Two of the greatest theories about our world and universe were formed in the first half of the 20th century. The General Theory of Relativity describes space, time and gravity and it explains our universe on the very large scale. Quantum theory describes how particles interact and describes things on the scale of the smallest particles in the universe. Both theories have proven to be accurate in all current experiments. The problem is…at least one of them is incorrect in our current understanding. One of the most fundamental features of Relativity is that nothing can go faster than the speed of light. One of the features of quantum theory is that all quantum events have a probability to occur. One of these events describes a situation where ‘entangled particles’ can be separated and that when something happens to one of the particles there is a probability that the information about the first particle can be instantaneously transferred to the other particle, in other words faster that the speed of light. This contradiction is one of the most important open questions about of current view of physics. The goal of a ‘Unified Theory of forces’ would take our understanding of the force of gravity (Relativity) and make it work with our understanding of the weak force, the strong force and electromagnetism (Quantum Theory).

(The Great Courses – Science in the Twentieth Century)