Physics is the study of the fundamental nature of the physical world. It studies the nature of the most fundamental of the physical things that make up existence and how they interact with one another. In other words, physics studies the nature of the physical world and the mechanisms by which it works.
Note the use of the word “matter”. Everything in physics ultimately describes the attributes of physical objects and how they interact with one another. There is nothing in physics which does not reduce to this.
Whatever aspect of existence you study in physics, ultimately you must refer to physical entities. This is what I call the “primacy of physical existence”. Which is another way of stating the primacy of existence, but focusing on the fact that the primary constituents of existence are all physical.
What does it mean for something to be physical? What is a physical entity and are they primary?
To say that something is physical is to say that it exists and does not exist as a concept. It is something which exists not as a concept but as a separate entity. It is not an attribute, it is not a relationship. Attributes are aspects of entities and relationships also pertain to entities. To be a physical entity is to exist as a non-conceptual existent with attributes and with relationships with other things that exist.
A simple way to look at this is to say that matter is that which has shape (credit to Bill Gaede for this simple formulation). That which does not have shape is not matter and is a concept.
Note, that by “shape” I do not mean that it has to be assigned a shape. I simply mean that the object has to have physical extension. It has to be some physical object with definite boundaries.
These physical existents are primary to physics. Everything in physics ultimately describes the attributes of physical entities, relationships between physical entities or the actions of physical entities. There is nothing else for physics to deal with.
Every concept in physics describes the actions of some physical thing. Actions are the actions of entities, not of concepts. If a so-called theory describes the actions of concepts, then it is not physics. At least, it is not rational physics. And it is certainly not a valid theory.
Take electricity. You can talk about current, charge, voltage all you want, but in order to understand any of this, you must eventually bring in the physical object known as the electron and so forth.
In order to describe how the atom works, you have to bring in the physical objects that make up the atom.
This is why we do not understand energy. We often do not treat energy as if it was describing the properties or actions of physical objects. We treat it as though it is somehow an alternative to matter. As though it was not an abstract description of some aspect of matter.
But, energy is not some form of an alternative to matter. We cannot turn matter into energy and vice versa. Because energy is a concept, it describes properties or actions of matter. And you cannot turn matter into a concept or concepts into matter!
Some would counter by postulating that the ultimate constituents of reality may prove to be energy and not matter. But that is not possible. We do not know what energy is, but we do know that it is an abstraction which describes some attribute of matter. That or it describes something matter does.
Physics must, of course, include a great many concepts. But those concepts describe the nature and relationships of matter. That is the business of physics, describing matter. Not of describing moving concepts. Not describing concepts which are not able to be reduced to a description of matter and its relationships/interactions.
Physics is not the same thing as mathematics. It is the study of the physical world and the mechanisms of how that physical world works. It is not the study of mathematical constructs and how they are related.
Physics describes the nature of physical objects and not the relationships of mathematical constructs. Physics does not end at mathematical descriptions. That is where we begin to understand matter and its relationships.
But we have to tease out the facts of reality the mathematics is describing. We must not end with mathematics, we must use it to find out the nature of what we are describing. Mathematics hints at the mechanisms of nature. But mathematics is not how nature works.
We do not describe how something works by throwing up an equation. If I ask you “How does gravity work?”, you cannot answer that question by pointing to a blackboard of mathematical equations. If I ask how magnets work, you cannot explain how by saying “it is fields” and then claiming that a field is just a mathematical equation.
If your “physics” is just a bunch of mathematical equations then you have failed to do physics. What you have is math. It might be good math, it might accurately describe reality. It might enable predictions, but it is still not physics. Physics is about physical objects and their mechanisms. Not mathematical description and predictions.
Note that mathematics is essential to physics. It is no accident that very little progress in physics was made until calculus and other advanced mathematical techniques were developed. But why is this?