The SI Units of Science

SI Units are the standard units we use to describe various quantities found in nature. In order to describe nature at all, especially with mathematics, we must first be able to give numerical values to the phenomena, events, and qualities of what we observe. In other words, we must be able to refer to quantities such as the mass of an object, or to it’s volume – that is, how much space is within in.

We must be able to refer specifically to the strength of an objects magnetic field, or the quantity of the electrical current running through a wire. Thus in order to both refer to these physical quantities and measure them, we must have standardized units associated with each so that we have a scale and name for this quantity for us to refer to is as and measure it as. That is where our use and derivation of units, specifically the SI Units, come into play

SI Units & The Metric System

The SI Units are the units of the Metric System, which is the major unit system used worldwide, and the standard for science. The metric system is used by virtually all of the countries of the world, except the United States, who still use the English system of units for measurements of length and weight. (These quantities are measured in units of miles, feet, inches, and pounds, for example. Canada also uses feet and inches as the common unit for measurement of height of people specifically, although otherwise they use the metric system.)

The Metric system’s units are also known as SI units, which is an acronym based on the original French name of the system, the Système International d’unités (International System of Units). The other major system of units is the old English units, known also as the Imperial System. The Imperial System can also be referred to as the foot-pound-second (fps) system, for the base quantities relevant to mechanics, as opposed to the centimeter-gram-second (cgs) system that the Metric System uses, which can also be called the MKS system (for the meter-kilogram-second system).

The units are somewhat arbitrary, because we can choose any number of ways to measure something and so invent units to measure it. For the most part, there is really no inherent superiority of one unit over another. Feet and inches are just as good for measuring the height of a person as meters are. In some ways they are actually preferable in the Americas simply due to familiarity. Most of us have used them our whole lives and have a far more intuitive understanding of them within a comprehensive understanding of scale. For example, I know that 5 ft is relatively short, 6 ft about average, and 7 ft to be quite tall. Yet I cannot say the same for height measurements in meters. I don’t know how tall I am in meters, nor how tall “quite tall” or “relatively short” is in meters. So in general, there is no inherent superiority of one unit over another, instead it being a matter of familiarity and preference in most cases.

However, the metric system is certainly superior because it is a base 10 system, which is a far cleaner and more efficient system for unit conversion. This is why it is the standard of science. Using a set of prefixes we can change the scale of a quantity by a factor of a thousand each time, meaning we only have to move the decimal around, as opposed to multiplying by some random numbers. For example, there are 1000 meters in a kilometer. So if we traveled 1.2 kilometers, then we can easily determine how many meters we traveled: 1.2 km x 1000 m/km = 1,200 meters. However in the Imperial System, there are 3 feet to a yard, and 1760 yards to a mile. So if we traveled 1.2 miles, we traveled 1.2 Mi x 1760 yards/Mi = 2112 yards.

With a calculator the calculations are just as easy, more or less. However when we get into values of planetary, solar system, or cosmic scale, as well as down into atomic scale, the Imperial System loses its edge considerably. For estimation as well, the metric system is miles better due to its efficiency and simplicity. [We will cover the Metric system, Metric prefixes, and unit conversion in depth in the next article.]

The true potency the SI Unit system of standardized units is due to the fact that it enables scientists around the globe to communicate in the same language. Interestingly, science in general and mathematics specifically are the first internationally common language, spoken by professionals across the globe. The language may be mathematics, with the numbers being its letters, but the grammar is the SI Units of the metric system.

Physical Quantities of the SI Unit System

What then do these SI units measure? All of the values I referred to above – mass, volume, electrical current – are known as physical quantities. Measurements for physical quantities are expressed in terms of units, which are standardized values that measure certain properties of a thing.

For example, we have specific units that we use to describe the property of length, which range from light-years (the distance light travels in one year) used for comic distances, all the way down to meters, centimeters, millimeters, and even more minute divisions down to nanometers which are a billionth of a meter, and even below.

Each one of these are essentially different units we use to express the physical quantity of length. These units are fixed, such that the 100 m sprint doesn’t change from year to year depending on what the local region specifically calls a “meter”. Or so that the distance Las Angeles to Shanghai doesn’t change depending on the day or who is measuring.

In physics this is especially important because this standardization literally makes physics possible. It is the standardization of these fundamental units which allows us to make comparisons between quantities, phenomena, and objects, and thus to discover relationships between quantities or phenomena, which is basically the whole point of physics. For it is this process of investigation which ultimately allows us to describe and understand nature and reality. Moreover, the descriptions of these basic quantities allows us to derive even more sophisticated quantities.

The Seven Base Quantities

In the physical universe, there are only seven base physical quantities. Furthermore, it is from these seven base quantities, which are known in the SI unit system as the base units, that all other physical quantities can be expressed by combining these base quantities algebraically in different ways. Everything that can be discussed and studied in physics, and in science for that matter, boils down to seven fundamental quantities, which interestingly touch on all of the fundamental phenomena of matter and nature as well.

Now while the units that we have used to describe these fundamental quantities is mostly arbitrary (we use the SI units for this because it is convention, there’s really no deeper reason for this choice than that) the quantities themselves are absolutely not arbitrary. They describe the fundamental aspects of reality, of the cosmos, of creation, upon which everything else that we know about creation is based. The seven base quantities which we use to describe everything that we know of reality are:

These quantities of mass, time, length, temperature, electric current, and luminous intensity form the basis of our description of reality. In essence, they describe the size of things, how heavy or massive they are, how long something takes, how hot something is (or in scientific terminology, the kinetic energy of the atoms of a body or substance) as well as the amount of substance, or the quantity of electrons stored within or passing through a material, as well as the quantity of photons (or the energy of the photons) being radiated from a body.

These seven quantities describe, in essence: size, weight, passage of time, quantity, heat, electricity, and light. This is the basis of our knowledge and description of reality.

SI Units For Derived Quantities

Furthermore, we can combine these base quantities and their base units as well. By doing so we can derive more specific and complex quantities such as area, or surface area, or volume. Measuring length twice in two perpendicular directions, we measure the area or surface area of something, perhaps of land or a field, or the surface area of a baseball or bowling ball – or the Sun for that matter. These are an advancement of one single concept, that base quantity of length. In the SI unit system, the base unit of length then is meters (m) as the table above displays.

If we combine length and time (which we could also refer to as distance instead of length) then we can arrive at a more advanced and specific concept, that of speed. Speed is a measurement of the distance that something traveled per some unit of time. So in the standard SI units of science, speed is measured in meters traveled per second, thus (m / s ). [The forward slash “/” in physics and mathematics means “per” or “divided by”.]

We can also arrive at even more complex concepts based on further derivations of the base quantities. From the derivation of length called volume, which is essentially the third power of an objects length, or the third dimension of length, ${m}^{3}$ or more generally ${L}^{3}$, we can also combine that derived quantity with the base quantity of mass, which is measured in the SI unit of kilograms (kg). When we consider volume and mass together, we synthesize both concepts resulting in a whole new derived physical quantity, a new way of looking at at describing physical reality, and a new unit to go with it.

This derived quantity is density, which is an objects mass per volume. This is measured in kg / m³. Which mean’s the number of kilograms of mass found within a meter cubed volume of a substance. Angles also can be considered as derived quantities, at least when they are measured as radians, because they are defined as ration between arc length and radius, essentially two lengths, though one of which is curved along a circular path.

As we progress we will explore many examples of derived quantities. All that is important now is an understanding that all physical quantities stem from these base quantities. We will explore all of the fundamental derived units as we come across them contextually.

I was bored when I first had to write this section. However, it is actually fascinating once you think about it. Once you consider that everything we know about reality is founded on a measurement and description of 7 fundamental quantities, and that everything else we could describe are just intricate and complex combinations of those quantities. Also the fact that there are also 7 notes that make up music, meaning that there are seven basic frequencies of sound, and that there are 7 base frequencies of light. This is a curious relationship and synchronicity that will require some thought, as it speaks to an inherent order within nature.