Mass is the quantity of matter that a physical body contains.
Most of us are familiar with the concept of inertia mass, which is a measure of a body’s resistance to acceleration. But what exactly is mass and how is it created? To answer such fundamental questions, modern physics turns to quantum field theory, a framework in which the universe is permeated by invisible fields. According to this theory, every fundamental particle is not an isolated object but an excitation, or ripple, in an underlying quantum field. Electrons arise from the electron field, photons from the electromagnetic field, and quarks from their respective quark fields.
Question
How does the excitation or fluctuation of a quantum field arise?
Answer
According to the Heisenberg Uncertainty Principle, a quantum field in a vacuum cannot simultaneously have both an exactly zero field value and an exactly zero rate of change. As a result, the vaccum field unavoidably contains tiny quantum fluctuations. These fluctuations are sometimes described as corresponding particles appearing and disappearing randomly for extremely short periods of time. However, real particles require stable excitations and therefore a definite enery source to exist. Their formation is triggered by physical processes that inject energy into the field, such as particle collisions in accelerators or the decay of an unstable nucleus. According to current cosmological theory, the energy that created the original fundamental particles of the universe came from the enormous energy density present in the early universe, particularly during the transition out of Cosmic Inflation.
In this picture, mass is no longer viewed simply as an intrinsic property of matter. Instead, much of the mass of elementary particles emerges through their interaction with a special quantum field known as the Higgs field. Unlike ordinary fields that may vary from place to place, the Higgs field exists everywhere in space, even in a perfect vacuum.
A particle’s mass is determined by how strongly its associated field, such as the electron field, couples to the Higgs field. This coupling, which can be thought of as a kind of “overlap” between mathematical structures in an abstract space, is an intrinsic property of the particle and does not depend on whether the particle is moving or stationary. Even a particle completely at rest still possesses rest mass and corresponding rest energy, described by Einstein’s relation: . Particles that couple strongly to the Higgs field acquire greater mass, while those that couple weakly remain light. Particles such as photons do not couple to the Higgs field at all, which is why they are massless and travel at the speed of light.
The existence of the Higgs field was confirmed in 2012 with the discovery of the Higgs boson at the CERN’s Large Hadron Collider. The Higgs boson is an excitation of the Higgs field itself, much like a ripple on the surface of water reveals the presence of the water beneath. Its discovery provided strong evidence for the field and, hence, for the mechanism through which elementary particles acquire mass.
Question
How does the Higgs field differ from a gravitational field?
Answer
The Higgs field is a scalar field that gives elementary particles mass through their interaction with it. Gravity, however, is not a force field like the Higgs field; it is the effect of curved spacetime caused by mass and energy, and it governs how objects move rather than how they acquire mass.
However, the story of mass does not end there. Most of the mass of ordinary matter, such as protons and neutrons inside atomic nuclei, does not come directly from the Higgs field. Instead, it arises from the enormous binding energy associated with the strong nuclear force that confines quarks within protons and neutrons. Through Einstein’s relation, energy itself contributes to mass. Thus, the mass of the visible universe is ultimately a manifestation of both quantum fields and the energy stored within them.
Question
Why does a proton have a much greater mass than an electron?
Answer
The electron is a fundamental particle, while the proton is a composite particle made of three quarks bound together by gluons. An electron obtains its mass primarily through its interaction with the Higgs field. By contrast, only a small fraction of a proton’s mass comes from the Higgs-generated masses of its quarks. About 99% of the proton’s mass arises from the enormous energy associated with the strong force that binds the quarks together. According to Einstein’s relation, energy contributes to mass, making the proton roughly 1,836 times more massive than the electron.
In daily life, we measure almost everything using inertia mass. Such measurements are made in relation to the kilogramme, which is now defined by the Planck constant. Prior to Nov 2018, the kilogramme was defined by the mass of a platinum alloy cylinder called the International Prototype Kilogramme (IPK), which is stored in France. The IPK contains octillions of atoms and has inevitably gained or lost mass over time through oxidation. Therefore, it cannot be used to accurately measure the mass of atoms.
To circumvent this problem, scientists decided to measure the mass of an atom or isotope relative to that of another atom or isotope, just as the mass of everyday objects was measured in relation to the IPK. The question then arises: which isotope should be chosen as the reference mass, and why?