The Great Unsolved Problems in Physics: Theory of Everything

| June 19, 2013 | 0 Comments

Undoubtedly, one of the most important unsolved problems (that can be found on this list) is whether it is possible to unify all of physics into a single theory — a theory of everything. A theory of everything would fully explain and link together all known physical phenomena and would, at least in principle, predict the outcome of any possible experiment.


Many candidate theories have been proposed throughout the years, however, to find the roots of the problem we have to go back to the ancient Greece. The ancient Greeks were famous for having a wide variety of schools of philosophy, the influence of which is still felt in present day philosophy and science.  Interestingly, many of the leading philosophers believed that the world around us could be understood simply by thinking about it rather than experimenting. Having this in mind, Archimedes stands out as a unique persona — his claim to fame was his influential work in mathematics, engineering, physics and astronomy.  Archimedes was possibly the first scientist, who attempted describing nature with axioms (or principles) and then deducing new results from them — a method, which is at the root of theoretical physics. Such an idea in turn lead Archimedes to attempt explaining everything around him starting with only a few axioms. A similar approach is pursued by modern scientists looking for a theory of everything.

Another important milestone in the history of physics surprisingly came from another ancient Greek philosopher Democritus. The most important contribution to the world of philosophy and physics by Democritus was the so called atomistic philosophy, the central idea of which is that the only things that exist in the universe are atoms and the void. Now, by “atoms” Democritus meant indivisible particles, which came in an infinite variety of shapes and sizes. The importance of this idea hides in the fact that Democritus believed that everything in nature could be explained by interactions of atoms in the void.  Even though this idea of atoms is different from our current understanding of elementary particles, it came to play a very important role in physics almost 2000 years after Democritus.

Pierre-Simon Laplace

An idea similar to atomism became popular again in the 17th century, after the tremendous success of Newton’s laws, which could explain, in principle, almost any mechanical situation. In fact the success of Newton’s ideas was so big that Laplace famously said that if one was to know all the forces acting on the system and all the position of the particles, in principle, it would be possible to describe what would happen to the system in the future in a single equation. In principle this statement meant that, according to Laplace, mechanics (described by Newton’s laws) and the law of gravitation could describe everything in nature. This shaped a view of reality consisting only of matter, which interacted according to the laws of Newton, and nothingness. The problem with such an approach, of course, was that the electric and magnetic forces (which were already known at that time) couldn’t be explained. In addition, the nature of gravity couldn’t be explained as well.

The beginning of the 20th century brought the world two pinnacles of modern physics: quantum mechanics and general relativity. Einstein’s general relativity explained gravity, whereas quantum mechanics explained the inner workings of the microscopic world. Following the success of general relativity Einstein aimed to unify the two known forces (at that time) of nature — electromagnetism and gravity. This great aim occupied the last decades of Einstein’s life, and, along with other scientists such as Edington and Kaluza, Einstein greatly contributed to the future theories. However, overall, the unification of electromagnetism and gravity was unsuccessful. In addition, another blow to the plans of a unified theory occurred when two new fundamental forces were found — the strong and the weak forces.

Quantum mechanics and general relativity: still incompatible

Today one of the main aims of modern science is to unify all the fundamental forces into a single theory. The electromagnetic force and the weak force have already been unified back in 1967-1968 — a contribution by  Sheldon Glashow, Steven Weinberg, and Abdul Salam, which was awarded with the Nobel prize in 1979. This in turn lead physicists to a search for the so called grand unified theory, which would unify the electromagnetic, weak and strong forces. So far, however, unifying weak and strong forces has been unsuccessful. Thus physicists searching for a complete theory of everything work in two fronts — unification of weak and strong forces and unifying quantum mechanics with general relativity.

The two most prominent attempts to unify gravity with quantum mechanics these days are loop quantum gravity and string theory. The first one is a theory, which  attempts to describe the quantum properties of gravity by quantizing space time. String theory, on the other hand, aims to explain particles and forces as vibrations of one-dimensional strings. Despite the ever-growing popularity of these approaches, no universally accepted theory of everything has been found yet. At present, most of the scientists share the opinion that more knowledge about dark matter and dark energy, as well as more experimental data from particle accelerators are needed to move forward in the quest for theory of everything.

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