They might have been referring to relativistic effects. Wikipedia has E^2 - (pc)^2 = (mc^2)^2 http://en.wikipedia.org/wiki/Mass_in_special_relativity http://en

They might have been referring to relativistic effects. Wikipedia has E^2 - (pc)^2 = (mc^2)^2 http://en.wikipedia.org/wiki/Mass_in_special_relativity http://en.wikipedia.org/wiki/Mass_in_special_relativity