Friday, April 20, 2007

The Elusive Photon

I'm sure most of you have heard some of your classmates mention "photons" in class, or at least people mentioning how light is also a particle, and also see me ignoring them.

So here I will address the question: what is a photon?

First of all, the reason why we know so clearly that light is a wave arises from this wonderful ingenious experiment (I might have mentioned this in class) known as the Young's Double Slit Experiment. I won't go into the details of the experiment, but you can check it out yourself here. But what the experiment demonstrates very clearly is that light undergoes a process known as (you can check it out too)interference. This process is one very special phenomenon associated with waves, much like reflection and refraction are special phenomenon associated with waves.

So a whole generation of physicists studied electromagnetic waves and made many conclusions about how electromagnetic waves should behave, and sure enough almost all phenomena associated with light could be easily explained by the fact that light is a wave.

One would think that the answer is immediately apparent from the experiments: light is a wave, and no more arguments. But there were a set of three physics problems at the beginning of the 20th century that were thought to be minor issues at that time. They defied the physicists' attempts at explanation.

One of them was known affectionately (or rather unaffectionately) as the ultraviolet catastrophe. A simplified version of the problem goes like this: when light was assumed to be an electromagnetic wave, it was predicted that if a person stands in front of a stove, he would be exposed to an infinite amount of radiation, and would therefore instantly burn to death, which is of course not true! Such a gross error definitely needed prompt correction.

The other two problems are the famous Photoelectric Effect and the less famous Compton Effect . In both cases, simply ignore the math.

All three of the problems had one thing in common: they could all be solved if we allow ourselves to envision light existing as billiard ball-like entities. Don't concern yourself with the technical details here, unless you are willing to crack your heads, but these three problems were all resolved by assuming that light existed in little chunks at a time called quanta (hence quantum physics). You can think of it as the light ray being composed of billions and billions of microscopic particles called photons.

What's the difficulty here? Well, fundamentally particles and waves are really different. For one thing, in a wave, the energy is continuous throughout the wave. When you shine light on a surface, the energy arriving from the light will gradually accumulate on the surface, and heat it up. But if you think of light as little billiard balls, then energy transfer comes in short "spurts": when a billiard ball hits the surface, the energy is immediate transferred to the surface, so there is a sudden "spike" in the total energy of the surface.

What is even more troubling is the fact that waves undergo refraction and interference, whereas particles absolutely do not. How can we explain the wave-like properties of light if we treat them as particles? On the other hand, how do we explain the three problems without resorting to particle-like properties?

The problem gets even worse!

After awhile, physicists were actually able to detect wave-like properties in electrons, which we definitely thought were particles. They were able to perform Young's double slit experiments on electrons, and showed that electrons can behave as waves too! How can that be?

A French prince named Louis de Broglie came up with a most proposterous suggestion, that turned out to be right. He suggested that everything in the world had two "faces": wave-like behaviour, and particle-like behaviour. And he really meant everything.

So even you can behave like a wave, undergo interference and all, but because of your incredibly great mass (in comparison to the electron, for example) the wave-like properties are not detectable, and the particle-like properties dominate.

It may not sound sensible at all, but if you read more about this incredible theory and its history and how the ideas behind quantum physics first developed, you would see that we were forced into a corner, and were basically forced to concede something that makes little common sense, but yet was right, because that was what the experiments were saying.

How can something be both particle and a wave? The modern understanding, is seriously mind-blowing. I don't truly understand it on some level, but what I understand is this:

Before you observe anything, you will know absolutely nothing about that thing, and so you will never know whether something is a particle or a wave until you decide to look at it. Now, in order to find out if something is a wave or a particle, we must make an observation, which involves some kind of measurement. It turns out that depending on what we choose to measure and what the experiment set up is, you will get different conclusions! If you design an experiment to measure the wave-like properties of something, you will detect its wave-like properties. If you design an experiment to measure the particle-like properties of something, you will detect its particle-like properties!

Sounds silly right. But it is a lot more ingenious and subtle than it sounds. On top of that, it works pretty damn well too.

An excellent example of this idea would be the extremely famous Schrodinger's Cat . You can read it yourself here, or click on the external links: there are some that give a very quick introduction.

For those who are interested in pursuing this further, you can check out the book "Who's Afraid of Schrodinger's Cat?", or the comic-like book "Introduction to Quantum Physics": the series with introductions to a lot of different subjects in comic form.

Like in all things complicated in physics, skip the math. It can wait. You can still understand most of this stuff at a basic level if you read the right stuff!

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