On Particle

In any science field, it is essential to identify what we are trying to study. For example, in Chemistry, we study matter (its structure, the interaction between different substances, etc.). Similarly, in Physics, we need to identify what we are trying to study.

In Physics, we are trying to study the motion of objects. So, what are objects? It is undeniably hard to truly define an “object” clearly, and Philosophical questions will eventually emerge during the process of defining it. So, I shall use examples to “define” what an “object” is. Everything we can touch is identified as an object. For example, a basketball is an object, a car is an object, etc.

Now, since we are trying to study the motion of an object. What is… the motion of the object? The motion of an object is, simply defined, its change of position as time advances. For example, a car is traveling from the bottom of a hill to the top of the hill in an hour. Now, the car’s position changed from the bottom of a hill to the top of the hill, and the change happened in an hour. That is motion. From the definition of motion, it naturally follows that even a rotation, which does not really seem like a change in position, is a motion. For example, a rotating box’s edges change their position from pointing West to pointing South. Although they are essentially just changes in position, I shall call one “changes in position” (like a car that goes from A to B) and rotation (like a car… flipping).

It is obvious that an object can both change its position and rotate at the same time. Naturally, as we study the motion of the object, we will have to study both its change in position and its rotation about its own axis, and that is a difficult task to do. There is one solution to make it simpler: we only consider one thing, rotation or changes in position. However, there is one problem with this approach: which kind of motion to study?

To answer that question, we have to start tying Physics to reality. Let us examine what we care more about in reality: when we throw a basketball, we usually care more about its change in position than its rotation (though if you want a beautiful curved shot, rotation is mandatory). However, the same does not apply when we are navigating through heavy traffic jams or a tight alley on a scooter. For example, in Viet Nam, there are a lot of scenic tight alleys, but they are a pain to ride a scooter through: if you cannot maneuver the bike correctly (what I mean by that is, you have to know where the front and rear of the scooter are, so you can rotate your bike to fit), you will not be able to fit the bike into the alley, let alone ride through it. So… what is the problem here? Why do we care about changes in position in one case and rotation in another? As you may have noticed, in the case of the basketball, the basketball is essentially a very tiny dot in space, but in the case of the bike and the alley, the bike is very big compared to the alley. So, the problem has to do with the size of the object we are considering. And, you may have noticed another subtle point there, too: the rotation of the bike has two “main” concerns: the position of the front of the scooter and the position of the rear of the scooter. It is practically the study of the change of position of two things, not just one thing, like a basketball. So, it is clear that the change of position is the kind of motion we have to study first, both because, practically, it is easier, and technically, it is the stepping stone of rotation.

As we have discussed, whether or not the rotation of the object is of concern or not depends on its size. So, to neglect the rotation, we have to choose an object whose size is very small compared to the environment. That is also the definition of a particle: a point that is so small compared to its environment that it can be approximated by a point. Unfortunately, there is another thing we need to be clear about: does a point… have mass? We usually see that the bigger a thing is, the more it weighs. For example, a car is far tinier than a yacht, and “consequently,” the yacht is heavier than a car. The “true” reason behind it is more subtle than that, but it is enough to raise the question for now. So, since a particle is something that has its size really, really small, does it also have mass? Let us take a look back at our example before: a flying basketball. If you are tall enough and you try to catch the basketball, will you still feel its weight? Of course, you would still feel it, so it seems that a particle can also have mass. Now, let us reason with our definition: a particle is, by definition, a thing that is really small compared to its surroundings. However, that does not mean the particle itself is essentially small. For example, even a yacht is just a tiny dot – a particle – when we look at it from a flying plane, because it looks really small compared to the sea. In that case, we still know for sure one thing: the yacht still has weight. So, a particle still has weight, even though it is really small.