Concept Exchange Society


Wednesday, June 12, 1996 6:30 pm

Once again at Glenn Johnston's house.

Presentation: Marvin Chester

A Physical Demonstration
of This Quantum Physics Finding


The world is made of identifiable things.
Here is a salt shaker. There is the moon. A pebble. Made of molecules. There are particles of light - called photons. Of sound - called phonons. All identifiable things. Made of identifiable things.
Identifiable things are but bundles of identifiable attributes. The attributes make the identification. Attributes, themselves, have identity.
The idea of identity is so elemental that one does not examine it. But that the universe falls into identifiably distinct elements is remarkable. Why isn't it all a vast soup?
Perhaps it is! Identity may be an illusion.

The thought derives from this remarkable finding of experiment:
Identity is lost when something is not observed.

Although it is not at all intuitive the concept acquires sensible meaning on reflection. To observe is to identify; hence to grant identity. What is counter intuitive is that without observation identity is lost.
A particle does not carry its destiny with it. If we assume that it does we get contradictions to experiment. Experiment shows us that until an attribute materializes in measurement, particles do not have the attribute!
My presentation is a simple physical demonstration of this effect: a case where identity is lost if not captured by observation.
In particular a photon will be shown to lose one of its identifying attributes if it is not observed.

For the success of the demonstration these two physical findings must be appreciated:

A. Photons are particles of light. We know that light consists of particles because they are countable. Detectors, also called counters, count them. Light comes in units. The more there are, the brighter the light.

B. Besides the property of color, photons carry another property called polarization. Although color forms a continuous spectrum of possibilities, polarization has only two possibilities in its spectrum. Two polarizations - say horizontal and vertical - comprise the entire spectrum possible. Any other direction in a plane may be resolved into these two.
Just as a glass prism is a device used to display the entire spectrum of color that a photon may have, there is something that displays the entire spectrum of polarization that a photon may have. A calcite crystal does this.

I will demonstrate how this property of polarization, which any photon possesses when measured, can be lost when it is not measured.

The physical significance of this effect is monumental. The structure of quantum theory was built in order to accommodate it. The philosophical significance is equally stunning.

Here's how the demonstration will go.

A: Light consists of particles. They can be counted.

B: These particles have a two state property. It's called polarization.

Take a piece of polaroid and cut it in two.

The pieces may be oriented with respect to each other.

The perpendicular arrangement shown is called 'crossed polaroids'.
I will bring polaroid sheets to the meeting for a hands on demonstration.
Put a light source behind two crossed polaroids. Light passes through both only when the polaroids are not crossed.
We all know that light has the properties of color and intensity. This simple crossed polaroid observation establishes that light possesses a further property with only two states - a spectrum of only two possibilities. Here is the argument:

So the total flux of light consists of only two parts: what does not penetrate the first polaroid plus what does not penetrate the second. All the light can be resolved into what is polarized one way plus what is polarized perpendicular to that way. These two cover all possibilities - the complete spectrum of possibilities.

A calcite crystal displays the polarization spectrum. Such crystals are found not uncommonly in nature. They go by the name of iceland spar.

This crystal exhibits double refraction. It resolves an entry beam into two exit beams: one direct polarized (the ordinary ray) and one perpendicularly polarized (the extraordinary ray).
I will bring some crystals with me to show the effect.

Here is the promised loss of identity demonstration in three steps.

A skew entering photon emerges either as a one or a one - at random. The probability that it emerges one or the other depends only on the skewness of the entry angle.

Replace the two counters of #1 with a reversed calcite crystal of #2.

What emerges from the back-to-back pair of calcite crystals?

In fact it doesn't happen!

With detectors present the photon is observed to be resolved into or by the crystal. (#1)

With detectors absent we observe that it is not resolved. The skew nature of the photon is preserved. (#3)

Conclusion: Polarization identity as marked by the detectors is lost when the detectors are absent!

Detailed explanations will be offered at the meeting.

Return to Concept Exchange Society
Return to marvin

Marvin Chester


© m chester 1996 Occidental CA