From the philosophers of antiquity to the theological school of the Middle Ages to the modern theorists working at the edge of scientific research had only one thought in their minds-to provide a contradiction-free and consistent explanation of the world.
The physicists in particular are engaged in finding the fundamental properties of matter, its movement and the laws that guide its movement or motion. The most difficult part of this endeavour is bringing together the larger and the smaller perspectives. Behaviour of gross matter on a bigger scale was explained by various theories devised by scientists from Newton until Einstein.
But the behaviour of the very small particles is being guided by quite a different set of rules which were discovered during the first few decades of the twentieth century starting with Max Planck and then by a galaxy of physicists of which the leading light was the Danish physicists Niels Bohr, who had long been engaged in a war of words with Albert Einstein to justify the quantum mechanics, because Einstein abhorred the highly probabilistic nature of the quantum laws. But strangely, both the microscopic and macroscopic laws together explain the material world rather in an elegant fashion. But still some gaps between them remained unaddressed.
The ultimate search was to fill these gaps. Robert Matthews of The Guardian writes on these attempts of the physicists to present a single model that at once takes care of the very big, the very small and the forces that come between the two groups.
Albert Einstein spent 30 years of his life searching for it, but failed. Today many of the world's most brilliant theorists are engaged in the hunt for it.
Some claim to have glimpsed it, while others insist it's a delusion. Such is the quest for the holy grail of physics: the theory of everything.
If the quest succeeds, it will one day be possible to write down a single set of equations that describes the properties of all the particles and forces that make up our universe. The nature of space and time, even the origin and fate of the entire universe, will all be there, captured in the arcane language of the universe: mathematics.
As such, the theory of everything will fulfil the age-old yearning of both mystics and physicists to reveal the underlying unity of the cosmos. Creating such a theory is the greatest intellectual challenge ever attempted by scientists. And central to that challenge is the marriage of the two most sophisticated and successful theories ever devised: quantum theory and general relativity.
Quantum theory: The start of the search for the theory of everything can be timed with surprising precision - to the early evening of Sunday October 7 1900. On that day, physicist Max Planck of the University of Berlin had been struggling with a mundane-sounding problem about heat. Put simply, Planck was trying to understand how the radiant energy given off by hot objects varies with temperature. Experimentalists had come up with some formulas that seemed to work but, as a theorist, Planck wanted to find a single, elegant formula that would work in all circumstances.
To his frustration, he found he could only succeed by making a ludicrous assumption: that heat and light are not continuous, but emerge from the object in packets.
Planck called these packets "quanta", from the Latin for "how much", and found a simple formula linking their properties. In so doing, he became the father of quantum theory, the laws governing these packets of energy - and, it turned out, far more as well. For quantum theory is now known to govern everything in the cosmos, from the smallest subatomic particles to the nature of space and time.
When applied to atoms, it explains everything from the colour of dyes to the workings of microchips, and underpins the processes of life itself. Its equations predict the existence of a host of bizarre phenomena such as "tunnelling", by which particles can magically pass through apparently insuperable barriers, a process central to radioactivity and the working of flash memories.
The existence of antimatter - the exact mirror image of ordinary matter - was first predicted using quantum theory, and led to the construction of brain scanners that can watch the mind at work. Transistors, light-emitting diodes and the tiny lasers at the heart of digital communications are all the result of insights from quantum theory. Cosmic insights:But for those engaged in the quest for the theory of everything, it is the cosmic insights of quantum theory that are the focus of attention. One of the most radical of these centres on the fundamental forces at work in the cosmos, such as electromagnetism and gravity. According to quantum theory, each force has its own exchange particle, responsible for transmitting the force from place to place. Some are heavier than entire atoms, while others have no mass at all, but all belong to a specific family of particles known as bosons. On the other hand, quantum theory puts all the particles affected by these forces - electrons, protons, neutrons and the rest - into a second family, known as fermions. Any theory of everything must be able to heal this fundamental schism, and reveal the underlying unity between matter and forces.
There is, however, a huge hurdle in the path of any such unification, and it takes the form of the most familiar of all fundamental forces: gravity.
According to quantum theory, this force is transmitted via massless exchange particles known as gravitons. In principle, it should be possible to create a quantum theory of gravity simply by applying the same techniques that have worked for all the other forces. The problem is that our most successful theory for gravity - Einstein's general relativity - shows it is not just another force.
According to Einstein, what we call gravity is actually the curving of the very fabric of space and time by mass. As such, taking gravity to be just another force that operates through space and time is a fundamental error. The reality is nightmarishly more complex, as quantum theorists soon discovered. Gravitons carry the force of gravity, but that very act affects their properties, producing a ferocious feedback loop. As a result, theorists found that even simple problems in quantum gravity produced nonsensical results.
For decades they made virtually no progress in creating a theory of quantum gravity, and turned their attention to understanding the other fundamental forces. Bizarrely, it was a failed attempt to understand a force at work in the atomic nucleus that revolutionised quantum gravity theory - and the search for a theory of everything.
String theory: During the late 1960s, physicists were attempting to make sense of the properties of particles that feel the so-called strong force, which holds together the atomic nucleus. Known as hadrons, these include the familiar protons and neutrons, along with a host of more esoteric particles. While looking for a pattern in the chaos, an Italian physicist named Gabriele Veneziano cobbled together a formula that seemed to capture many of the properties of hadrons. Other theorists then tried to explain the success of the formula, and made an astonishing discovery: it worked only if hadrons weren't like tiny billiard balls, but more like strings, which could stretch, shrink and vibrate.
Attempts to extend Veneziano's formula led to another discovery of profound importance for the theory of everything. The formula became far more powerful if there's a underlying unity or "symmetry" between the particles making up matter, like electrons and protons, and the exchange particles that transmit forces from place to place, like photons.
When combined with string theory, this aptly named "supersymmetry" led to the creation of a whole new understanding the strong force, known as superstring theory. Yet there remained a huge problem: superstring theory predicted the existence of a massless exchange particle utterly irrelevant to the strong nuclear force. This persuaded many theorists that, for all its attractions, superstring theory just wasn't up to the job it was designed for. They abandoned the idea - not least because a rival theory for the strong force had emerged with more successes, and none of the problems.
But in 1974, theorists John Schwarz and Joel Scherk at the California Institute of Technology made a discovery that would put superstring theory at the heart of the quest for a theory of everything. They pointed out that the massless particle that had killed off interest in superstring theory could be the graviton - the key particle in a quantum theory of gravity. Suddenly, a theory of everything no longer seemed like a pipedream. Superstring theory appeared capable of describing all the forces - including gravity - and revealing the underlying unity of every type of particle.
At the time, few took notice - not surprisingly, given the dismal failure of previous work on quantum gravity. That all changed a decade later when Schwarz and Michael Green of London University found evidence that superstring theory might be free of the nonsensical results that had killed off so many previous attempts to marry quantum theory with gravity.
Almost overnight, many theorists dropped what they were doing to work on superstring theory. They found themselves pondering a mind-bending realm where particles and forces were multidimensional vibrating strings, utterly unlike anything in conventional physics.
By the late 1980s it was clear that, despite being a major advance, superstrings weren't the whole story. While there can be only one theory of everything, theorists uncovered many different superstring theories, and no clear way of choosing between them. Superstrings seemed to be just a shadow of something even grander.
In 1995, string theorist Edward Witten of the Institute of Advanced Study, Princeton, unveiled what many now regard as the first glimpse of that ultimate theory, perhaps the theory of everything itself. Witten showed that the superstring theories may be just different facets of a single, overarching idea, which he dubbed M-theory.
M-theory: The "M" may represent "Mother", "Mysterious" or even "Magic", but the connection with superstrings is clearest if it stands for "Membrane". The various superstring theories then emerge as merely the multidimensional "edges" of 11-dimensional membranes, seven of which have contrived to stay unobservable in our four-dimensional universe.
Today, M-theory is regarded as the best candidate yet for what Einstein sought and more besides: a single, unified description of all the fundamental forces, and the particles on which they act.
Yet it remains less of a theory than a possibility, and leaves many mysteries unanswered. No one knows why just four of M-theory's 11 dimensions remain observable. Nor is it clear that when the basic equations of M-theory are found, they will be free of the deadly mathematical diseases that have wrecked every previous attempts to create a theory of everything. Perhaps most worrying of all, M-theory may never lead to just one, unique set of equations that describe the one, unique universe that we live in.
M-theory may ultimately prove to lack the power to serve as the theory of everything. But at the very least, it has given us a glimpse of a cosmic unity beyond the dreams of any mystic.
