Sigma Radiation (Σ or σ) is a form of energy or radiation that has been found to be present in each of the main cast's (Jeff, Evan, Vince and Steph) blood, as well as in a piece of cloth speculated to have come from Slenderman or Slenderman's suit.
Sigma Radiation was first mentioned in the Corenthal/Adam Roberts Notes which are the research notes of a physics student at The University of Florida who tested a scrap of fabric found by Dr. Corenthal outside the home of Milo Asher, a Slenderman victim. After testing the fabric the student found an unusual energy signature that the imager he used assigned the greek letter sigma (Σ) to describe. In the notes he described the fabric as a "scientific abomination" and could only further speculate that it contained trace amounts of carbon. He also noted that Dr. Corenthal asked him to design a device that could detect the unique radiation, which many speculate to be the Black Box Device.
After the Black Box Device was found, during the video "The Corenthal Connection (Jessie's back)" the cast was unsure what to do with it until Evan accidentally discovered that it reacted to his blood. According to the story relayed by Evan in the video "Sigma" he accidentally ran past the device during the events of "One step forward, two steps back," when he was bleeding from the Rake attack, and noticed that the device reacted to his presence. Later in the same video he tested both his own blood, and Alex's using the device and found that it only reacted to him. A few days later Jeff announced via twitter that everyone had tested themselves and that he, Evan, Vince and Steph all tested positive for the presence of Sigma Radiation according to the device.
In a comment on the YouTube page for the video "Sigma" Jeff speculated, "Apparently 'alpha' radiation is 'gentle' enough to be blocked by layers of skin or sheets of paper... reading over the physicist's notes (from the Florida university), it doesn't seem like this is 'normal' radiation. Personally, it seems like the branding of 'radiation' is simply the closet they could have gotten to naming it."
What the presence of this radiation means is uncertain at this point, but it has been speculated to be evidence for the Iteration Theory.
- It is possible that Sigma has the property of inducing fear in those exposed to it. From a neurological point of view, Sigma may affect the human brain in such a way that fear is the response it finds most appropriate.
- Dr. Corenthal seems to be able to sense Slenderman when he isn't physically present, possibly due to an ability to sense for Sigma. (This sense seems to be exhibited by the character of Karl in TribeTwelve as well, though that does not make it canonical to EMH.)