The four forces that determine the structure of matter. In descending order of strength, they are the strong, electromagnetic, weak, and gravitational forces. The strong force (strength 1) holds the atomic nucleus together and has a range of 10^–15 m. The electromagnetic force (strength $\frac{1}{137}$) has an infinite range. The weak force (strength 10^–6) is involved in neutrino interactions inducing beta decay and has a range of 10^–18 m. The gravitational force (strength 6 × 10^–39) has an infinite range.