I’ve now finished the last installment of the Mass and Energy article, whose first two installments were completed last week. If you’ve read the first two, skip down past Figure 5 to the section entitled “Where We Are So Far”. The third installment summarizes what’s been explained about energy, momentum and mass, introduces the concept of the mass of a system of two or more objects, and then goes on to illustrate all of the issues in a very famous system: a Higgs particle decaying to two photons (particles of light), as viewed by three different observers.
More articles providing a bit of additional side information, and others addressing more subtle questions for which the Mass and Energy article was a prerequisite, are coming soon.