A picture is worth a thousand words – or so goes the saying. What if I told you that energy is like a picture and trying to describe it would somehow make us run out of appropriate words.
This is perhaps the hardest question in all of Physics is this: What is Energy? What is Time is a close second, by the way. Of course, we all know what energy means. But when we have to define it in words it ceases to be so obvious. Perhaps words aren’t adequate enough to define energy. Perhaps the definition for energy exists only in the language of mathematics. That is to say, energy can only be defined in the form of equations; translating that equation into English or any other language loses some of its meaning. There is always loss in translation so we’re limited by the fact that the equations hold the truth about energy and only these equations will ever convey the exact meaning. That said, we can still make sense of what energy is and what it does.
Energy is a physical quantity. What that means is that it is a characteristic of some physical phenomena that can be observed and measured. It is a property of a material or immaterial entity which allows that entity to change its state. By material we mean anything that is made up of matter – in other words, if we were to break that entity down into its constituents, we’ll see that it’s composed of the basic building blocks of matter, namely, the quarks and electrons. (Please see my blog on matter for more details on this.) By immaterial we mean anything that belongs to the force carrier category. These are called the bosons and there are (so far) five of them: the photon (carrier of the electromagnetic force), the gluon (carrier of the strong nuclear force), the Z and W particles (carriers of the weak nuclear force) and finally, the most recent addition to this list, the Higgs particle (the one which bestows mass on all the fundamental particles in nature). So all these little particles put together are what make up the Universe as we know it. And energy is that thing which allows these particles (whatever combination or configuration they might be in) to change their states. This implies that energy is what make things possible. An electron is orbiting the nucleus of an atom at a given radius. If that electron wants to orbit the nucleus from further away then it needs energy to do so. So it absorbs energy from a passing photon. It absorbs the photon’s energy and it is able to jump to this higher state. The photon, as a consequence, disappears. It gives itself up for the electron. On the other hand, if the electron wants to orbit nearer to the nucleus then it has to give away energy. It releases that energy in the form of a photon and can then jump down to a lower orbit. Again, energy is the key element here which allows this electron to change its state. As the electron jumps to a lower orbit, therefore, it emits this photon.
Energy is essential for such things to happen. Without energy there wouldn’t be anything at all. For the Universe itself to come about, there needed to be energy. The Big Bang is the most energetic event that has ever occurred. It was the source of the Universe. What’s interesting is that, all this energy that was released in the Big Bang is distributed all over the Universe. If there’s one principle we need to be aware of about the Universe it’s the following: when it comes to energy, nothing is lost or destroyed or even created; it only flows from one state to another, it is only transformed. This is known as the Principle of Conservation of Energy. It is so fundamental, so important that it is the foundation on which everything rests.
The fact that energy can be transformed implies that it exists in different forms. Nuclear, electric, thermal and sound are some examples. All the different forms of energy can, however, be grouped in two main classes. One is kinetic energy and the other is potential energy. Kinetic energy is the form of energy associated with motion. Potential energy is associated with configuration. Some forms of energy are combinations of those two main classes. For example, thermal energy is related to how the constituent particles of a system are arranged and how they are moving about. (Please see Thermal for more details on this.)
To have an idea of the importance of energy we only have to consider life of Earth. Every life form on Earth is sustained by energy. The energy is ultimately sourced from one place. That’s right: the Sun. The Sun provides warmth and light; two necessary ingredients to keep the circle of life rolling on Earth. If we look at the food chain, we see that everything is supported by the flora. The flora is of course entirely dependent on sunlight without which photosynthesis would not be possible. The nuclear energy inside the Sun is transformed into light and heat (both of which are of the same nature, by the way – please see Spectrum) and this is then transformed into food by photosynthesis. This food, or chemical energy, then feeds into the food chain, sustaining life on the planet. When a life form dies it will decay and release that chemical energy back into the food chain. This energy, which came from the Sun, is thus used and transformed and recycled in the great circle of life. The Sun, itself, owes its existence to the clouds of galactic dust and particles which coalesced to form stars like the Sun. The energy of the Sun had its own source which, in turn, had a source of its own. If we linked up all those different sources from all the different stars and galaxies, they will all ultimately lead to the Big Bang. And this is the beauty of our existence. This is our elegant Universe. It shows how we are all connected. Every life form, every inanimate object, from bacteria to galaxies, energy runs through our very existence and binds us all.