Photon
Classification: boson
Fundamental: yes
Mass: massless, as far as we can tell
Interactions: electromagnetic
Spin: 1
Lifetime: stable
To conclude our Christmas tour of particles, there is one more force carrier we must meet. The strong force has its gluons, the weak force has its Ws and Zs, gravity even theoretically has its gravitons, and the electromagnetic force has its photons. These massless packets of energy interact with all charged matter and carry around electromagnetic fields. Since they are massless, photons are very easy for matter particles to emit and absorb, which is why all charged matter always interacts electromagnetically with its surroundings. Since photons are not self-interacting, they can travel away from the particles that create them, allowing the electromagnetic force to be observed both micro- and macroscopically. This makes photons the only vector boson that physicists can detect directly.
In everyday occurrences, photons are what makes up light, or all forms of electromagnetic radiation. The visible light we see by, the infrared heat we feel, the microwave radiation we use to heat food, the ultraviolet radiation that gives us sunburns, all are from photons. The apparent difference in behavior is due to the different amounts of energy carried by the photons involved.
The study of light in some ways has an even longer history than the study of fundamental particles. Sir Isaac Newton developed theories that treated light as a stream of particles, which geometric or ray optics are based on. For macroscopic applications like figuring out how lenses work, geometric optics describe the behavior of light extremely well, and students still learn how to draw geometric wave diagrams. But Newton's theories of light didn't hold for too long, because it was found that light also bent around corners and interfered with itself, which are behaviors of waves, not of particles. Light acts like a wave when it interacts with things about the same size as its own wavelength. Treating light as a wave describes all sorts of interference and diffraction effects, and when Maxwell's equations of electromagnetism predicted electromagnetic waves that traveled at the speed of light, the physics community hailed this as a great unification of different fields. Light was an electromagnetic wave.
But that theory didn't hold up either as the quantum nature of matter began to appear in experiments. Take the black body problem, for instance. A black body is an ideal object that absorbs all energy that hits it and emits energy in a continuous spectrum dependent on the object's temperature. In the late nineteenth century, Max Planck was studying various experimental measurements of black body-like radiation and trying to come up with a mathematical way of describing them. Other physicists had tried this, but had run into a problem: if the energy emitted by a black body is continuous and can come in infinitesimally small amounts, what stops the black body from emitting an infinite amount of energy, which is physically impossible? Planck eventually took a statistical approach to the problem to create a formula that matched the experimental results. The formula worked, but it implied that matter could only emit energy in discrete amounts. Planck didn't like it.
This was in 1901, and well, the quantum nature of matter was being apparent, so the grumbling wasn't too bad. But in 1905, Einstein hypothesized that light really was quantized and intrinsically came in packets with specific energies. He did this to explain the photoelectric effect, or why electrons kicked out of metals by light always have energies that can be related to the wavelength of the incident light but not the amount of light hitting the surface. This theory was not popular; Planck himself said accepting this theory would throw out all the progress made by the previous century of work on electromagnetism.
In 1923, Arthur Compton published a paper explaining the Compton effect, or how the momentum of electrons interacting with light changed. He could describe the changes in terms of particles bouncing off each other, not as particles interacting with waves. At the same time, he published measurements of the wave-like properties of x-rays. It took another fifty years to completely put the issue to rest, but light really is made up of photons, wave-particles that carry discrete bits of energy. While the electron is the matter particle that makes life as we know it happen, electrons do their work by exchanging photons. We need them, too.
As Christmas in my family is a time to remember and appreciate those who bless our lives, a time of stars and candles, I chose to finish with light. Merry Christmas!
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