Mass and energy appears to us in conventional thinking as two entirely different entities.
We all know, or at least seem to know, what mass is. We know that it is somehow inextricably connected to weight. A heavier object has more mass. It is also usually connected to size, but this is of less relevance, as a large amount of cotton for example, would have less mass than a sand heap of the same size. In scientific terms, mass is defined through the property of inertia, the more mass an object has, the more its inertia. This means that the more the mass of an object, the harder it is to get it to move and to stop it once it is moving. We can understand this easily when we consider how much easier it would be to push a small car and get it to move, or to stop it when it is moving, than it would be to start or stop a huge truck, for example.
Energy is a bit more illusionary. It is defined as the capacity of an object to do work. It is unseen, and it is only its effects that can be experienced, when it is let off as heat or light for example. We can also see its effects when an object moves or does any work.
Energy and work thus seems too entirely different things. While mass of an object is what makes it loathe to move, so to say, energy is what gets it moving and doing work. Mass is something we can touch, something which we can carry around, but energy is a concept and only its effects are experienced.
Therefore, it is quite perplexing to know from Einstein’s equation, E=MC2;, that energy and mass are so inextricably interrelated.
A common misunderstanding of E=MC2 is that it shows that mass can be converted into energy and vice versa. But this is not so.
What the equation means is that mass is the same as energy, and mass and energy can be expressed as each other. When an atom bomb bursts and gives off 21.5 kilotons of energy, the amount of matter left behind loses 1 gm of weight. This does not mean that 1 gm of matter in the bomb has been converted into 21.5 kilotons of energy. What it means is that 1 gm of matter in the bomb can also be expressed as 21.5 kilotons of energy and vice versa. The bomb when it bursts can be said to have lost both 1 gm of mass and 21.5 kilotons of energy. If we were to explode the bomb in a large box which absorbed all the 21.5 kilotons of energy, we would find that the box now weighs 1 gm more.
The real way to understand E=MC2 is that it unifies mass and energy, in that we can express a certain amount of mass as its equivalent of energy and vice versa. Energy and mass are like two sides of a coin, the same phenomenon appears as mass on one side and as energy on the other.
This unification of mass and energy, besides its implications for physics, also has very important ramifications for philosophy. Since matter and energy combines together to form this material world, what it means is that all things are unified, that there is a single commonality behind this world, which is expressed as matter on one side and energy as the other, and which is the root of all the phenomena in this world. Thus E=MC2 brings us closer to the monism of Advaita Vedanta philosophy of Hinduism.