One of the most misunderstood and subtle concepts in physics is that of ENERGY. The confusion about energy arises from several basic misconceptions. To fully understand energy it helps to begin with some key facts that are known about energy and then to look at some of the consequences of these important facts.
ENERGY is the property of a physically identifiable object (such as a brick) or a field (such as a magnetic field) which allows it to do measurable work on another object or field.
Wealth is to the economic world - as energy is to the natural world.
For example, wealth can take many forms. It may be in the form of cars, jewels, real estate, money in the bank, or potential earnings. Nothing in the economic world can happen unless money is transferred (wealth is transferred). The currency of the economic world is the dollar.
Similarly, energy can take many forms. It may be in the form of kinetic energy (heat and motion), gravitational or chemical potential energy, or even in the form of mass. Nothing in the natural universe can change unless energy is transferred. The currency of the natural world is the joule.
Another subtle, but very important similarity between wealth and energy can be noticed whenever an event, such as the purchase of an automobile, occurs. We never actually see the dollars transferred, we simply assume the transaction has taken place based on the effects we observe, namely the new automobile owner driving the car off the car dealer's lot. Similarly, when we observe a bowl of hot soup cooling down, we don't actually "see" joules of energy being transferred to the surrounding air, but we assume that this is happening based on the effects that we observe (soup getting cooler and the surrounding air getting warmer).
In 1905 Albert Einstein made a remarkable discovery. As a result of a theoretical study, he was able to show that mass was in fact just a very compact (although very different) form of energy. Einstein predicted that if matter were converted to energy the amount of energy released would be tremendous.
In fact he showed that the relationship between matter and energy could be described by the equation:
E = mc2
where E is energy released (in joules) when m kilograms of matter is converted into energy. c is the velocity of light in a vacuum, namely, 3 x 108 m/s therefore, c2 is 9 x 1016, that is, 9 followed by 16 zeros or 90 000 000 000 000 000.
Here is a truly amazing discovery! Most of the energy in the Universe is in a few familiar forms. The first major sources of energy are from matter (stars and galaxies etc. E = mc2) and kinetic energy (1/2 mv2... expansion of the Universe with energy due to the velocity v of a mass m). We can, in principle, get work out of these objects, therefore this is mathematically defined as positive energy.
The second source of energy comes from gravitational potential energy. Since we must put work into objects to produce gravitational potential energy this must be mathematically considered as negative energy.
Finally, there has recently been discovered an exotic form of energy which appears to be causing the expansion of the Universe to undergo acceleration. This "dark energy" appears to dominate the Universe. This "dark energy" is extraordinarily weak and diffuse. It probably cannot be detected with gigantic accelerators. It cannot be detected in galaxies or even clusters of galaxies. It can only be detected on the scale of the Universe itself... and yet "dark energy" accounts for more than 2/3 of the composition of the Universe.