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Quantum Gravity

The task to formulate a consistent quantum theory of gravity has occupied physicists since the first attempt by Leo Rosenfeld in 1930. The general expectation is that a quantum theory of gravity is needed in analogy to quantum mechanics in which the classical instability of atoms has disappeared. The origin of the universe cannot be described without such a theory so cosmology remains incomplete without quantum gravity.

A theory of gravity is also a theory of space time. Quantum gravity should thus make definite statements about the behavior of space time on the smallest scale. Because of its geometric nature gravity interacts with all forms of energy. Since all non-gravitational degrees of freedom are successfully described by quantum fields.

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Acceleration of Gravity

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Gravity is one of the four fundamental forces of nature which is responsible for giving weight to objects with mass and attracts them to the ground when dropped.

Gravitational theory was first given by Galileo Galilei in the late 16th and early 17th centuries. He showed that all bodies are accelerated by gravitation at the same rate.

Acceleration due to gravity

The strength of the field of gravitation is numerically equal to the acceleration of the body in that field and is denoted by g.

At the earth’s surface, g = 9.8 m/sec2

All planetary bodies are surrounded by their own field of gravitation which is responsible for exerting an attractive force on all objects. Newton’s third law postulates that the Earth experiences an equal and opposite force to that acting on the falling object, means that the Earth also accelerates towards the object until the object hits the earth.

According to Newton’s law of universal gravitation,

F = mg

Where, F is the force, m is the mass and g is acceleration due to gravity.

Gravity provides centripetal force enabling the bodies on the surface to move in a circular motion.

Loop Quantum Gravity

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Quantum loopgravity is string theory's biggest competitor. It gets press than string theory, in part because it has fundamentally more limited goal. Loop quantum gravity performs this feat by trying to quantize a space itself - in other words treat it like it comes in small chunks.

Loop quantum gravity makes some definite predictions which may mean that it could be tested well before string theory can be.

Scientists use loop quantum gravity to actually match Einstein's classical theory.
  1. The areas and volumes in loop quantum gravity are always in finite, discrete units,
  2. In the Barrett-Crane model of loop quantum gravity, the probabilities for quantum geometry to evolve into different histories are always finite.
  3. Including gravity, in a loop quantum gravity theory that contains matter theory, like the standard model involves no infinite expressions. If gravity is excluded we have to do some tinkering to avoid them.

String Theory

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The major alternative to quantum general relativity is string theory. Its starting point is completely different. Whereas quantum general relativity focuses on a separate quantum description for the gravitational field, string theory seems to implement the idea that the problem of quantum gravity can be dealt with only within a unified quantum theory of all interactions.

The main characteristics of string theory are
  1. The appearance of gravity is inevitable. The gravitation is an excitation of closed strings and it appears via virtual closed strings in open string amplitudes.
  2. String theory has an important ingredients the concepts of gauge invariance super symmetry (SUSY) and the presence of higher dimensions.
  3. It provides a unification of all interactions.
  4. String perturbation theory seems to be finite at each order, but the sum diverges.
  5. Only three fundamental constants are present: h, c and the string length ls; all other physical parameters should in principle be derivable from them
  6. A net of dualities connects various string theories and indicates the presence of an underlying non-perturbation theory (M-theory) from which the various string theories can be recovered in appropriate limits.

Loop Quantum Gravity vs String Theory

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Despite the possible harmony between the two fields, at the moment they are competitors for research funding and attention. Loop quantum gravity vs string theory have conference and loop quantum gravity people have their conference, and rarely shall the two conferences meet.

Part of the problem is one of sociology. Many string theorists even in research papers use phrases that make it clear they consider string theory to not only be their preferred theory, but to be the only theory of quantum gravity. By doing this, they often dismiss LOG as even being an option. Some string theorist have indicated that they are completely unaware of any viable alternatives to string theory.

Hopefully these physicists will find a way to work together and use their results and techniques to provide real insights into the nature of our universe. But so far loop quantum gravity like string theory is still stuck.

Quantum Theory of Gravity

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The last decade has seen a remarkable growth in the theoretical effort devoted to the construction of a quantum theory of gravity. The unification of gravity  and quantum physics has always been understood to be a fundamental question.

It is known that in general relativity, gravity is treated as space time geometry. So in most traditional quantum gravity, gravitational field is represented by space-time metric and space time is always curved when there exists non trial gravity. In quantum gauge theory of gravity, in order to set up a perturbatively renormalizable quantum gravity a completely different picture is adopted to treat gravity and space time.

In quantum gauge theory of gravity, gravity is treated as a kind of fundamental interactions which is its transcendental starting point. In other words it is a physical interactions of physical fields which is transmitted by gravitons.
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