The concept of “mass” is quite confusing to people. This is partly because it comes up in many different contexts, partly because the definition has changed somewhat over the centuries, partly because what one learns in pre-university chemistry and physics classes is badly out of date, and partly because the way Einstein’s thought evolved caused confusion (and even disagreement) in the physics community, which even now hasn’t settled.
So a few articles are needed to help keep things straight.
- Very important for anyone wanting to understand something about particle physics: There are two definitions of “mass” that appear in popular science books, magazines, and even in many first-year university textbooks (including the one I learned from), and they are inconsistent with one another, giving completely different interpretations of the famous equation E = mc². In particle physics we only use one of these interpretations, and to understand this website and other articles about particles, you need to make sure you’re clear on this point. Click here for a detailed explanation. For a shorter discussion, click here.