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The irruption of two new forces in the 1930s and 1940s (the Weak and Strong forces) was not a serious blow to the science community. Indeed the advances in understanding the structure of the atom demanded them. Some preliminary work on radioactivity had been done by Marie Curie. A much fuller understanding of radioactivity came with knowledge of the Weak force.

The Weak force is responsible for some kinds of radioactive decay and the Strong force is the binding agent in the atomic nuclei. The nucleus of an atom consists of positively charged protons and electrically neutral neutrons. Why don't the protons repel each other like two similarly polarised magnets? Because the short range Strong force overcomes the electrical repulsion. These forces patrol a beat smaller than an atom. Without the technology to investigate very small structures, these forces could not have been studied, unlike electromagnetism and gravity, which have been known about, in some sense, since the beginning of culture.

In a quantum mechanical version of electromagnetism, Weak and Strong forces all fit within the framework of quantum theory. The only force that has never been successfully 'quantised' is gravity. Gravity is weaker than the other forces, but infinite in range. It is the only relevant force in cosmology, controlling the motion of stars and galaxies. In General Relativity, Einstein described gravity in terms of the geometry of space and time.

The concept of spacetime was a major unification, bringing together two disparate strands of physics and mathematics. Although gravity is so familiar to us, it provokes the most serious headache in the unfolding history of unifications within physics.