What is a Photon?
A photon is a force carrier particle. Elementary particles (electrons, protons, neutrons, and such) can only interact using force carriers. In the race analogy, photons are like fruit; they are responsible for electromagnetic interactions.
Electromagnetic Interaction
Electromagnetic means having both electrical and magnetic properties. When physicists talk about interaction, it typically refers to the exchange of energy via photons. Keep in mind that visible light is only a small range of frequencies where photons exist.
Electromagnetic interaction between particles has three forms: forces within atoms, forces between atoms, and electromagnetic fields and waves.
Forces Within Atoms.
This force causes the electrons to bind with an atom's nucleus (the positive nucleus attracts negative electrons, similar to the attraction between north and south poles of magnets).
Forces Between Atoms.
The friction when tires roll, the pressure of squishing your thumb and forefinger together, and a chair holding you up occur because of changes in energy. The energy changes because electrons, or atoms, reposition themselves as material is deformed upon contacting itself or another material.
Electromagnetic Fields and Waves.
This interaction is responsible for electric or magnetic fields, and electromagnetic waves that travel (light, x-rays, microwaves, and such). All these forms of waves are basically the same, differing only by wavelength.
Quantized Behaviour
An electromagnetic wave must carry one or more whole units of energy (never arbitrary amounts). The units of energy carried are called quanta. Thus a single photon is an electromagnetic wave carrying one quantum of energy. Since it is a wave, it has a frequency; a higher frequency means a more energetic photon. Let's revisit the rope analogy to explore this concept.
Imagine making waves in a long strand of rope by swinging your arm up and down. The faster you move your arm, the more waves move along the rope. You have to work harder to make more waves, which means you have to put more energy into the rope. Since a photon behaves like a wave, when it has a high frequency it must have lots of energy! However, unlike the rope, a photon can only take on certain fixed frequencies. This is like being allowed to make 20 waves in the rope each second, or 30 waves, or 40 waves, but never 25, 37, or 42. The energy you put into the rope must be in quantized units.
Electrons will only absorb or emit photons of specific frequencies to perform a quantum leap. From the race analogy, this is similar to saying that runners are so picky that they will only eat fruit of a precise, fixed weight (for example, an apple weighing 140 grams is edible, but 142 grams is not).
Now that we know the role photons play, and how selective they are about their energy levels, the two remaining items we need to understand are spin and polarization. Both are tricky properties that play a role in quantum entanglement.
Spin
There are two types of spin covered in this section: particle spin and photon spin.
Particles
All particles have a property known as spin. The following describes the spin of typical particles, excluding photons. Once the notion of spin has been highlighted, the property of spin as it relates to photons will be addressed.
Although there is no exact classical analogy, it helps to picture spin in terms of a globe. Figure 4.1 shows a tiny globe that you can imagine rotating around its axis as it travels through space. (But note that particles do not actually rotate like a globe.)
The concept of spin can be clarified by showing an experiment that detects this property. Figure 4.2 shows the behaviour of a particle when passing between two magnets.
Figure 4.2. Particle Spin Detection.
In the above figure, a particle is shot from the Emitter like an arrow from a crossbow. When the particle passes through the influence of the two magnets (the S and N rectangles), it will change course depending on its spin. If the particle is spinning up, it will deflect along the high path to the detector; if spinning down, it will travel the low path. This behaviour is caused by the particle's intrinsic angular momentum, which is a fancy term for spin.
The spin of a photon has slightly different behaviour than other particles.
Photons
A photon's axis and its direction of motion are directly linked, making it impossible to change one without changing the other, much like a gyroscope. In all cases, a photon's axis must be 90 degrees to its motion. Since photons travel at light speed, their spin is limited to two states: clockwise or counter-clockwise. These states correspond to left-handed and right-handed photons.
The last property of photons that we need to explore, polarization, is related to spin.
Polarization
Polarization is the direction that photons oscillate. In physics, oscillate means to vary between alternate extremes, often within a set time limit. For a photon, its polarization is the orientation of its axis, which in turn is linked to its direction of motion. The following sections explore the idea of how photons and waves are related to polarization.
Polarized Waves
In Figure 4.3, a rope is being swung haphazardly, which makes not only horizontal and vertical waves, but all types in between. Note how the rope's length physically limits the size of waves that can be created.
The size of light waves is different from rope in that it is restricted by the waves' frequency, and consequently their energy. The direction of light waves, like the rope, has no restriction; most light (from the sun or incandescent bulbs) travels randomly, similar to the rope movement in Figure 4.3. Passing light through a specially constructed material will filter out photons that do not have a specific polarization.
Figure 4.4 continues the rope analogy to illustrate the concept of filtering waves. When the same haphazard motion from Figure 4.3 is used to whip waves through a picket fence, only vertical waves will pass through the slit. Consequently, someone watching from the other side of the fence will only see vertical waves in the rope.
Imagine placing a second fence after the first whose slats are horizontal. When waves try to pass through the first fence, only vertical ones succeed. The second fence, being horizontal, blocks all vertical waves. The result is that someone watching from the other side of the fences will see no waves in the rope. By using two fences with slits rotated 90 degrees to one another, all the rope waves are elimiated.
The same effect happens with photons, which can be demonstrated by a simple experiment.
Experiment 1
To see the effect of polarization on light, try these simple steps:
- Find two polarized sunglasses.
- Hold one pair horizontally.
- Hold one pair vertically.
- Press a horizontal lens against a vertical lens.
- Look through the pressed lenses.
As shown in Figure 4.5, the outer lens will block all vertically aligned photons, while the inside lens blocks the horizontal ones. The result is that no light completes the journey beyond the second lens.
In practice, some light may make it through. This could be due to flawed polarizing material in the lenses, or because the lenses are not exactly 90 degrees (perpendicular) to each other.
Experiment 2
Polarization is not as simple as the previous experiment would lead you to believe. For example, place a third lens between the two lenses in the first experiment. Now rotate the third lens and watch what happens. Colorado University has an in depth explanation of polarization.
