Sunday, February 10, 2008

Quantum Mechanics: particles or waves?

Quantum Mechanics! It's everyone's favorite physics topic, outside of cosmology. Few people understand it, and no one understands it fully. I am admittedly a little reluctant to get into this topic because I've only had an introduction to quantum mechanics so far in my formal education. Well, I at least understand it better than your average science journalist (though I'm probably worse at explaining it).

Most every explanation of Quantum Mechanics starts with a question. Is light a particle or wave?


How could scientists possibly be confused about this? Well, both particles and waves transmit energy and information between here and there. A particle is simply a point that moves from here to there. A wave is a disturbance that transfers from here to there through a medium (at least, we used to think it required a medium). In the above example, the ocean is a medium for an ocean wave. The ocean wave is a fluctuation in water level that transfers between this patch of water and that patch of water. It does not require that the water itself move from here to there.

Particles and waves behave very differently. Particles have a definite position and come in whole-number quantities. Waves do not have a specific position, and may come in any quantity. Waves can move around obstacles. Waves have a property called frequency, which is a measure of how quickly it fluctuates up and down. Waves can interfere with each other such that a fluctuation up and a fluctuation down cancel each other out. There are many more differences, since the mathematics that govern particles and waves are completely distinct.

Though particles and waves are completely different, it is not immediately obvious which one describes light. Historically, scientists just couldn't figure it out. Pythagoras thought light was a particle, Aristotle thought it was a wave. Newton thought it was a particle, Young thought it was a wave. Cocktail Party Physics probably gives a better treatment of the history than I could.

In the early 20th century, there was very good evidence on both sides. Maxwell's equations predicted the existence of electromagnetic waves that behave just like light. Einstein used the photoelectric effect to show that light comes in discrete quantities (this is, by the way, how he got his Nobel Prize). Physicists had long assumed that light is one or the other, so the ambivalence of evidence was confusing to them.

The culmination of the debate was a single experiment that simultaneously showed that light had properties of both particles and waves. This experiment was called the double slit experiment... and is explained on the next page.