So the the process that is inside nuclear weapons and current nuclear power plants. So it relies on the same underlying physical principle but it's exactly the opposite of which actually the names imply fusion means bringing things together fusion means splitting things apart. So fusion requires the heaviest instead of the lightest and the most unstable versus the most stable elements so this tends to be uranium or plutonium primarily uranium so take uranium so uranium two thirty five is one of the this is one of the heaviest unstable elements and what happens is that this is and fusion is triggered by the fact that one of these subatomic particles the neutron which has no electric charge basically gets in proximity enough to this and triggers an instability effectively inside of this what is teetering on the border of instability and basically splits it apart. And that's the fission right fissioning and so when that happens because the products that are and and roughly splits in two but it's not even that it's actually more complex it splits into this whole array of lighter elements and nuclei and when that happens there is less rest mass. Uh, uh, laugh than the than the original one, so it's actually the same. So it's again, it's rearrangement of the strong nuclear force that's happening, um, but that's the source of the energy. And so in the end, it's like so this is a famous graph that we show everybody is is basically it turns out every element that exists in the periodic table, all the things that make up everything have have a remember you asked a good question, it was like so should we think of mass as being the same as stored energy, yes, so you can make a plot that basically shows the relative amount of stored energy in all of the elements that are stable and make up basically the world, okay in the universe, and it turns out that this one has a maximum amount of of stability or storage at iron, so it's kind of in the middle of the periodic table because this goes from you know this it's roughly that, and so this what that means is that if if you take something heavier than iron like uranium which is which is more than twice as heavy that and you split apart if somehow just magically you can split apart its constituents and you get something that's lighter that will because it moves to a more stable energy state it releases kinetic energy that's the energy that we use kinetic energy meaning the movement of things so it's actually an energy you can do something with and fusion sits on the other side of that because it's also moving towards iron but it has to do it through fusion together so this leads to some pretty profound differences as I said they have some underlying physics or science proximity