Re: Where does the matter go in 'Mass Defect'?

 Hi,

 Your observation about Carbon are right, actually, the mass of a proton 
is 1.00728 amu; a neutron is 1.00866 amu. The standard is that one atom of 
carbon 12, the isotope of carbon with 6 protons and 6 neutrons, has a mass 
of exactly 12 amu. if you add up 6 protons and 6 neutrons, you get more 
than 12 amu. Good point. The mass of 6 protons and 6 neutrons is 12.0956 
amu, to be precise--but the mass of a carbon nucleus is less than the sum 
of its parts. You think, it is impossible. However it is possible, it can 
be explained by 'binding energy' on nuclear physics. What does "binding 
energy" mean?  The mass of any nucleus is less than the sum of the 
separate masses of its protons and neutrons. In other words, sticking 
protons and neutrons together somehow causes some of their mass to vanish 
into thin air. Einstein showed that mass and energy are really two 
different forms of the same thing; the "vanishing" mass of the protons and 
neutrons is simply converted to energy(E=mc^2, where m, mass, c= the 
velocity of light.). .That's the idea behind fusion. The "binding energy" 
of a particular isotope is the amount of energy released at its creation; 
you can calculate it by finding the amount of mass that "disappears" and 
using Einstein's equation. The binding energy is also the amount of energy 
you'd need to add to a nucleus to break it up into protons and neutrons 
again; the larger the binding energy, the more difficult that would be. 

The answer of your another question ('are photons the matter that is being 
gained/lost?')is:

Actually, Photons create matter. Einstein's equation, E=mc^2, formulates 
the idea that matter can be converted into light and vice versa. The vice-
versa part, though, hasn't been so easy to bring about in the lab. 

Best wishes,