Why are small stars red? Why are small stars so common? Why are small stars so interesting for the hunt for life?
What did Hubble really discover? Why does redshift imply an expanding universe? Why is the night sky dark?
How does gravity bend the path of light? Does it seriously act like a lens or is that just a metaphor? What does it take, and how do we know? What can we learn from this strange physics? I discuss these questions and more in today’s Ask a Spaceman!
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You're at a picnic and you see an ant crawling along the blanket over all the little hills and valleys because your blanket isn't perfectly flat. The ant knows exactly where it wants to go. It sees the strawberry or the pickle sandwich, or I'm not exactly sure about what you pack for a picnic, but you get the idea. The ant thinks it's going in a straight line. It knows. It saw the strawberry in the distance, and it's going to go right for it. No hesitation. But. It's forced to follow the contours of the blanket. It has to go up, down, maybe sideways. It's making a beeline for the food, but the blanket makes it follow an ant line. Get it? No? Nobody? Okay, well, you know what? Either way, good jokes or bad jokes, you should contribute to Patreon. Go to patreon.com slash pmsutter to learn how you can keep all this outreach going. Maybe... If more people contributed to Patreon, I would be able to write better jokes. But you know what? This is all you're going to get. Actually, I'm just kidding.
All of you are incredibly generous, and I appreciate all the support. That's patreon.com slash bmzutter. The word of the day that we all need to come away from this episode with is geodesic. Geodesic. You may have heard that word before in other contexts. Like if you think about airlines and flights and the path that a flight will take, it connects one city to another. Say you're leaving from New York and you want to go to Paris. a straight line in three dimensions a straight line in three dimensions would send you in a tunnel through the earth that's not exactly feasible so instead we're going to stick to the surface of the earth and we're going to follow we're just we're going to set our compass heading and the plane is going one direction it's not going to turn left it's not going to turn right it's going to go right to paris but then when i look at that in three dimensions it's a curved path Because the plane has to follow the surface of the Earth, because it's not going to go off into space, and it's not going to start digging through the rocks and the dirt in the ocean.
It's going to follow a straight line, but the Earth is curved, so it's going to follow a kind of straight line. It's going to follow what we call a geodesic. In cartography, in flights, these are also called great circle roots. The straightest line possible. I guess the definition of geodesic. And you can extend this from thinking about just the surface of the Earth in a globe to anything. Like the ant crawling along the surface of a very complicated blanket. It may not turn left or turn right, but it still has to go up and down. It has to follow all the contours. Geodesic. A definition I like to have in my head is a line that is as straight as possible under the circumstances. A line that is as straight as possible under the circumstances. And in our universe with gravity, the discussion is all about geodesics. In the framework of general relativity, space-time itself is bent. Space-time itself is warped and flexed like a blanket at a picnic, has hills and valleys. Any source of gravity, any mass or energy causes a distortion in the fundamental fabric of space-time.
Now, it's kind of hard to imagine because that's a four-dimensional thing and our brains aren't usually equipped for thinking in four dimensions, but it's still a thing. We can use lower-dimensional analogies like blankets or like the surface of the Earth to get the point across. In general relativity, which is how we understand gravity in our universe. Space-time is warped and bent and flexed, and other objects are forced to navigate this. Other things like light are forced. Even though light would love to go in a straight line, it can only go in as straight a line as possible under the circumstances. So if space-time in a particular region is bent, light must follow that bending. So gravity... The presence of a massive object can deflect the path of light. Just like curvature in a blanket at a picnic can deflect the path of an ant trying to get to that strawberry. That must be a really good strawberry. It's worth it. This is one of the earliest predictions of general relativity and provided one of the first tests of general relativity where Einstein said, hey, light passing near the sun.
will get deflected a little. So if you look at a star, a distant star, that's very, very close to the edge of the sun, it will be in a little bit of the wrong position because the path of light from that star, as it gets close to our sun, will get tweaked just a little bit and it will go in a slightly different direction than straight. And of course, the famous 1917 Eddington expeditions led by Sir Arthur Eddington to get some photographs of stars. Usually it's hard to take a picture of a star near the sun because the sun is kind of bright. But during an eclipse, which blocks out a lot of the light from the sun, you can get those photographs. You can take some measurements of stars very, very close and you can see this effect. He saw the effect as predicted by general relativity. Yay. Instant celebrity Einstein. So gravity can bend this path of light. What else bends the path of light? Well, tons of stuff, right? In normal everyday experience, we're bending light all over the place and it's usually caused by refraction.
Light has different speeds in different stuff. So if light travels from one medium to another, like from air to water, air to glass or whatever, if it comes in at an angle, that angle will change. And I'll admit, it's super easy to blow my mind. And here's one example of where my mind gets easily blown. You take a clear glass, put some water in it, stick a straw in it, and you look through the glass at the surface of the water at the straw. The straw looks bent. where it meets the water. Even though the straw isn't bent at all, it's bent because the light coming through the air and coming through the water are different, travel at different speeds, and so it makes all the angles messed up. It looks like the straw is broken, even though it's perfectly straight. This is also why it's hard to go spearfishing. Or if you're just standing, like stand in knee-deep water and there's some fish around you and you decide to grab them, the fish is not going to be where you expect it to be because the path of light got bent coming from the water and into the air.
This is also the physics behind lenses. I'm sure many of you are incredibly familiar with lenses because you have them inside your eyes, or you might be wearing them over your eyes in the form of glasses. What will a lens do? Well, a good lens will focus, magnify, enlarge images. It will bend light in just the right way to make it very, very useful. A bad lens will not do that. If you think of what happens with a funhouse mirror or a broken glass, it will distort images because it's bending the path of light in an unuseful way. If you put a good lens between your eye and a distant object, it might be easier to see that object. It'll bend the light in a nice, friendly way. If you put a bad lens between you and a distant object, the image will get all twisted and distorted. Either way, there is a connection between what you see and what the lens is made of and how it's shaped. If you take a good lens that's magnifying an image, enlarging it, making it focus, and you break it with a hammer or you heat it up or you melt half of it, it will change how you see that image.
Or if you make it out of different material, different kinds of glass, if you sandwich two pieces of glass together or whatever, if you change what the lens is made of, you're going to change what you see. Now, does gravity, we're talking about gravitational lensing, by the way, Now, does gravity really act like a lens or is that just a metaphor? Well, it depends on the lens. It certainly doesn't act like a pair of bivocals. But if you take the base of a wine glass, this is just one of those weird coincidences in the universe. If you take a base of a wine glass and you look through that base... It's very similar to the lensing caused by a small compact spherical object. If you were to say, look at a distant black hole, and that's a source of gravity, it will bend the path of light around it. Light will be forced to take different paths around that black hole. So you'll see a very strange and odd pattern of the starlight surrounding that black hole. If you look through a base of a wine glass at the same spot, you'll see a very similar distortion.
So do it yourself. I don't recommend breaking any wine glasses. This is one of the rare times I do not recommend doing this in public. Do this in the privacy of your own home. Go find yourself the base of a wine glass and start looking through it. You'll see arcs. If you line up things just right, you'll see circles. You'll see the same image multiple times. You'll see some cool stuff. Now, in general, gravitational lenses are much, much more complicated, and we'll get into that. But you could, in principle, ask your friendly neighborhood glassblower to reconstruct any gravitational lensing situation you might see on the sky, pretty much. By varying the density, the thickness, the materials, the curvature of the glass, you could reconstruct things that I'm about to talk about, things you can see in our actual universe that are generated by gravity, which just gave me a pretty cool idea for an art science project. But that's another thing. We see gravitational lenses in the universe. We literally see them.
We have pictures of them. We usually see them in very massive systems. That's the easiest to pick out, like a giant galaxy cluster. And we see it not in the cluster itself, but with images of galaxies that are behind the cluster. Those images get distorted. At least that's the conclusion. When you see like a long, snake-like, curved, odd-shaped galaxy, it's either gravitational lensing or there's some really funky-shaped galaxies in our universe that we have no idea how they exist. You take your pick. Take your pick. Here's what you do. Look at a galaxy cluster. Galaxy cluster, by the way, is a massive, bustling metropolis. They're massive, bustling cities of a thousand or so galaxies. Very, very massive. The largest, most massive, gravitationally bound structures in the universe. They have lots of stuff. And they can act like a big, giant, honking lens. You got a lot of gravity in one place. It is going to mess with the light. It is going to mess with any light passing near or around it.
They're also very, very large compared to the galaxies that actually make up the cluster. So a thousand galaxies might live in a cluster, but the galaxies themselves are very, very small compared to the entire volume of the cluster. Like my favorite analogy, bees in a swarm or birds flocking together. If you see a flock of birds... You, of course, can see the individual birds. That's how you know it's a flock. But you can also see through them and past them in the gaps to the sky beyond. It's like that looking at a galaxy cluster. We see the members themselves. We can also look through the galaxy cluster and see a bunch of background galaxies. Galaxies that have absolutely nothing to do with the cluster. So there's a bunch of galaxies in the galaxy cluster. There's a bunch of other stuff, too, like gas and dark matter that are acting like a lens. It's exactly as if you put a giant piece of glass there. Except we're doing this with gravity, not refraction. And there's a bunch of galaxies behind the cluster that have absolutely nothing to do with it.
The light from those background galaxies will get distorted. Just like if you took a big honking piece of glass and put it up and looked at the sky, you would see a bunch of distortions in the sky. That's exactly what we see. And you can play a game. You can look at these distorted, funhouse mirror-like galaxies when you're looking through the cluster. And you can look in just an empty patch of sky with no cluster there, where it's just a whole bunch of galaxies. They're far away. There's no lens between us and those galaxies. So those galaxies look totally normal and well-behaved. Then the galaxies behind the cluster look super messed up. You can compare and contrast and do a lot of math. The easy mode answer is you get the total mass of the cluster. The expert mode answer is you can figure out the distribution of matter in the cluster. The distribution. You can figure out. This is so cool. You can figure out the contents of a galaxy cluster. One of these massive bees. Where the matter is.
What it's made of, how it's distributed, where it's concentrated, where it likes to hang out, based on how it's distorting the light from the background galaxies. Is it perfect? Of course not. There's noise. There's uncertainty. There's modeling error. It's not perfect by far, but it's pretty dang good. Just like imagine this thought experiment using real glass. Imagine we don't have gravitational lenses. We just have actual lenses, pieces of glass floating around the universe or in your room, just in your room. Like imagine someone gave you a piece of glass and you want to figure out what that glass is made of. If it's lumpy or if it's perfectly smooth, what would you do? You would hang it up and you'd look through it. And you compare, okay, I see the pattern of my wallpaper looks like that. And then when I don't look through the lens, I see that pattern of wallpaper. You could figure out what the lens is made of. I mentioned this in the dark matter episode. This is one of the best uses of these water, so-called strong gravitational lens systems, where we can really plainly see the distortion of background galaxies.
One of the best uses is, for that point to the evidence of dark matter, or the point in the direction that our universe is filled with dark matter. Because when you look at the lens system, there's no way that the stuff that lights up, the individual galaxies or the gas, can explain the lensing distortion that we see. We know that there's something else going on with clusters of galaxies because we see its effect on the light as it passes through it through gravitational lensing. We saw this with the bullet cluster. There's other examples with amusing names like the train wreck cluster. Feel free to look those up on your own or ask me and I'll do a whole show on them. This lensing effect doesn't just distort the shape of background galaxies. Just like a lens doesn't just distort the shape. It can also brighten things. It can also magnify things, just like a lens can. Why does it brighten and magnify? Well, it's because multiple rays of light can get bent and focus just like a lens does.
So instead of just a tiny portion of the light reaching us in our observatories and our telescopes from a very, very distant galaxy, some rays of light that that object is emitting that would normally never hit us, would miss us by a country mile, get bent. Their paths get bent and instead get aimed right for the Earth. So we get bonus light. We get extra light that usually wouldn't be pointed at us because of the presence of a gravitational lens. That can brighten it, that can magnify it. This is how we use to detect incredibly distant objects. The most distant galaxies and quasars are detected using this lensing technique. Normally, the most distant galaxies in our universe would be far too faint to see. We just can't see them. They're too far. They're too faint. But when we have a lucky chance, when we look through a gravitational lens, when we look through a massive thing like a cluster at an even more distant object, we get a boost. We get a brightening. We get a magnification that we normally wouldn't.
And this slingshot method, this leapfrog method, sometimes it's called this gravitational lensing method, allows us to probe the furthest reaches of our universe. There's one other application of strong lensing that's super cool. You can use it to measure the expansion of the universe. Let's say you're watching. Let's say you're watching a distant galaxy that's being lensed by a closer cluster. So you're watching this background galaxy and something happens. Changes its brightness, maybe a supernova goes off, the aliens wave hello, whatever. Something in that galaxy changes and that information is going to travel, right? The light from that galaxy is going to travel from the galaxy to us. Usually without a lensing, it would just take one path. It would just aim straight for us and that'd be it. But because of the lensing, there are multiple paths of light. Right. Multiple paths. Some some go straight through the cluster. Some go around this way. Some go up that way and then down this way and then over twist around a little bit before finally aiming to us.
They have different paths, so they have different times. The light from a distant galaxy will take different amounts of time to reach us depending on the path. Some have the fast path, some of the slow path, some of the really slow path. And if you know the brightness of something and it's traveling through different distances, you can measure the expansion of the universe. It's been used a few times. It's not like the number one technique for measuring the expansion of the universe, but it's there. It's just another thing, another piece of evidence that we live in an expanding universe. On the opposite side of the spectrum from strong lensing is something we call weak lensing, which is, as you can imagine, a little bit weaker. And to visualize this, I want you to imagine you're sitting in a giant cathedral or concert hall, just a vast room, vast open space with light that surrounds you from the edges. So they're incredibly far away. That light is incredibly far away from you on that far away wall, that far away ceiling and suspended around you in the air.
are countless individual lenses, pieces of glass, jewels. They're so tiny that you can't even see the individual ones. They're just effervescent. Like, they're hardly even there. And you can't see. It's not like you can see a distant light and it's immediately obvious that it's being lensed. It's just a super tiny effect. Well, how could you figure out the properties of these lenses? How could you figure out how many there are? What shape they are? How they're able to bend light? Well, if you knew what that distant background light looked like, you can compare it to what you see and figure out the properties of the lenses. Obviously, this would be very weak. It wouldn't be very strong, so it'd have to be a statistical analysis, not like just a raw picture. Like we have pictures of strong lensing systems where we see, oh yeah, man, that galaxy is getting totally lensed. It's totally obvious because we see that galaxy being stretched out into a long, thin arc or a ring where it's all wibbly wobbly.
That's strong lensing where it's immediately obvious in a photograph what's going on. With weak lensing, it's something else. Now, instead of looking at massive galaxy clusters, we're looking at the effects of galaxies themselves. Much smaller things, much more subtle things. Stuff where you have to do a massive survey and do some deep statistical analysis to figure out the lensing that's happening. If you take this to the extreme, we are surrounded by light on all sides. It's the cosmic microwave background. And that light... that originated 13.8 billion years ago has been filtering through the universe for those 13.8 billion years. And it's been encountering structures in our universe that light will pass by galaxies or a cluster here and there. And it will tweak. It will shift a little bit left, shift a little bit right, shift a little bit up or down. It will change. And we've been able to use the cosmic microwave background to build up a map of the total matter structure in our universe between us and that background light.
And now I know some of you may remember that when a whole deal about how the universe is homogenous and isotropic, how it looks the same in all directions and on big enough scales, the universe looks the same. But now I'm saying we're doing a technique where we're like, why should this lensing signal? Why should this subtle weak lensing from the cosmic microwave background be anything at all? Why is there signal there? Well, one is it's a super duper tiny effect, well below the scale of what we call homogeneity. The CMB itself is almost entirely uniform. One part in 10,000 is different at all. And then a tiny fraction of that actually picks up this weak lensing signature from passing through all the structure, all the universe, the stuff between us and the cosmic microwave background. Also, to make any decent lensing signal at all, you need big structures like clusters. You need walls. You need filaments of galaxies to make any decent signal. And that only comes late in cosmic evolution, which means relatively nearby.
Remember, when we're doing cosmology, the nearby galaxies... are the most recent galaxies to form. Because light takes time to travel across our universe, the further back we go, the further out we go in the universe, the further back in time we go. So the most distant galaxies that we can see don't have a lot of structure to them because it takes time for structure to evolve. We only see the cosmic web, we only see the structure of the universe in our relatively nearby patch, The further and further out we go, the further back in time we're observing and the less structured it is. So it's a relatively nearby effect that's creating this lensing signal. So looking at the lensing from the cosmic microwave background is useful and we've done it. We've totally done it. You can find maps of Planck lensing signal, for example. where that tells us something interesting about what the universe is made of and how it's organized. Just like if you had a whole collection of lenses around you, you could use the background light to figure out what those lenses are made of and how they're arranged.
We're just doing that with that whole entire universe. But that gives us one kind of information. It's the light from the cosmic microwave background projected, filtered, traveled through the entire universe. But what if you could do this in shells? What if you could break it down like onion layers? Like, we're at the center of the onion. The cosmic microwave background is that outermost dry skin, the thing you have to rip off before you start cooking. That gives us a picture of the universe, of the whole thing, but it's relatively limited. What if you could do it at onion layers? Like, what if you could take one layer at a time? What if you could look at the weak lensing signal out to one distance, say halfway to the edge, figure out what the lensing distortion is between us and that halfway distance, then go out a little bit further, just a little bit further, figure out the lensing signal, the effect of any light from that layer between that layer and us, then you could take a difference.
That difference would tell us what that layer is made of and how it's arranged. And do this multiple times, we can build out a three-dimensional picture of the universe. including the dark matter, that is absolutely essential. Because if we just want a map of the visible matter, we would just go out and count the galaxies. This gives us a measurement of the dark matter itself. And it's the galaxies themselves that are acting as the layer. So you start with one layer of galaxies at a certain distance. Say we're going to go, I don't know, 10 billion light years out. That'll be our layer. We'll look at all the galaxies at 10 billion light years out. The light from those galaxies has filtered through 10 billion light years of stuff to get to our detectors. So those galaxies, their shapes will be just a little bit different. Maybe just a little bit stretched out. Maybe just a little bit twisted. Super, super tiny thing. Very difficult to measure. Relies on statistics. This method is still in the relatively early stages, but it's potentially incredibly powerful.
If you repeat this process, okay, now not 10 billion years. Now I'm going to look at 5 billion years. Galaxies that are 5 billion light years away, I'm going to see how their images are slightly distorted. Now 6 billion years. Galaxies, calling all galaxies at 6 billion light years away, I'm going to see how your image is slightly distorted. Now 7, now 8, now 9, now 10, now 11, now 12. Layer by layer by layer, you can build out a map in principle. of the growth of structure over time. Where you can see, where you can map out how our universe has evolved in 13.8 billion years. From its very early stages where there weren't a lot of structures, where galaxies were not clumped together, to the very late stages where we have a lot of structure in our universe. Using that, you can map out what dark matter and dark energy are. Because if you change ingredients in the recipe of the universe, you get different universes. You get different growths of structure. Both of these things influence the growth of structure in our universe, both dark matter and dark energy.
And by mapping out how that structure has grown, you can figure out the properties of dark matter and dark energy. That is an incredibly powerful method. That is still in its relative infancy. We're just now getting a handle on the statistics and the methods and the detections. But upcoming missions like the Dark Energy Survey, like WFIRST, the NASA mission, like Euclid, the European Space Agency mission, this is what they're going to do. They are weak lensing missions. They are going to look for this tiny, tiny shift. in the properties of galaxies, in their images, to figure out how much stuff is between us and those distant galaxies and use that to map out the universe, even the parts that we can't see. Lensing isn't just used for cosmology. We have our strong lenses, where we look at individual galaxy clusters and we use that to crack open inside and figure out what they're made of. We have weak lensing, where we're mapping out the arrangement of galaxies on the very largest scales.
We can also hunt for much smaller things. Much, much smaller things. So small, it's called microlensing. And let's say you look at a star. You just happen to pick a random star in the sky and you stare at it long enough. If something tiny, like a brown dwarf or a black hole, passes between you and that star, that tiny thing will lens the light from that star. The light from that star will get bent just a tiny, tiny, tiny bit. Far too small and far away to see rings or an arch or a big distortion, but you will see a brief bump in the brightness. Because in that moment, in that moment where the tiny thing is perfectly aligned between you and the star that you're looking at, then the starlight, just like with strong lensing, gets to follow multiple paths to reach your eye. So instead of just getting some of the light, you get a little bit more of the light that you normally wouldn't get. Just like a lens can brighten an object, you get a bump in brightness. And you can find it. You can see it.
It allows you to detect small, dim, distant things. Just waiting for these chance alignments. That's all it takes. I mean, it's super rare, to say the least, for one of these chance alignments to happen. But if you stare at enough of the sky long enough, it's bound to happen. In fact, we have seen it. We have detections of these kinds of events. They're very, very brief. They're like jewels glinting in the sunlight. But it allows us to look for small black holes, brown dwarfs, rogue planets, just all the stuff that doesn't necessarily glow brightly that's floating around the universe. More specifically, our galaxy, because stars in other galaxies are too far away for this to apply. But we can do it nearby. We can do it in our own galaxy. And we can use it to hunt for planets. If you're staring at one star... and another star passes in your line of sight, then that other star will cause a lensing event. You'll get a temporary moment of brightness from that background star. If the star that's between us and the one we're looking at has a planet in orbit around it, then it will have a slightly different lens.
It's like a lens with a small imperfection in it. And the event, the microlensing event, will look a little bit different. The brightness pattern will not look the same during that event. And it's enough. It's different enough that you can tell it apart. You can tell that the star you're using as a lens to look at a very far background star has a flaw in it, has an imperfection, has a little bit of extra weight where it shouldn't. And that's the signal of a planet. And we've actually done this. We've been able to use it to find planets. And it will become the number one planet detection method by far in the next decade. The NASA WFIRST mission that I mentioned that's going to be a weak lensing survey of galaxies to do cosmology. Well, it's doing lensing anyway. It's going to hunt for planets. It's going to hunt for planets using exactly this technique, and we expect to find millions of planets. Because this mission can stare at so many stars for so long, it will see countless chance alignments, and it will capture planet after planet after planet.
So that's how powerful gravitational lensing is. We can use it for strong lensing to peek inside galaxy clusters. We can use weak lensing to map structures in our universe. And we can use micro lensing to find even planets. Just like if we had a set of glass lenses. scattered throughout our universe. We can use it to figure out what those glass lenses are made of, what the stuff behind them are made of, what our universe itself is made of. Thank you very much. Thank you also to my top Patreon contributors this month, Justin G, Matthew K, Kevin O, Justin R, Chris C, and Helgen B. You too can contribute. Go to patreon.com slash pmsutter to learn more. And of course, go to astrotours.com. I'd love to see you on a fantastic trip to a fabulous part of a faraway and exotic land. And also, why don't you join me for Space Radio? That's our weekly radio show here at WCB, spaceradioshow.com. You can send more questions to hashtag AskASpaceman on Twitter and Facebook. Follow me directly. I'm at Paul Matt Sutter on those venues.
You can also email AskASpaceman at gmail.com. Visit AskASpaceman.com for all the show notes and everything. And there's a place to question, blah, blah, blah. You know the deal. I'll see you next time for more Complete Knowledge of Time and Space.
What strange creatures inhabit the so-called “particle zoo”? Why is it a zoo instead of something simpler? Is there anything that connects the forces and particles of our universe? I discuss these questions and more in today’s Ask a Spaceman!
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We've all heard stories of dystopian worlds, whether in the future or from our own past, like stories of the oppressors and the oppressed, of the haves and the have-nots, of the rulers and the ruled. And we have examples of these stories from medieval Europe, or we have stories of the far future, of, oh, it's going to be a horrible future. We hear a lot of these stories. I'm going to tell you another one of these stories of such a dystopian realm. But this world doesn't take place in the past. It's not from history and it's not in the future. This world is right here. It's all around us. But it's hidden from us. It's buried underneath layers and layers of powerful forces and energies. And only the most powerful experiments reveal the true nature of this reality. In fact, we knew hardly anything about it until about 100 years ago. We finally had the techniques developed where we could probe the subatomic world. Where we could probe the actual atomic world. It was only like 100 years ago or so that we realized that atoms were a thing.
And that there's stuff beneath atoms. Atoms are not the most tiniest, building blockiest things in the universe actually made of stuff. And what goes on beneath the level of atoms is crazy complicated. I mean, I'm not even joking. It is insane how complicated this stuff is, but I want to tell you a little story to get across the essence, the main essence of what's going on down there. We'll start at the top, the royal family, the top dogs, the big cheeses, the most powerful, rich, influential, dominant players of the subatomic world. It's the quarks. The Quarks are like any royal family. They're large. They're extensive. They're constantly infighting. There might be some examples of inbreeding. Like if you've seen like a family tree of royal families, you know what I'm talking about. There's claims to the throne. There's petty intrigue. All sorts of juicy court politics. in the quarks, amongst the quarks. The royal couple, the two at the top, themselves are called up and down. Up and down, the names we give to two of the quarks that sit at the top of the heap.
Between them, between those two, they form the most common, stable agglomerations in the universe, the protons and neutrons. Protons and neutrons are made of quarks of these royal couples, but it's not just two. For some very weird and complicated and deep reasons, this royal couple, they actually form triplets to form protons and neutrons. So they bind together. If you have two ups and a down bagged together, that gives you a proton. If you have two downs and an up, that gives you a neutron. And these quarks, these up and down quarks, rule the protons and neutrons. These are like the impregnable fortresses of the subatomic world. They are long-lived. They are stable. They are difficult to crack apart, protons and neutrons. But they don't get that strong from the qualities of the quarks themselves. They get that through the strong nuclear force themselves. The up and down quarks, these top two of the royal family, they rule by the power of the strong nuclear force. The strong nuclear force is so strong, its binding is so strong that it doesn't only form protons and neutrons, but it forms nuclei themselves, atomic nuclei.
It's able to bind together protons and neutrons. Two protons have the same charge. They would normally hate each other, but the strong force is able to overwhelm that. And It's because of this strong nuclear force that the strong nuclear force binds together the quarks, these ruling couple, the up and down quarks. And it's so strong that it leaks out of that range of the proton neutron is able to bind them together. So you can imagine the protons and neutrons are like the palaces, the fortresses, the keeps, the most powerful, the strongest part of the castle. And if you string a few of these castles together, like a few watchtowers together, you can get a whole castle complex on top of a hill. And that's the atomic nucleus. And like I said, it's only the most violent reactions that can bring down the walls, that can tear down the walls formed by the strong nuclear force that keep the up and down quarks safe in their cozy little palaces. The strong force is over 100 times stronger than any other force in the universe.
Over 100 times stronger. You just can't mess with the strong nuclear force. But it does have a trade-off. Even though that strong nuclear force can build these immense, powerful, strong castles, its range is limited. Inside the nuclear fortress, the strong force dominates all reactions. But outside, if these up-and-down quarks, these king and queens that are ruling the subatomic world, if they want to communicate or influence anything else outside of their fortress... They need to use other forces. The strong force provides not just the glue, the connection between the quarks, but also the mass. The mass of a proton isn't in the mass of the quarks. If you add up the mass of the quarks, you don't get the mass of a proton. You get something much, much smaller. But energy is mass. Mass is energy. We learned this from relativity. And This is how we get the mass of a proton is through the energy of the strong force itself. That's how strong it is. So it's like the castle isn't just the king and queen.
It's not just the up and down quarks. It's the walls and the bricks and the mortar. It's all the stuff. And it's the strong force that's actually putting that up. And so the mass of the castle is much more than the mass of the people that live in it because of the walls itself. Outside this castle, outside this castle is the great teeming masses of the peasants, of the underclass, living in poor little hovel villages clustered around those castle walls. And I'm talking about the Leptons. The leptons are the ones that do the work. If you've ever heard of chemistry, it's because the leptons are doing the thing of chemistry. If one castle, if one nuclei wants to communicate with another castle, it exchanges leptons. It will throw them back and forth, left and right. It will give up leptons, acquire leptons without a second thought. Doesn't care about the welfare, about their needs, their wants, their desires, their hopes, their dreams. They're just there to get the work done while the up and down quarks sit safe inside their castle walls.
The leptons are the ones doing the work. There's three leptons. The electron, the muon, and the tau. The electron I suspect you may be more familiar with. The muon is just like the electron, but bigger. And the tau is just like the electron and the muon, but bigger still. And the reason you don't encounter the muon and the tau all that often, why you tend to find electrons much more often, is that there's a rule in our universe, the rule in our universe, and especially in this realm, is that only the lightest survive. If you get too big, if you get too fat, if you get too wealthy, if you get too rich, you will be cut down. You'll be taken down. You are unstable. In this world, only the poorest survive. Only the smallest and weakest survive. If you're too massive, you are unstable. You will be torn apart into more fundamental particles. So the muon, even though it's exactly like the electron, it's like a peasant that's starting to get a little rich. Maybe they have a few businesses. Maybe they own a ship or something and they become a merchant.
That's not going to be allowed. They're going to be cut down. They're going to be taxed. They're going to be violent mobs. They're going to be unstable and they'll be torn apart into more fundamental particles. The electron is the lightest of the leptons. It's the poorest. It's the weakest. And so that's the one that gets to survive. This rule applies to the quarks too. I mentioned the whole quark family. There's actually six quarks. Six quarks, but only two of them, the up and down, are the ones that we encounter in protons and neutrons. The other ones, top, bottom, strange, and charm, are much more massive, hence they're much more rare. So even though up and down quarks get to rule as the royal couple, Paradoxically, they get to rule because they're the weakest ones, because they're the least massive ones, the least powerful ones, because anything bigger, any other heavier quark, if it forms, will immediately be cut down to size, just the same as it happens with the leptons. So you know what? In the subatomic world, it's the small guys ruling other small guys.
And that's just how it works. So you have a castle. Formed from the strength of the strong nuclear force, inside of it are the ruling family of the up and down quarks. Outside of the castle walls are the electrons, the leptons. So how does – if the strong force can't extend beyond the castle walls – How can the up and down quarks communicate with the leptons, with those electrons working the fields? How can they coordinate them? How can they tax them? How can they communicate with other castles over the horizon, over the hill? They have to use another force. They use the royal messengers. They use the photons. The photons are the carriers of the electromagnetic force. The photons are massless. The photons travel at the speed of light. The photons have infinite range. Of course, the farther you are away from the source, the weaker it will be, the fewer photons will reach you. But in principle, and even in reality, right now there are photons reaching us from the distant edge of the universe.
Photons are born, they travel, they die. They have only one goal, is to carry that electromagnetic force. They are able to communicate outside the castle walls so they can exchange messages between the royal family and the peasants outside. They can communicate from one castle to another, one atom to another, one atomic nuclei to another. They can travel across the universe carrying their very, very important messages. of either repulsion or attraction, doing what photons do. But not all particles can listen to a photon. These royal messengers, as important as they are, not everybody hears the message. If A particle is uncharged. If it doesn't have any electric charge, it is invisible to the royal messengers, to the photons. The photons can only communicate with a particle if that particle has some sort of electric charge. So what is the royal couple to do if they want to rule from their castle? They can't communicate with every particle in the universe outside their castle walls. What are they going to do? Well, they turn to their secret spy network.
They turn to gravity. Gravity is also massless. Gravity also travels at the speed of light. Gravity also has infinite range. And gravity sees everything in the universe. All matter. All energy. Everything. Nothing escapes the ever watchful eye of gravity. But gravity only communicates like a good spy in whispers. By far, gravity is the weakest force, 10 to the 40 times weaker than the strong nuclear force. 10 to the 40 times weaker than the strong force. That is weak. So that's the trade-off. Even though gravity sees everything, charged, uncharged, big, small, nearby, far, gravity sees everything, but its influence is very weak. It's very tiny. The up-and-down quarks in their castle keeps, in their fortresses, they can technically see everything in the universe. They can respond to everything in the universe. but not very strongly, because these spies only whisper. We have our castle keep with the strong nuclear force inhabited by the royal family of the quarks. We have our peasants on the outside, coordinated by the photons.
We have gravity, the extensive spy network, living on the fringes of society. are the ultimate untouchables, where even the peasants will turn their noses down at these unfortunate. These are like the leper colony of the medieval world, of the subatomic medieval world. These are the neutrinos. The neutrinos, once again, have three families. The electron neutrino, the muon neutrino, and the tau neutrino. They are leptons, just like their cousins, the electrons, but much less significant. They don't get to live in the villages. They don't get to cluster around castle walls. They're forced to travel, to roam the countryside. They can't be bound to anything, which can be bad, but also kind of good. Because they don't have any charge, they don't follow orders from the royal messengers, the photons. They ignore the quirks as much as they can. They can even come and go through the castle walls. That's how stealthy they are. That's how, like, they're just simply not noticed. Nobody cares if a neutrino is around.
Oh, did you feel that? What was that? Oh, that was nothing. I don't know. Maybe it was a neutrino. I didn't even see it. Whatever. Let's get back to feasting. Neutrinos are so lowly. that for a long time, we didn't even think they had mass at all. And it's only relatively recently that we discovered that neutrinos do have a little bit of mass. As invisible as they are, they do sometimes cause trouble. They do sometimes act as saboteurs because they can come and go through the castle walls without too much notice because everybody ignores them. Sometimes they do interact with a quark. And when they do, it causes havoc. One of the reasons they are so slippery is they don't even have fixed identities. They can wear masks. You never quite know what kind of neutrino you're looking at. You can look once and you say, that's an electron neutrino. You finally notice it. You finally pay attention to it and it's an electron neutrino. You look again and it's a tau neutrino. Travels a little bit across the room.
Now it's a muon neutrino. The neutrinos are strange beasts indeed. They can transform their identities. They can cycle through these three identities as they travel, as they propagate. That's how slippery they really are. There's this rigid hierarchy in the subatomic world. We have the collections of protons and neutrons, which we call atomic nuclei. The protons and neutrons are incredibly stable when they're bound to those nuclei in their fortresses with the electrons in the villages outside. Occasionally, order does break down. In this rigid, stratified, oppressive society, a neutron can defect. A clump of quarks that we call a neutron can escape the castle walls. Roam out into the countryside. Sometimes an electron can make it through the front gates and start roaming around the castle and eating all the food supplies and putting on gowns and suits. I don't know what you do inside of a castle. And sometimes a neutrino can completely slip through the defenses unnoticed. Who's there to do the dirty work? Who's there to clean up the messes? Well, the royal messengers, the photons, certainly aren't going to do it.
They're much too proud for that. They've got a job to do. The strong force is there, but it's too busy keeping the castle walls glued together. And besides, it has limited range. Gravity, gravity is all pervasive, but it's too weak to really do anything. There is one more force. to handle the rogue particles, say neutron defects from the castle, to deal with the occasional neutrino or electron incursion. And those are the Patreons. That's right, the loyal Patreons keep order in line and keep society functioning. Go to patreon.com slash pmsutter to learn how you can support this podcast and all my education and outreach activities. I really do sincerely appreciate all the support all of you have given me. over the years, and that's patreon.com slash pmsutter. Anyway, who does the dirty work that nobody else is willing to do? It's the special forces. The W and Z bosons. That's how special they are. They don't even have cool names, like photons or gluons. They're just W and Z, the carriers of the weak nuclear force.
The bosons, these W and Z bosons, the special forces are capable. They have a very, very special power that nobody else has. They're capable of transforming one kind of quark into another. They see an up quark, they can turn it into a down quark and vice versa. What does this mean? This means that they can transform protons into neutrons and neutrons into protons. This means that if an electron enters the atomic nucleus, enters the castle walls, or a neutrino does, they can use that to their advantage. They can use that to flip a neutron to a proton, a proton to a neutron. They're the only force capable. Of that very special exchange. Think about that. The weak nuclear force, probably the worst named force where it's just not getting a lot of it's not getting a lot of justice. It should be called the special nuclear force. I'm going to start. We should start right now just calling it the special nuclear force because it's how cool it is. What other force can communicate with quarks and communicate with leptons? Who? Nobody.
Well, I know the electromagnetic force can do it if they're charged, but who else gets to talk to neutrinos? Who can talk to a top quark one day, an electron another day, and a neutrino another day? It's the weak nuclear force. That's how cool it is. But that's why they really are the special forces. They do the dirty work that nobody else is willing to do. There is, in this world, a resistance. The anti-particles. All particles, the leptons, the quarks, everybody, have a mirror version of themselves where everything is the same, but the charge is reversed. Charge here doesn't always mean what you think it does. It can mean electric charge. I'm going to give you another example of a different kind of charge later on in the show. But you just take like all the mass is the same. The spin is the same. Everything is the same. But you flip the charges and you get an antiparticle. Matter and antimatter used to be in parity in the very early universe. There were equal amounts of matter and antimatter.
But something happened we don't fully understand. Feel free to ask. In the early universe to tip the scales to give the universe more matter than antimatter. Nowadays, antimatter is only occasionally formed here and there. From high energy reactions, whether in our laboratories or in energetic events in the universe, occasionally you will get antimatter formed. And when that happens, they are incredibly destabilizing, incredibly destructive. The last thing this structured, ordered society wants... is an antimatter particle floating around. Because as soon as an antimatter particle meets a normal matter particle, boom, a tremendous amount of energy is released. And you don't want that. You don't want energy released near your castle walls or in your peasants. As much as you hate the peasants, they still need to do the work of farming the fields and communicating with other atomic nuclei. So you got to keep them around. Would hate for them to go. You certainly can't have your castle walls destroyed because then that means you disassociate as an atom and anti-particles have the energy to do it.
So it's a good thing for these up and down quarks. ruling from their palaces and castles and their strongholds and their keeps and their fortresses, that there isn't a lot of antimatter around. There's one last aspect to this subatomic world. The shadow government. See, the up and down quarks... Think they're in charge, sitting inside of their protons and neutrons with their strong nuclear force, almost entirely resistant to any other force, to any other energy. But you know who's really calling the shots back there? The Higgs. The Higgs is the real power behind the throne. The Higgs is what gives both the leptons and the quarks, both the electrons, top, down, up, bottom, strange, charm, tau, muon, all of them. It's what gives them all their mass. The interaction of an electron with the Higgs field is its mass. The interaction of an up quark with the Higgs field is its mass. Without the Higgs, this whole society would break down. Nothing would make sense. The particles would not have their distinct abilities anymore.
and identities without the Higgs being present. And simultaneously, the Higgs allows for the splitting of the forces. If without the Higgs, we wouldn't have a separation between the electromagnetic and weak nuclear forces. We wouldn't have a separation between photons, the royal messengers, and the special forces, the WNC bosons. They would all be the same and it would be its own thing. And we wouldn't have the rich chemistry and interactions that we know in our real universe without the Higgs. So yeah, it may look like the up and down quarks are the rulers of the thrones, but there's someone right behind them whispering in their ear telling them what they need to do. This story... of the up and down quarks, the other quarks that really don't get to participate in normal everyday reactions, the electrons, the taus, the neutrinos are organized by what we call the standard model of particle physics. It's not really a theory per se, but like a framework of related theories. where the machinery for understanding all this comes from quantum mechanics and comes from special relativity.
You marry those together, you get theories called quantum electrodynamics, quantum chromodynamics, and this is how we understand the electromagnetic force, the weak nuclear force, and the strong nuclear force. There are many things, as it is a huge accomplishment of 20th century physics to work out what is going on at these incredibly tiny scales. But As much of a success as it is in something worth celebrating, something where researchers have received multiple Nobel Prizes for over the decades, there are things that the Standard Model does not explain. Things like neutrino mass. Things like gravity. Things like the nature of dark matter and dark energy. There are known physics that we have yet to incorporate into the Standard Model. What will it look like in the future? How can we extend the Standard Model or possibly replace the Standard Model with a whole new picture? We have no idea yet. That's a separate show. Besides the physics that we know that exists but we haven't incorporated into the Standard Model, like gravity, like neutrino mass, like dark matter, there are some other kind of overarching mysteries about the Standard Model.
Like, why are there three generations of matter? Why are there always three sets of particles? Why do we always get triplets of particles that have the same properties but different masses? Like the electron, the muon, the tau. Like the electron neutrino, muon neutrino, and tau neutrino. Even the quarks themselves, there's six quarks, but they're split into two families, subfamilies, groups, dynasties, whatever. Up, charm, and top all have the same charge, same properties, but different masses. And then down, strange, and bottom all have the same charge, but different masses. Why? Why? Who picked that? Why do we get that one? Why is that our universe instead of something else? We don't know. Why do we have four forces of nature instead of more? Why is there this huge discrepancy where the strong nuclear force is over 100 times stronger than anything else and where gravity is 10 to the 40 times weaker than anything else? Why? We don't know. There are a lot of mysteries in this universe.
The Standard Model is a huge success, painted this wonderful picture of our universe, but we don't fully understand it. And we don't fully understand what else is going on. There's one other thing I want to mention in this show before I go. And I couldn't figure out how to shoehorn it into the little mini narrative, the little picture of the medieval oppressive society that's happening at the subatomic world. And so here it is just kind of awkwardly tacked on to the end. And that's something called color charge. I did mention earlier that in anti-particles, when you take a particle and you keep everything the same, but you flip the charge. And it's not always just electric charge. There's something else called color charge that particles can have. And it's a horrible name, but we're going to go with it. Just like the electromagnetic force talks to particles via its charge, the amount of charge on a particle tells you how much it will respond to the electromagnetic force. Well, through experiments, we found out that there's something else that gives particles the ability to respond to the strong nuclear force.
And we decided to name a color because you need three of them to make a whole set, like primary colors, like red, green, blue, you need to mix together to make white. Well, you need three quarks to mix together to make a proton or a neutron. But there's complications. There's complications because you can also put a quark up with an anti-quark and they cancel each other out to give white. And you can solve a bound particle because a strong nuclear force is really complicated like that. Really, the strong nuclear force should be called the color force. It's what binds quarks together to form protons and neutrons and other groups of particles. It's so strong it spills out to bind them together to make atomic nuclei. The color force, not the strong nuclear force. I just wanted to mention that it really, really the strong nuclear force deserves its whole episode. But I wanted to just paint that little mini picture so you have some idea of what's going on. If you want to talk about color force, color charge, strong nuclear force, feel free to ask.
I'd be happy to do a whole entire episode on it. I just wanted to put that in so you have a complete picture of what's going on in the Standard Model. As depressing, as horrible, as dismal as that world is, that's the world we live in, subatomically speaking. I'd like to thank my top Patreon contributors this month, Justin G., Matthew K., Kevin O., Justin R., Chris C., and Helga B. Go to patreon.com slash pmsutter to learn more. Also, Astro Tours are a go. You need to go to astrotours.co. That's astrotours.co for more information. We have tickets available for the cruise, the Caribbean cruise with Fraser Cain and and the Atacama Dark Sky Expedition in December of 2018. Go there right now and make a reservation and then decide later where all the money is going to come from. It's going to be an amazing trip. I'm excited for both trips. They're going to be fabulous experiences. And I'd really love to share it with you. And space radio is also a thing. Go to spaceradioshow.com. We have tons of fun every week.
You need to try out that show if you haven't already. And of course, thank you to the listeners who asked the questions for this episode. We've got Alessandro M via email asking for an overview of particle theory in the Standard Model. Roger on the website asking about particle letter soup. Martin N on Facebook, what's beyond the Standard Model of particle physics. At Dan Chin on Twitter, please go into more detail on the forces. What are they? And at Pozoker on Twitter, can you please do an episode on the strong and weak nuclear forces and how they can help me be a better person? I missed that part. I missed that part about being a better person. You're just going to have to figure that out on your own. You can ask questions by going on Twitter and Facebook using the hashtag AskASpaceman. Also go to the website, AskASpaceman.com. You can also follow me directly on Twitter. My name is at Paul Matt Sutter. That's also good on Facebook too. You can go to YouTube. Go to YouTube.com slash PaulMSutter for...
All sorts of cool videos. Super fun. Doing lots of collaborations recently. Lots of cool stuff. If you can't donate, then I beg you to go to iTunes where you can give the show a nice little rating and tell other people about it. See you next time for more complete knowledge of time and space.