What is This Thing Called String Theory?

by Max Andrews

We’ve all probably heard of string theory. I’ve seen specials devoted to it on PBS, it in major science and philosophy books/papers, dialogues with skeptics, and even in [the greatest film of all time… ever] Good Will Hunting. It’s a very complex and confusing field of research. My hope is that my summation here will help give a introductory grasp of the material.

The spontaneous breakdown of symmetries[1] in the early universe can produce linear discontinuities in fields, known as cosmic strings.  Cosmic strings are also common in modern string theories in which the most fundamental reality are astronomically tiny vibrating strings (either closed or open depending on the interpretation of the mathematics).[2]  The combination of the string/scalar landscape with eternal inflation has in turn led to a markedly increased interest in anthropic reasoning.  In this multiverse scenario life will evolve only in very rare regions where the local laws of physics just happen to have the properties needed for life, giving a simple explanation for why the observed universe appears to permit the evolutionary conditions for life.  It is argued that such anthropic reasoning can give the illusion of intelligent design without the need for any intelligent intervention.[3]  There are at least four ways we can understand the different universes described by string landscape.[4]

The first version depicts various universes as simply occupying different regions of space.  This is most simply realized in chaotic inflation.  Inflation fields don’t vanish but different universes are created in different parts of the universe/multiverse.[5]  The scalar fields in different inflating patches may take different values, giving rise to different values for various effective coupling constants.  The second version describes universes as different eras of time in a single big bang.  What appear to be constants might actually depend on scalar fields that change slowly as the universe expands.[6] The third version depicts universes as different regions of spacetime.  This can happen if various scalar fields on which the constants of nature depend change in a sequence of first-order phase transitions.[7]  In these transitions meta-stable bubbles form within a region of higher vacuum energy and in these bubbles there are bubbles that form with a lower vacuum energy.[8]  It has been suggested that in this scenario the geometry of the universe is small for anthropic reasons and possibly large enough to be detected.[9] The final version treats the universes as wave functions (a part of quantum mechanical Hilbert space), which lead to superpositions of wave functions in which any coupling constant not constrained by symmetry principles would take any possible value. Stephen Hawking argues that a simple “minisuperspace” model of this boundary condition leads to a wave function, which can be interpreted as a superposition of quantum states, which are peaked around a family of classical solutions of the field equations. These solutions are non-singular and represent oscillating universes with a long inflationary period. They could be a good description of the observed universe.[10]

[1] This refers to the symmetry of particles.  For every particle there is a corresponding symmetric particle.  Physics has a translational symmetry, which means that the laws and values of physics are the same at every location in the universe.  If an observer were to travel from one point to a much farther distant point the observer we see no change in the physics.  A broken symmetry introduces change—a non-absolute uniformity.  The breaking of symmetries creates complexity in the laws of nature in the outcome of laws.  There’s a symmetry and uniformity between the strong and weak nuclear forces, which have been unified as electromagnetism by James Clerk Maxwell.  A typical example of vital symmetry breaking is that which gives rise to the balance between matter and antimatter in the early universe.  However, there is an asymmetry between the quantum and the large (a la gravity). String theory is the attempt to unify all of physics. Barrow, The Constants of Nature, 282-84.

[2] Edmund J. Copeland, Robert C. Myers, and Joseph Polchinski, “Cosmic F- and D-Strings,” Cornell University Library arXiv: hep-th/0312067v5, 1-3 (accessed May 9, 2012).

[3] Alan Guth, “Eternal Inflation,” in The Nature of Nature, 495.

[4] This can also be used to compare and contrast with Tegmark’s hierarchy.  These versions are proposed by Steven Weinberg in “Living in the Multiverse,” in The Nature of Nature, 553-54.

[5] Renata Kallosh, et al, “Chaotic Inflation and Supersymmetry Breaking” Cornell University Library arXiv: 1106.6025v2, 8 (accessed May 9, 2012).

[6] Weinberg, “Living in the Multiverse,” 553.

[7] L.F. Abbott, “A Mechanism for Reducing the Value of the Cosmological Constant,” Physics Letters B 150 (1985): 427. Weinberg, “Living in the Multiverse,” 553.

[8] Jonathan L. Feng, et al, “Saltatory Relaxation of the Cosmological Constant,” Nuclear Physics B 602 (2001): 307.

[9] Ben Freivogel, et al. “Observational Consequences of a Landscape,” Journal of High Energy Physics 3 (2006): 39.

[10] Stephen Hawking, “The Quantum State of the Universe,” Nuclear Physics B 239 (1984): 257. Weinberg, “Living in the Multiverse,” 553.

2 Responses to “What is This Thing Called String Theory?”

  1. I’ll be honest. Very little of this made sense to me. Too technical for those not familiar with this particular realm of scientific investigation.

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