One of my favorite things about physics is that it lets us think concretely and carefully about the basic nature of our experience. What is space? What is matter? What is time? What's a particle? Why are there waves everywhere? Physics helps us convert these squishy conversations on rooftops over smoke banana peels into robust mathematics. The math doesn't require persuasion or a clever choice of words. It's either right
or it's wrong. So instead of answering a question about the nature of the squish universe, we get to poke and prod our mathematical description of it. Sometimes that gives us real insight, like when relativity tells us that distance and velocity have to be relative rather than absolute, because that's the only mathematical model that describes the universe we see. Other times it surprises us by telling us that we
might just be asking the wrong question. Maybe the things we thought were important just reveal our fundamental misunderstandings of the universe and how it works. Today we'll be asking what physics can tell us about energy, What is it? And more importantly, where does it come from? Welcome to Daniel and Kelly's Extraordinary energetic Universe.
Hello. I'm Kelly Wiener Smith.
I study space and parasites and I just got over having the flu for a week and I really could have used energy.
Hi. I'm Daniel. I'm a particle physicist and I love how Kelly opens the podcast with so much energy every time.
Okay, well we both have a lot of energy.
Speaking of energy, do you rely on caffeine? What is your favorite form of liquid energy?
I do rely on caffeine. I'm drinking coffee right now here. You go, Oh oh for you asmr. Folks. Yeah, yes, I have caffeine in the mornings, but I can't have any really after lunch or interference with my sleep. I'm at that age where sleep is not necessarily a given.
Yeah. Same.
I used to drink coffee all day long. I like didn't drink water in college.
That doesn't sound like a good idea.
You know, I got a lot done, I got a whole lot done, but I could still fall asleep. But now I also if it's after lunch, forget it. I can't have coffee anymore either.
Remember, folks, this is not a health podcast. Do not take our advice.
Noah, no, please don't don't follow our lead. When you were a kid, did you ever try jolts?
Jolt was the kind of thing that was so forbidden in my family. Like my mom was very controlling about the kind of food we could eat, you know, she like counted potato chips, this kind of thing, So junk food definitely off of this jolt. She saw that as like battery acid, Like there was no way she would allow us. Did you try jolts?
Yeah? Do you remember pixie stick?
Yeah?
So there used to be like a megapixie stick. It was like a giant plastic tube like the thickness of my thumb, and it was just filled with.
Sugar and food coloring.
And we had like a party once where we just like had those pixie sticks with jolts.
And I got pretty sick.
And jolts made me feel horrible the way too much caffeine makes me feel now.
But I tried it once, and it was one thing as a kid.
Where I was like, I shouldn't do that again. Anyway, I had a lot more fun.
It sounds like it does sound like you had more fun, but you also got sick. But I have a basic biology question for you about caffeine. I mean, I understand if you eat a pixie stick, it's sugar. Sugar has energy. You could potentially get high and a rush and feel energetic from that. But caffeine, how does that give me energy or make me alert? Is it really energy from the caffeine that my body's using. Is it releasing some energy stores in my body? Or is it some much
more complicated neuro answer, dot dot dot. It depends.
Probably, it depends. I don't actually know.
I didn't prepare the answer for that coming in here. I believe it binds to certain receptors in your brain. My job in college was barista. I worked at like four different coffee shops, and so I've had like shirts with the caffeine molecule on the front. So yeah, I think it binds with some receptors in your brain. And I don't know how that results in you feeling awake, but it works.
It pulls on some levers and ropes of the Rube Goldberg machine that is my mind.
That's right, that's right, and thank God for it. I don't know how I would have gotten through college without it. At some point I was working at a coffee shop because I couldn't afford all the coffee I wanted to drink, but if I worked at the coffee shop, I could afford it and pay my rent. So anyway, I had a problem maybe, but I also had fun.
Well. Energy is a really fun topic because it's something that is colloquial and we talk about it in our lives and whether you feel energetic and whether you drink caffeine whatever, And it's also something deeply important to physics and the universe. So it's one of these topics that people have like a personal feeling about, like where energy is, where it comes from, and what it means. And it's also really pervasive in popular science. You know, equals MC squared,
energy is mass. People tell us there's a lot of nonsense out there about energy. So I thought it'd be good to have an episode where we dig into some details about what energy is and where it comes from and what we know and don't know about it.
Yes, and you always do a great job of getting us back on tracks. That was fantastic, and the other people who keep us on track are our amazing audience, and so we asked them where does energy come from? And if you want to answer questions for us, you can write us at questions at Daniel and Kelly dot org. But let's go ahead and here from our audience, where does energy come from?
Energy comes from heat, which comes from photon. I have no idea.
Is it a mathematical trick, is it just an accounting system?
Who else? Nobody knows where energy comes from, and nobody's ever gonna know. It is the magic of the universe.
I don't think energy comes from anyway, because you can't have not energy.
Well, it comes from fields that permeate the entire universe. Energy comes from hydrogen fusion caused by gravity, So I'd have to say gravity.
Yes, it comes from the Big Bang and has just been circulating in different forms ever since. Or powered Now, if you're talking about the energy that powered the Big Bang, or energy required to keep all the quantum fuels fluctuating and alive, I imagine a physicist is better suited to provide insight.
Hint, hint, So I guess that MLGS comes from symmetries in nature. Obviously the Big Bang has a big role in it, but it might not be the whole thought.
Since you can't create energy, I assume it originates from the Big Bang, and before then who knows.
I guess it's always better around.
It just changes form after form.
Perhaps what we're calling space time is actually energy. Oh well, I really am stumped by this one. I really don't know. All the energy in the universe today came from the storehouse of energy released in the Big Bang.
Some energy comes from the Big Bang, and some comes from dark energy, but most energy in this universe comes from eating your waities.
Have we not always had the same amount of energy?
I would say all energy originated from the Big Bang and everything since then it's just been conversions of different forms. But then you can ask where did that come from? And where does dark energy come from?
And I don't know. There's some really philosophical answers in here, and I loved listening to these.
Well, I mean, that's what I've come to expect from the audience.
It's like always a nice combination of like philosophical some people who get like exactly the right answer or get really close, and then some people who know they don't know and so they.
Just go all in on being hilarious.
I always love listening to the answers.
This comment that you can't have not energy, I had to pause that and think about it for a while. I was like, Wow, that sounds really profound, but I'm not even share what it means.
Like a lot of philosophy, I thought that they were trying to say, if there wasn't energy, none of us would be here, which doesn't really the answer the question where it comes from, but just something had to have made it exist.
I don't know, what do you think?
I didn't know? Yeah, what is not energy? Is that energy equals zero. It's true, you can't have zero energy in the universe because of quantum mechanics. Yeah, fascinating, Yeah, really fun answers here and a lot of assumptions. You hear that energy doesn't come from anywhere because we've had the same amount the whole time. Because people have this conception which we're going to take apart and debunk a
little bit, that energy is conserved. The energy has to come from somewhere because you can either create nor destroy it, and therefore it can only flow.
So yeah, when I was looking through your outline and I saw energy is not conserved, I was like, what, I'm sure I've heard that a lot of times, and so, you know, I think a lot of us are looking forward to having our preconceptions about energy sort of cleared up.
Yeah, exactly. But before we get into the mind blowing revelations about the nature of energy, we have to be clear and careful about what is it we're talking about anyway. Are we talking about how the earth keeps warm? Are we talking about what happens when you eat your wheaties? Are we talking about philosophical revelations about the fundamental nature of the universe. So the first thing we have to do, of course, is define what do we mean when we say energy.
Yeah, so as a biologist, I'm thinking ATP, I'm thinking at it at a chemistry level. We're as a physicist. Do you think about it like that as well? Or this is just a totally different topic.
Now ATP is like a chemical store of energy, so it's completely related to the topic at hand. But it's an example, right. It's like if somebody says to you, hey, what is a vegetable and you're like, an implant is a vegetable, And you're like, yeah, that's true, but what makes it vegetable? Why do we have them? Where do vegetables come from? Right? Showing you an a plant doesn't answer that question.
Right, and then I'll go on a long rant about how vegetables not a phylogenetic category and it's something humans made up, and so anyway, go ahead.
Oh my gosh, wow, oh I love when we accidentally stumble up upon a hidden Kelly rant. Love that. Wow.
Okay, let's go deeper, hurt, but not on vegetables. So what is energy? How does a physicist start to answer this question?
I like to answer this question by pointing out that we can invent any kind of concept. I mean, you can just make up a word called it, you know, bliblyon or whatever, and then define it and it could be a thing. And the question is like, does that reveal anything about the universe? Is it related to the physical universe in some insightful way, or is it just
some nonsense that you made up? And so, from that point of view, like everything in physics is something we made up, and a lot of it is just nonsense we then ignore because it wasn't useful. And some of it actually seems to reflect something that's happening in the universe and is therefore useful, and we think maybe in delightful about the fundamental nature of reality. Right, And so energy is that way. It is something we invented. It's
just like a concept we came up with. We talked recently on the podcast about entropy and the history of energy is similar predates entropy because entropy was defined as moving of energy the een and entropy comes from energy. But this is something people have been wondering about it for a long time, like how do you get a machine to work? And what's going on with heat a temperature? And so people came up with this concept of energy
one hundreds of years ago. It was actually leading It's first identified energy of motion, like things in motion have energy, and he called it this visviva, this living force, just energy of motion. He recognized that.
That was like a thing, but like an asteroid hurtling towards the Earth would have energy. But in what way is that a living force? Was he like specifically thinking about living organism.
No, he was just thinking about animation, not necessarily like something is alive, but in motion. And so I think he was thinking more about the motion than the actual life force or the definition of life in some sort of biological way. But that's something we still recognize, right. We call that kinetic energy today and kinetic energy is definitely a form of energy, like photons have energy, right, they zoom around the universe. They're moving really really fast,
So it's a type of energy. But again, this is like quoting the eggplant, right, it's not a definition of the concept. It's an example of the kind of thing, and it comes from history. And something that's especially fascinating and revealing about kinetic energy is that it's not conserved. Right. For example, you throw a ball up into the air, it slows down, right because of gravity. If you throw it straight up, it slows down, and eventually, when it
reaches the top of its arc, its stopped. So kinetic energy on its own is not something that's conserved in the universe, Like the universe doesn't seem to have a special relationship with kinetic energy. It's the way you could make up any other thing, like, hey, how many ice cream cones are there currently in the universe. That's a number that we don't expect to stay constant. Right, I eat one, it's gone. I've decreased the number. I make one,
it goes up. Kelly makes five thousand ice cream cones, it goes up a lot. Right, It's not anything meaningful or specific and kinetic energy is sort of the same way. Like when the ball gets thrown up into the air, it loses that kinetic energy. Where does the kinetic energy go. Well,
energy can just go up and down. In this case, we know it actually transforms into another kind of energy we call potential energy, because now that ball has gained altitude, which means it's moved up in the gravitational field, and so it's imbued now with something we call potential energy. And so now we have two kinds of energy kinetic energy, energy of motion, and potential energy, which is energy of position or arrangement.
Okay, do all different kinds of energy have the same units?
Yes?
Units, help me think about scientific phenomena.
Yes, all energy has the same unit. So you can use jewels, for example, as a unit of energy, and kinetic energy can be calculated in jewels, and potential energy also calculated in jewels, and you can go back and forth. Right, if you're standing on the top of a building, you have a lot of potential energy because you're high up, you have a lot of altitude, and you jump off that building. You turn that potential energy into kinetic energy because by the time you get to the bottom of
the building. You no longer have the altitude, so you have no potential energy, and you have a lot of velocity. Right, So you've turned potential energy into kinetic energy. And you roll a ball up a hill, or you throw a ball up in the air, kinetic energy turns into potential energy. And the same thing is true for example, like a spring. A spring is a way to store potential energy. And if you push on a spring and then you let it go, it will oscillate back and forth and back
and forth. And what's happening there is it's slashing kinetic energy into potential energy, back into kinetic energy, back into potential energy. So you know, like a simple model of you have a block of stone on a spring, it'll just sit there oscillating back and forth if there's no friction, turning kinetic and potential energy back and forth into each other. Which is also a really helpful way of thinking about
waves in general. Like whenever we're talking about fields oscillating, even quantum fields, that's what's happening is that they're sloshing back and forth between different kinds of energy. And so now we go from having just kinetic energy energy of motion to having two kinds of energy, which, if there's
no friction, do appear to conserve the total energy. So kinetic energy not conserved, potential energy not conserved, but their combination in simple systems with no friction or irresistance or anything does seem to be conserved, which suggests that it's like something may be more important to the universe.
Okay, all right, So for the examples that we've talked about so far, you've got like a ball rolling down a hill, or a ball being thrown up in the air or something that it seems like the energy for those activities came from the energy of the person who's making it happen. Is that an okay way to think about it? Both of those examples seem the energy is coming from the person who set the ball in motion.
Or carried it originally to the top of the building and dropped it.
Yeah, right, right, Okay, so we haven't really gotten to where energy came from because energy seems like it's coming from the person who started it.
Is that a fair way to think about it?
Yeah? I think that's fair. And there's something cool you can do there, which is sort of trace the path of the energy. Right, Like say the person threw the ball into the air, So you're right, it has potential energy which came from its kinetic energy, which came from the energy stored in that person's muscles, right, which converted some sort of chemical energy storage into the contraction of those muscle cells, which accelerated the ball. And that chemical
energy storage came from something that person ate, right. And that thing the person ate probably grew and maybe it was an animal and ate plants, or maybe it was a plant directly and that grew somehow by drinking light
from the sun. Right. And so you can play this game where you trace the energy back, and one of our listeners commented that a lot of the energy on Earth seems to come from the sun, which is true, right, There's like a little bit of a source of energy from inside the Earth, but most of the energy does come from the Sun. And playing this game tracing the energy back has inside of it an implicit assumption that you can trace energy because again, it has to come
from somewhere because it's conserved. It doesn't just disappear and it doesn't appear. And again I'm not saying that energy
is conserved. I'm saying that if energy were conserved, then you can play this game, which is really fascinating because it allows you to like, you know, sort of rewind the accounting of the universe, and it is kind of mind blowing to realize that, like, wow, almost all the energy that we use here on Earth was originally created in the sun by fusion and then gobbled up by plants.
All right, let's take a break, and when we come back, we are going to dig into whether or not mass could be considered a kind of energy.
All right, we're back.
So before the break, we were talking about how a lot of energy on Earth comes from the Sun, and so I'm thinking about you know, plants growing, putting on mass as you know, their chloroplasts, or turning energy from the Sun into energy for them, and then I eat them and put on a little bit of mass, just a little and use that energy to make a lot of ice cream cones, which makes people very happy. Should I think about mass as a third kind of energy.
It's tempting, right to think about mass and another kind of energy, because we talked about stored energy, chemical energy, and atp energy stored in protons as they get fused together in the sun. And you hear a lot that mass is another kind of energy. People say equals mc squared, right, and so therefore mass is energy and energy is mass. Yeah, so that's not really true, and the deeper answer is that mass is just an indicator of internal stored energy.
Think about some object and you can categorize all of its energy into two different boxes. One is the motion of that object, like, is the whole thing moving? If so, it's got some kinetic energy, right. Cool. The other kind of energy you can have is internal energy, like maybe there's a bunch of particles and they have bonds, or there's chemistry or something. All that internal stored energy is what we call mass. Mass is not a new form of energy. It's not a special kind of energy on
its own. It's an indicator of how much energy is stored inside something.
Okay, So like if you've got a big truck and a little truck and you set both of them going down the hill, the big truck is going to have more energy because it has more mass, and it's an indicator that it has more energy.
But it's yeah, exactly, if you measure the inertia of something right by giving it a push, you give something a certain amount of and then you measure its acceleration. That tells you it's inertia because F equalsma. Then what you're really measuring is how much energy is stored inside something. And you can make this very concrete, like take a rock that has a certain mass, shoot a photon at it.
What happens. The photon is absorbed by the rock, and where's that energy go It like goes into the bond somehow, it makes some rock molecule vibrate more, or if it's a gas that you're hitting that it makes those particles move faster. It's some sort of internal energy, right, it doesn't matter. The mass of that rock goes up when it eats that photon, which means like you lie in
the sun absorbing photons, your mass goes up. Or you know, you have an electric car parked in your garage and you've plugged it in and now you're adding energy to the battery. Its mass is going up, right, because that mass is an indicator of how much energy is stored inside of whatever kind. As long as it's internal stored energy, it contributes to the mass. So the mass is not on its own. A new kind of energy it's just an d cater of how much energy is stored inside
and that's what equals MC squared means. E. There doesn't mean total energy. It's not saying that all mass is energy and all energy is mass. E really means just internal stored energy, and there's a fuller equation that captures the full energy of the object as a term for momentum as well. So it really tells you that energy is internal stored energy as revealed by mass and another term for energy of motion energy of momentum.
All right, so mass is just a reflection of kineticum potential.
Energy, internal stored energy, which could be kinetic or potential. Yes, absolutely.
Okay, are there other forms of energy or are they all essentially just different versions of kinneticum potential energy?
Period It's a great question and the answer is yes. Kineticum potential energy are the two categories of energy, and there's nothing else. Right, So everything can be either kinetic or potential energy. And if it's internal stored energy, it could be kinetic or potential then we call it mass. And if it's just motion of the whole object, then that's kinetic energy of the whole shebang. And this helps you understand things like, well, how can photons have energy
if they don't have mass. People write to me a lot and say, like, photons have energy, right, Well, equals empc square, therefore photons have mass, right, Not true, photons don't have mass because again that equation equals empc squared. That e is just internal stored energy. Photons have no internal stored energy because they have no mass. But the full equation has a term for momentum in it, and photons definitely do have momentum, which is why photons also
have energy. So you can have energy without having any mass, right, because photons again are pure motion energy. And then you can play all sorts of fun games to confuse yourself by what this means. Like, you know, take a box, for example, and shoot a photon in it, and don't let the photon get absorbed, put back mirrors inside the box, the photon just bounces around. You still just have a photon in the box. Then what you've done is you've added to the mass of the by adding a massless photon.
People have this intuition that mass is like the amount of stuff in something, but it really isn't. It really just captures the internal store energy. Even if you add massless things to your box, then you're increasing the mass of the box. It's really kind of mind bending. It really makes you change your understanding of what mass is.
I need more coffee, Yeah, but I'm following you. Okay, So let's dig in more to this question about you know, when, if ever, is energy conserved.
So you were.
Talking about throwing a ball in the air and it goes from kinetic energy to potential energy, but it doesn't always get conserved. Can we talk about the history of why we used to think energy was conserved.
It's a really fascinating story. And you know, for a long time we just sort of noticed that energy seemed to be conserved. Like if you created a new quantity number of ice cream sandwiches or something, and then you notice that, weirdly, no matter what you did, the number didn't change, Like every time Kelly ate an ice cream sandwich, Daniel made one, there was this weird cosmic connection or
vice versa. Then you'd be like, something is going on here, right, Like the universe respects ice cream sandwiches or something.
I want to live in that use.
And that's sort of the case for energy. That In lots of situations that we studied, energy seems to be conserved, and so we assumed that it was same was also true for mass. For a long time, people assumed that you couldn't create or destroy mass. A lot of people still out there repeat that, even though now we know that it's not true. We know that you can convert mass into energy of motion or into potential energy that's not internal. I do that all the time at my
day job. Right when we smash protons together, we are turning their mass into new forms of energy. We're turning their energy into mass all the time, back and forth, and every time you absorb photons from the Sun, you're turning their energy into mass. But that's a hard thing to notice unless you have control a tiny part of
goals and you can see these things happen. Which is why chemists for a long time reasonably believed that mass was conserved, because they would measure the stuff going into a reaction and the stuff coming out of a reaction, and they never saw a change to within their measuring uncertainties. Now, of course, we know that if they had measured more accurately, they would have noticed very tiny changes in mass because
mass is very very dense stuff. Right. The C square there tells you that a big change in energy is required to make even a small change in the mass. Anyway, the same thing was long true for energy. People are like, okay, well mass is not conserved, we're going to give that up. But energy still, Energy definitely has got to be a conserved right, because people made all these measurements, and it seemed like the more accurately you measured it, the more
you discovered energy had to go somewhere. So in realistic studies, for example, say you throw that ball up into the air, what's going to happen. Well, it's not going to convert all of its kinetic energy into potential because some of it's going to get lost to air resistance. Or if you roll a ball down a hill, right, some of it's going to get lost to friction. That energy seemed
to creep out of those systems. But as long as you define the system to include those things, you account for friction and you account for air resistance, you're able to account for all of that energy. The balls flying up into the air and it loses energy to air resistance, but that energy then just goes into heating the air, or if you're sliding down a hill and you feel friction between your pants and the grass, then you're heating up the grass and your pants. That energy is still there.
And so for a long time people just noticed, even with more and more careful accounting, that energy did seem to be conserved.
So I remember that we were talking about Emmy notors on. I worked so hard to not say her name during the episode because I knew I'd get it wrong, But here I went and put myself in that position.
But anyway, that she had come up with a.
Really great law or theory that explained when things are symmetrical and when they're not. And so it sounds to me like we're going to determine here that energy is not symmetrical, and that Emmy would tell us that if she were here.
Yeah, it's really cool that we can do more than just measure the number of ice cream sandwiches and notice that they do or do not change and then conclude something. It's not just empirical. We actually now have theoretical ways to argue whether something should be conserved or not. And as you say, that's due to Emmy Nuther whose name I'm sure I'm also mispronouncing, and the number of mispronunciations
is not conserved. Like it just seems to grow, go up and up and up every time I hear this name, different pronunciations of it. So German listeners, please don't be shy right in and correct our pronunciations. We want to get this stuff right anyway. I mean, other told us that every time we have a conservation law in the universe. You're right, it's connected to some symmetry of the universe.
And symmetry has a sort of special meaning there. It says that you can transform your experiment, you can move it or rotate it in some way, and you should get the same answer. And so a famous and powerful example is momentum conservation. You know, alongside energy, we have this concept of moment momentum, which is similar to energy. Right, Momentum contributes to your energy. We said, there's internal storat energy and then this energy of momentum which come together
to make your total energy. And the famous and important example of that is momentum, which is something that is actually conserved in our universe. And when we say symmetry there, what we mean is that you can do an experiment and then transform the experiment according to some symmetry and do the experiment again, you should get the same answer.
So if you build a physics experiment and you do it in space, you know, you collide billiard balls or whatever, and then you do it ten meters to the left or one hundred meters to the right, or forward one thousand meters, you should get the same answer. So that's a symmetry of the universe. The universe doesn't care where you do your experiment. As long as you set up the conditions the same way nearby masses or whatever, you should get the same answer. That's a fundamental symmetry of
the universe. And that symmetry gives you conservation of momentum. So Emmy Nuther has this amazing conceptual bridge which you tell you what symmetry generates a conservation law. So conservation momentum comes from this symmetry of translation, where you don't care about where you are in space, where space is relative. There's no like official zero, there's no like golden numbers glowing in the universe. It doesn't matter where you do your experiment. And so now more than just saying hey,
look I noticed, momentum is conserved. You can say why it's conserved, and you can inspect that reason, say does that make sense? You know, we can say momentum is conserved because the universe has no preferred location. You can do your experiment anywhere, and we're like, all right, that makes sense and that's a good reason for momentum to be conserved. Okay, So what about energy? I mean, not there tells us that energy is conserved in the universe
if the universe's laws don't change with time. Right, So, momentum is conserved if the universe's laws don't depend on location, if you don't have like different laws of physics in different places, Energy is conserved if the laws don't change with time. If like this, the same rules of electromagnetism applied today and tomorrow and in a thousand years.
It feels like I want that to be there. But I think we've decided that energy is not concerved. Can you give me an example of how it's not conserved.
Yeah, it's fascinating because we assume that the laws of
physics are constant all the time. Think about just how simple it is to read about an experiment in a journal and say I'm going to reproduce that experiment and see if it's true, and I'm going to do the same thing and see if I get the same answer, and reproducing it later, and assuming you get the same answer, assumes the laws of physics shouldn't change, right, And it's really important to our whole process of science that the laws of physics were uncovering now are the same as
they were a thousand years ago or even a billion years ago. Like when we look at galaxies deep out into space in early times, we assume we know what the speed of light was back then, and how mass worked and all this kind of stuff. So the universe without any constant laws is very difficult the universe to do science in, right. And so it's not true that the laws of physics are just wially nearly changing with time.
That's not something we've seen. But there are a couple aspects of the laws of physics which do seem to be changing with time, and one of them is that the universe is expanding, so space itself, the frame in which we're doing all of this stuff, is not static.
And when Emmy Nuther says the universe laws have to be constant in time that includes space, right, it means space itself needs to be constant the way like conservation momentum requires you to be the same location in space, it's put in the context of the space in that same way, energy conservation requires you to have basically the same amount of space. It's not changing the laws of electromagnetism or you know, the speed of light or anything.
It's just saying, hey, the conditions of your experiment are not constant because your universe is expanding. And we've known for like twenty or so years that the universe is expanding, and not just expanding, but accelerating in its expansion, Like every year we are making more space between galaxy clusters and between ice cream sandwiches and everywhere in the universe.
Space is expanding. And we actually have a not so terrible explanation for why space expanding would mean that energy isn't conserved.
Oh, I want to hear about that after the break.
All right, we're back.
So you blew our minds a second ago by explaining that the expanding universe is why energy can't be concerned. It's crazy to me that something that's happening so far away but I guess also happening everywhere is impacting my day to day experience of energy.
But go ahead and tell us the crazy thing you.
Replaying, William, I think, well, that sounds kind of abstract, right, Like you're telling me the universe is expanding, and so in some sense the context of our experiments is changing. But why does that mean that energy is not conserved? Like is energy being created somewhere or destroyed somewhere. So let's tack into the details there, Like energy actually is
being created by the expansion of the universe. We talk about the expansion of space as creating more space, right, Like the distance between us and some cluster galaxy is growing, and not because we're applying some force to accelerate away from them. This is just like general relativity. Space is expanding, right, We're not measuring any acceleration. They're not measuring any acceleration with their accelerometers. But the distance between our galaxies is
increasing and faster and faster every year. So new space is being made. Well, what does that mean? For new space to be made? Space is filled with quantum fields, right, There's the electron field, there's electromagnetic field, there's fields for all the quarks, all these fields in space, and all those fields have non zero energy, like these are quantum fields, which means they can never go down to total zero.
They always have some energy in them. So you pop a new cube of space, boom, it comes with quantum fields that have energy in them. Right, So new space means new energy. So right there, making more space means you're creating energy, that doesn't It just.
Mean the energy gets spread more thin across this new space.
Because every chunk of space is the same and the rules of quantum mechanics have a minimum requirement of that energy. It's not something we deeply understand, but it's something that's required for the accelerated expansion of the universe. We know that dark energy behaves this way. We see, for example, as the universe expands, the density of dark energy doesn't change in the universe like the density of protons goes down. You have a certain number of protons, you know, some
huge number of protons in the universe. As the volume of the universe increases, the density of those protons drops exactly the way you would expect. Right you double the volume, that density goes down by a factor too. But dark energies. Density does not decrease. You double the volume, the density stays the same. Right, So dark energy behaves in this weird way that when you create more space, you also create more dark energy.
Where does it come from?
Where does it come from? Yeah?
How does that keep happening?
You ask where it comes from? And that's a question that only makes sense if it's conserved, if it has to come from somewhere, right, Like if I make an ice cream sandwich, do you say, hey, where did that ice cream sandwich come from? That seems to violate the loss of physics.
You're like, no, I wouldn't ask questions about a new ice cream sandwich, Dane, I would just enjoy it.
Right, go on. You know it's true you have to assemble it out of bits, But like the assembly itself didn't violate some law physics. You didn't have to destroy an ice cream sandwich somewhere else to make this one. Right, there's no limit on the number of ice cream sandwiches.
Thank the Lord, hallelujah.
Yeah, I know, we're all relieved. And so that question where does the energy come from? Only make sense if things have to flow. If you can just change the total energy in the universe or change under ice cream sandwiches. Then the question itself isn't actually as meaningful.
Right, So energy comes from nowhere. Energy can just like poop, pop up.
Energy can just pop up. Yeah, absolutely, and you can also disappear.
Right.
We talked about how the density of protons changes as the universe expands and the density of dark energy. So protons dilute in a way that makes a lot of sense, right, Like you double the volume, the density goes down by a factor of two. Interestingly, side note, same thing is
true of dark matter dark matter. As you double the volume of the universe, dark matter density drops by two, which is one reason why we think of dark matter as matter, because it dilutes the way it matter does, because radiation doesn't photons, As you double the volume of the universe, their energy density drops by more than two. Like you have a certain number of photons in the universe. Now you increase the volume of the universe, you have
the same number of photons and more volumes. You might think, oh, the energy density just decreases because you've increase the volume. Not true, because the expansion of space also red shifts those photons. It stretches their wavelength to rhetoric or longer wavelengths. Longer wavelength means lower energy. So you have a universe filled with photons, you expand that universe, their energy density drops faster than the energy density of matter or dark matter.
Where does that energy go? It doesn't have to go anywhere.
It just goes moves into the space that was just created.
I've solved it. Where's my Nobel price?
I love that idea. People are right to be about that a lot. They're like, Okay, energy is creative when you expand space and also destroyed. Do these two numbers add up, And it would be amazing if they did, right, And that would be a beautiful explanation, you know, plus seven thousand minus seven thousand boom. I think we figured
it out. These two numbers are vastly different scales. Like the energy and photons in the universe is tiny, like much less than one percent of all the energy and universe is in photons, but the dark energy of the universe is like two thirds of all the energy. So energy is increasing in dark energy much much more rapidly than energy is decreasing by redshift of photons. So we don't think that photons are like getting turned into dark energy or anything.
Okay, So bottom line here, we don't know where energy comes from. We know it's not conserved. Physicists have job security.
Yeah, we don't even really still have an answer to the original question of like what is energy? Right, still a description of something we've seen, and we have a little bit more theoretical footing now because we could, in principle define energy as the thing that would be conserved if the laws of physics were perfectly symmetric in time. Right, you can use Nuther's theorem as a definition of energy and say, oh, energy is that thing which would be conserved.
And that's kind of cool. I like that. It is very theoretically grounded, but it doesn't give you like a conceptual sense for like what is this thing? But unfortunately that sort of turns you back to the philosophy of it, like, well, what do you mean what is this thing? What kind of answer are you looking for when you ask what is energy? Part of the question is not satisfied by like, well, here's the description of the kinds of energy, here are
the rules that it follows. Here's how it's created, here's how it's destroyed. Here's how it's not concerned. But part of the question is not answered by that description.
Do you think I have no idea?
Yeah, I feel like there's something about it that's still unsatisfied, that's still like yeah, but what is it? You know, like, why do we have it? Where does it come from? It's a deep human question, but like the human questions we ask, aren't always the appropriate questions, aren't always the ones that answer the question. Sometimes we discover that something is different from how we expected it to behave and that means the kind of questions we should ask about
it are changing. Because energy isn't something that comes from somewhere. It's a feature of the universe, and it does seem to be important because under lots of context it is conserved, but it's not fundamental in that way, and so it could just be something that humans notice, something the humans like to calculate, something that connects with our everyday experience in a way that it's important to us, but isn't Like alien physicists, for example.
It only appears to be conserved on human scales because it's being lost at such low numbers it's hard to measure, but it's not even conserved on small scales.
It just looks that way.
Yeah, that's exactly right. And I think it's useful to think about this because it impacts other questions we have. Like a lot of listeners who wrote in Thought about the conservation of energy, and it talked about how the Big Bang had energy, and maybe all the energy in the universe just came from the Big Bang, right, And so I hope the answer today reveals that, like, no, there's some energy in the universe which was created after the Big Bang. Right, This expansion of the universe is
making energy, and that's post Big Bang energy. And also some of the energy of the Big Bang is gone now. Like you have the Big Bang, you have matter and anti matter created, they annihilate. You have a universe mostly filled briefly with photons. So photons did dominate the universe energy budget initially, but a lot of those photons are
now red shifted and their energy is gone. Like we talk on the podcast a lot about the cosmic microwave background radiation, this energy from the early universe that reveals so much about how the universe came together and what it means and how it rippled. But those photons are very very red. They're at a temperature of like two point seven calvin, very very cold. The plasma that made them was like three hundred thousand degrees calvin. Right, they were a glow of them, a very very hot plasma,
very energetic photons. Originally that energy is just gone now right, It didn't go anywhere. So a lot of the energy in the universe didn't come from the Big Bang, and a lot of the energy of the Big Bang is now gone, it's fizzled out.
Do we have less energy over time, more energy over time, or do we not know how this balance is working out.
We definitely have more energy over time because dark energy is much more dramatic, and so all the energy created by dark energy, the expansion of the universe vastly outweighs the energy loss due to red shifts of photons.
You would said that earlier.
But is that the only way that energy is lost or is energy lost while humans are doing all of these reactions down here, and is that impacting the like mass balance equation.
For the universe?
To my knowledge, the expansion of the universe is the only source of energy gain or loss. Right, You noticed that it's an underlying mechanism for both of these things, because it's the place that Norther's theorem is violated, right, It's the thing that doesn't respect the time symmetry. So the expansion of the universe redshifts those photons, breaking conservation of energy and creates news space breaking conservation of energy.
So anything that's sensitive to the expansion of the universe can be a source of the violation of conservation of energy. And I want to make another comment about the Big Bang, which is it seems like a little bit of a loss of power to explain and explore, Like if energy was conserved, then we could do this cool thing of tracing it back into the Sun and then where that came from, where that came from, all the way back
to the Big Bang. It's like we have a ledger, you know, the tell us how energy slashes around, and maybe by tracing that we could learn something. And that seems really cool, and it feels like, oh, oh, maybe we've lost that. Maybe we can no longer learn about the Big Bang by studying energy. Well, just now we can ask different questions, you know, we can ask how the energy for the Big Bang came about, Like we know that the universe was filled with hot, frothing energy
early on. Where did that come from? We no longer have to just answer that by saying, oh, it came from this other thing with equal energy. Now we have more ideas about how you can create all that energy from some earlier denser states. So, if anything, it opens up the examination to think differently about the origin of the universe. And you know, that's what we want. We want our understanding of physics to evolve and to give us new ways to think about the whole context of
reality and where it all came from. And so to me, that's exciting. It like removes blinders a little bit and gives us a broader sense for how the universe works and maybe how it all started.
I bet you are great at writing grants. Okay, we were wrong about that, but that's okay, because it's the fascinating thing is actually revealed now.
So please send me a million dollars to make a lot of ice cream sandwiches, because I have a lot to learn.
You know, please share those ice cream sandwiches. Daniel, Yes, Daniel needs this money.
It's important, everybody exactly. My cookies and cream project really top priority for national security.
That's right, that's right, all right.
Well, I learned a ton today actually, and I can't wait to tell my daughter actually about some of this stuff and blow her mind somehow.
I feel like she's going to find this really interesting.
Yeah. So energy is not conserved, which means it doesn't have to con from anywhere. But our understanding does seem to be growing, and it seems to em and a from these deep studies of how the universe works. So let's keep doing that.
Let's hope our understanding is expanding faster than the information we're losing.
All right, go off and enjoy ice cream sandwich. Everyone on us?
Oh, not on us, Not on us, Daniel.
Virtually on us, spiritually on us.
Don't send us three sets.
That's all right, emotionally on us.
There you go, Okay.
Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio.
We would love to hear from you, We really would.
We want to know what questions you have about this Extraordinary Universe.
We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.
We really mean it. We answer every message. Email us at Questions at danieland Kelly dot org.
You can find us on social media.
We have accounts on x, Instagram, Blue Sky and on all of those platforms.
You can find us at D and K Universe.
Don't be shy right to us.