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It has been shown that the theory of relativity can be applied physically to the functioning brain, so that the brain connectome should be considered as a four-dimensional spacetime entity curved by brain activity, just as gravity curves the four-dimensional spacetime of the physical world. Following the most recent developments in modern theoretical physics (black hole entropy, holographic principle, AdS/CFT duality), we conjecture that consciousness can naturally emerge from this four-dimensional brain connectome when a fifth dimension is considered, in the same way that gravity emerges from a 'flat' four-dimensional quantum world, without gravitation, present at the boundaries of a five-dimensional spacetime. This vision makes it possible to envisage quantitative signatures of consciousness based on the entropy of the connectome and the curvature of spacetime estimated from data obtained by fMRI in the resting state (nodal activity and functional connectivity) and constrained by the anatomical connectivity derived from diffusion tensor imaging. [ABSTRACT FROM AUTHOR]

Fundamental physics began the twentieth century with the twin revolutions of relativity and quantum mechanics, and much of the second half of the century was devoted to the construction of a theoretical structure unifying these radical ideas. But this foundation has also led us to a number of paradoxes in our understanding of nature. Attempts to make sense of quantum mechanics and gravity at the smallest distance scales lead inexorably to the conclusion that spacetime is an approximate notion that must emerge from more primitive building blocks. Furthermore, violent short-distance quantum fluctuations in the vacuum seem to make the existence of a macroscopic world wildly implausible, and yet we live comfortably in a huge universe. What, if anything, tames these fluctuations? Why is there a macroscopic universe? These are two of the central theoretical challenges of fundamental physics in the twenty-first century. In this essay, I describe the circle of ideas surrounding these questions, as well as some of the theoretical and experimental fronts on which they are being attacked. [ABSTRACT FROM AUTHOR]

Einstein's equivalence principle was initially the equivalence of an accelerated frame and uniform gravity. In spite of being often challenged, Einstein insisted on the fundamental importance of his equivalence principle to general relativity. It is shown that existing criticisms, starting from Synge and Fock, are due to misunderstandings and misconceptions in physics and/or inconsistent considerations. These include the misinterpretations of Pauli, Bergmann, Tolman, Landau and Litshitz, Zel'dovich and Novikov, Dirac, Wheeler, Thorne, Hawking, and others. It has been overlooked that Einstein's equivalence principle implies uniqueness of the gauge for a given frame of reference. The recent criticism by Hong has the distinction of starting from his intuitive, though inadequate, observation that "a homogeneous field is characterized by the fact that any part of it is representative of the whole." It is pointed out that his notion of uniform gravity disagrees with experiment on the gravitational redshift. His arguments concerning acceleration also disagree with special relativity, while repeating the same mistake of Landau and Lifshitz. Moreover, it is pointed out that the crucial role of Einstein's equivalence principle in general relativity is firmly established because the Maxwell--Newton approximation, which is rigorously derived in the theoretical framework of general relativity, is unambiguously supported by experiments. Thus the Schwarzschild solution is actually invalid in physics. [ABSTRACT FROM AUTHOR]

We address the question of the intrinsic relation between mass, gravity, spacetime symmetry, and the Higgs mechanism implied by involvement of the de Sitter vacuum as its basic ingredient (a false vacuum). Incorporating the de Sitter vacuum, the Higgs mechanism implicitly incorporates the generic relation between mass, gravity, and spacetime symmetry revealed in the frame of General Relativity for all objects involving the de Sitter vacuum. We overview two observational cases which display and verify this relation, the case known as "negative mass square problem" for neutrino, and appearance of a minimal length scale in e + e − annihilation. [ABSTRACT FROM AUTHOR]

The article discusses misconceptions related to general relativity. Misconceptions are mentioned related to topics including interpretation of the weak equivalence principle and the relationship between gravity and inertia; connection between gravitational redshift results and spacetime curvature; and Einstein equivalence principle and the ability to "transform away" gravity in local inertial coordinate systems.