Physics for Everyone: How Light Travels

In my last article, I discussed the basic nature of light. But there is still one aspect of this phenomenon which pervades and even defines our experience upon which I did not touch. An aspect that is still misunderstood by the majority of non-physicists. I’m talking about how light travels.

Yes, I mentioned in the previous article that it self-propagates by the interaction of the electric and magnetic fields that make up the photon. This is true, but it still leaves questions. How does it get from Point A to Point B? Does it travel in a straight line? Does it takes its time and take side-trips, visiting interesting landmarks and doing a little sight-seeing on the way? Does it spontaneously leap from Point A to B with no real travel in between? Does it always go at the same speed – does its speed vary?

Let’s start with the most basic of these questions – does light spontaneously leap from Point A to be with no travel in between. The short answer to this question is that, without entanglement, it would seem that it does not. Even virtual photons (photons exchanged by particles to communicate with each other) seem to travel the distance between the two particles exchanging them, just that distance is so miniscule it’s difficult to detect the existence of the photon before it is absorbed by the receiving particle.

But what about entanglement? What is that? Entanglement is something that happens when two particles are forced to interact with each other in such a way that all of their states are “lined up” and essentially matched, with the exception of their position. The two particles are then separated from each other. It turns out that they maintain a connection that allows information to be spontaneously exchanged without having to travel the distance between the two particles. Entangled photons have been demonstrated to spontaneously travel across short distances without having to actually move through the intervening space. These special cases would seem to be the only ones where light is able to travel from one point to another without moving through the space In between.

Leaving the next point: Does light always travel in straight lines, or can it bend and curve? What’s taught to most people throughout primary school is that the former is true and not the latter – in other words, that it is boring and always follows the straight and narrow path. In reality, this is not true.

How light moves is complex, and is a result of probabilities. Another interesting aspect of our universe (and the source of frustration for some people when talking with physicists), is that everything is the result of probabilities – there is no such thing as a sure bet in our universe – no 100%. So when trying to figure out what happens in a single event, a physicist has to consider all the possibilities of what could happen. With light, the path it takes from Point A to B could be a very circuitous one – it could go straight, or it could curve, or it could even do crazy loop-de-loops. What does happen is a result of the summation and “average” of all these possible paths. Ultimately, the photon “chooses” the average result that gets it to its destination the fastest. In most cases, at least in most cases that will be common in human experience, this will be a straight line.

BUT, even in human experience, there are cases where that line isn’t exactly straight – the path gets broken up into parts, and we can even actually see the light bending at certain points. This always happens when light passes from one medium into another, such as from air into water. The light will appear to go in a straight line until it hits the interface, make a sharp angle, and then continue on in a straight line once again. We’ve all experienced this when looking into a glass of water, or into a pond (and some of us have even tried to grab an object underwater, and missed, because of the bend of the light at the surface).

Why does it do that? What makes it bend at the interface of the two media?

The answer to this is the answer to one of the other questions – does light always travel at the same speed? The answer to that question is no.

What? What about this universal speed of light as a limit we keep hearing about? Well, that limit exists – nothing can move faster than the speed of light. But that speed is NOT a constant in all media. The speed at which a photon can move in one medium is different than that in another. But if light is the defining limit of speed, why does its velocity differ in different media? The answer to that is interference.

Remember how in the last article I mentioned that light is a wave? And that matter is also made up of waves? Well, when light moves through matter, it interacts with those waves – they in effect interfere with it, and slow it down. Different forms of matter (different materials) interfere with it in varying amounts –some more, some less. To add to that, some matter simply absorbs the light (the photons are “captured” by the atoms of the material). While some of the photons absorbed may be reemitted, this will also slow down the light, as the absorption and re-emission takes time. It is the combination of the interference and the absorption and re-emission time that slows down lights speed within that material.

Now, light still wants to take the shortest amount of time to travel from Point A to Point B. If those points are in different media (or there is a different medium somewhere in between them), then a straight line actually is NOT the fastest way for it to accomplish this. It needs to change course in order to accomplish its goal of getting between the two points in the shortest time possible. And thus, it bends.

But wait! It gets crazier. Not only might it bend at an angle, but it might actually CURVE in its course, if that curve produces the shortest time possible to get from one point to another. Crazy, I know, but it happens. We don’t really see that in our everyday experience, though, so it wasn’t until Einstein that physicists even considered that such could happen.

And the reason for that is that the space we live in is not flat, despite our intuitive experience. Space is stretchy – and very massive objects seem to have the ability to bend and curve space. And when I say very massive, I mean it – it’s not really a noticeable effect until you’re dealing with something at least as a star (we can measure it on a planetary scale, but it’s not noticeable unless you’re using very precise instruments). Einstein predicted that this would happen when he formulated his theory of relativity, and it took some time before we were able to actually observe it happening.

How did we observe it? Well, simple – remember that light wants to take the shortest time possible to reach its destination. If it passes through a region where space is curved, a straight line will actually make it take quite a long time to get there. But if it curves, it might actually go *around* the deepest bend in space, and get where it’s going a whole lot faster.

Imagine this – think of space as a giant thin sheet of rubber. Think of a star as a very heavy lead ball. Put that ball on the rubber, and it’s going to drag the sheet down and make a deep well, resting at the bottom of it. Now try to draw a line between two points on either side of (and outside of) that well. If you stay exactly on the sheet in a way that would be a straight line where the ball not there, you’d have to draw the line INTO the well, and actually end up drawing a line that is longer than if you curve around the edge of the well. And light will do the same thing – instead of going into the well, it will curve around it, follows the shortest average path. This effect is most noticeable in something called gravitic lensing, where we can see an object (usually a galaxy) that is behind another ultra-massive object (usually another galaxy), but because of lensing, or the light curving around the well created by that ultra-massive object, the object behind it appears to actually be beside it, and closer to us than it really is. Astronomers take advantage of gravitic lensing to observe extremely distant galaxies, as well as to measure the mass of closer ones.

There’s one last thing I’d like to touch on before I wrap up – and it explains just WHY light behaves the way it does, and why its speed is the defining speed of everything. To truly understand why light moves the way it does, and why NOTHING moves faster than light, one has to understand what light does. What its purpose is. The function of photons is to transfer information – they are basically the communicators of the universe. It is the photon that allows one particle to communicate with another, to say “this is what I am, and these are my states.” Without photons, particles would not interact with each other – and that’s also why “virtual” photons exist – they are exchanged when two particles get very close to each other. That information tells the particles *how* to interact with each other – whether to rebound, or bind, or do something else interesting. And since light is the information carrier of the universe, nothing can outrun it – because to outrun light would be to outrun the very information that is telling you how fast you’re moving. Just can’t be done.

And there you have it – how light travels. Not quite as simple as your high school physics teacher would have you believe. It’s a crazy thing that takes all possible paths, averages them, and then “picks” the shortest route to arrive in the quickest time it can.