## Foreword

by Dr Stavros Mouslopoulos, BA, PhD, Dip Ed

Richard Feynman used to say that physics is one scientific field that has not admitted any evolutionary question: “Here are the laws! we say. We don’t even think about how they got that way…”. Luckily today we are in a position to view such statement as an exaggeration (although it still holds true for the way of thinking of the majority of theoreticians). This book spans the history of physics from the classical (Newton) to the modern (Hawking), providing a glimpse into the evolution of ideas over the centuries since about the beginning of modern science. However, here let me take the top-down perspective.

Amidst the search for the nature of reality, the quest for the Theory of Quantum Gravity has led us to question even the most deeply rooted assumptions of the physicist’s dream - the ultimate unified theory, namely, the expectation that the theory itself will have the form of an immutable set of equations governing all the physical laws and thus determining and describing the world around us.

The need for such a shift in the scientific paradigm did not only come from theories such as Loop Quantum Gravity^{1} with its emergent laws of nature and the discrete nature of spacetime itself at the fundamental level. Perhaps unexpectedly, this shift was also shared by its seeming competitor, String Theory over the last two decades or so.

Given that String/M theory, which is formulated in 10 and 11 space-time dimensions, has to eventually reproduce the 4-d phenomenology as we know it, the compactification of the additional dimensions is inevitable. It is exactly this process of ‘fixing of the moduli’ of the Calabi-Yau compactifications that brings us to The String Theory ‘Landscape. The ‘Landscape’ is a direct consequence of the large (and perhaps infinite) number of Calabi–Yau manifolds, and a huge number of possibilities of stabilizing them, that has brought forward a vast number of solutions (vacua) for the theory:^{2} “The resulting four-dimensional vacua have all possible physical laws with all possible constants, and this has led to a radically new view of physics in which one argues that the constants in the physical laws that we measure in our Universe do not come from an underlying unified theory, but are environmental (anthropic) variables that are determined by where we are in this Multiverse."^{3}

The previous comment is not mentioned as necessarily a flaw of String Theory but certainly as one of the most obvious ways that the Theory has fallen short of its promises. At the same time, it may be interpreted as an indication that we should be looking towards fundamentally probabilistic and emergent laws springing from the ‘continuum’ of possible vacua. One should have in mind that previous questions may not be possible to completely address without a satisfactory resolution of the problem of measurement in quantum mechanics. The previous comments are naturally related to Black Hole physics which is another arena where the clash between classical and quantum physics appears and where both aforementioned theories not only have made progress on their own but also seem to converge surprisingly.^{4}

^{1} In Loop Quantum Gravity the smooth background of Einstein’s theory of gravity (General Relativity) is replaced by nodes and links to which quantum properties are assigned and space is built up of discrete entities- the loops.^{2} Here we refer to the possible ‘flux compactifications’ of String Theory to four-dimensions. For further reading, please refer to the review article by Iosif & Bena Mariana Graña, Comptes Rendus Physique, Volume 18, Issues 3–4, March–April 2017, Pages 200-206.^{3} See Note 2^{4} In the context of Loop Quantum Gravity the black hole entropy can be done approximately using spin networks and for large enough cases does give the area law for the entropy. In the context of String Theory, in specific cases, the area law can also be also derived by counting the string microstates. Furthermore, in String Theory, the black hole formation can be seen as similar to a phase transition when the Calabi-Yau shape goes through a space-tearing conifold transition.