A theory of everything (TOE) is a hypothetical framework explaining all known physical phenomena in the universe. Researchers have searched for such a model ever since the development of quantum mechanics and Albert Einstein's theory of relativity in the early 20th century.
Each of these pillars of modern physics describes its respective area of inquiry — the very smallest and the most massive things in the cosmos — with astounding accuracy, but both quantum mechanics and relativity fail when applied to each other's subject matter. So far, an overarching theory of everything has eluded scientists, and some believe the ultimate goal is unrealistic.
Einstein's valiant effort
Einstein began to search for a unifying theory in the 1920s, according to the American Physical Society (APS). He had never fully accepted the strange paradoxes of quantum mechanics, and he believed that the mathematics describing electromagnetism and gravity, the only two forces known at the time, could be combined into a single framework.
"I want to know how God created this world," Einstein told a young physics student named Esther Salaman in 1925. "I'm not interested in this or that phenomenon, in the spectrum of this or that element. I want to know His thoughts; the rest are just details."
But Einstein's quest proved quixotic during his lifetime. "Most of my intellectual offspring end up very young in the graveyard of disappointed hopes," he wrote in a letter in 1938. Yet he didn't give up, and while on his deathbed, he asked to have his latest notes on the theory of everything brought to him, according to the APS.
During the mid-20th century, physicists developed the Standard Model, which has been called the "theory of almost everything." It describes the interactions of all known subatomic particles and three of the four fundamental forces: electromagnetism and the strong and weak nuclear forces, but not gravity.
A model that also included gravity would be known as a quantum gravity theory. Some researchers believe that string theory is such a framework and fits the bill for a theory of everything. String theory posits that particles are actually one-dimensional, string-like entities vibrating in an 11-dimensional reality. The vibrations determine the different particles' properties, such as their mass and charge.
Or, maybe it doesn't exist
But other scientists consider the idea of string theory an intellectual dead end. Peter Woit, a theoretical physicist at Columbia University, has repeatedly scolded his colleagues for chasing what he considers an imaginary dream.
"The basic problem with string theory unification research is not that progress has been slow over the past 30 years," Woit wrote on his blog, "but that it has been negative, with everything learned showing more clearly why the idea doesn't work."
In his bestselling book "A Brief History of Time" (Bantam Books, 1988), physicist Stephen Hawking discussed his desire to help create a theory of everything (which was also the title of his 2014 biopic). But the famous scholar changed his mind later in life; he thought such a theory would be out of reach forever because human descriptions of reality are always incomplete, according to a 2002 lecture available on a website dedicated to the late physicist.
This fact did not sadden him but rather gave him hope. "I'm now glad that our search for understanding will never come to an end and that we will always have the challenge of new discovery," Hawking said. "Without it, we would stagnate."