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I’m bored so I thought I’d attempt an explanation of plasma colors in Halo canon.
Plasma bolts (projectiles)
One of the most common covenant weapons is the Plasma Rifle, which fires blue bolts of plasma. But why blue? -And why do the other weapons have different colors?
For the uninitiated, you should know that Plasma is the fourth state of matter, (after solids, liquids and gasses). Plasma exists naturally in all stars such as our sun, and can be created by simple human technology, such as arc-welding equipment.
Bungie showed great attention to detail by putting an electrical arc at the muzzle of each plasma weapon. It could canonically function similarly to the pilot flame on a human flamethrower, i.e. a catalyst to excite atoms in the plasma weapon.
It has been theorized that the plasma weapons create electromagnetic fields to contain and propel the plasma, And although this might make sense theoretically, I won’t go into the details of that. The inner workings and mechanics of the weapons can stay a mystery for now.
Our experience as humans gives us some intuition about the colors of light, but we need to make some distinctions to avoid making the mistake of comparing plasma to fire. A common mistake is to believe that a “red is less hot than yellow” or “blue is the hottest color”.
The reason this is the case with fire, is because the red-orange-yellow part of the fire is blackbody radiation emitted by the soot in the fire. Your body also emits blackbody radiation, but you need a thermal camera to see it.
However, the blue part of the flame is glowing because the hot atoms move from low- to high-energy states. When the atoms change back down to low-energy state, they release distinctive quantities (quantums) of energy, in the form of photons (light). The light emitted depends on the nature of the element, and thus different kinds of atoms will produce different colors of light. This is why burning copper can produce a greenish flame. Scientists use these physics to measure the composition of interesting materials, for example in metallurgy to analyze an alloy, or in astronomy to analyze the composition of stars.
More on this:
https://en.wikipedia.org/wiki/Emission_spectrum
https://en.wikipedia.org/wiki/Flame_test
https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/worksheet-specgraph2-sol.html
Stars emit light because of the fusion reaction occurring in them. Stars are plasma, and apart from the ongoing fusion reaction may serve as a good comparison for the plasma bolts seen in the Halo franchise.
In a given element, the distinctive quantum structure of the atom will release energy of a given pattern, in the form of light of a given wavelength. However, it is important to note that this does not say anything about the intensity of the energy. Thus, we can not gauge energy content or heat by the color of plasma.
This brings us to the actual Halo franchise, with the blue plasma bolt of the plasma rifle. Why is it blue? I’d say it’s first and foremost because blue is an ‘alien’ color, and fits the design language of the early covenant, but from a canonical point of view, I’ll hypothesize it is the most efficient energy-delivery system that the engineers (of several species) could put into a compact weapon platform.
It makes a lot of sense, then, that the Brute plasma rifle fires approximately the same bolts, but they glow a different color. During the story of Halo 2, the Elites were betrayed by the Prophets in favor of the Brutes. With the elites being given a cold-shoulder in the covenant internal affairs, it makes sense for infrastructure and logistics of their material flows to take a hit. At this point, the covenant is a massive multi-species allegiance in the midst of all-out intergalactic war with the humans. Much needed weapons are being mass manufactured, and suddenly the supply chain changes. In addition, the customer changes.
The original blue plasma rifle could be charged with energy and Xenon as the energy carrier/substrate. The wavelengths emitted by excited xenon-atoms create a distinctive blue color.
At the point of the Prophets betrayal, they could have easily shifted their supply chain towards other elements, like Neon or Cadmium, which can release a red color. -So it could have simply been by coincidence (canonically) that the brute plasma rifle fires red bolts.
To suit their tempers and battlefield habits, the new rifles were also made with a faster rate of fire, but were more easily overheated. As far as I know the individual bolts of blue or red plasma do the same amount of damage, which makes sense as explained above.
Now what about the Plasma Pistol? Well the plasma pistol is different for two reasons. Firstly, it has to be more economical, because it is the weapon given to less important troops like Grunts and Jackals (Jackals are somewhat proficient, but I understand them to be mercenaries? Please get back to me on that). And secondly, it has to be capable of firing a charged-up shot designed to eradicate energy shields. This presumably means the energy carrier that becomes the plasma has to be of a different composition. I hypothesize that the covenant uses a copper-based compound to create this plasma. Because copper is also highly conductive, potentially lending some credibility to the idea that charged Pistol shots can carry electromagnetic pulses over long distances. Copper releases green light by the mechanisms described above.
(Continued beneath).
