There are five most issues as the most recent theory of physics. The first three correspond to the three pillars of modern physics i.e. thermal physics, quantum theory and relativity theory. The fourth and fifth concern combinations of these pillars; and lead to speculations about the future of physics. These five headings will provide a way of introducing to the physics science at most, albeit not in the order in which they occur.
Thermal physics
Controversies about the foundations of thermal physics, especially the characterization of the approach to equilibrium, have continued unabated since the days of the field’s founding fathers, such as Maxwell and Boltzmann. Some aspects of the original controversies can be seen again in modern discussions. But the controversies have also been transformed by the development of several scientific fields; especially the following three, which have grown enormously since the 1960s:
(i) classical mechanics, and its offspring such as ergodic theory and chaos theory;
(ii) quantum thermal physics; and
(iii) cosmology, which nowadays provides a very detailed and so fruitful context for developing and evaluating Boltzmann’s bold idea that the ultimate origin of the “arrow of time” is cosmological.
Quantum theory
Since the 1960s, the physics community has witnessed a revival of the debates about the interpretation of quantum theory that raged among the theory’s founding fathers. In the general physics community, the single most influential author has no doubt been John Bell, not only through his non-locality theorem and the many experiments it engendered, but also through his critique of the “Copenhagen orthodoxy” and his sympathy towards the pilot-wave and dynamical collapse heterodoxies. But in more specialist communities, there have been other crucial factors that have animated the debate. Mathematical physicists have developed a deep understanding of the various relations between quantum and classical theories. Since the 1970s, there has been progress in understanding decoherence, so that nowadays, almost all would accept that it plays a crucial role in the emergence of the classical world from quantum theory. And since the 1990s, the burgeoning fields of quantum information and computation have grown out of the interpretative debates, especially the analysis of quantum non-locality.
Relativity theory
The decades since the 1960s have seen spectacular developments, for both theory and experiment, in general relativity and cosmology. But this Renaissance has also been very fruitful as regards foundational and philosophical issues. Mathematical relativists have continued to deepen our understanding of the foundations of general relativity: foundations which, as mentioned in Section 1, were recognized already in the 1920s as crucial for the philosophy of space and time. And the recent transformation of cosmology from a largely speculative enterprise into a genuine science has both brought various philosophical questions closer to scientific resolution, and made other philosophical questions, e.g. about method and explanation in cosmology, much more pressing.
Quantum field theory
Although there are relativistic quantum mechanical theories of a fixed number of particles, by far the most important framework combining quantum theory and special relativity is quantum field theory. Broadly speaking, the foundational issues raised by quantum field theory differ from quantum theory’s traditional interpretative issues, about measurement and non-locality. There are two points here.
(i) Although quantum field theory of course illustrates the latter issues just as much as elementary quantum theory does, it apparently cannot offer a resolution of them. The measurement problem and the puzzles about nonlocality arise so directly from the unitarity and tensor-product features of quantum theories, as to be unaffected by the extra mathematical structures has seemed to most workers to be wisest to pursue the traditional interpretative issues within non-relativistic quantum theory: if you identify a problem in a simple context, but are confident that it is not an artefact of the context’s simplicity, it is surely wisest to attack it there.
(ii) On the other hand, there are several foundational issues that are distinctive of quantum field theory. Perhaps the most obvious ones are: the nature of particles (including the topic of localization), the interpretation of renormalization, the interpretation of gauge structure, and the existence of unitarily equivalent representations of the canonical commutation relations.
Quantum gravity
Finally, we turn to the combination of quantum theory with general relativity: i.e., the search for a quantum theory of gravity. Here there is of course no established theory, nor even a consensus about the best approach for constructing one. Rather there are various research programmes that often differ in their technical aims, as well as their motivations and conceptual frameworks. In this situation, various foundational issues about the “ingredient” theories are cast in a new light. For example, might quantum gravity revoke orthodox quantum theory’s unitarity, and thereby en passant solve the measurement problem? And does the general covariance (diffeomorphism invariance) of general relativity represent an important clue about the ultimate quantum nature of space and time?
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