String theory: Hidden soul of harmony towards a unified theory

THE IMPRESSIVE edifice of modern physics firmly rests on two foundations: Quantum Theory and the General Theory of Relativity. But these two fundamental theories of modern physics are incompatible with each other. String Theory is the only convincing candidate for a theory that resolves this incompatibility.

The effects of quantum theory become noticeable at very small distances. For example, the physics of molecules, atoms, nuclei, and elementary particles can be correctly described only within the framework of quantum theory.

Even though the formalism is rather abstract and far- removed from everyday experience, it has by now come to play an essential role in many practical applications.

It is at the heart of much of the modern technology that has entered our living rooms, including the semiconductor chips in computers or the lasers in CD players. General relativity describes gravity at large distances. For example, the physics of the solar system, neutron stars etc.,

But the beauty of its theoretical structure is so compelling that it is widely recognized as one the greatest achievements of the human mind.

String theory

A correct fundamental theory must incorporate both though the two theories are inconsistent with each other. General relativity is a satisfactory theory of gravity in situations where quantum effects can be ignored.

But a complete theory must be a quantum theory of gravity. Earlier attempts to formulate such a theory failed.

The glaring contradiction remained one of the most important unresolved problems in theoretical physics for over half a century.

String theory is finally revealing to us the glimpses of a more majestic framework that can successfully resolve this contradiction. String theory is currently the most promising candidate for a unified theory of all forces including gravity.

To appreciate this better it is necessary to put it in a historical perspective.

Chemistry has reduced all of matter to a hundred or so types of atoms, called ``elements''. But the atoms themselves turned out to consist of smaller, more elementary particles interacting with each other. Elementary particles are thus the indivisible elements that the world is made up of. They interact with each other via four basic forces- gravity, electromagnetism, the weak nuclear force and the strong nuclear force.

Of the four basic forces, gravity stubbornly refuses to be incorporated into the standard model of particle physics.

Since at large distances quantum effects are negligible, general relativity is a very successful and adequate description of gravity.

But at very small distances, this description must be supplanted by a full-fledged quantum theory of gravity. This is where string theory comes in.

String theory posits that the fundamental constituents of matter are elementary strings. As with a musical string, this basic string can vibrate. Each vibrational mode of the string can be viewed as a point-like elementary particle.

Thus, according to string theory, an electron is a tiny loop of string vibrating in a particular way as it moves around. The loop looks like a point because it is extremely tiny.

This deceptively simple idea was found to have many far-reaching and surprising consequences: Not only the electron, but all elementary particles can arise as different vibrations of this single elementary string.

The theory has been able to place the force of gravity on the same footing as the other three forces and is naturally incorporated within a quantum framework. In string theory, the description of interactions is somewhat different from the standard model. Since both the photon and the electron are simply the same string vibrating in different modes, the emission of a photon from an electron appears as splitting of a string into two strings.

Conversely, the absorption of a photon by an electron appears as joining of two strings into one. The same is in fact true for all other interactions because all force carriers are different excitations of the same string. Therefore, all interactions in string theory take place by splitting and joining of strings.

Unification

All particles arise as different vibrations of the same elementary string. Thus, there is no fundamental distinction between `particles of matter' and `particles of force'.

Moreover, all interactions are completely specified by specifying the rule for the splitting and joining of the elementary string. It follows that `matter' and `force' are simply different aspects of the same fundamental entity and are thus unified. For the same reason, all fundamental forces including gravity are also unified.

Compactification

The world around us appears to have only three dimensions. Each object has length, breadth and height. String theory, on the other hand, predicts that the world should have nine dimensions. Consider the following analogy to understand this.

If we take a long, thin wire, then for all practical purposes, the wire appears to have only one dimension - its length.

Of course, we can find out that the wire is really three dimensional by viewing it under a magnifying glass.

Similarly, in string theory, it is possible that the six extra dimensions curl into a tiny ball. This process is called compactification. If these extra dimensions are sufficiently small, then they would not be noticeable to us and the world would appear effectively three-dimensional. For string theory to describe the real world, the size of the curled up six dimensions would have to be at least ten thousand times smaller than the atomic nucleus.

Stringy geometry and duality

Another feature of the theory is that very large distances can be exactly equivalent to very small distances. This extraordinary equivalence is known as duality.

This is possible because unlike point particles, strings are extended objects and therefore cannot be completely squeezed into a point.

There is a minimum length in string theory called the string length which is at least ten thousand times smaller than the atomic nucleus. If we probe the theory at distances much smaller than the string length, it looks exactly identical to the theory at distances much larger than the string length. As a result, in string theory there is no physical meaning to arbitrarily short distances.String theory is beginning to address some of the long- standing puzzles in quantum gravity. One particularly striking application of these ideas is to the quantum physics of black hole.

A Black Hole is an exotic astrophysical object, whose gravitational pull is so strong that whatever falls inside can never come out. It appears black because even light cannot escape its enormous gravity.In a seminal paper, Hawking showed that when quantum effects are taken into account, a black hole is not really black because it emits a steady stream of particles.

In an exciting recent development in string theory, it was shown that a black hole is indeed very much like an ordinary hot object. At least for a class of black holes, the entropy and other properties of a black hole can indeed be understood in terms of its internal states. This is considered one of the convincing successes of string theory.

Yet, at present, we do not even know the full equations of the theory and how the standard model in all its details would follow from it.

Atish Dabholkar, Sunil Mukhi

& Spenta Wadia

Tata Institute of Fundamental

Research, Mumbai

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