The equation E=mc2 was originally written as m=E/c2 in the Journal published by Einstein, as it was used to define the true meaning of mass. While studying we all thought that this formula leads to the conclusion that mass can be converted into energy. But we have been thinking about it all wrong.
In correct sense it states that mass is actually a property exhibited by energy. Let’s take example of two similar windup clocks of one is windup and running and another has stopped. If we have a scale which is sensitive and weight the two clocks the one which is windup and running would have slightly more more mass. The change is not subtle but it raises the question why despite being similar the one windup and running has much more mass.
So this leads to the formula m total = m rest +m extra . So you would start to thing about what is this m extra. This is the additional mass exhibited by the energy. Don’t get into the idea that this mextra is always positive. It also has negative value but you would say how can mass be negative. Let’s take up example of H2O. The mass of H2O is less than the combined mass of hydrogen and oxygen atom. An you may see that if hydrogen and oxygen combine in different way than the mass loss is different. The most simple explanation to this question is that the when hydrogen and oxygen atoms combine some of their mass is converted and released in form of energy. From here on we are thinking about it all wrong. There is force of attraction between the nucleus and the each other electron and when they get close to each other this gives rise to what we call negative potential energy. So you must be thinking how the energy can be negative. Firstly you need to understand that energy cannot be negative but we consider it negative as we take it from reference. So lets take an example of droping a stone from a height.When we drop the stone it start gaining speed and this speed gives rise to kinetic energy. The kinetic energy of the stone when it hits the ground gets converted into sound, heat and other form of energy. So we can say that some part of the potential energy is converted into kinetic energy. So the total energy linked with the stone and earth system has decreased. So when we consider it’s potential energy we say it’s negative as compared to the system when the stone was at a height.
So if you understood the concept than in case of atoms they release this energy and hence they have less overall energy. And as I have told yo earlier that energy has the property mass. So with decrease in energy there is decrease in mass.
So, you can now understand the true meaning of E= mc2. For further research the Einstein original paper is here.