Entropy: Why the Universe Exists Today

Written By: Kesavan Rangarajan

Entropy is one of the most important topics in physics. It is the reason the universe exists and the reason why it will die. It may be the reason why time flows forward and, also, why you are alive right now.

You may have learned about entropy as a measurement of disorder, which is partially true, yet pretty misleading. More accurately, entropy is a measure of energy configurations’ probability. For example, you have two solids - each composed of 4 atomic bonds that store indivisible amounts of energy - called quanta. With a total energy of 8 quanta, there are a few ways the energy can be distributed between the two solids. In each solid, however, the exact position and orientation of the quanta is called a microstate, and you can have thousands of them while maintaining the same overall distribution of energy. With each microstate having an equal probability of occurring, we see that due to the number of microstates, some of the overall energy distributions have a higher probability of occurring. In this case, the widest distribution of energy has the greatest entropy. In general, thus, entropy is a measurement of the distribution of energy. When energy is more spread out, it holds a greater entropy.

Animation of the distribution of quanta (The Most Misunderstood Concept in Physics)

The second law of thermodynamics states that entropy tends to reach a maximum state of disorder. In other words, energy tends to spread out over time. This law is fundamental to understanding most phenomena in the universe. It explains why hot things - which are hot due to their compressed energy - cool down, why cool things heat up, and why gases expand.

Here’s a couple things to think about: If energy tends to spread out, how are there still structures in the world? How are there hot places, but also cold places? How does life even exist? If entropy is always increasing, how is the Earth stable?

Well, if Earth was a closed system, all the energy it has would spread out, destroying all structures, diffusing heat, and even destroying life. Fortunately, we have the Sun. The Sun provides the Earth with condensed energy and low entropy. This energy is far more useful than the high entropy energy the Earth radiates outwards. As disordered as it is, the Earth and the Sun’s energy transfer still follows the second law of thermodynamics, which is the reason why structures and life are able to exist on Earth.

Animation of the sun’s release of low-entropy energy and the Earth’s radiation of high-entropy energy. (The Most Misunderstood Concept in Physics)

Works Cited

The Most Misunderstood Concept in Physics, Veritasium, 1 July 2023, https://www.youtube.com/watch?v=DxL2HoqLbyA. Accessed 6 Oct. 2023.

What Is Entropy? - Jeff Phillips, TED-Ed, 9 May 2017, https://www.youtube.com/watch?v=YM-uykVfq_E. Accessed 6 Oct. 2023.