The Nature of the Universe - Part III - Energy

 

In the previous two posts, I talked about matter and fundamental forces. In this post I'll discuss the concept of energy and its relationship with mass and matter.

 

Energy

Energy is a measurement of the strength of Forces available to a Particle. For example, the energy of a single Photon for visible light is 4 x 10^-19 joules or 9 x 10^-20 calories. (If you are health conscious, you would be happy to know that this is a very small amount of energy - drinking a single drop of beer gives a person around 1 joule of energy) Given that it takes around 1 calorie of energy to raise the temperature of 1 gram of water by 1 °Celcius, it would take the absorption of all energy of 9 x 10²⁰ (900 million trillion) photons by 1 g of water to raise its temperature by 1 ° C. Of course water does not absorb all photons - most light passes right thru water and some is reflected back. This means that most photons pass right thru water and some are reflected back. Since energy of very few photons is actually absorbed by water, it would take a lot many more photons to raise the temperature than 9 x 10²⁰. On the other hand, a completely black, opaque material which absorbs all light would absorb the energy of all photons and would get warmer faster.

Similarly, when you hold a ball in the air it is being pulled down by the gravitational force of Earth - we say that the ball is experiencing the force of gravity. It is said to have potential energy which when you leave the ball converts to kinetic energy - the ball gains speed as it accelerates to the ground. In fact energy keeps changing from one form to another all the time.

 

Conservation of Energy

While energy keeps changing from one form to the other, the total amount of energy is always the same. So when the ball is held at a height, all its potential energy gets converted to kinetic energy. Of course some potential energy gets converted to heat because of friction with air, but the total energy is always maintained. You cannot create net new energy, nor can you destroy energy.  This is called the Law of Conservation of Energy.

 

Mass-Energy Equivalence

The law of conservation of energy is actually slightly inaccurate. It turns out that net new energy can actually be created out of mass. For example, when an electron and a positron (anti-particle of the electron) collide, both are annihilated and photons are produced. Now photons do not have mass, but they have energy. Hence there is loss of mass and creation of energy. It is a bit hard to visualize this loss of mass and appearance of energy. The way to think about it is that mass can manifest itself as energy and energy can manifest itself as mass. We call this Mass-Energy Equivalence.

In terms of magnitude, the relationship between mass and energy is given by Einstein's famous formula: E=mc². Here E is energy, m is mass and c is the speed of light in vacuum. If we take m = 1 kg, then the energy released would be 9 x 10¹⁶ joules. Remember, drinking one drop of beer gives one joule of energy. Here we are talking of 9 trillion joules! This is almost the same as 20 million tons of TNT.

So the correct formulation of the law of conservation would be as the Law of Conservation of Mass-Energy: The sum total of mass and energy in the Universe is always constant. You cannot create net new mass-energy, nor can you destroy mass-energy.

 

 

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