Three Americans won the Nobel Prize in physics Tuesday for revealing how forces in the atomic nucleus keep it from flying apart — a discovery that has brought science one step closer to a “grand unified theory” of how the universe operates at the subatomic level.

The Royal Swedish Academy of Sciences awarded the $1.3 million prize to David J. Gross, H. David Politzer and Frank Wilczek for a 1973 breakthrough that explained the unusual properties of the strong force, which binds the fundamental particles known as quarks into protons and neutrons.

Their work has helped science get closer to “a theory for everything,” the academy said.

The academy said the three physicists discovered “something that, at first sight, seemed completely contradictory.”

They found that unlike electromagnetism and gravity, which grow more powerful as two particles get closer to each other, the strong force actually weakens as two quarks converge. It is as if the particles were connected by a rubber band that pulls them together more tightly as it stretches.

Wilczek, 53, and Politzer, 55, were still graduate students when they did their prize-winning work — Wilczek at Princeton and Politzer at Harvard; Gross, now 63, was a young professor at Princeton. Wilczek is now a professor at the Massachusetts Institute of Technology, Politzer is at the California Institute of Technology, and Gross is at the University of California at Santa Barbara.

Their achievement cemented the theory of quantum chromodynamics, or QCD, which describes the interactions of quarks and other subatomic particles inside the nucleus.

“All of us have talked about this for a long time as a very significant piece of work,” said Chris Quigg, a theoretical physicist at the Fermi National Accelerator Laboratory. “There’s a before the work that these people did and an after, and the after is much more glorious.”

Before their explanation of the strong force’s properties, Quigg said, the issue of how mass arises in protons and neutrons, and thus in virtually all of the universe’s visible matter, was considered beyond scientific inquiry. Afterward, it was relatively easy to show how mass arises from the interactions between quarks in an atom’s nucleus.

Perhaps more significantly, the establishment of QCD helped demonstrate the possibility of uniting the electromagnetic, strong and weak forces — the weak force governs radioactive decay — into a single “grand unified theory” of particle physics.

Some even see the possibility of going further by drawing in Einstein’s theory of general relativity, which describes how gravity works and predicts the existence of black holes, wormholes and other far-out phenomena. The work of Wilczek, Gross and Politzer brought science one step closer to that “grand dream,” the Swedish academy noted.