## Thursday, December 09, 2004

### Quantum Geometry

Mathematics is not the rigid and rigidity-producing schema that the layman thinks it is; rather, in it we find ourselves at that meeting point of constraint and freedom that is the very essence of human nature.
- Hermann Weyl

I know I said I would post the discussion between Susskind and Smolin again for refreshing but I wanted to post the issue of Quantum Geometry first and then move there.

My area of research is superstring theory, a theory that purports to give us a quantum theory of gravity as well as a unified theory of all forces and all matter. As such, superstring theory has the potential to realize Einstein's long sought dream of a single, all encompassing, theory of the universe. One of the strangest features of superstring theory is that it requires the universe to have more than three spatial dimensions. Much of my research has focused on the physical implications and mathematical properties of these extra dimensions --- studies that collectively go under the heading "quantum geometry".

Quantum geometry differs in substantial ways from the classical geometry underlying general relativity. For instance, topology change (the tearing" of space) is a sensible feature of quantum geometry even though, from a classical perspective, it involves singularities. As another example, two different classical spacetime geometries can give rise to identical physical implications, again at odds with conclusions based on classical general relativity.

If one did not understand where this geometry will begin, then it does not make much sense for a person to consider the mathematics that will arise from this situation?

The Elegant Universe, by Brian Greene, pg 231 and Pg 232

"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of genral relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."

So I have shown I thnk the importance of the math involved and how it might address the quantum nature of the world in small things. We find, we can be quite comfortable in looking at the achievemets of Einstein, in leading us to a good perception about things on a cosmological scale. But moving back to the "quantum geometry," what are we describing here?

Quantum gravity is perhaps the most important open problem in fundamental physics. It is the problem of merging quantum mechanics and general relativity, the two great conceptual revolutions in the physics of the twentieth century. The loop and spinfoam approach, presented in this book, is one of the leading research programs in the field. The first part of the book discusses the reformulation of the basis of classical and quantum Hamiltonian physics required by general relativity. The second part covers the basic technical research directions. Appendices include a detailed history of the subject of quantum gravity, hard-to-find mathematical material, and a discussion of some philosophical issues raised by the subject. This fascinating text is ideal for graduate students entering the field, as well as researchers already working in quantum gravity. It will also appeal to philosophers and other scholars interested in the nature of space and time.

The same vigor with which string theory/M theory is attack for is fundamental points about the nature of the geometric world is no less important then what achivements and attempts are made by Rovelli. Each aspect of the societal influence theoretists and physics people engage in, is part and parcel of the individuals who are, hands on with the Elephant.

Edward Witten

Reflections on the Fate of Spacetime