Einstein derived a formula given the relation between mass and energy as E = mc2. Here 'm' is the mass, 'c' is the velocity of light and 'E' is the energy. This equation has given equal states to mass and energy. It states that a mass m has energy E given by the above formula. Since the velocity of light is equal to 3 x 108 ms-1 energy equivalence of 1kg of mass is 9 x 1016 joules. This is interesting because 1kg of mass can produce tremendous amount of energy. But for this 1kg of mass has to disappear. How is it possible? A portion lives for more than 1032 years (highly stable particle). If neutrons decay, they become protons and hence the number of particles in a nucleus remains the same. Hence even though mass contains lots of energy it cannot be liberated. Therefore to annihilate matter we need antimatter. But antimatter is not found in our part of the universe. We can only produce them in our laboratories.
A positron has same mass as an electron but with the opposite charge. So, when an electron and positron are brought together they annihilate and their mass gets converted into energy.
The reverse of this process is also possible. It is called as pair production. A photon can collide with massive nucleus under high electric field and split it into an electron and a positron. Here the energy is converted into matter.
