"Time Reborn" (2013)

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During the 20th century many physicists reached the conclusion that time is
only an illusion. Smolin reaches the exact opposite conclusion: that time is
the only thing that is fundamental.
Smolin also attacks the notion that timeless truths exist.
Therefore it is not possible to calculate the future: knowing the exact state
of the universe allows you to calculate the future state of the universe only
if the laws of the universe are known, not if they change.
Smolin had already offered a theory of how the laws of the universe may change
in his book "The Life of the Cosmos" (1997), basically inspired by Darwin's
biological evolution: universes reproduce creating black holes which in turn
create new universes, each with slightly different laws of nature.
Smolin adopts a view called "relationism": to define something you have to
define its relationship with everything else.
Newton was not a relationist, Einstein was.
The first relationist was Leibniz.
Everything is defined in terms of relations to something else. Even the mass
of particles depends on their interaction with other particles.
A network of interactions yields reality as we know it, including space.
Space is emergent. Time is not.
Alas, it is very difficult to study a system of interactions. Therefore
physicists isolate a subsystem of the universe and, standing outside it,
study it. One particular object that stands outside the subsystem under study
is the clock.
Whatever one concludes from a study of such a system is predicated on the
assumption that the external clock is not affected by what happens inside
the system.
Smolin shows that Newton's theory made time inessential: the history of a system is a curve in configuration space. There is no time in a configuration space. That curve is already determined by the initial conditions and by the (timeless) laws of nature. Einstein's Special Relativity further reduced the power of time because it questioned the meaning of "now" and of causality (the sequence of events and the possibility that one influences the other): everything is relative. Minkowski represented Special Relativity in a four-dimensional spacetime in which time is simply the fourth dimension. However, Einstein's General Relativity is a different kind of theory in which space and time are purely relational: the geometry of spacetime changes in time.With the discovery of the expansion of the universe, cosmology was born, which is basically historiography applied to cosmic bodies. Bryce DeWitt, John Wheeler, Charles Misner and Stephen Hawking mixed quantum mechanics with General Relativity and created quantum cosmology. The equations of quantum cosmology dispose of time altogether. The problem, again, is that the clock that quantum cosmology uses sits outside the universe to which it is applied, and is not supposed to be influenced by what happens inside. Most physicists rescue time by assuming that time "emerged" during the life of the universe. But people like the British physicist Julian Barbour ("The End of Time", 1999) took the equations of quantum cosmology literally and argued that only moments exist. Some of your moments happen to contain memories of other moments and create the illusion that some moments followed others, but this flow of time is just an illusion. In reality each moment exists forever and is independent of every other moment. Einstein himself once wrote "The distinction between past, present and future is only a stubbornly persistent illusion". Why does our universe have these laws of nature and why did it start with those specific initial conditions in the Big Bang? These questions are no longer purely philosophical: both loop-quantum theory and string theory have numerous solutions, only one of which gets implemented in our universe. Smolin argues that all the laws that physics assumes to be universal are actually just approximations: we verified them only in a subset of the whole universe, and under the assumption that their interaction with the rest of the universe is negligible. Each law of string theory or loop-quantum gravity explains at the same time too much (predicts the existence of too many universes) and too little (it cannot explain its own existence). Smolin believes that a new paradigm is required, and believes that time will be central to this new paradigm. Einstein never believed that Quantum Mechanics was the final theory of the universe. He insisted that there were "hidden variables", whose discovery would make Quantum Mechanics deterministic. He wasn't alone. Louis DeBroglie presented such a theory in 1927: by adding a "quantum force", he made Quantum Mechanics deterministic. VonNeumann's book contained a proof that hidden variables were impossible, but his proof was flawed. Then David Bohm reinvented DeBroglie's theory. Bohm developed a perfectly valid theory that satisfied Einstein's hypothesis of hidden variables, but this theory shows that Einstein's Relativity is wrong about one fundamental thing: the relativity of simultaneity. In Bohm's theory there is a preferred observer. Decades later, the British physicist Antony Valentini proved that this fact applies to every hidden-variables theory ("Hidden Variables and the Large-Scale Structure of Spacetime", 2005): if Einstein was right about hidden variables, then Einstein was wrong about the relativity of time. Smolin believes that it is possible to salvage General Relativity while surrendering the relativity of time. There are several theories that assume time is fundamental. First of all, there is "Shape Dynamics", sketched by Julian Barbour to incoporate Mach's principle into General Relativity ("The Physical Gravitational Degrees of Freedom", 2005) and successfully elaborated by Henrique Gomes, Sean Gryb and Tim Koslowski ("Einstein Gravity as a 3D Conformally Invariant Theory", 2010), who proved that it is equivalent to the standard formulation of General Relativity as conceived by Richard Arnowitt, Stanley Deser and Charles Misner ("Dynamical Structure and Definition of Energy in General Relativity", 1959). Stephen Hawking expressed Mach's principle as: "local physical laws are determined by the large-scale structure of the universe". Einstein was inspired by this principle but his assumptions implicitly violated it. Time is universal in Shape Dynamics. It turns out that Shape Dynamics and General Relativity are dual theories: we can use one or the other and obtain the same results, just expressed in a different language. This duality may or may not be related to Juan Maldacena's "gauge/gravity duality", a consequence of the holographic principle ("The Large N limit of superconformal field theories and supergravity", 1998). Other attempts at restoring absolute time in General Relativity were made by the Czech physicist Petr Horava ("Quantum Gravity at a Lifshitz Point", 2009), the Southafrican physicists George Ellis and Rituparno Goswami ("Space Time and the Passage of Time," 2012) and by the Taiwanese physicist Hoi-Lai Yu ("General Relativity Without Paradigm of Space-time Covariance, and Resolution of the Problem of Time", 2012). Correspondingly, there are theories that space is not fundamental but emergent. For example, the Danish physicist Jan Ambjorn and the German physicist Renate Loll ("Discrete Lorentzian Quantum Gravity", 2000) initiated the theory of "causal dynamical triangulations", which were perhaps the first quantum cosmologies that on large scales were solutions of Einstein's equations, and Fotini Markopoulou formulated "quantum graphity" ("Quantum Graphity", 2006). Smolin argues that an absolute and real time can also explain why the universe is full of structure and complexity. He ends the book admitting that one colossal mystery still eludes every scientific theory of the universe: consciousness. It's something we all know that it exists (because it's inside us) but it is not predicted by any of the equations of science. The day the last human dies, an alien visitor would be able to use human science to reconstruct the entire past of the universe, including the body of each of us, but with no clue that these objects were also conscious. 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