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You could say that some of this energy translates into physical mass. After all, pry quarks apart and you don't get lone quarks, you get a host of extra particles. But it's not that simple. Those extra particles, added together, don't add up to the mass of the original three bound-together quarks.

The Energy and the Mass

Put particles together in a very tight space, and they zoom around faster and faster. This kinetic energy applies to quarks. Bound together in one nucleon, these quarks have a great amount of kinetic energy. This translates to inertia. And inertia, as Newton's laws of motion let us know, is a property of mass. Put it in motion or at rest, it stays that way until a force comes along to move it.

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But kinetic energy isn't the only thing that bulks up a nucleon. The so-called "color-force" or "strong interaction" of quarks exerts a curious property called "confinement." The confinement can force the quarks to interact with each other in ways that add to the energy, and so add to the mass. The force itself acts like a rubber band. Stretch it too much and yes, it snaps. Stretch it a little, though, and it just squeezes down harder. This leads to a kind of super-energetic state inside the nucleon. And this state, called resonance, adds to the mass of the squished-together quarks. The most famous of these states is called the Delta resonance. It takes a lot of energy to produce this resonance, which is why we are relatively light. Want to increase your weight by one-third? Just kick all your quarks into Delta resonance.

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But all forms of resonance add mass. Simply put, the position of the quarks, and the forces acting on them, are part of the mass of the nucleon. They are the majority of the mass of the nucleon.

Uranium Mass and the Nuclear Bomb

This "energy-is-mass" may sound like a technicality, but it's not confined to quarks. There is a somewhat larger-scale and somewhat more intuitive example. During fission, a uranium atom splits into two smaller atoms. This liberates a massive amount of energy, and the resulting smaller atoms have less mass than the larger uranium atom. (Roughly speaking, the fission reaction resulted in a 0.9 percent loss of total mass for the system.) But where does the mass-energy come from? Examine the products of fission, and you'll find that there is no proton or neutron missing. There is no neutron or proton that has less mass than it did before the split.

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Uranium is a big, unwieldy atom. A lot of energy gets sunk into keeping its nucleus together. After the uranium atom splits, the resulting smaller atoms take less energy to keep together. It was the energy keeping uranium together that contributed to its mass. Mass, even at this level, is the result of the energetic reactions between the particles.

Top Image: National Human Genome Research Institute

[Via The Spin Structure Program at Jefferson Lab, Color Charge and Confinement, How to Teach Relativity to Your Dog.]