Transcript from Epic of Evolution: Life, the Earth and the Cosmos (BEP 210A)
January 26, 2000 - Lecture by Claude Bernard
So a wave is a train of disturbances of various kinds. For sound the disturbance is the compression of air molecules; for electromagnetic waves it’s regions of strong electric and magnetic fields. This train of disturbances moves along. Let’s say it’s visible light and you look at the wave arriving at your eye. What you respond to is the frequency with which these disturbances arrive, how often they arrive. And that’s what determines the color you see. If it’s a lower frequency for example you see red; higher frequency, violet. Now, all electromagnetic waves all travel at the same speed. This whole pattern is moving along at the same speed, and it’s what we call the speed of light. Not just visible light, but all electromagnetic waves travel at the speed of light, which is roughly 300,000 kilometers per second or 186,000 miles per second. And what you observe when you look at it is the frequency. How often the disturbances arrive come determines the color. But how often they come is determined, in turn, by how far apart these disturbances are. If the disturbances are close together in space then they’ll arrive very rapidly one after another. If the disturbances are far apart then they’ll arrive with more time in between, i.e., with a slower frequency. Okay, any questions on that?
So you can describe a wave by saying how rapidly the disturbances will arrive: that’s the frequency. Or you can describe it by saying what the distance is between neighboring disturbances. That’s what’s called the wavelength: the distance between neighboring disturbances.
As was implicit in what I just said, there’s a relation between the wavelength and the frequency. If something has a high frequency, it’s going to have a short wavelength. And if it has a long wavelength, it’s going to have a low frequency. Long wavelength corresponds to these disturbances being far apart so they arrive less rapidly one after another. Therefore the frequency will be low. That’s part of the description of this character in our drama: electromagnetic radiation. Electromagnetic waves have not only frequency but also wavelength.
We need some more information about electromagnetic waves. People first realized that light was made of waves in the 19th century. But what they didn’t realize --- this was only discovered in the 20th century --- is that these waves cannot have just any arbitrary amount of energy.
Instead, the waves come in little packages, and the packages of electromagnetic waves are called photons. That has to do with what’s called quantum mechanics. I’m not going to be talking a lot about quantum mechanics, but it is important to know the word and to have some idea of what a photon is. Photons are packages of electromagnetic waves. The important thing about the packages is that high frequency waves come in packages with high energy, whereas low frequency waves are the opposite. Low frequency waves come in packages with low energy. Now, that’s going to be important for us.
Let me just say a couple of words just in the context of what we’ve been talking about to help you understand a little bit about photons. The fact that high frequency electromagnetic waves come in high-energy packages is why ultraviolet rays, X-rays, and gamma rays can be dangerous to living creatures. The photons of ultraviolet (UV) light have enough energy to break apart proteins and DNA in living cells, damaging the cells, and possibly causing mutations or cancer. What’s true for UV light is all the more true for X-rays and gamma rays, which have still higher frequencies and hence have photons with even more energy. Of course, this is also the reason that X-rays, for example, can be useful. The photons of X-rays have enough energy to go right through your skin and muscle and only get stopped by bones, which are much denser. So X-rays can be used to show the position of your bones. And gamma-ray photons are even more powerful and can be used to kill cancerous tumors in “radiation therapy.” Their photons carry a great deal of energy so each package is very powerful. For visible light, and even more so as the frequency gets lower and lower, the packages have less and less energy, and have less and less power, and less and less destructive power in the sense of biological effects. It is highly unlikely that microwaves could cause cancer, for example, because microwave photons don’t have enough energy to break up DNA or proteins.
The last thing I want to say about electromagnetic waves --- putting them into the context of the Big Bang and the expansion of the universe --- is to tell you about a different way to understand the redshift of the light from far-away galaxies. We’ve talked about how the light coming from distant galaxies is reduced in frequency. I explained that by the Doppler effect: The galaxies are moving away from us, and that reduces the frequency of the electromagnetic waves they emit. There’s another, equivalent way to understand why it is that the light from distant galaxies comes to us with lower frequency than when it’s emitted. It’s equivalent, but it’s also very useful. In fact, for thinking about the Big Bang it’s often more useful than the idea of the Doppler shift, although they’re just two different ways of looking at the same thing.
The idea is the following: Imagine a very distant galaxy. The light from it that we see now must have been emitted a long time ago. A long time ago the entire universe was much smaller than it is now because it’s been expanding all that time. As the universe expands, space expands, so electromagnetic waves that are moving through space are actually expanded as the universe expands. So if a photon or electromagnetic wave is traveling through space and space is expanding, the wavelength of the photon will be changed. It will expand with expanding space. No force holds two neighboring disturbances in the electromagnetic wave together. So when space expands the distance between any two disturbances in the electromagnetic wave also expands. That means that if light was emitted a long time ago when the universe was a lot smaller, the expansion of the universe in the meantime has stretched out the wavelength of the photon. The wavelength of this photon must be a lot longer now when it gets to us than it was when it was emitted. It’s been expanded with the expanding universe. And if the light arriving at us has a much longer wavelength than when it was emitted, it will have a much lower frequency. Remember, frequency and wavelength are related inversely. High frequency means short wavelength, low frequency means long wavelength. So when the photon got stretched out by the expansion of the universe it became redshifted; its frequency decreased. Yes, sir.
[Student: Does that mean that the stuff that we’re viewing now, when you say redshifted and blueshifted, was actually UV or X-ray or gamma rays, and it’s now in the visible spectrum?]
Yes, it can be, although it depends on from how far away it’s come. We can see out to galaxies whose frequencies are shifted by a factor of 10 or so. So we might see as visible light waves that were emitted X-rays, but not very high up in the X-rays. At least that’s the greatest shifts we have observed so far. And things that were violet light will be shifted way down here and so forth. And again, the further we look out the greater the redshift, which we can explain in either of two ways: (1) The further we look out the faster things are going away from us, therefore the greater the redshift. (2) The further we look out the longer it’s taken the light to get to us, thus the greater has been the expansion of the universe in the meantime, hence the greater has been the stretch in the wavelength and the greater has been the change in the frequency. One last point: Photons that are high frequency have high energy. Photons that are low frequency have low energy. So when these photons are redshifted that also means they arrive with lower energy than they had when they left. The same photon which started off maybe as a very high frequency, short wavelength object has been stretched out to a long wavelength object with a lower frequency, and therefore also a lower energy than when it left. Okay, now?
[Student: I thought nothing could either gain or lose energy. I mean would that kind of go against the laws of physics if it’s either losing or gaining in wavelength, I mean if it’s causing it to lose or gain energy?]
Well, we actually can understand it. The statement is not that nothing can lose or gain energy but that if something loses energy something else must gain it. Well, it’s sort of subtle when space itself is expanding, and part of the explanation I’m leaving for the very last lecture. Let me give you an example. I’m not sure this will be that helpful to you but let me try. Let’s talk about gas which compressed in a cylinder, it’s very hot and it has a lot of energy. If you let it expand it will push on the piston, and it will do work pushing on the piston as it expands, and its energy will decrease because it did work on the container that holds it. And that’s in fact basically how refrigerators work -- you compress a gas and let it expand and it’ll cool off. The same basic idea is happening here. As the universe expands, work is being done in expanding the universe and that work is coming out of the objects in it whose energy is decreasing. So we can account for where that energy went.
So the photons themselves lose energy as the universe expands. And things that have lower energy are cooler. Temperature describes the average energy things have. As the photons expand out they get cooler. That’s another way of understanding why it is that as the universe expands its temperature decreases. Or vice versa, as we look back in the past when the universe was highly compressed, its temperature was a lot higher. So if you think about the photons it’s easy to understand. If you look back in the past the universe was compressed. All these photons had much higher frequency, much smaller wavelengths, and therefore carried much greater energy than they do now. So the temperature was that much higher. You had another question.
[Student: When you said everything happened so long ago that the light was emitted, what about us and the Andromeda galaxy? As we’re moving towards each other is it blueshifted instead of redshifted?]
Yes, absolutely. It is blueshifted. In fact, that’s the only way we know that we’re moving towards it. The light of the Andromeda Galaxy is indeed blueshifted. Not by a whole lot. Its speed is not that great. It’s something like 140 kilometers per second. It’s pretty fast but it’s not enormous on the scales of things we’re talking about. And yes, the light from Andromeda is blueshifted. Okay, so that’s all I want to say about electromagnetic waves.
Let me now talk about another of my cast of characters. Electromagnetic waves are what we call radiation. The other key ingredient in the universe is something we normally call matter. (Sometimes the distinction really between radiation and matter is not always so entirely clear but for our purposes I’ll try to not get into too much of the subtleties.) Matter comes in a variety of forms just like electromagnetic waves come in variety of forms. Matter is made of particles, little pieces. Let’s start with things that you know. You know if you observe at a finer and finer scale, look with instruments at smaller and smaller distances, eventually you’ll come down to atoms, which make up everything around us. And as you probably know --- I don’t know where you learn this these days, probably in second grade --- atoms have electrons going around a center, which is called the nucleus. I’ll talk more about this in my next series of lectures. This is kind of a rougher view now and we’ll go into more detail later on.
The nucleus is not just a blob. It’s made out of other things. The nucleus is made out of two objects, called protons and neutrons. And by the way, almost the entire mass of the atom is contained in its nucleus. The electrons are very light compared to the heavy nucleus; most of the mass of an atom is concentrated in the nucleus. The nucleus is very, very small, a tiny thing in the center of this atom. Now, what holds atoms together is the electrical force. The electrons have a negative charge and the protons have a positive charge, and, as you probably know, opposite charges attract each other electrically. The neutrons have no charge, so do not help in holding the atom together. As far as holding the atom together, what matters are the protons. The protons, which have a positive charge, attract the electrons, which have negative charge, and which are orbiting around the outside. And what determines what element we’re talking about is the number of either electrons or protons. Atoms of normal materials are neutral, which is to say they have the same number of electrons as protons. So they have a total charge of 0: some number of protons with positive charge, and the same number of electrons with negative charge.
Let me give you just a little table of a few common elements that will be players here. Let’s start with hydrogen, which is the simplest. The chemical symbol for hydrogen is H. Let me write a little table here: protons, neutrons and electrons. And the most common form of hydrogen just has one proton, no neutrons and one electron. It’s the simplest atom, just a single proton with a single electron orbiting around it. That’s the most common form of hydrogen. There are other forms of hydrogen so I’m going to leave a little space here. I don’t want to talk about them yet but you can leave a little space. I have two more forms of hydrogen to put in.
The next element is helium and again I’m going to talk about the most common form of helium first. It has two protons and two neutrons and two electrons. So in the center are two protons and two neutrons and then going around it are two electrons. Let’s just do a few others: carbon, oxygen, and iron. Fe is the symbol for iron. O is the symbol for oxygen. C is the symbol for carbon. And carbon, the most common form has six protons, six neutrons and six electrons; oxygen has eight protons, eight neutrons and eight electrons; and iron 26 protons, 30 neutrons and 26 electrons. Iron is getting to be a fairly complicated atom. Now, I’m not going to ask you on a test how many neutrons are in the iron nucleus. This is just to give you some orientation to the kind of things we’re doing.
Now, all elements can come in a variety of forms. What determines which element we are talking about is how many protons and how many electrons it has. In a normal atom, which will be neutral, there will be the same number of protons and electrons, but the number of neutrons can vary. Since the neutrons have no charge, if you change the number of neutrons it does not affect the number of electrons that go around the outside. For example, there’s a variety of hydrogen called deuterium. It’s also sometimes called heavy hydrogen and it still chemically acts like hydrogen because it still has one proton and one electron, but it has an extra neutron in the center. And there’s even another form of hydrogen, which is even heavier. It’s called tritium and it has one proton and one electron as always, but it has two neutrons in the center. Tritium is an unstable nucleus. The nucleus of tritium is unstable and it quickly will decay, break apart. If you go out and look at seawater (water, which is H2O, has two hydrogen atoms and one oxygen atom) you will find most of the hydrogen atoms in seawater are ordinary hydrogen. A few of them will be heavy hydrogen (or deuterium), and you won’t find any tritium in general because tritium will just decay away. Unless it’s been made by some other process very recently, it will have decayed away. These different types of the same element are called isotopes. So hydrogen has three common isotopes: hydrogen, deuterium and tritium.
Helium has two important isotopes for us. I’m not going to talk much about any other isotopes. All elements have various varieties. The normal helium is often called helium-4. Put a 4 up there because it has total two protons and two neutrons in its nucleus so it has four nucleons. (Nucleon is just a general term for either protons or neutrons.) But there’s another variety of helium called helium-3. It has to have two protons in order to be helium and therefore also two electrons, but it only has one neutron. And that’s called helium-3. Okay, questions?
[Student: You said that tritium often just decays. Are there just protons, neutrons and electrons floating around independently or what happens to the decayed tritium? Where do the parts of that go?]
Actually I can’t remember the most common decay chain, [but I looked it up after the lecture –CB]: Tritium decays because one of its neutrons turns into a proton plus some other stuff by what’s called the “weak interaction.” This change, which is also what happens to a “free” neutron --- one not in a nucleus --- takes place as follows: a neutron turns into three particles: a proton, an electron, and something else called a neutrino, which I’m not going to talk about much today but which I’ll eventually come back to. When a neutron in tritium is replaced by a proton, the tritium nucleus becomes the nucleus of helium-3. So the answer to your question is that the nucleus of tritium decays into the nucleus of helium-3 plus an electron and a neutrino. This can happen because a neutron has slightly more mass than a proton (and hence a tritium nucleus has slightly more mass than a helium-3 nucleus), and that extra mass is converted into the mass and energy of the electron and the energy (and mass, if any) of the neutrino. The helium-3 nucleus only has one electron (from the tritium) orbiting it initial, but it will soon pick up another electron. (Note that there is an extra electron floating around from the decay.) In the same way, a neutron sitting by itself is not stable either and will decay after an average of about 11 minutes into a proton, an electron, and a neutrino. So neutrons don’t exist for long by themselves but they can exist stably inside the nucleus. And the first example of a stable nucleus with a neutron in it is deuterium, which has one proton and one neutron. Yes?
[Student: Can you talk about the purpose of the neutron, if it doesn’t affect the number of electrons in the atom.]
Do neutrons have a purpose? Well, yeah, actually they do. That’s a good question. (Of course it’s not clear that things must have a purpose, and that’s maybe something we could talk about in section: Do things really have to have a purpose or are they just there because they’re there?) And it brings up another point that you might worry about. I said that an atom is held together because the protons in the center attract the electrons around it by electrical forces. That’s what holds an atom together, but what about the nucleus? The nucleus has protons in it, all of which are repelling each other by electrical forces, and neutrons, which have no net electrical force at all. Why doesn’t the nucleus break apart from the repulsion, the outward pressure? Well, it turns out that there’s another force in nature. I won’t talk about it much now, but I’ll come back to it in the next set of lectures. It’s called the “strong force.” There’s a good reason it’s called the strong force because it’s strong enough to hold the nucleus together against the repulsion of the protons repelling each other. And both protons and neutrons are affected by the strong force. In fact, as far as the strong force is concerned they’re basically the same. The strong force doesn’t care whether a particle is a proton or neutron. And therefore the presence of neutrons is kind of a leavening. You get some neutrons in the nucleus, you can keep the protons a little bit further apart than if it was just protons. If there were just protons the repulsion due to having the same charge would make it break apart. So, for example, you cannot make a nucleus with seven protons and no neutrons. That’s just too concentrated, all those plus charges next to each other; it will fall apart. However, if you stick in a bunch of neutrons, seven neutrons, in there to keep the protons a little bit further apart, then you can make something called nitrogen. So the neutrons are crucial in that way to allow the nucleus to be stable. Other questions?
Okay, now, the protons and neutrons aren’t the end of the story. We now know that protons and neutrons themselves are made of smaller things. Protons and neutrons are made of things called quarks. And there are actually three quarks inside a proton, and also three quarks inside a neutron. I’m not going into details of quarks at this point --- I will later in the course. Quarks have various types, and, depending on what combination of quarks is inside, you will have either a proton or a neutron. So what types of quarks are inside determines whether it’s a proton or it’s a neutron. But for the moment that’s not important to us. What’s important is that protons and neutrons each have three quarks of some type inside. It’s sort of beside the point, but you could ask “well, if protons have something smaller inside of them what’s beyond that?” The answer is we don’t know. As far as we know for sure there’s no smaller part to a quark. That’s not to say there isn’t any. We just don’t know of any. There may be, there may not be. There are various hypotheses, none of which has been experimentally proven. So we don’t know if quarks are the smallest thing or if there can be smaller things inside. At the very, very early history of the universe, when things get very compressed and very hot, the question of whether there are components inside the quark or not would make an important difference. And since we don’t know if there are components, we can’t say what happens beyond that point when things are very compressed and very hot. That’s one of the reasons I put question marks before the time t = 10-32 seconds.
All right, so quarks are inside protons and neutrons. There are some names that go with these things. Quarks and anything that’s made out of quarks are called “baryons.” Baryon comes from a Greek root meaning heavy --- my Greek is non-existent so I can’t exactly say what the root is. And the reason is that quarks make up protons and neutrons, which are much heavier than electrons (about 2000 times heavier). Protons and neutrons make up the nucleus, which is the heaviest part of the atom. So a baryon is anything that is made out of quarks. The distinction that physicists usually draw is between baryons and what are called leptons. Leptons are just the opposite; the Greek root there means light. Our example of leptons is electrons. And there are other examples. I briefly mentioned something called a neutrino, which is another example of a lepton. We’ll get back to that later. For the moment, electrons are enough. So this is just nomenclature: baryons, leptons.
Okay, that’s not the whole story by any means, but even for us there’s something else I have to tell you, which is that every single one of these particles that I’ve talked about also has something else, another particle called its antiparticle. Each particle has an opposite called it’s antiparticle. And for example, the antiparticle of the electron is called the positron. It actually has its own name, but an equally good name is to call it an antielectron. And what is an antiparticle? Well, some basic properties are opposite: for example, since an electron has negative charge, the positron has positive charge. But otherwise a positron is just like an electron. It has the same mass as an electron. It has other kinds of properties, the same “spin” as the electron (how it spins around is the same as an electron), but its charge is opposite. And it was actually predicted (before it was observed) by the quantum theory of the electron, which was developed around 1930 by Paul Dirac. Dirac showed that when you combine the quantum theory of the electron with the requirements of relativity, the electron must have an antiparticle. This antiparticle was discovered a few years later and was called the positron.
So electrons have antiparticles called antielectrons or positrons. Protons also have antiparticles. It doesn’t have a special name, it’s just called an antiproton, and whereas a proton has positive charge, an antiproton has negative charge. Otherwise it’s the same as a proton. An antiproton has the same mass as a proton; it just has opposite charge. Neutrons also have antiparticles. Of course since a neutron doesn’t have any charge the antineutron also doesn’t have any charge either, but they still are opposite in a way I’ll explain. So neutrons have antineutrons. And quarks, not surprisingly, have antiquarks. And the fact that particles always have their opposite in this sense of antiparticles, is actually fundamentally and deeply embedded in our theories of how particles behave. By everything we know it seems to be required that for every particle there be an antiparticle.
Well, what are the properties of the antiparticles? Well, one I already discussed: they have opposite charge from the particle. The other property that makes antiparticles interesting is that if a particle hits its antiparticle they annihilate, they disappear into a bunch of energy, which in this case is generally electromagnetic waves. So for example, if an electron, which is normally abbreviated e- to show it has a negative charge, hits a positron, which is an e+, they’ll disappear and two photons will be produced. In fact they are such high frequency (therefore high-energy) photons that they’re gamma rays, so usually we write this equation as e- plus e+ goes to two gamma, i.e., two gamma rays (very high frequency and therefore very high-energy photons). Now, how is it possible? Electrons and positrons are matter. They have mass, they have weight. How they can disappear and just create light, which has energy but it doesn’t have any mass. Well, it was explained by Einstein in the famous equation, E=mc2. This equation says that energy can be converted into mass and mass can be converted into energy. So a certain amount of mass is by this formula equal to a certain amount of energy and this “c” here is the speed of light, which as you know is a very big number. Light goes very fast, so this equation tells us that a certain amount of mass is equivalent to a very large amount of energy. There’s a lot of energy stored in mass. And here’s one way of getting it out: if you take an electron and allow it to hit a positron, they’ll both disappear and be replaced by pure energy in the form of two photons, high energy electromagnetic waves.
The process can go also the other way. If you have two photons hitting together, they can make an electron and a positron. It also happens. You can also have something like this: 2 gammas (photons) à e- and e+. Or the processes can have arrows going both ways: e- and e+ ßà2 gamma. So energy can make mass and mass can make energy. And the same is true of any of these particles and its antiparticles. They all have the same kind of reaction possible. A neutron hitting an antineutron will also make two photons and similarly for a proton and an antiproton. For an antiproton, usually we just put a bar on top of the name, that’s just another way that we indicate that it’s an antiparticle. So physicists would say P + P bar can go to two photons, or two photons can go back and create a proton and an antiproton. And similarly, neutron plus antineutron can go back and forth between two photons. And even a quark and an antiquark can do the same. So there are antiquarks just like there are anti everything else. In fact an antineutron is made out of three antiquarks, and an antiproton is made out of a different three antiquarks. So it’s a little bit complicated, but that’s the way nature is. It has antiparticles for every particle.
Now, here’s an interesting fact: our universe seems to be mainly made out of what we call particles (protons, neutrons, electrons) not antiprotons, antineutrons and antielectrons. How do we know that? Well, we know it on the Earth just by observing what we have, all the electrons that we find in atoms are negatively charged. We don’t find any antimatter on Earth. The only place we find it is in very high energy experiments that physicists do to create the stuff, for example by slamming photons together or by equivalent procedures. But we don’t find antimatter occurring naturally on the Earth, and there’s very good evidence actually that the universe as a whole is purely made out of what we call ordinary matter (protons, neutrons and electrons) and not antimatter. How do we know? Well, from these very reactions. We know our solar system is made out of ordinary matter just by looking. So if the matter ended somewhere and there was antimatter after that, then the places where the two touched each other would have annihilations of matter and antimatter, and that would produce a very distinctive signature. Because every time a proton annihilates with an antiproton it produces two photons with a very specific amount of energy, an energy equal to the equivalent amount of mass that is lost. So what we have to do is look out in the sky and see if we see any of these photons of this very specific energy indicating that matter is hitting antimatter. And we don’t. From nowhere. Not from our galaxy. Not from other galaxies. Nowhere in the universe do we detect any of these collisions between matter and antimatter, which leads us to suspect --- although it’s not 100 percent proven --- that the entire observable universe is in fact made out of ordinary matter.
And then the question is why is the universe made of matter and not antimatter? How did that happen? Because from the laws particle physics, matter and antimatter are almost equivalent to each other, so why did it happen that our universe ended up being made out of matter and not antimatter? And that’s one of the things that the theory of the Big Bang actually explains. The Big Bang combined with what we know about the forces and interactions of particles, can actually explain why our universe is made up primarily of matter and not antimatter, and it also explains why the amount of matter is what it is. Or more precisely, it explains why the ratio of the amount of matter to the amount of energy in photons is what it is. That’s actually something that comes out of the theory of the Big Bang. We don’t know all the details of exactly how it takes place but we have a pretty good idea. I’ll say a little bit about it on Friday --- I don’t think I’ll get to it today --- and then I’ll also come back to it later in the course. Where does this matter come from? Why so we have matter and not antimatter?
Well, that’s the introduction to the players. Let’s actually start the play. Remember I said I was going to start it at a certain time, where I felt comfortable that we knew enough about the physics that it was worth telling you things as if it’s true. And I also indicated that the longer the expansion goes on the more convinced we are that we know exactly what was happening. The earlier we’re talking about the more there are possibilities that there could be changes in the picture when physicists learn more. I tried to pick a place that was fairly safe, but it’s not 100 percent safe. Some of the early things in our picture could change. So we start off with this beach ball with a radius of about 30 centimeters. That’s about a foot. That’s my beach ball. And the time I said is about 10-32 seconds and the temperature was this enormous amount, three times 1026 degrees in absolute temperature scale, or Kelvin temperature scale.
What did the universe look like in this beach ball at this time, at this enormous temperature? Well, it was a hot soup of all these particles. As you increase the temperature of things, what you tend to do is break them apart. A simple example is water. Water at room temperature is a liquid with the molecules next to each other. What happens when you heat the water? Eventually you boil it and the molecules shoot off out of the container and become a gas. In a gas, molecules are far apart from each other. This is a general feature: At lower temperatures things condense, stick together. At higher temperatures things have a lot of energy, they go shooting off by themselves, and are separated from each other. Even if there are forces between them pulling them together, objects will separate if you give them enough energy by heating them to high temperature. For example, in water there are forces between the water molecules that hold them together in a liquid. But if you heat the water enough, the energy of the molecules is great enough to break those forces and they go flying off in all directions: you have a gas.
The same thing happened to matter in a more extreme way in the Big Bang. At this enormous temperature everything “boiled.” Atoms didn’t exist because the electrons, which are normally attached to the nucleus, were boiled off from the atoms, flying in all directions. Electrons were just flying around freely, not connected to nuclei. Their energy was much too high to be held by the nucleus. So the electrons are going zooming around and the nuclei can’t hold them. Also the nucleus can’t hold the protons and neutrons anymore. There are no nuclei at that point. And even protons and neutrons don’t exist at this point, only quarks. The quarks are flying around freely, broken out of the protons and neutrons. There are no protons and neutrons, just quarks. I picked a time where we think it’s quarks, and we don’t think we have to worry about maybe the quarks themselves being broken up. As I said, that’s one of the reasons why we can’t go back further. But at this time what we have is a soup of everything broken up. It has quarks. It also has antiquarks. In fact, the same number of quarks and antiquarks. They’re just flying around freely. Everything is at very high energy. There are also photons; these photons have enormous energy because it’s enormously hot, and photons can hit into each other all the time and make quarks and antiquarks. Even if at one particular instant there happened not to be any quarks and antiquarks (which is not what happened but let’s imagine there weren’t any) then photons would immediately make them by hitting into each other. And in fact, we had quarks, antiquarks, photons, electrons and antielectrons (positrons) and just about the same number of all of them.
So this early universe was in a very simple situation. It’s what we call equilibrium: a completely mixed up, uniform combination. Think of a bathtub. If you put in hot water on one end and it’s cool on the end you’re sitting, that’s not equilibrium. There are two different temperatures in there. But if you wait long enough it will come to equilibrium, completely mixed, and the whole thing will be lukewarm. And in the same way here, everything was mixed up --- uniform (homogeneous) --- and everything was at the same temperature. From our understanding of physics we can show that there were not only all these particles but there were basically the same number of all of them -- photons, electrons, positrons, quarks and antiquarks. And the time when quarks dominated, and antiquarks went away, is not yet. Right now there’re the same numbers of every particle. It’s actually a very simple time.
Okay, let me say a few more sentences. This stuff is really fascinating. But you could ask the question, “Where do you physicists get off talking about what the universe looked like when it was the size of a beach ball, when you can’t even predict the weather two days from now?” It’s a really good question. The answer is that the physics of the Big Bang is a lot simpler than the physics of the weather. It’s further from your experience, but the Big Bang is basically a situation of equilibrium, everything mixed up very well. And we know very well how to think about equilibrium situations. As the universe expands the equilibrium changed but in a very controlled, understood way; whereas the weather is really hard. It has a lot of variables. And the weather is very different in different places: it’s very far from equilibrium. Predicting the weather is much harder than understanding the Big Bang.
[Note: Later we’ll talk about departures from equilibrium in the Big Bang due to the rapid expansion and cooling of the universe. But that is a very simple departure from equilibrium, and the universe remains basically homogenous.]