Samuel Beckett’s Guide to Particles and Antiparticles
Note: the following is the third in a series of five blog post that I was invited to contribute to the blog ribbonfarm on the topic of particles and fields. You can go here to see the original post, which contains some discussion in the comments.
I was 12 years old when I first encountered this quote by Samuel Beckett:
“Every word is like an unnecessary stain on silence and nothingness.”
That quote impressed me quite a bit at the time. It appeared to my young self to be simultaneously profound, important, and impossible to understand. Now, nineteen years later, I’m still not sure I understand what Beckett meant by that short sentence. But I nonetheless find that its dark Zen has worked itself into me indelibly.
The Beckett quote comes to mind in particular as I sit down to write again about quantum field theory (QFT). QFT, to recap, is the science of describing particles, the most basic building blocks of matter. QFT concerns itself with how particles move, how they interact with each other, how they arise from nothingness, and how they disappear into nothingness again. As a framing idea or motif for QFT, I can’t resist presenting an adaptation of Beckett’s words as they might apply to the idea of particles and fields:
“Every particle is an unnecessary defect in a smooth and featureless field.”
Of course, it is not my intention to depress anyone with existential philosophy. But in this post I want to introduce, in a pictorial way, the idea of particles as defects. The discussion will allow me to draw some fun pictures, and also to touch on some deeper questions in physics like “what is the difference between matter and antimatter?”, “what is meant by rest mass energy?”, “what are fermions and bosons?”, and “why does the universe have matter instead of nothing?”
Let’s start by imagining that you have screwed up your zipper.
A properly functioning zipper, in the pictorial land of this blog post, looks like this:
But let’s say that your zipper has become dysfunctional, perhaps because of an overly hasty zip, and now looks more like this:
This zipper is in a fairly unhappy state. There is zipping a defect right in the middle of it: the two teeth above the letter “B” have gotten twisted around each other, and now all the zipper parts in the neighborhood of that pair are bending and bulging with stress. You could relieve all that stress, with a little work, by pulling the two B teeth back around each other.
But maybe you don’t want to fix it. You could, instead, push the two teeth labeled “A” around each other, and similarly for the two “C” teeth. Then the zipper would end up in a state like this:
Now you may notice that your zipping error looks not like one defect, but like two defects that have become separated from each other. The first defect is a spot where two upper teeth are wedged between an adjacent pair of lower teeth (this is centered more or less around the number “1”). The second defect is a spot where two lower teeth are wedged between two upper teeth (“2”).
You can continue the process of moving the defects away from each other, if you want. Just keep braiding the teeth on the outside of each defect around each other. After a long while of this process, you might end up with something that looks like this:
In this picture the two types of defects have been moved so far from each other that you can sort of forget that they came from the same place. You can now describe them independently, if you want, in terms of how hard it is to move them around and how much stress they create in the zipper. If you ever bring them back together again, though, the two defects will eliminate each other, and the zipper will be healed.
My contention in this post is that what we call particles and antiparticles are something like those zipper defects. Empty space (the vacuum) is like an unbroken zipper, with all the teeth sewed up in their proper arrangement. In this sense, empty space can be called “smooth”, or “featureless”, but it cannot really be said to have nothing in it. The zipper is in it, and with the zipper comes the potential for creating pairs of equal and opposite defects that can move about as independent objects. The potential for defects, and all that comes with them, is present in the zipper itself.
Like the zipper, the quantum fields that pervade all of space encode within themselves the potential for particles and antiparticles, and dictate the rules of how they behave. Creating those particles and antiparticles may be difficult, just as moving two teeth around each other in the zipper can be difficult, and such creation results in lots of “stress” in the field. The total amount of stress created in the field is the analog of the rest mass energy of a particle (as defined by Einstein’s famous , which says that a particle with large mass takes a lot of energy to create). Once created, the particles and antiparticles can move away from each other as independent objects, but if they ever come back together all of their energy is released, and the field is healed.
Since the point of this post is to be “picture book”, let me offer a couple more visual analogies for particles and antiparticles. While the zipper example is more or less my own invention, the following examples come from actual field theory.
Imagine now a long line of freely-swinging pendulums, all affixed to a central axle. And let’s say that you tie the ends of adjacent pendulums together with elastic bands. Perhaps something like this:
In its rest state, this field will have all its pendulums pointing downward. But consider what would happen if someone were to grab one of the pendulums in the middle of the line and flip it around the axle. This process would create two defects, or “kinks”, in the line of pendulums. One defect is a 360 degree clockwise flip around the axis, and the other is a 360 degree counterclockwise flip. Something like this:
As with the zipper, each of these kinks represents a sort of frustrating state for the field. The universe would prefer for all those pendulums to be pulled downward with gravity, but when there is a kink in the line this is impossible. Consequently, there is a large (“rest mass”) energy associated with each kink, and this energy can only be released when two opposite kinks are brought together.
(By the way, the defects in this “line of pendulums” example are an example of what we call solitons. Their motion is described by the so-called Sine-Gordon equation. You can go on YouTube and watch a number of videos of people playing with these kinds of things.)
In case you’re starting to worry that these kinds of particle-antiparticle images are only possible in one dimension, let me assuage your fears by offering one more example, this time in two dimensions. Consider a field that is made up of arrows pointing in the 2D plane. These arrows have no preferred direction that they like to point in, but each arrow likes to point in the same direction as its neighbors. In other words, there is an energetic cost to having neighboring arrows point in different directions. Consequently, the lowest energy arrangement for the field looks something like this:
If an individual arrow is wiggled slightly out of alignment with its neighbors, the situation can be righted easily by nudging it back into place. But it is possible to make big defects in the field that cannot be fixed without a painful, large-scale rearrangement. Like this vortex:
Or this configuration, which is called an antivortex:
The reason for the name antivortex is that a vortex and antivortex are in a very exact sense opposite partners to each other, meaning that they are created from the vacuum in pairs and they can destroy each other when brought together. Like this:
(A wonky note: this “field of arrows” is what one calls a vector field, as opposed to a scalar field. What I have described is known as the XY model. It will make an appearance in my next post as well.)
Now that you have some pictures, let me use them as a backdrop for some deeper and more general ideas about QFT.
The first important idea that you should remember is that in a quantum field, nothing is ever allowed to be at rest. All the pieces that make up the field are continuously jittering back and forth: the teeth of the zipper are rattling around and occasionally twisting over each other; the pendulums are swinging back and forth, and on rare occasion swinging all the way over the axle; the arrows are shivering and occasionally making spontaneous vortex-antivortex pairs. In this way the vacuum is never quiet. In fact, it is completely correct to say that from the vacuum there are always spontaneously arising particle-antiparticle pairs, although these usually annihilate each other quickly after appearing. (Which is not to say that they never make their presence felt.)
If you read the previous post, you might also notice a big difference between the way I talk about fields here and the way I talked about them before. The previous post employed much more pastoral language, going on about gentle “ripples” in an infinite quantum “mattress”. But this post uses the harsher imagery of “unhappy defects” that cannot find rest. (Perhaps you should have expected this, since the last post was “A children’s picture book”, while this one is “Samuel Beckett’s Guide”.) But the two types of language were actually chosen to reflect a fundamental dichotomy of the fields of nature.
In particular, the previous post was really a description of what we call bosonic fields (named after the great Indian physicist Satyendra Bose). A bosonic field houses quantized ripples that we call particles, but it admits no concept of antiparticles. All excitations of a bosonic field are essentially the same as all others, and these excitations can blend with each other and overlap and interfere and, generally speaking, happily coincide at the same place and the same time. Equivalently, one can say that bosonic particles are the same as bosonic antiparticles. In a bosonic field with many excitations, all particles are one merry slosh and there is literally no way of saying how many of them you have. In the language of physics, we say that for bosonic fields the particle number is “not conserved”.
The pictures presented in this post, however, were of fermionic fields (after the Italian Enrico Fermi). The particles of fermionic fields — fermions — are very different objects than bosons. For one thing, there is no ambiguity about their number: if you want to know how many you have, you just need to count how many “kinks” or “vortices” there are in your field (their number is conserved). Fermions also don’t share space well with each other – there is really no way to put two kinks or two vortices on top of each other, since they each have hard “cores”. These properties of fermions, together, imply that they are much more suitable for making solid, tangible matter than bosons are. You don’t have to worry about a bunch of fermions constantly changing their number or collapsing into a big heap. Consequently, it is only fermions that make up atoms (electrons, protons, and neutrons are all fermions), and it is only fermions that typically get referred to as matter.
Of course, bosonic fields still play an important role in nature. But they appear mostly in the form of so-called force carriers. Specifically, bosons are usually seen only when they mediate interactions between fermions. This mediation is basically a process in which some fermion slaps the sloshy sea of a bosonic field, and thereby sets a wave in motion that ends up hitting another fermion. It is in this way that our fermionic atoms get held together (or pushed apart), and fermionic matter abides.
Finally, you might be bothered by the idea that particles and antiparticles are always created together, and are therefore seemingly always on the verge of destruction. It is true, of course, that a single particle by itself is perfectly stable. But if every particle is necessarily created together with the agent of its own destruction, an antiparticle, then why isn’t any given piece of matter subject to being annihilated at any moment? Why do the solid, matter-y things that we see around us persist for so long? Why isn’t the world plagued by randomly-occurring atomic blasts?
In other words, where are all the anti-particles?
The best I can say about this question is that it is one of the biggest puzzles of modern physics. (It is often, boringly, called the “baryon asymmetry” problem; I might have called it the “random atomic bombs” problem.) To use the language of this post, we somehow ended up in a universe, or at least a neighborhood of the universe, where there are more “kinks” than “antikinks”, or more “vortices” than “antivortices”. This observation brings up a rabbit hole of deep questions. For example, does it imply that there is some asymmetry between matter and antimatter that we don’t understand? Or are we simply lucky enough to live in a suburb of the universe where one type of matter predominates over the other? How unlikely would that have to be before it seems too unlikely to swallow?
And, for that matter, are we even allowed to use the fact of our own existence as evidence for a physical law? After all, if matter were equally common as antimatter, then no one would be around to ask the question.
And perhaps Samuel Beckett would have preferred it that way.