# Energy conservation when exchanging meson

In Prob 1.2 of Griffith's "Introduction to Elementary Particles" 2nd ed., it says that:

However, I don't understand why it states that "they must temporarily violate the conservation of energy by an amount $$mc^2$$ (the rest energy of the meson)". If my understanding is correct, the "missing" energy was just temporary stored in the meson while exchanging, and therefore the energy is still conserved if we consider the system including the 2 interacting protons and the meson exchanged. So, is the energy conservation law really violated in this case? If true, where and why it is violated?

• – PM 2Ring Mar 11 at 8:42
• Yes, I know that the virtual particle need not obeys the energy conservation. However, in this particular problem, it just seems doesn't violate the conservation of energy by an amount $mc^2$ if we consider the meson (though we know it's not exactly the case in reality) – pinchun Mar 18 at 6:56
• the "missing" energy was just temporary stored in the meson That doesn't really help though, because the (rest) mass of a pion is much larger than the mass difference between a neutron & a proton. – PM 2Ring Mar 18 at 7:12