The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory is a book by Briane Greene that explains in non-technical terms the principles and unification attempts of string and superstring theories. The book explains the theories as a way of unifying the probabilistic theory of Quantum Mechanics according to the standard model and the Einstein's deterministic theory of General Relativity.
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The most popular website for book notes. SparkNotes was created by students for students, and their summaries are accessible and extremely well-written. Besides the usual plot overview, chapter summaries, and character analysis, SparkNotes differentiates itself with a section discussing themes, motifs, and symbols.