Red lightsabers are the weakest, according to a Star Wars study conducted at the University of Leicester regarding lightsaber colors. Decades of Star Wars lore has dictated that the red lightsabers favored by The Sith offered greater offensive capabilities than the softer colors favored by The Jedi. This also seems sound logically, given that The Dark Side of The Force is dependent upon aggressive emotions. It follows then that Dark Jedi would desire a weapon catering to a more offensive combat style, favoring powerful strikes over finesse.
History and common sense, however, have taken a backseat to hard science. A study at the University of Leicester's Centre For Interdisciplinary Science explored the question of lightsaber strength relative to color. Surprisingly, their findings indicate that the red lightsabres favored by The Sith are physically the weakest of all lightsaber colors.
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Luke Willcocks determined this by first calculating the strength of Qui-Gon Jinn's green lightsaber. Utilizing the scene in The Phantom Menace where the character (played by actor Liam Neeson) used his weapon to cut through a metal door as a base-line, Willcocks calculated the energy output required to accomplish this task. Presuming the door to be made of materials of comparable strength and density to those of an Earth space shuttle, Willcocks' determined that a standard lightsaber produces an energy output of 6.96 MW (megawatts) - two magnitudes of power lower than the output of a small nuclear generator!
How then does color factor into the power output? Presuming that lightsabers produced pure photonic energy, Willcocks calculated the spectral wavelength of each primary and secondary color and the effect that changing the wavelength to yield a specific color would have upon the power output. Willcocks concluded that shorter wavelengths would result in a more powerful photon blade and that purple lightsabers would be the most powerful of all. Willcocks' calculations suggest that the purple lightsaber of Jedi Master Mace Windu (played by Samuel L. Jackson) would be nearly twice as powerful as that of a Sith Lord like Darth Vader, capable of burning through a starship blast-door in 8.2 seconds.
It is worth noting that Willcocks' study was partly conducted in the sense of fun. The University of Liecester has a Science Module that encourages aspiring scientists to attempt to explain scientific phenomena in popular culture through their writings. The goal is to teach students how to write, critique, and publish serious scientific papers, while also giving them experience in writing in such a manner as to be understood by non-professionals. While Willcocks' study is fascinating, it is not considered canon by The Powers That Be At Lucasfilm.
Despite this, Willcocks' theory as to the inherent weakness and instability of Sith lightsabers is born out by some recent Star Wars stories. The novel Ahsoka revealed that Jedi must bond themselves to a special kyber crystal, which forms the basis of their lightsaber. Dark Jedi are unable to forge this bond and must either steal another Jedi's lightsaber or take it from their dead body, before forcing their will upon the crystal and bending it to serve them. This process weakens the crystal, causing it to "bleed", resulting in its red coloration. It has also been determined that Kylo Ren's lightsaber uses a cracked kyber crystal and the cross-guard his lightsabre generates is necessary to vent excess energy and prevent the hilt from exploding. One wonders what Star Wars: The Last Jedi will have to say on this subject.
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Source: The University of Liecester
