Protons and neutrons are built from smaller particles called quarks, held together by force-carrying particles called gluons. But the inside of a proton or neutron is far more dynamic and strange than that simple picture suggests. Rather than three quarks sitting quietly in a shell, the interior is a churning environment where particles constantly pop in and out of existence, energy behaves like matter, and the force holding everything together gets stronger the harder you try to pull it apart.
The Three Valence Quarks
Each proton contains two “up” quarks and one “down” quark. Each neutron contains two down quarks and one up quark. These are called valence quarks, and they determine the particle’s electric charge. An up quark carries +2/3 of a proton’s charge, while a down quark carries -1/3. Add those up for the proton (2/3 + 2/3 – 1/3) and you get a charge of +1. Do the same for the neutron (-1/3 – 1/3 + 2/3) and you get zero, which is why neutrons are electrically neutral.
That symmetry is the only real difference between the two particles. As physicist Gordon Cates of Jefferson Lab put it, “The only difference between a proton and neutron is that you take every up quark and turn it into a down quark, and you take every down quark and you turn it into an up quark. Otherwise, in many respects, they’re the same object.”
Gluons: The Force Carriers Inside
Quarks are bound together by the strong force, the most powerful of the four fundamental forces in nature. The particles that carry this force are called gluons, and they work nothing like anything in everyday experience. While a photon carries the electromagnetic force but has no electric charge itself, gluons actually carry the type of charge they transmit. This means gluons interact with each other, not just with quarks, creating an incredibly complicated web of interactions inside every proton and neutron.
Gluons are constantly being exchanged between quarks, split into pairs of particles, and recombining. They account for a surprisingly large share of what’s happening inside a nucleon. In fact, gluons carry roughly half of a proton’s total momentum, meaning much of the “stuff” inside a proton isn’t matter at all. It’s the energy of the force field binding the quarks together.
The Quark-Antiquark Sea
Beyond the three valence quarks, the interior of a proton or neutron is filled with a constantly fluctuating “sea” of additional quarks and their antimatter counterparts (antiquarks). These sea quarks are generated when gluons spontaneously split into a quark-antiquark pair, which then annihilate back into gluons or interact with other particles before disappearing. This process happens continuously, meaning the number of particles inside a proton at any given instant is not three. It’s far more.
These sea quarks are real enough to be detected in experiments. When physicists fire high-energy electrons at protons, the scattering patterns reveal the presence of these extra quarks lurking at low energies. The sea quarks don’t change the proton’s overall charge or identity, since they always appear in matter-antimatter pairs that cancel out, but they do affect the proton’s internal structure in measurable ways.
The Proton Spin Puzzle
One of the most surprising discoveries about the inside of protons came in the 1980s, when an experiment at CERN revealed that the spinning motion of valence quarks accounts for, at best, about a quarter of the proton’s total spin. This was shocking. Physicists had assumed the proton’s spin (a fundamental quantum property) came directly from its three quarks spinning. It didn’t.
So where does the rest come from? Decades of experiments, including work at Brookhaven National Laboratory’s particle collider, have shown that gluons play a significant role. The sea quarks contribute too, and in unexpected ways. Recent data showed that up antiquarks contribute more to spin than down antiquarks, even though there are more down antiquarks present. The orbital motion of quarks and gluons, essentially how they move around inside the proton rather than how they spin on their own axes, likely accounts for another chunk. The full accounting is still one of the open puzzles in nuclear physics.
Why Quarks Can Never Escape
One of the strangest properties of the strong force is that it gets stronger with distance. Pull two quarks apart and the force between them increases, like stretching a spring. Between any quark pair sits a “flux tube,” a narrow string of gluon energy that connects them. This tube has a roughly constant cross-section, so the energy stored in it grows linearly with its length. The farther you pull, the more energy you pump into the system.
Eventually, if you stretch this tube far enough, something remarkable happens. Instead of the quarks breaking free, the energy in the tube becomes large enough to create a brand-new quark-antiquark pair. The tube snaps, and you’re left with two pairs of bound quarks instead of one. You never get a free quark. This phenomenon is called color confinement, and it’s the reason no isolated quark has ever been observed in any laboratory. Every quark in nature is locked inside a composite particle.
The term “color” here has nothing to do with visible color. It refers to the type of charge that quarks carry under the strong force. Just as electric charge comes in positive and negative, strong-force charge comes in three varieties that physicists labeled red, green, and blue. Every observable particle must be “color neutral,” meaning its internal color charges cancel out, just as a mix of red, green, and blue light produces white.
How Small Is All of This?
A proton measures about 0.84 femtometers across (0.84 millionths of a billionth of a meter). To put that in perspective, if an atom were the size of a football stadium, the proton at its center would be smaller than a marble. And the quarks inside that proton? They have no known size at all. As far as current experiments can tell, quarks are point-like, meaning they behave as if they occupy zero space. All of the proton’s measurable size comes from the cloud of gluons and sea quarks, plus the energy of the strong force field binding everything together.
The neutron, despite being electrically neutral overall, has internal charge structure. Calculations from quantum chromodynamics (the theory of the strong force) show that the neutron has a positive core surrounded by a negative outer region. The charges of its quarks aren’t evenly distributed; they’re arranged in a way that cancels out only when you measure the whole particle from the outside.
Where Mass Really Comes From
Here’s perhaps the most counterintuitive fact about proton and neutron interiors: the quarks inside are almost massless. The three valence quarks together account for only about 1% of a proton’s total mass. The other 99% comes from the energy of the gluon field and the kinetic energy of quarks zipping around inside. Through Einstein’s famous relationship between energy and mass (E=mc²), all that internal energy manifests as the mass you’d measure on a scale. In a very real sense, most of the mass of every atom in your body comes not from matter, but from the energy of the strong force.
Neutrons on Their Own
Inside an atomic nucleus, neutrons are stable. But a free neutron, one that’s been knocked out of a nucleus, will decay in about 15 minutes on average (with a measured lifetime of roughly 885 seconds). It breaks apart into a proton, an electron, and an antineutrino, releasing a small amount of energy in the process. What’s actually happening at the quark level is that one of the neutron’s down quarks transforms into an up quark, turning the neutron (two down, one up) into a proton (two up, one down). The electron and antineutrino carry away the leftover energy and balance the quantum books. This process, called beta decay, is driven by the weak nuclear force, an entirely different force from the strong force holding the quarks together.