# Fermilab

## Inquiring Minds

All About Light Main Page  |  Classical  |  Relativistic  |  Quantum

 Quantum - Absent On a Meeting? Blame the Interference! Black Body  |  Photon Idea  |  Dual World  |  Photo Effect  |  About Photons We are Dual!  |  Interference  |  Photon@fnal
On the previous page I described the huge step in mankind's thinking, made by Planck and others when they postulated that every particle with a momentum p acts as a probability wave with a frequency .

On this page, we use this fact to excuse you from a very important meeting you forgot to attend. Let us start with a wave interference experiment, called the double-slit experiment. The experimental setup is very simple, as seen in the picture.

A light source shines a light on a wall with two holes. Then you measure the intensity of the light on the back screen. The light can get to the back screen via any of those two holes. There is a (path shift, and therefore also a) phase shift between the two light beams going through holes A and B and reaching a point P on the screen. Based on this phase shift, an interference occurs at point P. This can be either constructive or destructive.

Therefore at an arbitrary point P on the screen one observes either a bright spot (constructive interference) or dark spot (destructive interference). This phenomenon can occur only if there are waves.

If you repeat the same experiment with billiard balls instead of light, you will not see any interference.

Now wait a minute. Didn't I say that every particle is a wave? Therefore, shouldn't the interference pattern have been observed also for particles? There must be something wrong.

Planck and company did not get discouraged by these results. They knew that the distance between the peaks on the interference picture depends on the frequency of the incoming wave. The higher the frequency, the closer the peaks.

They claimed that the resolution of the apparatus is not fine enough to distinguish between the peaks of such high-momentum incoming objects as billiard balls.

Later on, physicists repeated the experiment with electrons by using high-resolution detectors, and indeed, they observed INTERFERENCE patterns. There were spots on the screen which classically should have had electrons present, but did not! This was explainable only by the wave properties of electrons.

Conclusion: If you forgot to go to an important meeting, here's your excuse:

Say you tried, but your interference pattern when you entered the conference room via the two doors was such that you happened to have a destructive interference in the conference room. I bet that no one would argue with this excuse! Quantum mechanics is to blame. One cannot cheat on Mother Nature! (P.S. Let me know if it worked :-))

On the next page, I show how we create and treat photons at Fermilab.

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