Background independence
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Background independence is a condition in theoretical physics, especially in quantum gravity (QG), 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.
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[edit] Manifest background-independence
This is primarily an aesthetic rather than a physical requirement. It is analogous to requiring in differential geometry that equations be written in a form that is independent of the choice of charts and coordinate embeddings. If a background-independent formalism is present, it can lead to simpler and more elegant equations. However there is no physical content in requiring that a theory be manifestly background-independent - for example, the equations of general relativity can be rewritten in local coordinates without affecting the physical implications.
[edit] String theory
Although physics of string theory can in principle be background-independent , the current formulations of this theory do not make this independence manifest because they require starting with a particular solution and performing a perturbative expansion about this background.
[edit] Background independent QG theories
[edit] String theory
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.
[edit] Loop quantum gravity
A very different approach to quantum gravity called loop quantum gravity has been claimed to be background-independent. However, this theory has difficulty reproducing Einstein's theory of general relativity. Furthermore, the physics of loop quantum gravity is only background-independent in a weak sense. For example, it requires a fixed choice of the topology and dimensionality of the spacetime, while any consistent quantum theory of gravity should include topology change as a dynamical process. Topology change is an established process in string theory.
[edit] History of background independent theories
This dichotomy between background dependent and independent theories is sometimes traced back as far as the antagonism 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 and 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] See also
[edit] External links
[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
