Imaginary numbers are one of the mysterious parts of mathematics, which most people have heard about - but not many people truly understand.
The first thing that will spring to mind for most when we talk about imaginary numbers, is the mysterious symbol $i$.
Now, we ask ourselves, what does $i$ actually mean? Well, $i$ is simply the number which is equal to $-1$ when squared, so:
$$i^2 = -1$$
Or you will sometimes see:
$$i = \sqrt{-1}$$
Now that we "know" what $i$ is, we can use this number to solve a lot of previously unsolvable problems - a notable case being electronics. In fact, you may have heard of $i$ sometimes being written as $j$. This is because in physics, the letter $i$ is often used to denote current:
Engineers use a 'j' to indicate the square root of minus one since they tend to use 'i' as a current. Mathematicians use 'i' for this since they don't know a current from a hole in the ground!
University of St Andrews
However, I suppose you might still be thinking this is all very well and good, $i$ is the square root of minus 1 - but what does it mean in
practice? Show me some evidence!
In fact, there is a
very good video on YouTube which explains how $i$ actually works in a very intuitive way. Here is a summary of what he is trying to say - you will soon see how exactly this fits into electronics.
$i$, just like any other symbol in our number system, is simply used to describe a quantity. $i$ is not all too dissimilar to negative numbers in that sense - it is equally as valid as the positive set of numbers and yet because we cannot "touch" or "interact" with $i$ very practically, we often lose sense of what it really is. Professor Arthur T. Benjamin of Harvey Mudd College sums this up in one of his lectures in the series 'The Joy of Mathematics'. Professor Benjamin challenges us to think about the ways in which the concept of simple negative numbers, which seem so obvious and necessary to us in a society where we use them everyday (profits and losses; increases and decreases etc.) might have seemed as alien as complex numbers like $i$ for example, to people of another age simply because they did not really use them in practice. How can you have a negative amount of rocks? Surely the only way to represent something in real life is using positive, tangible, whole objects? We now know this not to be true, since negative numbers are a necessity, but it is very much so that imaginary numbers are just the same - because most people do not really come into contact with them in a tangible sense, they lose sight of what imaginary numbers actually are.
Now, how can we understand imaginary numbers and how they fit into real life? Using something simple of course: circles!
Imagine an object travelling in one direction. We know from the laws of physics that this object might have a velocity in that direction. A velocity is a vector quantity and has magnitude, but more critically
direction. Say, we wanted to reverse the velocity of this object, i.e. make it travel at an equal
speed in an opposite direction. We need to keep the magnitude the same but rotate it through 180 degrees, or $2\pi$ radians - whatever floats your boat.
N.B. In fact, radians might be more sensible here - since they're more natural to use with circles (or else it would have been pretty superficial and unnecessary to come up with them!) - I might do a post about this later.
If we want to make an object travel in the opposite direction, once again from physics, we know that all we need to do is multiply its current velocity by $-1$. This makes sense in terms of the conservation of momentum for example, where we can see an explosion as valid when two particles with zero initial velocity then have velocities which are of opposite sign to one another. ($-1 + 1 = 0$ so the conservation of momentum applies)
So, we know that we can make an object travel in an opposite direction by multiplying its velocity by $-1$ - this gives its opposite. However, what if we wanted to make it travel in a perpendicular direction? Think about it - perpendicular means at 90 degrees (or $\pi$ radians) and since two turns by this number of degrees/radians would give you a full 180 degree ($2\pi$) turn, we simply need to multiply by the square-root of this half-turn. In other words the square-root of minus one.
But we need a name for this new value and guess what?
$$i$$
If you're still going to try to prove to me that $i$ "doesn't exist" now, then I'd like to hear how? $i$ seems to be essential - it must exist since the square root of minus one "doesn't" in the set of real numbers. In another world, maybe $i$ is the norm - in fact it could be if you looked at the problem from a perspective starting 90 degrees/ $\pi$ radians later!
Now we can draw up a new scale - with values of $i$ included. We can pretend each point is a state of motion of the particle. Those that involve 2 axes (real and imaginary parts) are called
complex numbers. But, you can also have vectors in complex space - vectors are just transformations relative to a point of origin. If there is no point of origin you can simply think of a vector as coming from the point (0,0) or $O$ as you may have seen it written in all those maths problems that now make a lot more sense. That's really all there is to it. So, the point $[1,2i]$ simply represents a magnitude in a fixed direction. It can also be written as $1 + 2i$ if you are representing the complex number relative to the origin.
Lets say we call that point $z$, so $z = 1 + 2i$ is the complex number (vector). Remember - you can add/subtract vectors (since they represent translations) so you can also do this with complex numbers. In short: if you represent the complex number as a set of coordinates, you've already defined the starting point (the origin), however if you define it as a vector equation - this will be assumed to have started from the origin, however you can also say it is relative to another point. Lets say this was $[4, 4i]$ - then the end point of the vector $z = 1 + 2i$ relative to
this point would be $4 + 4i + 1 + 2i = 5 + 6i$ or $[5, 6i]$ - a new complex number - and remember that you simply group the separate real and imaginary components to add or subtract. You can also perform operations on them similar to those when dealing with polynomials, where when equating or performing mathematical operations with two or more polynomials, you simply equate coefficients of the same type e.g. $x^2$. As with vectors, you could also represent a complex number in the form $[\frac{x}{yi}]$ - a vector translation.
Moving on: guess what we can use to calculate each part (the magnitude and the direction)?
The omnipresent trigonometry
Lets see - the direction and magnitude of a complex number is very similar to the direction of a vector in physics, except when talking about complex numbers the direction (or angle from the horizontal) is called the
argument and the
modulus (similar to magnitude in vectors). These two
components can also be combined into one, as you will see, to represent the complex number in a different form: the
trigonometric form. It all seems to link together.
So, lets take the vector "$z$" above again. If we want to represent it on a diagram (as a vector, we can simply draw it as above. However, if we want to simply look at the magnitude of that line (in practicality the
speed at which the point is travelling) we can use Pythagoras' Theorem.
As can be seen on the diagram (simply click to zoom) I have now split the complex number up into its two original components - the real and the imaginary. Now, before I tell you how to use the Pythagorean Theorem to work out the modulus (magnitude) I need to explain an insight I had into signs of numbers to you).
Signs of Numbers
How I see it, the signs of numbers are not actually part of the numbers themselves - numbers such as 4,5,6 which we take as the "normal" numbers are simply numbers of the
dimension (note this is not official - I simply use this description since I think it makes sense) $+r$, in other words
real, positive. I see this $+r$ as the descriptor of the number - used to describe which
dimension it belongs to. The exact opposite dimension is $-r$ or
real, negative. These
real dimensions, are not by any means the most important however - there are of course the
imaginary dimensions, which are equally as valid. These dimensions are exactly perpendicular to the
real dimensions as seen on the diagram, and is composed of both an
imaginary, poistive and an
imaginary, negative dimension. What I am trying to say is that when we describe numbers as "$2$" or "$-2$" we should really describe them as "$2r$" or "$-2r$", since we do this for the imaginary numbers by adding "$i$". Now I can continue with the explanation.
So, the diagram above should really have "$+1r$" in place of the "$1$" along the
real axis. We know however, that Pythagoras' Theorem states that we must calculate the modulus of the two components added together to find the magnitude. Now, when you calculate the modulus of an expression, you take each component in turn, take away its sign (which in this case we will be thinking of as the
dimension) and then finally square each part and take the square root of the overall expression. That is how you do it: the reasons behind this are in the Pythagorean Theorem itself. However, I suspect you want a proof of why $a^2 + b^2 = c^2$ in a triangle of hypotenuse $c$. Well, the geometric proof can be found
here. (There are many other possible proofs - I think I found website listing upwards of 40!)
So, now that we know how to find the modulus, we merely square the 'coefficients' of the two components. We therefore obtain:
$$2^2 + 1^2 = M^2$$
$$M = \sqrt{2^2 + 1^2}$$
$$M = \sqrt{5}$$
In fact, the generally accepted notation for the "magnitude" or
modulus of a complex number is simply... the modulus notation, so for our $z$ this would be:
$$|z| = \sqrt{5}$$
So, now that we have the length of the line (with no
dimensions - this isn't actually in the
real, positive form) we can calculate the argument - this is simply the
angle which the complex number 'vector' makes with the
real, positive axis.
To do this we use trigonometry. In fact, we can actually draw a similarity between the complex number vector we are working on and that of the many triangles which make the unit circle in a CAST diagram, often used in trigonometry - an interesting point to note.
We can see that the complex number's modulus is very similar to the magnitude of a force/velocity vector. Ask yourself now - how would you express a vector in terms of its two components? Well, it should of course be the two separate vector components (here components means the components including the angle
and the magnitude) added together. i.e. if you had a road and you wanted to get from its start to its end, you could simply follow a horizontal path then a vertical path, once you had completed the horizontal component. The total change in displacement would be the two displacements added together.
So, working out the angle, as you might've guessed, is just simple trigonometry. Since our components are
real, positive ($+1r$)and
imaginary, positive ($+2i$), it follows we can get a positive angle using the
tan ratio (I will do an article on trigonometry and how sin, cos and tan are all linked perhaps, as well as how both they and the logarithmic functions can be estimated using the infinite Taylor series), since:
$$tan(\theta) = \frac{opposite}{adjacent}$$
So...
$$tan(arg(z)) = \frac{2}{1}$$
$$tan(arg(z)) = 2$$
$$arg(z) = tan^{-1}(2)$$
Once again using the magnitude of the numbers only and not their "
dimensions".
You could of course work in radians or in degrees, but you must stick to your guns once you have picked one. Once again, I would advise radians since circles are involved - since it is more "natural" (since radians are in terms of $\pi$ and $\pi$ is at the heart of all circles).
So, what do we do now that we have used Pythagoras and Trigonometry to work out the modulus and argument? Well, we can now express the original vector in trigonometric form. Instead of each component having a magnitude and a
dimension, each component will now have a magnitude and
an angle which can be helpful if you need both the real and imaginary components in the same "form".
Think about how you might do this for a normal physics vector - you have the hypotenuse and the argument. You simply need to obtain an expression for the imaginary component using the $cos$ ratio and the real component using the $sin$ ratio.
$$sin(\theta) = \frac{opposite}{hypotenuse}$$
$$opposite (imaginary) = hypotenuse[sin(\theta)]$$
$$opposite (imaginary) = 5[sin(tan^{-1}(2))]$$
$$cos(\theta) = \frac{adjacent}{hypotenuse}$$
$$adjacent (real) = hypotenuse[cos(\theta)]$$
$$adjacent (real) = 5[cos(tan^{-1}(2))]$$
There we go, and to get from the origin to the end of the vector we just add the two components (doesn't matter about the order:
$$z = 5[cos(tan^{-1}(2))]
+ 5[sin(tan^{-1}(2))]$$
$$z = 5[[sin(tan^{-1}(2)) + cos(tan^{-1}(2))]$$
The above is the trigonometric form.
Now that you understand how to work with complex numbers a bit better: here's a little bit of insight into how they're vital to electronics:
I obtained the following images from
this website: you should go and check it out for a more expanded explanation.
Look at the moving diagram - its a model of how an Alternating Current Works - a current will not suddenly change from I to -I and back again, it will oscillate, with the sine curve modelling its motion in one plane. This plane can be regarded as our
real plane. This is shown in the last diagram - where physics equations are substituted in to the complex and real parts to get an overall expression for the
phase or the angle $\theta$.
It just goes to show how circles, trigonometry and circular motion are all inherently linked (article?).
However, simply doing this ignores a major part of the picture: the horizontal plane. Unfortunately our real numbers have all been used up on the axis modelling the vertical motion. But we can still include it using imaginary numbers. Do you see where this is going?
Imaginary numbers, as you have seen are linked to almost every important topic in mathematics: circular motion, vectors, diagrams, trigonometry, calculus (the change of the trigonometric curve over time in the AC), etc. etc. But one question: is it imaginary numbers that links all of these, or all all of these simply inherently linked themselves? I agree with the latter and I hope reading this Blog will convince you to agree with me.
Here's a nice video to finish off with by
Sixty Symbols of Nottingham University.
If nothing else, Philip Moriarty's confusion should reassure you.
Important note: I write this Blog on my own and undoubtedly I will make mistakes. Please do not hesitate to correct me in the comments if I am wrong or slightly misled about something, since this Blog is an essay in learning for myself as much as its readers. Discussions are just as good. I really appreciate your involvement. Any contribution is part of the Blog itself.
