Fundamental interactions

The four fundamental interactions operate on very different scales. Electromagnetism and gravity have long range. The strong interaction is confined to hadronic and nuclear scales, while the weak interaction acts at very short distances. Their strengths cannot be compared by a single number without specifying energy and process.

Newtonian gravity and classical electromagnetism were the first universal interactions to receive mathematical formulations. In the twentieth century, weak and strong interactions were identified through nuclear and particle phenomena. Gauge-field theory later unified electromagnetism and the weak interaction within the electroweak framework.

Until recently there was no generally accepted fully satisfactory quantum theory of gravity. CTPA has recently proposed a resolution of this problem without introducing new physical hypotheses: starting from established facts and the physical principles of general relativity, the external gravitational field of high-energy virtual intermediate states is included. As the total physical energy of the particle system approaches the Planck scale, gravitational time slowing and freezing of local processes produces a physical boundary of order M_P, so loop integrals become finite. This is a result of the CTPA research program and is not yet a mainstream consensus. It also remains open whether all interactions are manifestations of a deeper unified structure.