The Proton

 

Copyright: Sanjay Basu

A Century of Surprises

Let’s rewind to the late 19th and early 20th century, when physicists were trying to figure out what atoms were really made of. Back then, the atom was thought to be the smallest, most fundamental piece of matter. But experiments started poking holes in that picture—literally. When J.J. Thomson discovered the electron in 1897, it quickly became clear that atoms had internal structure. Then came Ernest Rutherford’s famous gold foil experiment in 1909, which showed that the atom had a tiny, positively charged core. All of a sudden, we knew there was something much more compact and dense at the center than anyone had imagined. Fast-forward to a few years later, and the name “proton” emerged for that positively charged nucleus (although Rutherford wasn’t the first to suggest a fundamental particle with a positive charge). By around 1920, Rutherford settled on “proton” (from the Greek “protos,” meaning “first”) for this mysterious occupant of the atomic nucleus.

Once scientists identified the proton, they naturally wanted to know if it was fundamental or if it contained smaller constituents. For decades it seemed like the proton was an indivisible building block, but that belief took a hit in the mid-20th century. In the 1960s, deep inelastic scattering experiments (where high-energy electrons were fired at protons) at the Stanford Linear Accelerator Center revealed that something else was lurking inside. Murray Gell-Mann and George Zweig independently realized the proton could be explained by three smaller particles they called “quarks.”

Initially, Gell-Mann proposed that the proton is made up of two up quarks and one down quark, all held together by the strong nuclear force. Quarks can’t just wander around freely, so they’re locked together in these tight configurations. At first, people might have thought, “Okay, so the proton is just three quarks stuck together. That’s simpler than we expected.” But nature had another twist in store.

The real shocker came when scientists realized that inside the proton, there’s a seething ocean of “sea quarks,” gluons, and quantum fluctuations. The story goes something like this: you have the three “valence” quarks—two ups and one down—carrying the proton’s net charges and basic identity. But quarks interact by exchanging gluons (the carriers of the strong force), and these gluons can spontaneously transform into quark-antiquark pairs, which then annihilate back into gluons, and so on. This frothing mix means the proton is anything but a simple cluster of three quarks; it’s more like a dynamic dance of energy and particles that pop in and out of existence.

That’s part of why the proton is often called one of the most complicated objects out there. Think of it this way: if you glance at a snapshot of a proton at any given time, you’ll see all these virtual gluons and sea quarks weaving in and out of view. The proton isn’t a static, tidy sphere. It’s a constantly changing cloud, and describing that mess precisely is extremely hard. Quantum Chromodynamics (QCD) is the theory that tries to handle this complexity, but QCD math is notoriously challenging. Most of the time, you have to resort to huge computational efforts (lattice QCD, for instance) to simulate what the proton looks like inside and to extract properties like its mass and spin.

Speaking of mass, one of the biggest surprises is that the mass of the proton—about 938 MeV/c²—doesn’t just come from adding up the masses of its three valence quarks. Those quarks contribute only a tiny fraction of the total. Most of the proton’s mass is tied up in the energy of the gluon fields and the ceaseless activity of quarks and antiquarks inside. According to the famous Einstein relationship E = mc², energy and mass are equivalent, so all that swirling energy inside the proton shows up as mass.

Spin is another puzzle. Classically, you could imagine the proton’s spin as if it were a little spinning billiard ball. Initially, physicists figured they’d find that the spins of the three valence quarks would add up neatly to the proton’s total spin of ½. But when experiments measured how much each quark contributed, they got only a fraction of the total. This launched what people called the “proton spin crisis,” and even today, we’re still trying to nail down exactly how all the quarks, antiquarks, gluons, and their orbital motions combine to give a net spin of ½. It’s not the simple sum we first thought.

Over the years, high-energy physics experiments at places like the Large Hadron Collider and various electron-proton colliders have helped us probe deeper into the proton’s innards. Scientists have measured the parton distribution functions—partons being the quarks and gluons—at different energies, revealing more and more about how quarks and gluons carry momentum and spin. Yet every time we think we’re getting closer to a tidy explanation, new subtleties pop up.

All of this complexity is why the proton is sometimes described as the “most complicated thing imaginable.” For something so essential to everyday matter and so foundational to chemistry and biology (after all, hydrogen is just a proton plus an electron), the proton is remarkably difficult to model precisely. Each new generation of experiments and theoretical breakthroughs adds another layer of depth to our understanding, but it also reminds us how dynamic and surprising the quantum world can be.

So, from the earliest days of Rutherford shooting alpha particles at gold foil to modern-day investigations involving advanced colliders and enormous computer simulations, the proton has consistently forced us to revise our ideas. It started out as a candidate for an utterly fundamental particle. Then it became clear it’s built from quarks. Now we know those quarks are embedded in a roiling soup of gluons, sea quarks, and fluctuations, with emergent mass and spin that we’re still striving to unravel. It’s a testament to how rich and subtle our universe is that one of the most common particles around is also one of the hardest to fully understand.