In this post I will share an introduction and description of the Standard Model of Modern Physics. It 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.” That would bring you roughly to the end of the 1800s in terms of humanity’s understanding of the universe :)
My short foray into Quantum Mechanics at university left a major impression on me. Something about the intricacy and chaos at the heart of all things, the search for those fundamental truths deeper and deeper into the universal fabric, and then the humble awareness that our existence is just a layer on top of that truth. 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 about 1/2 of the universe works. In particular, it focuses on how the universe works at the closest quarters we know: the quantum, sub-atomic levels. This is distinct from the other half of universal phenomena — what happens at large distances. Those phenomena are described mostly by gravity, and were the subject of Einstein’s famous model of Special Relativity. These two branches of cutting-edge modern physics, developed over the past 100 years, are generally studied separately (for now!).
The human understanding of quantum physics has been developing for thousands of years. We have always been intrigued by what could be happening at levels deeper than human perception. From the 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. In a sentence: The Standard Model is humanity’s most up-to-date attempt to combine (almost) everything we know about how the universe works into one elegant, comprehensive theory.
The early-20th century dilemma: atomic nuclei
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. During the two centuries prior, scientists had also developed a modern understanding of electricity and magnetism. Among other things, we learned that (1) electricity and magnetism are actually dual 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 difficult question: how does the nuclei of the atom, in opposition to what we know about electromagnetism, manage to stick 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. 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. We needed to discover explanations for these newly observed behavior of atoms and small particles.
Getting started: 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 of various shapes and sizes that interact with other objects across distances and 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, and drop some other small magnets nearby, they will either jump towards or away from the magnet. We describe this observation by saying that a magnetic “field” has exerted a magnetic “force” on some “particles” in the magnets.
One of the fundamental statements of the Standard Model is the generalization of this principle: that every interaction 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.
Two fundamental particles: fermions and bosons
In the language of the Standard Model, fields exert their influence on objects via the transmission of a “force particle.” This means the Standard Model groups all particles in the universe into two categories: (1) particles which make up the objects and matter around us — called “fermions”, and (2) particles which exist to transmit forces — called “bosons”. You may be familiar with the boson particle which transmits electromagnetism from one place to another: the photon.
By organizing all particles in the world into these categories, the Standard Model asks for the existence of fermions and bosons during every interaction we consider. In order to satisfy this requirement, and to answer those questions posed at the start of the 20th century, we need to descend one level deeper than the protons and neutrons of atomic physics.
What we eventually discovered was that the protons and neutrons of an atomic nucleus are actually made up of sets of smaller particles called quarks. It took years of mathematics and incredibly precise experimental tools to predict and verify that these smaller particles exist. But the interactions of those quarks, and the introduction of two “new” forces into scientific understanding, are able to explain those two major sub-atomic issues of nucleic stability and atomic decay.
Four fundamental forces: Visible and sub-atomic
The two forces that explain sub-atomic behavior are called the “strong” and the “weak” force. Along with gravity and electromagnetism, which were known before, this means that modern scientists have managed to group every interaction in the universe into one of four fundamental forces.
- We are all familiar with gravitational force: 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 familiar with the electromagnetic force: when you drop a magnet near another magnet, it either gets pulled in or pushed away. Both gravity and the electromagnetic force can be observed easily by our eyes.
- The two forces needed to explain sub-atomic behavior are less perceivable 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, gravity (the long-distance force) is explained by Einstein’s theories. The remaining three (short-distance forces) appear in the Standard Model.
Experimental quantum physics: predictions and verification
As we’ve mentioned, one of the core statements of the Standard Model is that every time we want to describe anything in the universe happening, we should look for a “field”, a “force”, and two types of “particles” — the “fermion” which experiences the force and the “boson” which transmits the force.
With this in mind, one of the great accomplishments of the new theoretical framework is that it has predicted the existence of particles which have then been discovered afterwards by experiments. Whenever you can predict that something should exist, and then lo-and-behold it appears, you earn some credit! An important measure for success of the Standard Model is that it is correctly able to describe the behavior of larger objects (e.g. neutrons) in terms of the behavior of elementary constituents (e.g. quarks).
Experiments to verify these hypotheses 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.
Recent discoveries: The Higgs Boson
The first versions of Standard Model, as developed in the mid-1900s, was not complete. Among the various issues in the beautiful theories was one glaring requirement: for all 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 improvement to the Standard Model that integrated a never-before-seen particle. It was predicted in the mid-1960s. Remember all the press about the discovery of a “Higgs Boson” about 10 years ago? Scientists spent years searching for this anticipated particle, hoping for the theory to be true… and found it in 2012.
Unsolved mysteries: Gravity
Identifying the Higgs Boson was a major step forward for the Standard Model, but issues still remain. For example, the model is not yet able to describe the complicated mathematics of gravity. And, we haven’t actually been able to observe any particles which carries the force of gravity — remember, this was a requirement for any force in the Standard Model.
Understanding the mathematical distinction between the three “close-quarter” fundamental forces (Standard Model) and the major “long-distance” force (gravity) is one of the major focuses of theoretical physics today. You may have heard of string theory, which is one branch which offers an approach to tackling these problems. The dream of scientists remains the same as it has been for thousands of years: can we find a way to describe everything in the world using a simple and elegant set of descriptions and rules.
Timeline of discoveries
The Standard Model in its complete statement served to unify many theories and observations over the 20th century. I wanted to share a quick timeline of some of these events to clarify the order in which particles, forces, and models were developed.