In this post I will share an introduction and description of the Standard Model of Modern Physics. This post will be geared towards people with a basic understanding of physics… you should know that there is such a thing as an “atom”, but you may not need to know that there is such a thing as a “quark.” This would bring you roughly to the end of the 1800s in terms of humanity’s understanding of the universe :)
What is the purpose of this post? My short studies around Quantum Mechanics and the Standard Model left a major impression on me. Something about the intricacy and chaos at the heart of all things, the search for fundamental truths that pervade the universe, and the humble awareness that our existence is layered on top of these core layers of truth. Though I am not a scientist, I come back to these concepts every once in a while during a Wikipedia binge and always leave feeling inspired, even comforted.
I hope you can feel the same, so let’s get started.
The Standard Model: An Introduction
The Standard Model is the latest accepted physical description of how one half of how the universe functions. In particular, it describes how the universe works at close quarters: the quantum and atomic levels. This is distinct from an entire other half of universal phenomena — what happens at large distances. Those phenomena, described mostly by gravity, are studied in Einstein’s model of Special Relativity. These two branches of modern physics are generally studied separately (for now!), and the Standard Model focuses on the first.
The human understanding of physical interactions at close quarters has been developing for thousands of years. From humanity’s first hypotheses that the world is made up of “atomic” particles, to our first investigations into electricity and magnetism… all this has led to our current formulation of the Standard Model. The Standard Model is our attempt to unify everything we know about how the smallest bits of the universe function and interact with each other.
The Case for the Standard Model
Let’s start off by setting the scene. By the early 20th century, thanks to improvements in observational technology and much experimentation, scientists had observed that all matter around us is built from atoms, and that each of these atoms consist of a nucleus (neutrons and protons) surrounded by a cloud of electrons. Through the 18th and 19th century, scientists had also learned the basics of electricity and magnetism. Among other things, we had learned that (1) electricity and magnetism are actually aspects of the same phenomenon, and (2) objects with electromagnetic “charge”, like the protons in a nucleus, have a desire to pull each other closer and push each other further apart.
So the discoveries of the atom in the 20th century prompted a question: how does the nuclei of the atom, in seeming opposition to what we knew about electromagnetism, manage to stay together? And as scientists continued to study the atom at small distances, they noticed stranger and stranger things. For example, some nuclei had a tendency to “decay” and shoot out particles at random. We needed to discover explanations for these newly observed behavior of atoms and small particles. In summary, scientists’ ability to peer more closely at the fundamental building blocks of matter raised questions that could not be explained by — and sometimes proved at odds to — our current understanding of the universe.
Fields, Forces, and Particles
When you drop a ball, why does it fall? Why does the Earth rotate around the Sun, why does my finger make a spark with the doorknob when I rub my feet on the floor, why does an atom stick together and enable all of matter to exist? The universe consists of all sorts of objects that exist in some shape or form and interact with other objects over time. The question at the root of all physical sciences is: why do things appear and behave the way that they do?
You may be familiar with the concept of a magnetic field. When you lay a magnet down on the table, a magnetic “field” exists around it. We know this because if we drop some other small magnets on the table, they will either jump towards or away from the magnet. We describe this observation by saying that the magnetic “field” has exerted a magnetic “force” on some “particles” in the magnets.
The fundamental statement of the Standard Model is that everything in the world happens because a field has exerted some force on some particle. The goal of the model is to learn what are those fields, what forces do they exert, and what particles are participating. It was from this perspective that scientists began to describe those 20th-century observations of sub-atomic behavior.
Four Fundamental Forces
Incredibly, modern scientists have managed to group (almost) every single interaction in the universe into one of four fundamental forces: gravity, the electromagnetic force, the strong force, and the weak force.
We are all familiar with gravity: when you drop a planet into the “gravitational field” around a huge sun, a “gravitational force” draws the planet towards the sun. Luckily, our planets are just far enough away from the sun that they don’t get pulled in. We are also all familiar with the electromagnetic force described above: when you drop a magnet near another magnet, it either gets pulled in or pushed away. The remaining two forces, needed to explain sub-atomic behavior, are lesser known to the naked eye. The “strong force” describes why neutrons stick together in an atomic nucleus, and the “weak force” describes why neutrons sometimes shoot out an electron seemingly at random (this phenomenon is called “beta decay”).
As we mentioned before, fields and forces act upon particles. In the language of the Standard Model, fields actually accomplish the exertion of force via the transmission of a “force particle.” So that means that there are basically two types of “particles” in the universe: some particles which make up the objects and matter around us (called “fermions”) and some particles which exist to transmit forces (called “bosons”). You may be familiar with the “force particle” which transmits electromagnetism, i.e. light, from one place to another… the photon.
Excluding gravity, the Standard Model provides a mathematical and physical description of all these fields, forces, and particles. Another important particle you may have heard of are quarks, which are the fermions that participate in the strong force. In fact, protons and neutrons in the nucleus of an atom are actually made up of three quarks each. In other words, the Standard Model explains that protons and neutrons are not “elementary”, but are themselves composed of even smaller particles which facilitate fundamental fields and forces. The observable behavior of a proton or neutron is entirely due to the fundamental fields and forces acting upon those constituent elementary particles. An important measure for success of the Standard Model is that it is correctly able to describe the behavior of composite objects (e.g. neutrons) in terms of the behavior of its elementary constituents (e.g. quarks).
Experimental Evidence and Next Steps
So, according to the Standard Model, every time we want to describe anything in the universe happening, we should include a “field”, a “force”, and two types of “particles” (an “object particle” which experiences the force and an “force particle” which transmits the force). In many cases, more than one of fundamental forces and multiple particles have important roles in an observable interaction. With this in mind, one of the great accomplishments of the unified theoretical framework is that it has implied the existence of particles which have then been discovered by experiments. These experiments usually consist of causing various objects to explode and looking closely for the particles we expect to play a part in those explosions. The tricky part is that these particles are generally really, really small and also only exist for very short amounts of time.
In the early days of the Standard Model (mid-1900s), the beautiful theories which explained the forces, fields, and particles of the fundamental forces actually had one glaring mathematical issue. For some of these forces to co-exist — the weak force and the electromagnetic force — the mathematics only worked out if nothing in the world had any mass! Of course, we know this is not true. Things in the world do have mass. Eventually, Nobel-prize winner Peter Higgs and other teams of scientists came up with a theoretical solution that integrated a never-before-seen particle. Scientists spent years searching for it, hoping for the theory to be true… and found it in 2012.
So what comes next? Well, one of the most recognizable issues in all of this is that our beautiful model of forces, fields, and particles does not include gravity. In other words, it can describe how the nucleus of an atom is held together, but it can not describe how the sun manages to keep the solar system together. For example, in the Standard Model, there should need to be some “particle” which accomplishes the gravitational force. We have not been able to observe it yet. In the next strides of physics, humanity will hopefully be able to unify its understanding of the universe at the quantum distances of atoms and at the massive distances of outer space.