#20MustKnowTheories 1/20 Theory of Everything It is called The Holy Grail of physics: A set of mathematical equations unifying all the forces and particles in the universe. The quest of this theory of everything convinced even Albert Einstein. Now, theoretical physicists believe that soon they will be able to give answers to the ultimate cosmic questions. Sunday April 17th 1955, Albert Einstein in his bed in the hospital of Princeton started the last calculations of his life. He wrote algebraic line after algebraic line. After resolving some basic equations, he put his work next to him to take a break. Few hours latter, the greatest scientist of the 20th century died. Near his bed there was his last vain attempt to create his dream of “a unified field theory”: a simple and coherent explanation of all the known forces in the universe. Einstein has searched unsuccessfully for such a theory for many years. Today, more than half a century later, his dream is perhaps about to become a reality. Some of the most eminent theoretical physicists in the world think that they elaborated a theory that takes in more than Einstein himself has considered. In its final form, it will explain not only all the forces of the universe and all the particles on which they act, but also the true nature of space and time. Its scope is so vast that it was called the “theory of everything”. This name does wince many theorists, even though they are captivated by this theory which can maybe answer questions like: “Why is our universe like it is?”; “Where does it come from?”; “What was before?” One must admits that its final form lacks clarity, even its biggest lines are extraordinary: hidden extra-dimensions of space, coutures in the fabric of the spacetime, superstrings, and entire new other universes different than our. The most striking thing is maybe that the origins of these concepts are found in a series of discoveries made in a laboratory 170 years ago. In the Royal Institute of London, during the summer of 1831, the great English physicist Michael Faraday led a pioneered series of study on the relationship between electricity and magnetism. He knew already that a flux of electricity though a cable produces a magnetic field. What Faraday wanted to know is whether the converse is true: can magnetism generates electricity? A missing discovery After some false beginnings, Faraday succeeded, building the first version of what we call today the Dynamo. But besides this amazing technological invention, he has made a big scientific discovery. Despite appearances, electricity and magnetism are in fact two different sides of the same phenomenon. This was the first clue of what will become one of the driving principles for those who are searching for the ultimate theory of everything: the universe possesses a fundamental unity, if we look at it in the appropriate way. The experimental knowledge of Faraday allowed him to have a glimpse of this unity, but he didn't have the intellectual tools to reveal its glory: mathematics. In 1861, the Scottish theorist James Clerk Maxwell succeeded to translate Faraday's discoveries into mathematical language. The results are now called Maxwell's equations of electromagnetism, which show explicitly the fundamental unity of electricity and magnetism. It was more than a big theoretical discovery. Maxwell's equations predicted the existence of invisible waves of the electromagnetic energy which are the basis of many of nowadays technologies from the micro-waves to the Internet. The prowess of Maxwell aroused an evident question: Can this cosmic unity be extended further to include the most familiar force, Gravity? It's the challenge that Einstein gave himself just after revealing in 1915 his new radical vision of gravity: General Relativity. According to Einstein, gravity is not a mysterious force which emanates in a certain way matter like Newton has described it; it is rather the result of the deformation of the fabric itself of the spacetime around us. Einstein had in the same time the faith and the intelligence necessary to unify gravity and electromagnetism. However he discovered soon that the challenge was much more difficult than what he has imagined. The first difficulty was to find a way to unify general relativity with Maxwell's equations. On closer look, general relativity had little in common with Maxwell's equations which consider electromagnetism like a “Field of Force” In 1919, Einstein sighted what he thought a major hint of the unification of these two disparate theories. Theodor Kaluza, a German mathematician, had demonstrated with a set of equations what perhaps will recap the two theories, but only if the universe has an additional dimension, a fifth dimension of space if we consider the three familiar dimensions of space plus the dimension of time. It was an incredible idea, but hard to take it seriously. After all, where is this extradimension? In 1926, the Swedish mathematician Oskar Klain suggested an answer. The extradimension could be wrapped, very small to be detected, like a tiny thickness of a hair gives the impression that it has only one dimension. Einstein had the intuition that Kaluza and Klein had a piste, but he couldn't make of it a unified theory of gravity and electromagnetism that he is searching for it with passion. He had always unresolved questions or absurd consequences. He tried many other vainly approaches. When he passed away in 1955, most of researchers were convinced that Einstein simply wasted his time with ingenious mathematics but completely empty. The feeling of failure was underscored by the fact that Einstein had completely ignored the two other new forces discovered after the beginning of his work. The first is the Strong Nuclear Force which bind together the protons and neutrons inside the nucleus surpassing the electric repulsive force between the positive charged protons. The second is the Weak Nuclear Force causing radioactivity. After Einstein's death, very few serious researchers tempted to resume his attempts to unify gravity and electromagnetism. The attention turned instead to unify electromagnetism with of the of the new discovered forces: the weak interaction. Like Maxwell has demonstrated, the trick was to find a mathematical description of the two which shows their hidden similarities. During the 1920s and 1930s, physicists believed that they have found a way to do it using Quantum Field Theory. Quantum Field Theory sees each fundamental force having its own carrier particle, for example, the photon in the case of electromagnetism. In the 1950s, physicists started exploring the similarities between the photon and the carrier of the weak interaction, W particle. Finding similarities was far from being simple, especially because W particle is infinitely massive compared to the massless photon. However, in the end of the 1960s, three theoretical physicists, Steven Weinberg and Sheldon Glashow in the USA, and Abdus Salam in England, developed independently theories according to which these two forces presented just different aspects of the same force “The electro-weak force”. Moreover, this unification predicted new subtle effects which could be tested experimentally. When these predictions started to be verified in the 1970s, physicists celebrated the first successful unification after Maxwell's more than a century ago. A truly cosmic vision At that time, Glashow at Harvard University, had started looking for other ways to reveal the underlying unity of the forces. In 1973, working with his colleague Howard Georgi, found a mathematical theory unifying electromagnetism and the strong and weak nuclear forces. This Grand Unification Theory opened a really cosmic vision of the fundamental forces of nature, showing that these three forces had been part of a unique “Superforce” which had ruled the universe just after the Big Bang. During the cooling of the universe, the three forces has been separated, creating the cosmos that we see today. Once again, the theory made some predictions, but this time the confirmation was hard to get. Theoretical Physicists discovered that the Grand Unified Theory misses a main ingredient, an element which produced another surprising unification. This is the so-called “Supersymmetry”. Discovered in the beginning of the 1970s, Supersymmetry is a mathematical property which unifies matter particles like electrons and protons with forces carrier particles like photons. Supersymmetry erases the apparent differences of these subatomic particles to reveal their basic unity. But in the same time, theoretical physicists discovered that Supersymmetry gives another clue, much deeper, to the Theory of Everything, a clue that suggests how to unify the superforce of the Grand Unified Theory with one that was still apart: Gravity. Some theoretical physicists had already tried to include gravity using Quantum Field Theory with the same philosophy which is to describe it with force carrier particles called Gravitons. However, like Einstein, they faced frightful mathematical problems and difficulties. Nevertheless, the most promising theorists of these heroic failures included all Supersymmetry, plus the idea that fascinated even the mighty Einstein half a century ago: extradimensions. What was missing is an ingredient which unifies Gravity with the other three forces without unchaining mathematical demons. This ingredient was Superstring. In 1984, theoretical physicists John Schwarz at Caltech and Michael Green at the university of London surprised their colleagues by claiming that they could unify gravity with the other forces without having the usual problems. They had a condition though: the particles are not considered like point particles anymore, ut rather like tiny entities called “Superstrings”, so much smaller than an atomic nucleus, these objects looking like tiny strings of energy should also possess Supersymmetry and they exist in six dimensions. Something even bigger It was an incredible assertion that prompted an army of theorists to know more about the Superstrings. In the end of the 1980s, it was clear that, despite this major advance, it's not the whole story yet. So that there can be only one theory of everything, theorists discovered five Superstring theories! And there were no evident clue to choose between them. Superstring theory seemed to be just a shadow of something bigger. In 1995,the eminent string theorist and mathematician Edward Witten, from the Institute of Advanced Study in Princeton, New Jersey, unveiled what many people in physics community the first vision of this ultimate theory, maybe even the theory of everything itself. Witten has shown that the five Superstring theories were all a rough description of a unique idea that encompasses them, he called it M-Theory. Some physicists said that M means “Mother”, “Mysterious”, or even “Magic”; but its link with Superstrings shows that M means rather “Membrane”. The five theories emerge as like the “Edges” multidimensional of membranes of eleven dimensions, seven of them are so wrapped to be visible. Today, so many theoretical physicists consider that M-Theory is the closest theory to The Holy Grail of physics: a unique description unifying not only gravity with electromagnetism, but also with the strong and weak nuclear forces and all the particles on which they act. The theorists are searching now in its complicated equations some clues of the puzzles of the beginning of the universe. A main stream actual line of research suggests that the Big Bang is the most recent of an infinite series of Big Bangs triggered by collisions of the multidimensional membranes of M-Theory. Lack of evidence The theorists are not all convinced that M-Theory is ready to probe such questions. They think that there are still many things lacking in the theory to have faith in its conclusions. Such skepticism is consolidated by calculations done by theorists in California, Joe Polchinski and Raphael Bousso, which show that M-Theory permits a huge number of possible universes- about . It is rather from the unique universe that we live in. Or maybe not? The theorists working on the events which happened at the Big Bang discovered that our universe could be just a small part of something much more bigger, called a “Multiverse”. If this is true, this means that the incapacity of M-Theory to predict one universe could be one of its biggest and first triumphs. But this emphasizes a more fundamental point concerning M-Theory: How could we say for sure that there are really other universes than our? Other critiques look simply at the absence of experimental evidence of the basic concepts of M-Theory like the existence of Supersymmetry or extradimensions. However, this could change soon Thanks to the searches which are done in the Large Hadron Collider LHC at CERN. Many theorists believe that this particles accelerator can prove the existence of Supersymmetry, a major component of M-Theory. Einstein said once: “What really interests me is whether God had any choice in the creation of the world.” If the LHC finds all the new particles predicted by Supersymmetry, this is going to be a huge step toward answering Einstein's question and accomplishing his quest for the ultimate theory of everything.