## Monday, June 30, 2008

### First video sequence of Common Sense Quantum Physics

Today I published my first video sequence presenting Quantum Physics intuitively. I hope to have another six or seven sequences mixing theoretical and experimental analogies.

Here is the videoscript:

Hello, I'm Arjen, the Common Sense Quantum Physicist. My goal is to bring Quantum Mechanics nearer to intuition. As an introduction, we'll look at a characteristic property of light : the polarization. Light may be polarized in some cases, that means that it can take a characteristic orientation.

For example, the sunlight reflected from this surface is polarized in such a way that it is filtered by these sunglasses if I wear them horizontally on my nose. If I turn my head, I am dazzled by the reflected light.

So, how could we explain this ?

Firstly, we need to know that a polaroid film is deposited on these sunglasses. A polaroid film is in fact a bunch of molecules that are arranged parallelly on the glass of the spectacles.

Secondly, we take advantage of a scientific representation of light. Light is composed of tiny particles, that we call photons. In quantum physics, a photon is represented by a little spinning arrow. One way to understand light is then to visualize it as a flux of little spinning arrows guided by a wave. When an arrow bounces from a reflecting surface, it affects its spinning direction. Before the reflection, the arrow is spinning in a random direction. The reflecting surface then rearranges that in a definite spinning direction and the polaroid film filters the photons depending on their spinning direction.

Let us simulate this polaroid filtering with ordinary objects.

Firstly, we have this safety barrier representing the polaroid film on the sunglasses.

Secondly, we have this rotating rod that represents the spinning arrow. If the rod is spinning perpendicularly to the rails of this barrier, it will nearly never pass the grid... If the rod is spinning parallelly to the grid, the probability is much higher. If it is spinning in any other direction, it is just a matter of probability.

So this experiment learns us two important things about the behaviour of the particles composing light.

Firstly, a photon is represented by a rotating arrow. The photon is a prototype of all quantum particles, in fact it is the simplest of all quantum particles. While in ordinary classical mechanics, particles are represented by points or spherical objects, like bullets or tennis balls, in Quantum Mechanics, the objects are represented by rotating arrows or rods or baseball bats, scientists say vectors. This constitutes the core of Quantum Mechanics. A very famous physicist, Richard Feynman, once presented Quantum Mechanics as the science of drawing arrows. You'll find that in this very clear presentation of Quantum ElectroDynamics : " All we do is draw arrows, that's all ".

The second important thing that we learn through this experiment is that quantum measurements are a matter of probability. The quantum rules do not give certainty about the result of an experiment. Quantum Mechanics only give odds about measurements under given conditions.

So remember these two important facts when dealing with light...

 photons are best represented by little arrows and

 measurement on these arrows is a matter of probability.

Next time, we'll look at how we may characterize the physics of quantum particles.