The anthropic principle is a group of principles attempting to determine how statistically probable our observations of the universe are, given that we could only exist in a particular type of universe to start with. In other words, scientific observation of the universe would not even be possible if the laws of the universe had been incompatible with the development of sentient life. Proponents of the anthropic principle argue that it explains why this universe has the age and the fundamental physical constants necessary to accommodate conscious life, since if either had been different, we would not have been around to make observations. Anthropic reasoning is often used to deal with the notion that the universe seems to be fine tuned.
The multiverse is a hypothetical group of multiple universes. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them. The different universes within the multiverse are called "parallel universes", "other universes", "alternate universes", or "many worlds".
In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus string theory is a theory of quantum gravity.
In cosmology, the cosmological constant, alternatively called Einstein's cosmological constant, is the energy density of space, or vacuum energy, that arises in Albert Einstein's field equations of general relativity. It is closely associated to the concept of dark energy.
Lee Smolin is an American theoretical physicist, a faculty member at the Perimeter Institute for Theoretical Physics, an adjunct professor of physics at the University of Waterloo and a member of the graduate faculty of the philosophy department at the University of Toronto. Smolin's 2006 book The Trouble with Physics criticized string theory as a viable scientific theory. He has made contributions to quantum gravity theory, in particular the approach known as loop quantum gravity. He advocates that the two primary approaches to quantum gravity, loop quantum gravity and string theory, can be reconciled as different aspects of the same underlying theory. His research interests also include cosmology, elementary particle theory, the foundations of quantum mechanics, and theoretical biology.
The characterization of the universe as finely tuned suggests that the occurrence of life in the Universe is very sensitive to the values of certain fundamental physical constants and that the observed values are, for some reason, improbable. If the values of any of certain free parameters in contemporary physical theories had differed only slightly from those observed, the evolution of the Universe would have proceeded very differently and life as it is understood may not have been possible.
Leonard Susskind is an American physicist, who is a professor of theoretical physics at Stanford University, and founding director of the Stanford Institute for Theoretical Physics. His research interests include string theory, quantum field theory, quantum statistical mechanics and quantum cosmology. He is a member of the US National Academy of Sciences, and the American Academy of Arts and Sciences, an associate member of the faculty of Canada's Perimeter Institute for Theoretical Physics, and a distinguished professor of the Korea Institute for Advanced Study.
Andrei Dmitriyevich Linde is a Russian-American theoretical physicist and the Harald Trap Friis Professor of Physics at Stanford University.
The black hole information paradox is a puzzle resulting from the combination of quantum mechanics and general relativity. Calculations suggest that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state. This is controversial because it violates a core precept of modern physics—that, in principle, the value of a wave function of a physical system at one point in time should determine its value at any other time. A fundamental postulate of the Copenhagen interpretation of quantum mechanics is that complete information about a system is encoded in its wave function up to when the wave function collapses. The evolution of the wave function is determined by a unitary operator, and unitarity implies that information is conserved in the quantum sense.
The Fabric of the Cosmos: Space, Time, and the Texture of Reality (2004) is the second book on theoretical physics, cosmology, and string theory written by Brian Greene, professor and co-director of Columbia's Institute for Strings, Cosmology, and Astroparticle Physics (ISCAP).
The string theory landscape or landscape of vacua refers to the collection of possible false vacua in string theory, together comprising a collective "landscape" of choices of parameters governing compactifications.
Cosmological natural selection also called the fecund universes, is a hypothesis proposed by Lee Smolin intended as a scientific alternative to the anthropic principle. It addresses the problem of complexity in our universe, which is largely unexplained. The hypothesis suggests that a process analogous to biological natural selection applies at the grandest of scales. Smolin published the idea in 1992 and summarized it in a book aimed at a lay audience called The Life of the Cosmos.
The Boltzmann brain argument suggests that it is more likely for a single brain to spontaneously and briefly form in a void than it is for the universe to have come about in the way modern science thinks it actually did. It was first proposed as a reductio ad absurdum response to Ludwig Boltzmann's early explanation for the low-entropy state of our universe.
Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experimental tools to probe these phenomena.
In cosmology, the cosmological constant problem or vacuum catastrophe is the disagreement between the observed values of vacuum energy density and theoretical large value of zero-point energy suggested by quantum field theory.
Renata Elizaveta Kallosh is a theoretical physicist. She is a Professor of Physics at Stanford University, working there on supergravity, string theory and inflationary cosmology.
The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos is a book by Brian Greene published in 2011 which explores the concept of the multiverse and the possibility of parallel universes. It has been nominated for the Royal Society Winton Prize for Science Books for 2012.
Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics is a 2008 popular science book by American theoretical physicist Leonard Susskind. The book covers the black hole information paradox, and the related scientific dispute between Stephen Hawking and Susskind. Susskind is known for his work on string theory and wrote a previous popular science book, The Cosmic Landscape, in 2005.
The Theoretical Minimum: What You Need to Know to Start Doing Physics is a popular science book by Leonard Susskind and George Hrabovsky. The book was initially published on January 29, 2013 by Basic Books.
