In a recent paper in the British Journal for the Philosophy of Science, Kosso discussed the observational status of continuous symmetries of physics. While we are in broad agreement with his approach, we disagree with his analysis. In the discussion of the status of gauge symmetry, a set of examples offered by 't Hooft has influenced several philosophers, including Kosso; in all cases the interpretation of the examples is mistaken. In this paper we present our preferred approach to the empirical significance of symmetries, re-analysing the cases of gauge symmetry and general covariance. Direct and Indirect Empirical SignificanceThe notion of symmetry that we are concerned with is defined with respect to the laws of motion. Given the laws, specified in terms of dependent and independent variables, a symmetry transformation is a transformation of these variables that preserves the explicit form of the laws. The issue we are interested in is the empirical status of such symmetry transformations. Galileo's famous ship experiment (Galileo [1967], pp. 186-8) provides an example of where (to an appropriate approximation) a symmetry transformation is both physically implementable and directly observable. The transformation is implemented via two empirically distinct scenarios of the ship at rest and in uniform motion with respect to the shore, and the symmetry is observed by noticing that, relative to the cabin of the ship, the phenomena inside the cabin do not enable us to distinguish between the two scenarios. Following Brown and Sypel ([1995]), 1 we maintain that the direct empirical significance of physical symmetries rests on the possibility of effectively isolated subsystems that may be actively transformed with respect to the rest of the universe. 2 This active transformation need not be physically implementable in practice (try boosting a planet, for example); the point is that we compare two empirically distinct possible scenarios at least theoretically, one containing the untransformed subsystem and one the transformed subsystem.The example of Galileo's ship also illustrates that observing a symmetry involves two observations, as has been discussed by Kosso ([2000] Budden ([1997]). 2 Notice that, since our objective is empirical significance, this goes beyond a purely mathematical active symmetry transformation. We discuss the case symmetries of the universe as a whole in the final paragraph of this section.As long as one can claim to be able to observe that the transformation prescribed by a particular symmetry has taken place, and that the associated invariance held, then one can claim to be able to directly observe the physical symmetry in nature.And he goes on (p. 87):To observe the transformation is to observe both the unchanged reference and the changed system.In other words, we first observe the transformation, which involves transforming a subsystem with respect to some reference that is itself observable, and we then observe that the symmetry holds for the subsystem (p. 86):observation of a...

We approach the physics of minimal coupling in general relativity, demonstrating that in certain circumstances this leads to (apparent) violations of the strong equivalence principle, which states (roughly) that, in general relativity, the dynamical laws of special relativity can be recovered at a point. We then assess the consequences of this result for the dynamical perspective on relativity, finding that potential difficulties presented by such apparent violations of the strong equivalence principle can be overcome. Next, we draw upon our discussion of the dynamical perspective in order to make explicit two 'miracles' in the foundations of relativity theory. We close by arguing that the above results afford us insights into the nature of special relativity, and its relation to general relativity.

The quantum theory of de Broglie and Bohm solves the measurement problem, but the hypothetical corpuscles play no role in the argument. The solution finds a more natural home in the Everett interpretation.KEY WORDS: De Broglie-Bohm; Everett; measurement; decoherence; consciousness."If the quantum theory is to be able to provide a complete description of everything that can happen in the world . . . it should also be able to describe the process of observation itself in terms of the wave functions of the observing apparatus and those of the system under observation. Furthermore, in principle, it ought to be able to describe the human investigator as he looks at the observing apparatus and learns what the results of the experiment are, this time in terms of the wave functions of the various atoms that make up the investigator, as well as those of the observing apparatus and the system under observation. In other words, the quantum theory could not be regarded as a complete logical system unless it contained within it a prescription in principle for how all these problems were to be dealt with." Bohm (1) , p. 583.

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his paper investigates what the source of time-asymmetry is in thermodynamics, and comments on the question whether a time-symmetric formulation of the Second Law is possible.