Why can't matter spontaneously turn into energy? What prevents this from happening? Energy is still conserved. Suppose we consider an atom as a system. The entropy of the system is very low. Now if the atom turned into energy, then the entropy would increase dramatically, is this correct? Also, I am thinking that the conversion for mass to energy is a reversible process. However, entropy increased, and in order to compact the energy back together again, you would also be decreasing the entropy again back to when it was an atom. I though that entropy always increases, I learned this from my thermodynamics class. So, if entropy always increases, then why won't matter spontaneously turn into energy?