The equation E = mc^2 is a pretty important equation in physics. Not many people would learn about it in a general physics class in high school or even college general physics class. However, it is probably the most known equation in the world. E = mc^2 is most popular because it describes the amount of energy released by a nuclear reaction (useful in both energy production and weaponry). It’s also an equation that tends to be a little misused or misinterpreted when it is not really understood. So rather than over-thinking what the relation E = mc^2 means and risk coming to incoherent, incomplete, or just plain wrong explanations for, it may help to break the components down a little to see what it says. The letter E stands for the energy an object has. However, the energy here is not an objects total energy, this doesn’t include the energy something has from gravitational or electric potentials or the kinetic energy the thing has from motion. Historically this has been called an objects ‘rest’ energy, it is the energy that the objects mass (the ‘m’ in the equation) contains when no other types of energy are considered. The c^2 in the equation is the speed of light in a vacuum squared, or multiplied by itself. This is a physical constant meaning that its value never changes. It’s simply a lot easier to write c than it is to write the value it stands for which is approximately 300,000,000 meters/second. Since c is just a constant value that never changes the only way to change the ‘rest’ energy of an object is to change its mass. The equation doesn’t tell us how or why energy and mass have this relation it just tells us “how much”.
Maybe the way the equation is written here is a little misleading as well. Typically when this equation is talked about, the subject of discussion is nuclear energy. It is true that when an atom splits into two or more constituents the mass of the two parts does not quite add up to the original mass. The small difference in mass is what is converted into energy during the reaction, and in some cases this can lead to an incredibly large amount of energy being released. What can be deduced from this is that energy and mass are related. If the original equation is taken and divided by c^2 then it reads a little differently: m = E/c^2. Now there seems to be a working definition of mass revealed. Mass is simply one form of energy! Energy is also known as a conserved quantity. This means that while it may change forms the total energy of the universe never changes. So this equation would also imply that mass is also conserved since it is merely a form of energy.
