Friday, January 25, 2008

Particulate This!

Everyone's heard of Feynman Diagrams (or maybe not! :p) but what exactly are these? To begin understanding such a pictorial diagram of quantum processes, one first needs an example, which is aptly shown below:
I haven't quite drawn the diagram in the conventional way (without the axes being labelled) because I want the meaning of the diagram to be explicit. In this diagram, the bottom left arrow depicts an electron moving to the right - notice that it moves upwards (in time) meaning it is travelling forward in time, and it moves right (in position) meaning it is travelling to the right in real life.

The bottom right arrow represents a positron (the anti-electron) - in this convention, all anti-particles are drawn with a reversed arrow, so while the electron has an arrow that points forward in time, the positron has an arrow that points backwards in time. You might ask: why so? Simply put, an antiparticle is a particle travelling forward in time! We'll have more on that later; for the time being, suffice it to say that this is the convention we're going to adopt.

You'll notice two dotted lines at the top, which corresponds to two identical photons - you might notice that there are no arrows at all. Want to fashion a guess? It's because a photon is its own anti-particle, and hence no arrow is needed to distinguish between a photon and an anti-photon at all. Hey wait a minute! Are you saying that photons can actually be travelling forward and backwards in time? Perhaps, for light itself defines the very limits of time travel, and maybe only a photon can experience true time travel without being altered itself.

So how do I read this diagram? Easy, if we use the conventional wisdom of our real world, we say that an electron and a positron travel towards one another, collide, annihilate, and their mass energy is converted into electromagnetic radiation in the form of two photons that travel away from one another.

But in modern Physics, there is an alternative viewpoint: an electron moves from the left to the right, and at one moment in time, emits two photons. The emitting of these two photons causes the electron to change its momentum in time, and causes it to move backwards in time, becoming a positron that goes on towards the right, but backwards in time.

Well, that's all for this post - till when I'm feeling awake again!

1 comment:

quirK said...

Hehe, Feynman rocks, and thanks for already demonstrating this to me earlier. =)