What is anything really?
In very general terms, that's what the science of physics allows us to ask. There is still so much we don't know about the universe, and new research is constantly surprising. The more we know, the more questions we find to ask. Understanding the answers to that question is the primary motivation of physics.
Most of operational physics (how things relate to each other) can be understood in terms of fundamental quantities. A fundamental quantity is an irreducible "thing". It cannot be described in terms of other "things". This is in contrast to derived quantities, which can be described in terms of other "things". Fundamental quantities are also called base quantities.
Fundamental quantities can be considered to be dimensions, but in a loose context. In physics, when we refer to dimensions, we usually refer to space and time (and theoretically higher similar dimensions), and not the dimension of the luminosity (see below).
The fundamental quantities:
- Time
- Space (or length)
- Mass
- Temperature
- Electrical current
- Luminosity
- Amount of matter
Each fundamental quantity has an associated unit in the SI system:
- Time: seconds (s)
- Space: meters (m)
- Mass: kilograms (kg)
- Temperature: degrees kelvin (K)
- Electrical current: ampere (A)
- Luminosity: candela (l)
- Amount of matter: mole
Time is perhaps the most abstract of the fundamental quantities, possibly because we experience it in a linear way; we can't get out of it. Space can be experience from a nonlinear perspective, and is more easily grasped as a distinct concept. The same goes for mass, which is very tangible. Temperature is experiential but its quantum definition veers into abstract territory (in simplistic terms, it is the amount of atomic vibration in a system). Like temperature, electrical current is an everyday experience, but gets stranger the more closely it is analyzed (the "flow" of electrons through a medium). Luminosity is straightforward (we can see how it changes). The mole is a stumbling block for students of chemistry, but ultimately makes sense as a measurement of items (particularly of matter).
Derived Quantities
All other quantities in physics can be expressed in terms of the fundamental quantities. Examples are velocity (space divided by time), acceleration (space divided by time squared), force (mass times space divided by time squared) or energy (mass times the constant representing the speed of light squared - aka. space divided by time all squared). Understanding this concept helps in understanding how all equations work, and how different "things" are related. In teaching physics this is an important concept to transfer - teaching students not just to work an equation, but really understand it in terms of how derived quantities relate to fundamental quantities.
The Future
There may be more fundamental quantities discovered in the future, or it may be found that fundamental quantities are actually derived from others. From our current perspective, this seems unlikely, but should not be ruled out entirely. That is part of the immense joy of learning about the universe; there is inevitably new and fascinating discoveries to be made.
