Chuang Liu

Physics seems to tell us that there are four fundamental force-fields in nature: the gravitational, the electromagnetic, the weak, and the strong (or interactions). But it also seems to tell us that gravity cannot possibly be a force-field, in the same sense as the other three are. And yet the search for a grand unification of all four force-fields is today one of the hottest pursuits. Is this the result of a simple confusion? This article aims at clarifying this situation by (i) reviewing the gauge-field programme and its conception of unification of force-fields, (ii) examining the various attempts at a gauge theory of gravity, and (iii) articulating the nature of "gauging" and using it to explain the difference between gravity and the other force-fields.

Over forty years after the foundations of the special theory of relativity had been securely laid, a heated debate, beginning in 1965, about the correct formulation of relativistic thermodynamics raged in the physics literature. Prior to 1965, relativistic thermodynamics was considered one of the most secure relativistic theories and one of the most simple and elegant examples of relativization in physics. It is, as its name apparently suggests, the result of the application of the special theory of relativity to thermodynamics. The basic assumption is that the first and second laws of thermodynamics are Lorentz-invariant, and, as a result, a set of Lorentz transformations is derived from thermodynamic magnitudes, such as heat and temperature.

This paper aims at answering the simple question, “What is spontaneous symmetry breaking (SSB) in classical systems?” I attempt to do this by analyzing from a philosophical perspective a simple classical model which exhibits some of the main features of SSB. Related questions include: What does it mean to say that a symmetry is spontaneously broken? Is it broken without any causes, or is the symmetry not broken but merely hidden? Is the principle, “no asymmetry in, no asymmetry out,” violated by SSB? What really distinguishes SSB from the usual types of symmetry breaking?

In this paper, a criticism of the traditional theories of approximation and idealization is given as a summary of previous works. After identifying the real purpose and measure of idealization in the practice of science, it is argued that the best way to characterize idealization is not to formulate a logical model – something analogous to Hempel's D-N model for explanation – but to study its different guises in the praxis of science. A case study of it is then made in thermostatistical physics. After a brief sketch of the theories for phase transitions and critical phenomena, I examine the various idealizations that go into the making of models at three difference levels. The intended result is to induce a deeper appreciation of the complexity and fruitfulness of idealization in the praxis of model-building, not to give an abstract theory of it.