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The physical reality of time,
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This review is from: From Eternity to Here: The Quest for the Ultimate Theory of Time (Hardcover)
The behavior of matter (or energy) in space and time is described by the laws of physics, but the puzzling thing about physical reality is that space and time behave differently. Space is the same in all directions and it never changes, but time has preferred direction; past to future and the cause-effect relationship runs parallel to this. There is no such thing as special place (space) in the universe but there is a special time. This is a mystery because physical laws governing the fundamental particles are mostly time-symmetric (it can function thermodynamically or anti-thermodynamically), but the time-asymmetry observed in many macromolecular processes is thermodynamic and it has an arrow of time. Examples include, a glass bottle breaking into pieces or hot water becoming cold are attributed to the second law of thermodynamics which seem to set this arrow of time. Thus the physical reality is not only governed by laws of quantum physics and relativity, but also by the second law of thermodynamics which requires that the entropy (a measure of disorderliness) of a closed system, such as this universe, increase with time. This implies that the past has more order than future, hence the state of orderliness was probably the highest (or the entropy was the lowest) at the origin of the universe (big bang). The problem of justifying this arrow is not so much showing that the entropy of isolated systems increased, but explaining why there was low entropy in the past. While inflationary theory proposed by Alan Guth explains many key features of the early universe but it doesn't explain low entropy.
In this book, the author looks for clues in several areas such as, properties of black hole; information-loss paradox and Hawking radiation, string theory, inflationary epoch, multiverse cosmology and baby universes. He argues that a classical de Sitter background (mother space-time where vacuum energy is positive) does not fluctuate, but the space would be expanding and quantum fields will be fluctuating in a classical fashion. But if quantum gravity is taken into consideration then de Sitter space is itself susceptible to quantum fluctuations and this result in not only stretching and bending of spacetime as required by general relativity but also they could splice into multiple pieces. These pieces first appear as bubbles of spacetime, and then they grow and splice off to form baby universes. The baby universe created in a background de Sitter space is inclined both towards its past and to the future, but each baby universe starts in a dense low entropy state and exhibits a local arrow of time as it expands and cools. The baby universes born in the past have an arrow of time pointing in the opposite direction to those in the future, but for each universe, the time is directed towards increasing entropy and the multiverse manifests overall time symmetry. The author's hypothesis sharply contrasts the idea that big bang represents the boundary to space and time, and it dispels the notion that space and time were created at this time. He distances himself from other physicists like Larry Schulman who suggests that the universe switched to a highly ordered state at about 380,000 years when the universe became transparent to light (1, 2). The essential features of thermodynamics in the arrow of time are discussed by others which include mathematical physicist Roger Penrose (3), physicists Robert Wald (4), and Larry Schulman (5).
This is an excellent review of the concept of time in terms of physics, cosmology and philosophy. You need to have basic knowledge of physics to understand and appreciate the core ideas of the author. Chapters 12-15 are most interesting and the author discusses certain aspects of cosmology and black holes that are not relevant to physics of time but his discussions are well presented and it is very interesting to read. The main hypothesis of the author, about the arrow of time presented in chapter 15 is largely speculative and it is unlikely that physicists are convinced with his argument, however the debate will continue.
1. Schulman, L.S., Source of the observed thermodynamic arrow. [...], Nov 17, 2008
2. Gold T., Am. J. Phys. 30, 403-410 (1962)
3. The Road to Reality: A Complete Guide to the Laws of the Universe
4. Wald, R.M., The arrow of time and the initial conditions of the universe. [...], July 21, 2005
5. Time's Arrows and Quantum Measurement