Please note that the speaker and topic of this seminar have recently changed.
According to the Invariance Principle, quantities which are invariant under the symmetries of our theories cannot represent physical elements of reality. But what justifies the Invariance Principle? Various metaphysical, semantic and epistemological arguments have been given, e.g. by Saunders (2007), Caulton (2015) and Dasgupta (2016). I will argue that all these arguments rely on what I call Quantity-Value Essentialism. This is the assumption that a quantity essentially is its values; if the values of a quantity aren’t real, then neither is the quantity itself. However, I believe that there is an alternative view, spelled out by Stalnaker (1979), on which (some of) the values of (some) quantities are conventional. On this conventionalist view, we can avoid the conclusion that only invariant quantities are physically real, potentially affording us the explanatory benefits of gauge-quantities such as the electromagnetic vector potential.