The furious movements of particles called gluons and quarks are responsible for most of your body’s mass With the onset of a new year, many people resolve to lose some weight. They often ask: Where did all that weight come from? Herewith, the answer. Albert Einstein’s E=mc² is the most famous equation in physics. It says that energy (E) is equal to mass (m) multiplied by the square of the speed of light (c²). Thus, mass can be converted into energy. Famously, that concept is realized in the sun and other stars as well as in nuclear reactors and weapons, wherein certain atomic nuclei morph into others with less mass, with the difference liberated as energy. But we can also read E=mc² the other way, as m=E/c², to suggest that mass originates in energy. And most of it does! That revelation is a great triumph of modern physics. Let’s break it down. Almost all the mass in atoms, and in ordinary matter generally, is supplied by the protons and neutrons within atomic nuclei. Protons and neutrons are themselves made from particles called quarks and gluons. But those quarks and gluons have very little mass of their own: Gluons have zero, and the relevant “up” and “down” quarks have only about 1% of a proton’s mass. So where does the rest come from? Inside a proton, its constituent quarks and gluons move around rapidly. All that motion means that the proton is a bundle of kinetic energy. And it’s precisely that energy of motion that supplies the proton’s mass, according to m=E/c². How can we be confident of that marvelous conclusion? Quarks and gluons can move around in other patterns besides those we observe in protons and neutrons. Most of these patterns of motion, known as mesons and baryons, are unstable, but dozens survive long enough to be studied experimentally and weighed. Not on a bathroom scale, of course: This “weighing” involves monitoring the mass and motion of the stuff the mesons and baryons decay into, and then using formulas from the theory of relativity to add it up. In the parallel world of ideas, we also have a precise mathematical theory of how quarks and gluons interact with one another, called quantum chromodynamics or QCD, that has passed many observational tests. It is challenging to solve the equations of QCD, but clever people, with the help of supercomputers, are getting good at it. They can identify the sustained patterns of motion, including the ones we call protons and neutrons, and they have calculated how much energy-and, thus, according to m=E/c², how much mass-each one contains. Theory and observations agree beautifully, inspiring faith in our understanding. If you’ve been distressed about your weight, maybe you’ll find comfort in knowing that it reflects your pent-up energy. Or maybe not. Sadly, since the quarks and gluons inside protons and neutrons can’t be coaxed to slow down, their energy can’t be starved or exercised away. There’s no subatomic recipe here for weight loss. The only practical way to shed your excess weight remains what it has always been: Your output of protons and neutrons must exceed your input. Dieting, obviously, decreases the input. And exercise, by speeding burnt fuel along, raises the output. Good luck!