Interpretations of Einstein’s equation differ primarily concerning whether E = mc2 entails that mass and energy are the same property of physical systems, and hence whether there is any sense in which mass is ever “converted” into energy (or vice versa). In this paper, I examine six interpretations of Einstein’s equation and argue that all but one fail to satisfy a minimal set of conditions that all interpretations of physical theories ought to satisfy. I argue that we should prefer the interpretation of Einstein’s equation that holds that mass and energy are distinct properties of physical systems. This interpretation also carries along the view that while most cases of “conversion” are not genuine examples of mass being “converted” into energy (or vice versa), it is possible that the there are such “conversions” in the sense that a certain amount of mass “appears” and an equivalent of mass “disappears.” Finally, I suggest that the interpretation I defend is the only one that does not blur the distinction between what Einstein called “principle” and “constructive” theories. This is philosophically significant because it emphasizes that explanations of Einstein’s equation and the “conversion” of mass and energy must be top-down explanations.



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This is an electronic version of an article published in International Studies in the Philosophy of Science.

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