Energy--What is it?

One of the most basic characteristics of our world and the universe around us is that of motion and activity. Change is everywhere and in everything. Movement is pervasive. On the large scale of things, living beings and inanimate objects roam over the surface of the Earth. The Earth circles the Sun, the Sun like billions of other stars revolves around the center of our galaxy, while our galaxy along with countless others careens through space. On the submicroscopic scale, the atoms and molecules that are the building blocks of all larger bodies are in ceaseless state of agitation. If there was no motion, our universe would collapse and cease to exist.

Things move because energy is expended. Things interact and change because they exchange energy with each other. Every known activity including all processes within living things can be described in terms of energy interchanges. Energy is ubiquitous--it makes the sun shine, the wind blow, wheels turn, water flow and fire burn. It may rest quietly in a gallon of gasoline, a jelly donut, a compressed spring, or in a dammed-up river. You have to invest some energy to do work--push a shopping cart, lift a book, or pedal a bicycle. Whatever work is done, energy is used. The faster the work is done, the more energy must be applied. We cannot make energy or destroy it; we can only use it. The total amount of energy in the universe is the same now as it was at the beginning of time. Only its distribution has and is changing.

Although we use the word energy quite casually and freely, does energy really exist? Not really, its not a thing like matter that has substance, weight and can be seen. Instead, it is a concept--a convenient way of thinking about and describing the effect of forces acting on matter to produce motion. It is an accounting system scientists use to keep tract of the transfer of movement between objects that has proved to be incredibly powerful in explaining our natural world. The idea of energy allows numerical values to be assigned to movement and change allowing us to track mathematically the interaction of things like their change in position, velocity, and the distances they move. It is because energy is conserved that it is such an important and useful concept. The fact that nature maintains the amount of this abstract quantity at a constant value through its many possible transformations from one form to another is quite amazing and useful from a practical point of view. The very essence of science is the belief that the total quantity of energy in the universe is constant and is conserved through its many transformations. Why movement or energy is at the essence of things is unknown. All science can say is that is the way things are and continue to use the concept of energy to probe, describe and shape our world.