Background independence

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Background independence is a condition in theoretical physics, especially in quantum gravity, that requires the defining equations of a theory to be independent of the actual shape of the spacetime and the value of various fields within the spacetime, and in particular to not refer to a specific coordinate system or metric. The different configurations (or backgrounds) should be obtained as different solutions of the underlying equations.

Although physics of string theory can in principle be background-independent , the current formulations of this theory don't make this independence manifest because they require the physicists to start with a particular solution; i.e. a predetermined background in which only small perturbations are described by the theory . A very different approach to quantum gravity called loop quantum gravity is manifestly background-independent. However, the physics of loop quantum gravity may not be entirely background-independent. For example, it may require a choice of topology of the spacetime, although many definitions of background independence do allow for topology choices.

The classical background-independent approach to string theory is string field theory. Although string field theory has been useful to understand tachyon condensation, most string theorists believe that it will never be useful to understand non-perturbative physics of string theory.

This dichotomy between background dependent and independent theories is sometimes traced back as far as to antogonism between Newton and Leibniz about absolute vs. relational space. Most physicists would claim that the choice of approach is merely philosophical so far as no different falsifiable claims follow, not unlike the question of interpretations of quantum mechanics. But philosophers of science Imre Lakatos Elie Zahar have argued that research programs can be driven by metaphysical questions and so adopting the view of background independence may lead to different results.

[edit] References

• L. Smolin, The case for background independence, hep-th/0507235
• C. Rovelli et al, Background independence in a nutshell, Class.Quant.Grav. 22 (2005) 2971-2990, gr-qc/0408079
• Edward Witten, Quantum Background Independence In String Theory, hep-th/9306122.
• J. Stachel, The Meaning of General Covariance: The Hole Story, in J. Earman, A. Janis, G. Massey and N. Rescher (eds.), Philosophical Problems of the Internal and External Worlds: Essays on the Philosophy of Adolf Grünbaum, University of Pittsburgh Press 1993, ISBN 0-8229-3738-7, pp. 129-160.
• J. Stachel, Changes in the Concepts of Space and Time Brought About by Relativity , in C. C. Gould and R. S. Cohen (eds.), Artifacts, Representations and Social Practice; Kluwer Academic 1994, ISBN 0-7923-2481-1, pp. 141-162.
• E. Zahar, Einstein's Revolution: A Study in Heuristic, ISBN 0-8126-9066-4