Sunday, June 24, 2012

Science Sunday: Light, You Magnificent Bastard

An animated gif showing a prism dispersing visible, white
light into its color components: red, orange, yellow,
green, blue, indigo (seriously? indigo?), and violet
Well now, it sure has been a while, hasn't it?  It's about time that I got back to this, no?

Last time on Science Sunday, I discussed the Sun.  I didn't discuss it in it's entirety though, because let's face it, there's enough going on with the Sun to warrant its own blog.  In fact, it already has it's own Scientific Journal!  Nevertheless, I did a quick overview of how it stays up against the constant threat of gravity without really discussing much of any of the interior physics, and I'm going to just continue to leave that as an open question for some later date.

What I originally wanted to do today was move on toward discussing larger populations of stars beyond our own Sun and those like it (and believe me, there are many).  It seemed like the next logical step to me: start with one really well known star, and use that as a jumping-off point to talk about stars in general.  However, about one-tenth (if even THAT much) of the way through writing that post, I hit what I perceive to be a massive roadblock.  The way that astronomers categorize stars has everything to do with the light that they give off, and it's really pretty damn stupid to talk about this categorization and go onward from there without actually talking about what light IS!  It's somewhat like saying "hey, let's talk about all of the different types of cars there are, basing the types on the engines they house," with you all like "hey that's great and all, but what in the hell is an engine?"

I'm supposed to be educating here right?  Right!  So let's get on with the edumacation* regarding that most magnificently brilliant of things in the Universe: the phenomenon of light!  The most blindingly magnificent of bastards!

What?  What is this?  This is nonsense!
Dammit man, put this into plain English!
So the question is: what is light?  The short(ish), somewhat confusing answer is that light is the manifestation of electric and magnetic waves propagating in phase with one another, with their amplitudes directed perpendicularly to one another as well as their direction of motion, through space, and is also a massless particle containing all of the energy of those aforementioned waves in one discrete little packet moving at the same speed as those waves.  In other words, the short answer is complete nonsense.  Even to me! And I wrote it!  True understanding comes from the long answer, so let's instead get neck-deep into that long answer.  To begin with, in order to understand the nature of light you have to know a little bit about charged particles.

Hey Electron, Why Must You be So Negative?
An electron, which is negatively-charged,
shown with electric field lines pointing towards it.
You don't know 'bout that yet.  We'll get to that later on in our program
I had used the electron in the previous Science Sunday post to talk about fusion without really talking all that much about the electron itself, so here we go. The electron is your basic charged particle.  So basic in fact that fundamental units of charge are based upon that of the electron.  I really wish that I could give an adequate description of what a "charge" is, but it's a lot like defining "mass".  I'll put it like this: in a similar way that mass is how much physical "stuff" an object possesses, charge is how much electric "stuff" an object possesses.

Top (attraction): Q1 - Man, my outlook is so negative!  Q2 - Come closer and let me add some positivity to your life.  In fact, the more negative you are, the more that I want you next to me!  Q1 - I can dig it.
Bottom (repulsion): Q1 - Man, my outlook is so negative!  Q2 - Dude, everything's already negative for me.  Get away, I don't need yours too.  Q1 - Yeah...well...I don't want you around either!  Q2 - Fine!  Q1 - FINE THEN!
Much unlike mass, charges come in two varieties: positive (+) and negative (-).  The way in which this matters is that similar charges (i.e. (+)(+) or (-)(-)) repel each other, while opposing charges ((+)(-)) attract each other.  This repulsion/attraction manifests as an electric force (push or pull) between the two charges. Regardless of whether it's repulsion or attraction, the strength of the force between the two charges increases as the amount (more negative or more positive) of either charge (charges q1 and q2) increases.  It also decreases as they get further apart.

You Turn Me On With Your Electric Field
Every electron existing somewhere sets up what's called an electric field (from here on, called the E-field).  The way to think about this is that the E-field is the area of influence of that electron.  If you had a stronger electric charge (say, TWO electrons *gasp*), then together they'd set up a stronger E-field (twice as strong, in fact).  Electric fields are typically shown in pictures as lines radiating away from some central charge, with arrowheads pointing toward the charge if it's negative and away from the charge if it's positive.  Therefore, a negative and positive charge together (in perfect harmony) set up an E-field like the one shown in the picture to the left.  For the sake of simplicity for later let's imagine that  charges q1(+) and q2(-) are nailed in place, such that they set up an unchanging E-field.  This field, has some strength, E, based on the charges that set it up.  Ok.  That framework is set.
Oh, how pretty

Now, if you can, stretch your imagination to place a freely-moving third charge (q3 (+)) within our beloved E-field.  q3 experiences a force based on the strength of the E-field (surprised?), and the strength of q3, where the strength of the force on q3 increases if either the strength of the charge or the strength of the field increases.  The glorious thing about the field is that when q3 is placed within this field, it'll move in whatever direction the field is pointing.  The amount of this movement, again, is dictated by the force it experiences within the field (and curiously enough, not the mass of the charged object).  Lovely.

Word, You're Not Confused Yet? Challenge Accepted
Now it's time to talk about magnetic fields (from here on, called B-fields.  Don't ask me why they chose "B"), and I tell you now that I hope I don't screw it up.  It's actually a lot easier to understand if you understand the mathematics, but bleh.  Whatever.

Above, I talked about two charges creating a static (i.e. unmoving, unchanging) E-field.   Well, if the strength or the direction (or both) of the E-field changes, then something kinda crazy happens: the changing E-field creates a B-field!  The B-field that's created interacts with your charged particles in a somewhat different way than your static E-field, but the weird thing about it is that at its base the E-field and the B-field are just two parts of the same phenomenon of electromagnetism. The way to think about it that makes the most sense is that magnetic fields are emergent properties of electric fields when electric fields can't keep still.  An analogy that I find useful is that of a fan with lights on its fins (below).  When the fan is static, all that you see is lights on one of the fins.  However, when the fan is turned on and the fins start to spin, a pattern emerges that wasn't there before.  The underlying structure of the fan with lights on the fins is the same, but in motion you get what appears to be a different phenomenon!

Now that I see it, this is an almost perfect analogy, haha.  The same thing happens between electric and magnetic fields!  When you start spinning, shaking, oscillating (this becomes REALLY IMPORTANT), or otherwise changing E-fields, B-fields emerge.  B-fields can appear to be an entirely different phenomenon, but they're really only the product of changing E-fields!

And Now I Drop The Bomb on Your Brain Meats
If you change an E-field at a constant rate, you set up a static B-field.  Now, if you change the RATE at which you're changing your E-field, you change the B-field that you generate.  If you then change the rate at which you're changing your B-field, you set up an E-field!  What?! YES! It's true! A changing E-field produces a changing B-field, and a changing B-field produces a changing E-field!  Example in the figure below.

Honestly, it doesn't matter at this point which is the
E-field and which is the B-field.  The point is that
changing fields change each other

At this point, if by some stroke of luck or extreme patience you're still reading this blog post, you're probably wondering to yourself, what does ANY of this have to do with light?  Well, funny you should ask.  That animated gif above is actually a representation of the oscillating E-fields and B-fields that compose a wave of light.  The sizes of the arrows represent the strength of each field at a given point in time, and the direction of each arrow is...well...the direction of each field at any given point in time.  As the fields oscillate back and forth, they do so with some physical length for the completion of one oscillation cycle called a wavelength.  The faster the frequency of oscillation, the shorter the wavelength, and vice versa.  Additionally, the shorter the wavelength, the higher the energy of the light being produced.

As you can probably imagine, there's a whole range of wavelengths that a given wave of light can possess.  Perhaps...perhaps even a whole spectrum of wavelengths for these electromagnetic waves.  We should probably give a name to that.  How about the electromagnetic spectrum?
There's...there's a lot of different types of light

Interesting thing here is that the light that we see, the visible light (red through violet), represents only a veeeeeeery small portion (greatly exaggerated here) of the total amount of light present in the Universe.  Light ranges from the Radio wavelengths (at the lowest energy end) up through and beyond Gamma Rays, and everywhere in between.  Different objects/phenomena produce different types of light (a subject that we'll delve into in other Science Sundays), and different processes produce different intensities of that light.

As astronomers, light is our tool.  It is how we measure the Universe and what we use to model just what in the hell might be going on out there.  As I talk more and more about astronomy and the stuff that we see out in the Universe, I'll reference light as sort of just this thing that shows us what's out there.  But, we shouldn't ever lose the knowledge of what produces light in the first place and what it's composed of.  It matters!  Hmmm...maybe I should do a post sometime about the different ways in which light is actually produced in the Universe.  Maybe I will.  But not next time!

Next time, star stuff!

*Beware, we about to get all technical n' whatnot up in here.  I'm going to throw an equation or two at you because I expect you to be adults that won't crap their pants at numbers and other math on the screen.  Because I'm on vacation, all of my facts and figures either come from my faulty memory or from somewhere on the internets (read: Wikipedia and Google).  Any conversions and calculations that may have been done were done almost entirely on my iPhone, and may be a little (or a lot) off due to rounding and general laziness.  This is certainly not an ultimately-reliable source for facts and figures, just an astronomer rambling about stuff that gets him excited.  Enjoy for your own pleasure, but cite at your own peril!  In fact, seriously, don't cite at all.  If you're citing this as a source, you need some serious, serious help.

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