Midi-chlorians were intelligent microscopic life forms that lived symbiotically inside the cells of all living things. When present in sufficient numbers, they could allow their symbiont to detect the pervasive energy field known as the Force. Midi-chlorian counts were linked to potential in the Force, ranging from normal Human levels of 2,500 per cell to the much higher levels of Jedi. The highest known midi-chlorian count belonged to the Jedi Anakin Skywalker (over 20,000 per cell), who was believed to have been conceived by the midi-chlorians.
Midi-chlorian counts were measured through a blood test; the Jedi used this method to locate Force-sensitive children before their Order was purged by the Galactic Empire. With the rise of the Empire, research into Jedi and the Force was banned, and though midi-chlorians continued to be tested for, sometimes by the Empire itself to root out hidden Jedi and other Force-sensitives, knowledge of them was diminished and inquiries into them were branded as illegal medical research. Midi-chlorians were only rediscovered after a New Jedi Order was founded.
When not forbidden, studies of midi-chlorians occurred among those who could master the Force and those who could not. While medical teams worked to understand the relationship between midi-chlorians and the Force, Jedi healers performed their own studies of the organisms. Even more esoteric studies were conducted by the Dark Lord of the Sith Darth Plagueis, who discovered a way to manipulate the midi-chlorians to create new life.
Midi-chlorians were intelligent microscopic life-forms that served as organelles within all living cells, existing in a symbiotic relationship with the beings they inhabited and comprising a collective consciousness amongst themselves. Present in all life, midi-chlorians were isomorphic on every world that supported life. Midi-chlorians, in fact, were necessary for life to exist. They also allowed for a connection with the pervasive energy field known as the Force; in sufficient numbers, midi-chlorians could allow their symbiont organism to detect the Force, and this connection could be strengthened by quieting one's mind, allowing the midi-chlorians to "speak" to their symbiont and communicate the will of the Force. Counts as low as 2,000 midi-chlorians per cell provided no sensitivity to the Force; an average Human had less than 2,500 per cell, while a mildly Force-sensitive being such as Nova Stihl had a count in excess of 5,000. Jedi had especially high midi-chlorian counts, and of them Anakin Skywalker was said to have the highest ever recorded at over 20,000, even higher than the powerful Jedi Master Yoda. Though Skywalker later lost a good deal of his organic body, his cells continued to teem with midi-chlorians.
The magnitude of the midi-chlorian count served as a measure of one's potential in the Force, though there were other inheritable characteristics that could influence Force ability as well. Indeed, though Force ability often meant a high midi-chlorian count, it was not always the case. While some medical theories postulated that the midi-chlorians created the connection between more macroscopic organisms and the Force, some Jedi believed that, contrarily, the midi-chlorians were created by the Force to serve as the link between it and other life. Midi-chlorian counts did not indicate an upper limit to Force ability; the possibility existed for a Jedi to achieve a connection to the Force on an equivalent level to a Jedi with a higher count.
Midi-chlorians could be influenced by the Force to create new life, a technique developed by Dark Lord of the Sith Darth Plagueis. There was even a possibility that they could induce conception in a Human; Anakin Skywalker was believed to have been conceived by the midi-chlorians, with no biological father. This was considered impossible by many Jedi, however, and the idea received only shock when it was first proposed to the Jedi Council.
Though tested for, midi-chlorians did not usually figure into the medical treatment of most beings, and they were linked to no known pathology. Chemicals used in medicine, such as anesthetics, could thin the midi-chlorians; if it was vital they be preserved, a being could be operated on without anesthesia. The drug bota, on the other hand, was thought capable of boosting the effects of the midi-chlorians, enhancing an individual's Force-sensitivity. Blood rich in midi-chlorians could be transfused into an injured organism to keep it alive, but this would not transfer any of the blood donor's Force-sensitivity.
Midi-chlorians could be detected through a blood test by measuring their concentration in a being's red blood cells, though such tests were not perfect and were prone to fault. Midi-chlorian tests were part of a standard medical check-up, recorded in a blood analysis. From at least 1,000 BBY to 19 BBY, blood tests for midi-chlorians were the primary way by which the Jedi Order found Force-sensitive children. Some Jedi comlinks could have midi-chlorian samplers attached to them to measure blood samples, though these results would need to be interpolated through a computer, such as that found aboard a ship, to get a count. Darth Vader had an entire blood-testing system in his meditation chamber, which he not only used to test for infections, but also measure his midi-chlorian count.
While most research on midi-chlorians was conducted by Jedi healers, the connection between midi-chlorians and the Force was studied in non-Jedi medical circles as well, with theories advanced as to the relation between them. On Vjun, high midi-chlorian counts were a mark of status, possessed by the more prestigious families. As a result, interest in the midi-chlorians remained high on that world.
Experimentation on midi-chlorians happened occasionally as well. Nobles on Vjun conducted forbidden experiments on the life-forms in an attempt to increase Force sensitivity, the results of which drove the population of Vjun insane. Darth Plagueis, Dark Lord of the Sith, sought a way of influencing the midi-chlorians with the Force to create new life from nothing, and he may have succeeded before his death at the hands of his apprentice.
The Force Detector was one of the tools developed to measure midi-chlorian counts. Based on plans of the scientist Jenna Zan Arbor, the Force Detector consisted of a pair of paddles connected to a control pack; the paddles were placed around the subject, at which point the control pack would read out a list of statistics, including midi-chlorian count. It also generated a blue nimbus around a wireframe hologram of the subject; though this correlated to Force-sensitivity, it was not a direct measurement of the subject's midi-chlorian concentration.
Before the creation of the Jedi Order around 25,783 BBY, a number of Force sects including the Followers of Palawa, Chatos Academy, and Order of Dai Bendu studied the Force scientifically, including the science of midi-chlorians, on worlds from Had Abbadon to Ondos. Circa 3,956 BBY, the Rakata, a species that had almost lost all trace of Force-sensitivity, conducted their own scientific inquiries to find a genetic basis for the Force, which they hoped would allow them to become Force-sensitive once again. How the Jedi detected Force-sensitives in their early millennia was lost to time, but shortly after the Battle of Ruusan in 1,000 BBY, midi-chlorians were either discovered by them for the first time, or rediscovered after having lost knowledge of them in an earlier era. Blood tests for midi-chlorians were instituted to find recruits with a connection to the Force, a method used by the Jedi for almost a thousand years until the purge of the Jedi in 19 BBY. In an event that would have repercussions a very long time in the future, a prophecy was made in these early days of a Chosen One who would bring balance to the Force and was said to have a high concentration of midi-chlorians.
On the world of Vjun, a high midi-chlorian count was synonymous with status, leading to a surge of interest in midi-chlorian phenomena on that planet. When Vjun came into contact with the Galactic Republic and Jedi Order, the most eminent families—those with the highest counts—sought to protect themselves from Jedi recruitment by augmenting their own abilities. The seventeenth Viscount Malreaux of the esteemed House Malreaux, along with a consortium of others, worked to genetically manipulate the midi-chlorians and thereby enhance the counts of Vjun's population. They were successful, but at a cost; when the entire planet became Force-sensitive with no mental training to handle it, the populace went mad and murdered each other.
In the decades before the Battle of Naboo, Dark Lord of the Sith Darth Plagueis commenced his own experiments with the midi-chlorians. Plagueis sought to develop a means of creating life from nothing by using the Force to directly manipulate the midi-chlorians found in living beings' blood. Plagueis believed that any being created in this manner would be the living embodiment of the Force; when he informed his apprentice Darth Sidious of his plans, though, Sidious realized that Plagueis intended to create his replacement and, shortly thereafter, killed his master. Before dying, Plagueis may have used his technique; Sidious, however, did not learn how to influence the midi-chlorians to do his bidding.
In 32 BBY, Jedi Master Qui-Gon Jinn was stranded on the planet Tatooine, where he encountered the slave boy Anakin Skywalker. Jinn sensed the Force strongly in Skywalker, and when he learned from the boy's mother Shmi Skywalker that Anakin had had no father, Jinn realized that there was something special about him. That night, Jinn took a blood sample from a scrape on Skywalker's arm and tested it, telling the boy that he was checking for infections. Jinn had his Padawan Obi-Wan Kenobi run a midi-chlorian test on the sample; Kenobi was surprised to find Skywalker's count was off the chart—over 20,000, exceeding that of even the powerful Master Yoda, or any other Jedi. Jinn later brought Skywalker to Coruscant and took him before the Jedi Council. There he informed them of Skywalker's unusually high midi-chlorian count and stated his belief that the boy had been conceived by the midi-chlorians. Jinn was referring to the prophecy of the Chosen One; while the Council recognized the reference, they did not agree with Jinn's conclusion, although some Jedi nonetheless believed Skywalker had the potential to become the most powerful Jedi ever. The lore of the Sith Lords claimed that Skywalker's conception had been a result of Plagueis' manipulations of the midi-chlorians.
By these latter years of the Republic, midi-chlorians were a matter of public knowledge, though some found the idea of them to be unclear. In 22 BBY, the Spotts TradeChip Company released a set of tradechips of famous Jedi Knights, featuring data, including midi-chlorian counts, garnered from what the company called the "best information available." However, the chips attracted criticism from the Order for their inaccuracies, as did the midi-chlorian counts provided; Yoda's, for example, provided the wildly inaccurate number of four million. Spotts defended its product, claiming that all mistakes were minor.
Later that year, Count Dooku, leader of the Separatist movement, revealed to the Jedi that Plagueis' apprentice Darth Sidious secretly controlled the Galactic Senate. Seventeen months later, the Jedi Ronhar Kim conceived of the idea to test the midi-chlorian counts of the senators, believing that the Force-sensitive Sith would be revealed in this way. Kim approached Supreme Chancellor Palpatine with his plan, but Palpatine, who secretly was Sidious, discouraged him, advising him that it would be politically impossible. Upon learning that Kim had not spoken of his idea to the Jedi Council, Palpatine arranged for the Jedi to be killed in a trap, and Kim's plan was never implemented.
In the closing years of the Republic, studies of midi-chlorians were conducted by Jedi healers and medical personnel alike. During the Clone Wars, Republic defector Ko Sai was one of those scientists who sought to uncover the mysteries of the midi-chlorians and Jedi genome. The Jedi Order in its later years established a taboo over empirical studies into the Force and its tangible aspects, believing it could not be scientifically studied. When the Order was disbanded and Galactic Empire declared, though, the taboo was broken. The scientists of Emperor Palpatine treated the Force as a purely physical phenomenon, breaking it apart into the tangible components of midi-chlorians and energy. The Force Detector was one of the results of these studies, and it was used by the Inquisitorius in the Great Jedi Purge. During the reign of the Empire, midi-chlorian tests were performed routinely on Inner and Mid Rim worlds to root out Force-sensitives and Jedi in hiding. Such individuals were rarely heard from again. In response, an underground trade of drugs and blood treatments sprang up that could supposedly fool a test or lower one's count; however, they were largely ineffective.
After the rise of the Empire, Palpatine ordered a complete ban on all information related to Jedi or the Force and had such data purged from data banks galaxy-wide. Statute OB-CPO-1198, covering illegal medical research that included studies of midi-chlorians, was also enacted. With the Jedi's records on midi-chlorians no longer available for study and any information on their relation to the Force kept out of medical libraries, medical professionals knew little of the nature of midi-chlorians, even as they continued to test for them. When Kornell Divini, a doctor on the Death Star, measured an abnormally high midi-chlorian count from one of his patients, he put out a request on the MedNet for any further information on midi-chlorians. Divini's search triggered warning flags put in place by the Empire, attracting the attention of the Emperor's servant Darth Vader, and Divini was arrested under the provisions of OB-CPO-1198.
When Luke Skywalker established his New Jedi Order in 11 ABY, he had no knowledge of midi-chlorian tests and as a result was forced to devise his own tests for finding Force-sensitives. The method was rediscovered by the Order at some time before 40 ABY.
Midi-chlorians were first conceived by George Lucas as early as 1977. In this time the first Expanded Universe products were being created, including the ongoing Marvel Star Wars series and Alan Dean Foster's novel Splinter of the Mind's Eye. Lucas sat down with a member of his staff, Carol Titelman, to dictate a number of guidelines for these works, explaining various concepts of his universe. Amongst them were an explanation of midi-chlorians, which Force-sensitive beings were said to have more of in their cells. However, Lucas did not feel he had the time to introduce the concept of midi-chlorians. The idea would not appear in any Star Wars product for twenty-two years; Lucas chose 1999's Star Wars Episode I: The Phantom Menace as the opportunity to first mention the midi-chlorians, explaining why some were sensitive to the Force while others were not, an issue that he had left unresolved since the original film Star Wars. Lucas incorporated the explanation of midi-chlorians into the film as part of Anakin Skywalker's journey towards understanding the Force. That Lucas had planned the midi-chlorians as far back as 1977 was hinted at on the DVD commentary of The Phantom Menace, but the details would not be fully revealed for another eight years, coming to light in 2007's book The Making of Star Wars: The Definitive Story Behind the Original Film, the hardcover edition of which had, amongst its appendices, Lucas' notes for the Expanded Universe authors.
Midi-chlorians in The Phantom Menace are part of a recurring theme throughout the movie, that of symbiotic relationships. They were loosely based on mitochondria, organelles that provide energy for cells; like midi-chlorians, mitochondria are believed to have once been separate organisms that inhabited living cells and have since become part of them; even now, mitochondria act in some ways as independent lifeforms, with DNA all their own. Lucas established this relation with mitochondria, in particular the necessity of midi-chlorians for life to exist, as a metaphor for society; namely, he says all parts of society must get along in much the same way the midi-chlorians and their symbiont do. In 2006—as a tribute to this similarity—a newly discovered species of bacteria residing within mitochondria was named Midichloria mitochondrii after the midi-chlorians. Its discoverer, Nate Lo, wrote to Lucas requesting permission to use the name, and was granted it.
Though midi-chlorians would not make a public appearance until 1999, pre-Phantom Menace Expanded Universe materials hinted at an individual's biological connection to the Force. In Jedi Search, Lando Calrissian carries out a search for potential Jedi for Luke's new academy by using a device that can detect affinity to the Force. In The Thrawn Trilogy, two organisms are mentioned, the vornskrs and the ysalimiri, that have evolved the ability respectively to use the Force to hunt prey and block the Force.
On April 1, 2006, several entries relating to George Lucas' Willow universe were added to the StarWars.com Databank as an April Fools' Day joke. The update page names the planet Andowyne as the possible homeworld of the midi-chlorian.
The non-canonical comic Tag & Bink: Revenge of the Clone Menace tells of other effects of Darth Plagueis' manipulations of the midi-chlorians: Tag Greenly and Bink Otauna became Force-sensitive, while a red sun and its planet, whose only survivor would go on to become a hero on Earth, was destroyed—references to Krypton and Superman, respectively.
In the introduction page to Star Wars Tales 10, Dark Horse Comics editor Dave Land goes inside a Jedi Knight with the help of droid 2-1B in search of midi-chlorians. He finds they look like assistant editor Philip Simon, then orders them back to work.
Midi-chlorians have had some negative reception amongst fans; some see them as adding hard science to the alleged "mysteriousness" or spirituality of the Force and dislike what they see as a new concept. Others, however, believe that having a physical aspect to a mystical Force calls upon real-world religious traditions, feeling that the mythic qualities of the Force have been strengthened by midi-chlorians. A misconception exists that rather than being indicators, midi-chlorians actually are the Force or create it; there is no canonical basis for this belief. Steve Perry, who used midi-chlorians in his novel Death Star, opined that they were "less than inspired." George Lucas, on the other hand, considers the two aspects of the Force separately, treating the midi-chlorians as the practical, biological side, distinct from the spiritual and metaphysical side of the Force.