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 fundamental nature of the physical world, of matter 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.
Please note that “matter” is not synonymous with “particles”. By matter, I simply mean physical stuff. Physics is about the fundamental physical stuff of existence and how it interacts.
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 mathematical shape. I simply mean that the object has to have physical extension. It has to be some physical object with definite boundaries.
A gas cloud is made out of atoms/molecules bumping into each other. A gas cloud has physical extension, hence it is “physical” in the sense being used here.
A blob of dough has a shape, even if we have no idea what to call that shape. It is a material object with physical extension and not a concept.
Here shape is not used to refer to specific geometric forms such as circles, squares or, say, polyhedral objects.
Here shape is defined as:
The external form, contours, or outline of someone or something.
There, to say that physical objects are things with shape, is to say that something has an external form and boundaries. That is, it has physical extension.
If it has physical extension, if it has boundaries, if it confined to specific regions/places than it is physical. If it is an entity that exists and is not an abstraction, it is physical.
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.
Once you start, say, talking about electrons as “probability clouds” you are no longer doing physics. This is bad mathematics and attempts to turn probability, a valid mathematical abstraction, into some kind of physical thing.
What about black holes? They are described as infinitely dense “regions of spacetime”. But what is a region of spacetime? Is it a physical object?
No. Spacetime is some a combination of two mathematical abstractions, space and time. They are not physical things and neither is spacetime.
Space relates to the relationship between the positions of objects. Time is measurement of motion/change.
So if we talk about regions of spacetime, are we talking about physical objects? No. We are talking about abstractions.
Besides, physical objects cannot be infinitely small. Nor can they be infinitely dense. Everything that exists has a finite size and by implication, a finite density.
So, this black hole “physics” is not really talking about a known physical thing or things. It is a bunch of assertions about a mathematical point which sucks matter into it. How is this physics? It is not.
What about gravity? Gravity is treated as the curvature of spacetime. But this is not a physical explanation. It does not reduce gravity to properties or actions of physical entities, as it should.
But space and time are abstractions, so to try to explain gravity by something that happens to abstractions is not physics.
Remember, physics is about the properties of and the interactions of physical objects. That is the nature and actions of physical entities.
Not about abstractions which somehow do things and somehow cause interactions such as gravity. Once you try to reduce gravity to something that abstractions do, you are no longer doing physics. You are abusing mathematics.
We do not understand energy. For some reason 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.
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 or action 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 explain 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. No, equations are not how gravity works. That math is simply a statement of quantitative relationships.
Quantitative relationships do not explain the mechanism by which something works. Mathematics is just that, a description of quantitative relationships. Mathematics is not an explanation of causal mechanisms.
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.
Let us look at the concept of fields. What is a field? If you look into this enough, you will find that a field is alleged to be a bunch of numbers attached to points in space.
So, a field is a bunch of numbers attached to space? What is space? An abstraction describing relationships. But physics tends to treat space as a mathematical abstract.
So a field is a bunch of numbers attached to a mathematical abstraction? That does not get us any closer to understanding what these fields really are. It does not get us any closer to understanding fields in terms of physical objects and how they interact.
Fields are not just numbers stuck to space. These field equations describe some kind of interaction between physical entities. Fields are not made out of matter as such, but they seem to describe attributes of matter and/or their interactions.
Read more about fields and how they are mathematical descriptions, here.
But if we decide that they are numbers attached to space, it is unlikely we will figure this out.
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?
Remember what we said mathematics was and what its purpose was. Mathematics is the science of method that allows us to identify attributes and relationships. In particular, those that cannot be directly perceived.
Much of physics deals with these attributes and relationships. Much of physics pertains to attributes and relationships which we cannot directly perceive. But which we can measure using mathematics.