Are there more than four forces of nature? Is dark matter connected to dark energy? What the heck are dark photons? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
What does it mean for there to be a force of nature? We're surrounded by things that are constantly, well, happening. You drop your keys and have to pick them up. The chair creaks when you sit in it. The sun feels good on your skin.
What's that smell? The world around us is alive and buzzing with activity. It appears as if reality is made of objects that are constantly interacting in a number of weird and wonderful ways. So what's the difference between an interaction and a force of nature? The forces of nature are fundamental.
This is a very, very important word. It means that this force, a cause of an interaction cannot be broken down into smaller bits. It itself can explain a variety of interactions. It explains a bunch of phenomena. It can't be broken down.
It is important. It is essential. It is critical. It is fundamental. What we call a force of nature is our theory of an interaction.
We see something happening in the universe, whatever that thing is, and we ascribe a force to it. And the force is what we use to explain the interaction. You drop your keys. That is some sort of interaction between you, your keys, and the ground. The forces, the fundamental forces are the things that are able to explain that interaction in as few assumptions as possible.
In a single day, you might encounter roughly a bajillion interactions, but how many fundamental forces are responsible for those interactions? Is it one? Is it 10? Is it a thousand? Is it a bajillion?
The first force to be recognized as a force was gravity as we saw in that episode I did on Newton a while ago, and it was Newton who named it as a force. He said, look. There's all these interactions that we see, falling apples, orbiting moons, tides, etcetera, etcetera. I think I can explain it all with a single force, and I will call that force gravity. Previous to Newton, there are just a bunch of unrelated interactions happening.
Newton was able to wrap it all up in a single force. The same thing happened in the eighteen hundreds with James Clerk Maxwell. There were a bunch of interactions, magnets sticking to metals, electricity running down wires, etcetera, etcetera. So all this stuff was happening, and Maxwell was able to wrap it all up into a single explanation and use a single force, the electromagnetic force, to explain it. As bonus points, he kind of invented light.
I will get to that when I someday do an episode on James Clerk Maxwell, which I've been wanting to do for a few years. And after that, we had two forces, gravity and electromagnetism. And just these two forces were able to explain the variety of every interaction we saw in the universe. Think how powerful that is. That's what it means to be a fundamental force is that's power, where you can write down a simple equation like Newton's gravitational law or Maxwell's equations for electromagnetism and explain basically everything.
So as we enter the twentieth century, there were two fundamental forces, gravity and electromagnetism, and then we started to find weird stuff. First were the atomic nuclei. We realized over the course of a bunch of experiments that the nuclei of atoms, the cores of atoms were just that. They were cores. They were heavy.
They were massive. They were made of lots of particles, and all those particles were either neutral, which we call the neutrons, or positive, which we call the protons. That was a bit of an issue because protons being positively charged should repel each other. Neutrons just don't care. They're just sitting around like, oh, no.
None of my none of my business. I'm just here for the free food. But the protons should hate each other. They should have repelled. Everything we knew about the universe, gravity, and electromagnetism couldn't explain the nucleus of an atom.
Many decades later, we figured out what was responsible, a new fundamental force of nature, the strong nuclear force. And then came beta decay. So then we had three forces of nature, but then came around beta decay, which is just a neutron randomly deciding one day when nobody asked and nobody wanted just, you know what? I'm gonna be a proton, and boom, it becomes a proton, and it spits out an electron in something called a neutrino. Like, no big deal.
Just seriously, if you just stare at a neutron long enough, it will just turn into a proton. So, oh, you thought I was a neutron? Well, I'm gonna be a proton instead. Boom. Now what are you gonna do about it?
None of our forces of nature were able to explain it. We can't explain the transformation from neutron to proton with gravity, with electromagnetism, or with the strong nuclear force. We had to invent a new force of nature, the weak nuclear force, to do that thing. And here we are decades later, and we have our four fundamental forces of nature that explain everything we see in the universe. We've been down this road before.
All the activity, all the interactions, all the behavior boil down to four and only four forces. Let that sink in. Let that sink in from the deepest interactions in an atomic nucleus to the evolution of the universe itself, we can explain with four forces of nature. That's kind of powerful. We've tried really hard to find new forces of nature.
We've looked for variations in the strength of gravity. We've looked for variations in the speed of light. We've looked at atomic reactions and nuclear reactions and the behavior of nuclei and everything and everything and everything and everything and everything, and it all just works. It seems like the universe is made of four forces and only four forces. Almost.
You see, there's this really annoying thing about modern physics. We have the standard model, which is how we understand three of the four forces of nature. It's how we understand electromagnetism. It's how we understand strong nuclear. It's how we understand weak nuclear.
We understand all of these forces in a quantum context using something called techniques called quantum field theory. It's all super cool and super complicated, but it's the way it goes. It's the standard model from these three forces. The forces aren't exactly straightforward. They interact in very complicated ways, and we express that complication, the richness and the variety of how these three forces, actually express themselves in the standard model.
And then when it comes to gravity, we have general relativity. Not a quantum picture. It's a classical picture, curvature of space time, Einstein, all that good stuff, you know, relativity. But with those four forces, it's a rich and complex world, and we have a surprisingly brief description of it. That's how we know we're on the right track, by the way, when simple statements can explain many things.
You can write down Einstein's equations. You can write down expressions for the weak nuclear and strong nuclear force. You can write down Maxwell's equations for the electromagnetic force. You can write down the quantum counterparts, the quantum field theory descriptions of nature. And it's a relatively short statement.
It's like a a couple paragraphs of math, and you can explain everything. But the really annoying thing is that all the centuries of sweat and labor and understanding and digging and coming up with fundamental forces and working out the math, the generations of people that went into explaining the richness of the everyday world amounts to somewhere around 4% of all the matter and energy in the universe. 4%. Four %. It took us over three hundred years of scientific work, and we've been able to kind of sort of wrap our heads around 4%.
Most of the universe is dark. It's It's made of either dark matter, which is around 25, 20, 20 five percent of the universe or matter and energy budget in the universe. If you take a random chunk of the universe, 25% of it will be dark matter. It is a matter. As far as we can tell, it is a thing.
It is an object. It is made of particles. It is made of stuff the same way you're made of stuff, and it simply doesn't interact with light. It doesn't participate in the electromagnetic force, which is fine. No one's obligated to.
We don't know what that particle is. We don't know what it's made of. We don't know what created it, and it's just we don't know why it's such a big deal. And then the other two thirds to 70% of our universe is something we call dark energy. That is the accelerated expansion of the universe.
We have no clue what's going on. We know it's not a particle. It's not a thing you can hold in your hand. That's why we call it some sort of energy, but other than that, we really don't know what it is. And in case I haven't made this clear enough, in any of my previous episodes or this episode so far, when it comes to dark matter and dark energy, we have no idea what's going on.
We don't know what is responsible for the dark matter. We don't know what's responsible for the accelerated expansion of our universe. We don't know what 96% of our universe is made of. We know it's out there. We can get some broad strokes on how it behaves.
We know that dark matter clumps and that normal matter nestles inside of it to make galaxies. We got that covered. We know that dark energy causes the accelerated expansion of the universe, but other than that, we're, well, in the dark. And one of the biggest mysteries is the amount of dark matter and dark energy. If you take the total energy budget for the universe, if you add up all the things, all the sources of matter and energy, dark matter gives you around 25%, and dark energy around 70%.
Let's talk about the dark matter first. Why is it 25%? Why isn't it 1%? Why isn't it 62%? Presumably, there's some fundamental reason why it makes up 25% and not some other number.
And it might be connected to why normal matter only makes up 4%, but it's just some random number, so we don't really need to sweat about it. Right? Like, the universe could have picked any number it wants for the amount of dark matter, and it picked 25. Okay. Whatevs.
The universe just pulled that much dark matter out of a hat, and that's what we got. That's the universe we happen to find ourselves in. Why are there this many protons? Why are there this many dark matter? Where is this this much radiation?
It's just part of the game. Could have been something else, but it wasn't. So here we are. It just added to the list of numbers that we have no explanation for, which I really should do an episode on. So feel free to ask about some of the numbers that we have no explanation for, but just add it to the list.
We'll figure that out someday. It's just another mystery of the universe. But then there's dark energy. And there are two issues with the fact that dark energy is at 70%. The first issue is why is it not 0% or 100%?
Here's the thing with dark energy. Dark energy, as best we can understand, and and this is a simple way to model dark energy, we don't know if this is on the right track, but it's a good way to talk about dark energy, is as something called a cosmological constant, as something called a vacuum energy. It's it's a way of framing our discussion even though we don't really understand it, but it seems to operate in a way that has constant density. Like, if you have a box and you empty everything out, your box is now full of vacuum energy. It's now full of dark energy.
It has constant density, which means you get more of it with bigger volume. So if you take your box, your empty box, and you stretch it out and make it eight times the volume, you now have eight times as much dark energy as you did before because the density was constant, but the volume went up. So the total amount of dark energy also went up. Okay. So why is this a big deal?
It's a big deal because in the very early universe, when our universe was just a baby, our universe was smaller. The volume of our universe was smaller. The total amount of dark energy was basically zero because the universe was small. Its fraction of the energy density budget of the universe was around 0% because there wasn't a lot of it because the volume of our universe was small. And in the far distant future, our universe will be way bigger than it is today.
With every passing day, we're getting more and more dark energy into our universe because the density is constant and the volume is growing, so we're getting more dark energy every day. No. This is not a problem. Yes. You are allowed to create things out of thin air.
Feel free to ask the question why. Happy to do another episode on it. I I've touched on it in previous episodes, but I'd be happy to dig into it more for you. But for now, for our discussion here, let's just take it as a given and move on. We are getting more dark energy as time goes on.
That's just a fact. So in the far distant future, the amount, the fraction of stuff in the universe that will be dark energy will be basically 100%. Normal matter dilutes itself because we have so much matter in the universe, and it's getting thinner and thinner and thinner and thinner as the universe expands while the amount of dark energy grows. So the amount of normal matter stays fixed. It just gets thinner.
Its density drops while the amount of dark energy grows. So in the past, there was a lot of matter crammed into a small volume, not a lot of dark energy because it was a small volume. And so the percent, the budget that went to dark energy was basically zero, like point o o o 1%. And in the future, the far future, it'll be 99.999999% dark energy. Why are we at 70?
Why are we between these two extremes? This transition period where dark energy goes from basically zero to basically all of it is relatively short in cosmic history. The universe will spend if we fast forward a trillion years from now, the universe will have spent most of its existence completely and totally dominated by dark energy, nearly 100% dark energy. We are in a very strange transition time. Why?
Why is dark energy between these two extremes? Did we just get lucky? Did we just arise in consciousness and get to observe the universe, and we just so happen to get a lucky break? Like, wow. We caught dark energy in this transition zone.
How cool for us. It could just be a coincidence, but astronomers don't really like coincidences. It seems a little bit weird. Seems unlikely. So that's one issue with dark energy is the fact that it's stuck or that we happen to just so observe it as it's making the transition from nearly unimportant to absolute dominator in the universe.
Second, why does dark energy just so happen to be roughly the same as dark matter? Of all the numbers in the universe, which the mathematicians tells me is, quite a few, Why did the cosmos pick two numbers that were relatively close together? Yes. Dark matter is at 25% of the universe. Dark energy is at 70% of the universe.
I know 70 is a larger number than 25 and not very close to 25. But when it comes to astronomy where things can be billions of times different and usually are, why did the universe pick that? Why do we see dark matter and dark energy? This unknown component of the universe, both of them at roughly the same. Again, maybe it could be a coincidence.
The universe has this had this much dark matter to play around with. Its density is dropping with time because there's so much dark matter and it's getting diluted. So its fraction of the energy budget is dropping with time. It used to be the major player. It used to be number one.
But then dark energy with as time goes on, we get more and more dark energy with time. So it's going to be number one, and we just happen to witness this transition point. The relatively brief few billion years as dark matter winds down and dark energy winds up, and we just happen to arise in consciousness and see it happen, and aren't we lucky and aren't we special? Like the eclipses, like the moon and the sun just happen out of pure sheer cosmic coincidence to be roughly the same size on our sky so we get to enjoy eclipses. Are we just happening to watch a cosmic eclipse, or is there a conspiracy?
In science, we don't like coincidences. Every scientist is at heart a conspiracy theorist. Every scientist thinks nature is always listening and how to get that. Now what I I mean I mean, when we see things like this, our spidey senses go off, and we think there there's more to the story. Maybe dark matter and dark energy are connected.
Maybe there's an interaction. But what could that interaction be between dark matter and dark energy? Well, is it gravity? We've tested gravity on these scales. Dark matter is just behaving exactly as, general relativity would predict.
Dark energy is already folded into general relativity in our understanding of the expansion of the universe. Any interact extra interactions between them. As far as we can tell, gravity isn't gonna do the job. Electromagnetic force. Well, by definition, dark matter doesn't interact with the electromagnetic force or at least we don't think it does.
If it did, it wouldn't be very dark now, would it? So that doesn't seem to be the case. Strong nuclear, I mean, it only affects it only has an interaction range within atomic nuclei. That's its range. It's very powerful, but very limited, so that's not gonna do the job.
Weak nuclear, nobody cares about weak nuclear. It does nothing except transform protons into neutrons and vice versa. It's not doing anything interesting. If dark matter and dark energy talk to each other, if there is some maybe a reason why the dark matter and the dark energy are somewhat have roughly the same values, there's a reason, there's a connection, there's an interaction, maybe there's a fifth force, a new fundamental force of nature discovered in the same way that the strong nuclear and the weak nuclear forces were found by interactions that we couldn't currently explain. If they are talking to each other, there is a fifth force of nature.
We call it quintessence or fifth essence because it's a fifth force, and it works by introducing a new force of nature that only dark matter and dark energy get to feel. The reason that we haven't seen this force of nature before in any of our experiment experiments or observations is that it doesn't talk to us, which is okay. The electromagnetic force doesn't talk to dark matter. So maybe dark matter gets its own force that doesn't talk to us as payback. Maybe dark matter and dark energy talk to each other through a fifth force.
Okay. That's nice. This force is used to explain something we see, but that's not good enough. It has to make a prediction. That's the game of science.
Has to be different. Otherwise, we can just assume that the dark matter dark energy connection is really just a coincidence and move on with our lives. Just noting the existence of this coincidence isn't good enough because it could just be a coincidence and until us we know more or suspect more, that's that's all we got to go on. So if you wanna propose the new fifth force of nature, you have to make a prediction. How would our universe be different with the existence of this force?
How would this fifth force manifest itself? Could we detect it on Earth or the solar system or the galaxy? Probably not. Because anything that's connected to dark energy is only going to show itself on very large scales. It's only going to show itself on cosmological scales.
Inside of a galaxy or solar system on the earth, there's just too much other stuff going on. Like someone whispering in a crowded noisy bar. There's just too much other stuff going on to detect this Fifth Force. I got it. We need Patreon.
Go to patreon.com/pmsutter to learn how you can contribute to keeping this show going and to make a fifth force detectable. It's true true science. Science. Now in order to detect and measure this potential hypothetical fifth force, we have to study the universe as a whole. We have to do cosmology.
And in these models of a fifth force, dark matter and dark energy don't just have a coincidence that they're roughly the same, that this extra force between them creates what's called a tracker, where the amount of dark energy in the universe talks to and responds to the amount of dark matter in the universe and vice versa. They keep each other in check. Like, the reason we don't see dark energy being 100% of the universe is that it can't because it's somehow tied to the amount of dark matter. And the reason that we see roughly the same amount is that there's a reason for that. There is a new force that keeps them both in check.
But if this fifth force exists and is actually doing that and is causing dark matter and dark energy to talk to each other, then it should show up in other ways. I mean, you're introducing a fifth force of nature. It should do something other than this, and you can work out the math. How it shows up has to be very, very subtle. Otherwise, we would have seen it by now because we've gone looking for it.
And if there was, like, a very strong, very clear fifth force of nature, then when we study like, started making our basic observations of cosmology, it would have shown up. Like, if we started making our cosmic microwave background maps and it didn't agree with our theories whatsoever, that would be a clue that our theories are incomplete, that maybe there's a new force of nature. So it has to be subtle. It has to be subtle. Otherwise, we sort of seen it by now.
But you can work through the math of let's say, there is a fifth force in nature. You write down how this interaction looks, how the dark matter into dark energy talk to each other, and then you can see what kind of universe that would make. Maybe it alters the shapes of cosmic voids, the big empty spaces between the galaxies. Maybe it alters how frequently clusters merge together. Maybe it alters how the large scale structure of our universe evolves over billions of years.
And you can study this in simulation. You can make pretend universes on a computer, including all the forces of nature that we know about, plus a hypothetical fifth force, and see how it changes. See how it changes the evolution of voids. See if clusters of galaxies are slightly different than they were in a universe, without this fifth force. And yes, this gets into the weeds of careful statistical modeling.
And, yes, it's hard. And, yes, it's the only way to do it. It's hard to compare simulations to observations because, yes, we can make pretend universes, but we can't necessarily make those pretend universes pop out galaxies and stars and cosmic rays and supernova that we can compare directly to observations, but we can do a very good job. Our simulations are getting better and better every day, and observations are getting more and more detailed every day. But the end result is we can't find any evidence for a fifth force of nature between dark matter and dark energy.
We've tried. We've tried. And this is actually some of my own personal research years ago, investigating, how fifth forces of nature altered properties of clusters of galaxies and how they altered voids, the structures in the universe. So far, we have no evidence for a fifth force of nature between dark matter and dark energy. So right now, a fifth force involving dark energy isn't in the cards, at least not yet.
But what about a fifth force within dark matter all by its lonesome? Dark matter makes up 25% of the universe, makes up, like, 80% of all the matter in the universe. Is it big and dumb and simple? And then what we call normal matter has all the rich and complex and interesting chemistry, or is dark matter as big and complex and rich as normal matter? Does dark matter have extra forces that only talk to dark matter?
Is there dark matter chemistry? Is there a dark matter periodic table of the elements? Do any of these extra forces that might exist within dark matter also talk to normal matter? There are a lot of theories because theorists have had a lot of lonely nights to think creative things, but the simplest scenario, the simplest if we're gonna go down this road of introducing extra forces of nature in the dark matter, The simplest force is that there's a force like the electromagnetic force, but only for dark matter, which means it's carried by a special kind of photon that only talks to dark matter. It's like a photon for dark matter, dark photons.
I know. I'm sorry. It's not my fault that physicists come up with the absolute cheesiest names possible. They call these hypothetical particles dark photons, and we just have to live with it because we don't have any other choice. So if we can find dark photons, then that will prove the existence of a new fifth force that operates mostly on dark matter, but occasionally leaks over to the normal physics world.
Back in 2016, a research group claimed the detection of a new particle, for those of you keeping scores called x 17 because they had a massive 17 mega electron volts, which makes it about 34 times heavier than the electron. That's odd to find a particle with that mass because we thought we had found all the light particles. Light lighter particles are more stable and long lived, and we thought we had got them all. But, hey. Okay.
I guess we missed one, and this research group found it. We called it x 17. Later, a group of theorists realized that this mysterious x 17 particle could have just the right mass, that 34 times heavier than the electron, could be just the right mass to be related to dark photons. Like, if dark photons exist, they could give rise to a stable particle with just the right mass range, and we wouldn't have found it already in previous experiments because dark photons only ever really cross over from the dark matter world into the normal matter world. That was the theoretical picture that made this particle possible.
Lots of excitement a few years ago, and then nothing happened. Nobody was able to confirm the results, and the theory didn't lead in any new directions. Said, oh, if this x 17 particle is real, you can also go out and look at galaxy clusters and see something interesting there, or there might be some extra signal in X rays from over there. The possible hunt for a fifth force just kind of fizzled out. And I know that's a lame way to end an episode, but that's where we are when it comes to fifth forces of nature.
We've been trying as hard as we can to find tiny variations in gravity or the weak nuclear force or looking at the biggest scales in the universe, all striving to find a hook in interaction and effect that doesn't fit into the four known forces. And, yes, dark matter and dark energy are massive unsolved mysteries, and the solutions to those mysteries may involve new fundamental forces of nature or not. Forces of nature or not. Not is always an option. And right now, as lukewarm of an ending as it is, that's the best option.
I'd like to thank Leo e on email and just g on Patreon for the questions that led to today's episode. If you haven't bought my book, How to Die in Space, now is a good time. Look it up online on Amazon or at a friendly neighborhood bookstore. And if you have bought it, please please please leave a review that helps get the word out, positive reviews. And speaking of positive people, I'd like to thank my top Patreon contributors this month.
These are the top ones, Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Nuder Dude, Chris c, Robert m, Nate h, Andrew f, Chris l, Cameron l, Nalia, Aaron s, Kirk t, Tom b, Scott m, and Billy t. That's patreon.com/pmsutter to keep this show going. Speaking of keeping it going, hit me up with questions. Ask a spaceman@gmail.com. Ask a spaceman dot com or find me on social media at paul matt sutter.
And I will see you next time for more complete knowledge of time and space.