Unit I – Entropy: Why Can You Remember Yesterday, but Not Tomorrow?


Entropy is a critical concept in biology and chemistry. In both of these subjects you speak of the Gibbs Free Energy

\Delta G = \Delta H - T \Delta S

to determine if a process will occur spontaneously (if \Delta G < 0 then the process is spontaneous). The S in this equation is the entropy. But, what is entropy? Often it is simply defined as “disorder” as in the OpenStax Biology 2e textbook quote below:

“An important concept in physical systems is that of order and disorder (or randomness). The more energy that a system loses to its surroundings, the less ordered and more random the system. Scientists refer to the measure of randomness or disorder within a system as entropy. High entropy means high disorder…”

— Clark, M. A., Douglas, S., & Choi, J. (2018). The Laws of Thermodynamics. In Biology 2e (https://openstax.org/books/biology – 2e/pages/6–3 – the – laws – of – thermodynamics). OpenStax.

However, who decides what is disordered? Is the Jackson Pollock painting below disordered? You might think so, but the artist probably does not! Moreover, how do we quantify disorder? This section looks to answer these questions. Along the way we will see that disorder is a TERRIBLE definition for entropy. Hopefully along the way, you will see why and gain an appreciation for this facilitating topic.


Autumn Rhythm (Number 30), 1950 by Jackson Pollock
Autumn Rhythm (Number 30), 1950 by Jackson Pollock: https://www.jackson-pollock.org/autumn-rhythm.jsp#prettyPhoto.

Time – Why Can You Remember Yesterday and Not Remember Tomorrow?

Most of the laws of nature we know of are ambivalent as to the direction of time: they can be used to figure out what happened in the past or to predict what will happen in the future. Consider the video of the ball below. Is that video run backwards or forwards? How do you know?

As we will see, entropy is one of the few concepts we know of which can distinguish between past and future.


Why Are We Starting with Entropy?

If you’ve had a physics class before (or even if you haven’t!), you may have expected to start with position and speed or something similar. Not in this class! Since this is an admittedly unusual choice, I felt it warranted some explanation.

So, why are we starting with entropy? There are several reasons:

  1. It is interesting!
  2. All we need to know is how to count and some basics of probability.
  3. We can learn a lot about how physics can provide additional insight into the other sciences by studying entropy.
  4. It provides a neat perspective on what physics is really about – physics is so much more than just flying balls and so forth!
  5. Almost none of you have studied it before, making it an equitable playing field!


Share This Book