When someone asks us the dreaded question, “What’s your weight?” we often get the wrong answer. Not out of shame, or anything like that, but because, in our daily lives, we use the word “weight” differently from the way we use it in Physics. When someone asks us our weight, the person really wants to know our mass (measured in kilograms). In Physics, weight is the gravitational force of attraction that a body feels near the surface of a planet like Earth, and which must be measured in Newtons.
Whether in Physics or in other areas of human knowledge, the most basic concepts are sometimes more difficult to define than the more advanced concepts. This happens because the most elaborate ideas usually use rules already established by the most primordial concepts.
Concepts such as weight, mass, energy and time are called fundamental in physics, which makes defining them more difficult than it seems. Try stopping for a moment and asking yourself the question in the title of this text (“what is time?”), and notice how difficult it is to put into words something that would seem obvious before.
Each of these fundamental concepts can be discussed in a later text, given the depth of the subjects. In this particular one, we will focus on the concept of time. A common starting point is to think that time is represented by the sequence of events that happen in nature, like the Earth’s orbit around the Sun, which we use to measure the year, for example. However, we can delve deeper into the questioning to think about what really defines the passage of time.
For starters, Albert Einstein was one of the first to realize that time is relative. That is, it does not pass the same way, depending on who measures it. Saying this is not the same thing as the perception that time passes quickly when we are having fun or that time passes slowly when we are in the dentist’s chair, for example. Einstein’s ideas consist of time literally passing slower or faster according to the speed of the observer. For example, an observer with a higher speed perceives time passing more slowly than a slower observer. Such a difference is possible to be calculated and it is quite common to use such a calculation to measure our position through GPS satellites that are moving with very high speeds in the Earth’s orbit.
Also, we can imagine two people holding a stone and dropping it. If they are too far apart, they may disagree on which stone fell first. That is, not even the order of occurrence of events is absolute. Therefore, defining time as such a sequence of events is not good enough if we want to deepen our understanding of nature.
A concept widely used to try to define time is entropy. To understand it, we can imagine a sand castle that, over time, is gradually destroyed by the wind that can cause the grains of sand to become disorganized. However, the wind does not make the grains of sand on a deserted beach organize themselves into a castle. This degree of disorder of the grains of sand and, in general, of the entire universe, is called entropy. And one of the best-established laws of physics says that the entropy of the universe must always increase. So the universe is constantly getting more disordered, and the increase in entropy is what gives us the sense of the passage of time.
According to entropy, it is physically impossible to travel back in time to the past (where entropy would decrease) and perfectly possible to travel to the future, since entropy increases in that “direction”. So, as you can see, from this point of view, our notion of time is merely a consequence of the natural properties of our universe. This discussion is just one of the examples that can show us how deep the issues of our universe and our nature are. And that, although we don’t have definitive answers, and we probably never will, at the very least, the search for such answers can be a good way to pass the time.