## Physicall Entity or "object of study" for physics

Not including candelas, there are 7 unit in Physics (6 dimensions unit, 1 dimensionless) which means Physics studying 7 basic properties as its "object of studies", that is; mass, length, space, time, electrical current, temperature, amount of matter and angle.

We know each of this properties and derivative properties from it, such as speed momentum, energy and many more. But actually what we know for each of this properties (the "object of studies" for physics) is its behavior. For example, we know matter must have mass and occupying space, and mass bend space and time continuum. But do we know what exactly this mass that we studying for?

Is there any serious writing that sum up in shortly what is each of this properties that we study in Physics? What I want is not writing that sum up what we know about it, but instead theory that purpose what is it actually.

For example, what is mass, time, and space?
 Recognitions: Gold Member Science Advisor Staff Emeritus Welcome to PF! Unitlessness isn't a different base unit. Given a base unit like meters, you can form units like m2, m3, etc., and we consider these to be derived, not basic. By raising a unit to the 0th power, you get unitlessness. To further simplify things, you don't really need 6 units but 1. In relativity we normally pick units where G=1 and c=1. There is then only one base unit, and everything else is that unit raised to some power. Things like candelas and electrical units are not physically independent of the meter-kilogram-second units. The only reason they're taken to be independent base units in the SI is that their standards are easy to reproduce with high precision. Moving past the stuff about units, I don't think there is any well established and complete theory of what is meant by all the fundamental types of measurement in physics. For instance, time has a different fundamental status in quantum mechanics (where it's a parameter, not an operator like position) than in general relativity (where it's a coordinate, just like position), and nobody knows how to reconcile these two descriptions. If we could make a theory of quantum gravity, presumably it would accomplish this. Small chunks of what you request have certainly been achieved. For example, Einstein's 1905 papers on relativity clarified the previously hazy logical status of what it meant to measure time, synchronize clocks, etc. In the 19th century, statistical mechanics succeeded in showing that temperature was derived, not fundamental.
 The answer that I'm looking for theory that define object of study for physics the way like this. In order to be studied by physics, the object of study should exist in the way that it can not exist as the only one. Thus the properties of "amount" is born. The amount of the object of study can exist in infinity, because if the existence can exist in the amount of infinity, the size of the object can be also reaching infinite. Accordingly, infinite amount of matter in infinite size can be said exist as single entity that has infinite size. Thus the limit of amount is born. It can be said as; if it exist as the only one, than the size of it will be infinite and vice versa. Since the object of study for Physics can not be the only one, than the amount and the size of it will be limited. This point of view is fit accordingly with theory finite universe. I'm looking this kind of writing that explain what is the properties "amount" that we study in Physics. As we know SI unit for "amount is "mole". In my thinking, there is must some sort of law or theory that propose why there must be exist the properties of "amount". Any body can show me what theory/law is it?

## Physicall Entity or "object of study" for physics

Exactly my question is bcrowell.

Why up until now, we don't have "any well established and complete theory of what is meant by all the fundamental types of measurement in physics." ?

Regarding time as position or coordinate. Why one or more properties need to be measured as vector/coordinate? What law or theory that gave the need of vector/coordinate in Physics? We observe vector is exist, but why it exist? Isn't it vector or coordinate is simply derivative of length and angle?
 Physics isn't built from the bottom up. In mathematics, you start from basic axioms and build all sorts of laws out of them. But in physics, you start from measurements, and try to build models that approximate the measurements. These models aren't perfect. There's no ultimate justification for a physics model other than the fact that it works. If you are looking for a reason for physics, it's how we try to make sense of reality. All the units and algebraic structures we use are just tools and not necessarily reflective of the actual reality.