Quantum chromodynamics is the theory that describes how quarks and gluons assemble into protons and neutrons. In its equations, there is a strange angle, the parameter θ (theta). If it were not zero, the neutron would behave like a tiny electric dipole, but experiments show that this dipole is vanishingly small, meaning θ is less than 10⁻¹⁰. Why is it so small? This is the strong CP problem.
Physicists usually reason as follows: gluon fields can twist into special configurations, divided into topological sectors. It is from this global classification that the θ-angle emerges. The authors of the new work, however, remind us that this is just one possible mathematical description. They checked which QCD phenomena truly require such a global structure and which follow only from local laws — principles that hold at every point in spacetime. It turned out that everything important, anomalous identities and lattice calculations alike, works without global topology.
The conclusion is sobering: perhaps the strong CP problem is not a puzzle of nature but a shadow cast by an unnecessary mathematical assumption. If global classification of fields is not needed to explain experiments, then the question 'why is θ so small?' simply disappears. This does not mean the usual approach is wrong: it works perfectly. But its foundational assumptions deserve a second look.
If the parameter θ were even 10⁻⁹, the neutron would have an electric dipole moment that would have been detected in experiments long ago, and our understanding of the symmetries of the microcosm would be quite different.