Imaginary number

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In mathematics, an imaginary number (or purely imaginary number) is a complex number whose square is a negative real number or zero. The term was coined by René Descartes in 1637 in his La Géométrie and was meant to be derogatory. At the time, such numbers were thought not to exist, much as zero and the negative numbers were sometimes regarded by some as fictitious or useless.

1 Definition
2 Geometric interpretation
3 Are imaginary numbers "real"?
4 See also
5 External link

Definition

Any complex number can be written as $a + b i$ , where $a$ and $b$ are real numbers and $i$ is the imaginary unit with the property that:

$i^2 = -1\,$

The number $a$ is the real part of the complex number, and $b$ is the imaginary part. Although Descartes originally used the term "imaginary number" to mean what is currently meant by the term "complex number", the term "imaginary number" today usually means a complex number with a real part equal to $0$ , that is, a number of the form $b i$ . Note that, technically, $0$ is considered to be a purely imaginary number: $0$ is the only complex number which is both real and purely imaginary.

Geometric interpretation

Geometrically, we find the imaginary numbers on the vertical axis of the complex number plane, allowing them to be presented orthogonal to the real axis. One way of viewing imaginary numbers is to consider a standard number line, positively increasing in magnitude to the right, and negatively increasing in magnitude to the left. At $0$ on this $x$ -axis, draw a $y$ -axis with "positive" direction going up; "positive" imaginary numbers then "increase" in magnitude upwards, and "negative" imaginary numbers "decrease" in magnitude downwards. This vertical axis is often called the "imaginary axis" and is denoted $i\mathbb{R}$ .

In this model, multiplication by $- 1$ corresponds to a reflection about the origin, i.e. a rotation of $180$ degrees about the origin. Multiplication by i corresponds to a $90$ -degree rotation in the "positive" direction (i.e. counter-clockwise), and the equation $i 2 = - 1$ is interpreted as saying that if we apply $2$ $90$ -degree rotations about the origin, the net result is a single $180$ -degree rotation. Note that a $90$ -degree rotation in the "negative" direction (i.e. clockwise) also satisfies this interpretation. This reflects the fact that $- i$ also solves the equation $x 2 = - 1$ — see imaginary unit.

In electrical engineering and related fields, the imaginary unit is often written as $j$ to avoid confusion with a changing current, traditionally denoted by $i$ .

Are imaginary numbers "real"?

Despite their name, imaginary numbers are considered just as "real" as real numbers. (See the definition of complex numbers on how they can be constructed using set theory.) One way to understand this is by realizing that numbers themselves are abstractions, and the abstractions can be valid even when they are not recognized in a given context. For example, fractions such as $\frac{3}{4}$ and $\frac{5}{7}$ are meaningless to a person counting stones, but essential to a person comparing the sizes of different collections of stones. Similarly, negative numbers such as $- 3$ and $- 5$ are meaningless when keeping score in a football game, but essential when keeping track of monetary debts and credits.

Imaginary numbers follow the same pattern. For most human tasks, real numbers (or even rational numbers) offer an adequate description of data, and imaginary numbers have no meaning; however, in many areas of science and mathematics, imaginary numbers (and complex numbers in general) are essential for describing reality. Imaginary numbers have essential concrete applications in a variety of sciences and related areas such as signal processing, control theory, electromagnetism, quantum mechanics, and cartography.

For example, in electrical engineering, when analyzing AC circuitry, the values for the electrical voltage (and current) are expressed as imaginary or complex numbers known as phasors. These are real voltages that can cause damage/harm to either humans or equipment even if their values contain no "real part".

Specifically, Euler's formula is used extensively to express signals (e.g., electromagnetic) that vary periodically over time as a combination of sine and cosine functions. Euler's formula accomplishes this more conveniently via an expression of exponential functions with imaginary exponents. Euler's formula states that, for any real number x,

$e^{ix} = \cos x + i\sin x \,$