The Mobius Double Reacharound Virus is a ~ATH program which Sollux wrote and shared with Karkat. This code, when executed, immediately causes the user's computer to explode and places a curse on the user forever, along with everyone he knows and everyone he'll ever meet.
Examining the source code, the program appears to split itself into two interconnected programs, one red and one blue ("this" in programming refers to the program that code is in). It then imports two universes, one red and one blue. What follows is inscrutable: the indentation and coloration suggests that the ~ATH blocks overlap in a manner that is entirely unlike any normal programming language.
According to Andrew Hussie, the program represents the relationship between the trolls' and kids' universes:
- “Sollux's shades, the ~ATH code, etc help illustrate this, that [their universes] are bifurcated, interwoven realities.
Even Act 5 is a microcosm of this idea. It is a bifurcated act. Act 5.1 is the troll half. Act 5.2 is the kid half. 5.2 even begins with two sets of curtains, red and blue. Blue symbolizes the troll universe, red symbolizes our universe. First we crossed through blue, then red.” - — Andrew Hussie
The Homestuck map
for Act 5 mirrors this idea, showing fragments of the code as subheadings for various parts of the act, and beginning the act with “bifurcate Act_5 [”“Act_1”“, ”“Act_2”“];”
The map also shows that the code of the virus does not follow the standard red/blue color relationship. Although, as Hussie states above, blue generally represents the troll universe
and red, our universe, the heading of Act 5, “bifurcate Act_5 [”“Act_1”“, ”“Act_2”“];” has Act 5 Act 1 (the troll half of Act 5) written in red and Act 5 Act 2 (the kid half) in blue. In addition, the subheading of Act 5 Act 1 is “import universe U1;” and the subheading for Act 5 Act 2 is “import universe U2;”. Thus, in the code of the virus, “U1” is Universe A, the troll universe, and “U2” is Universe B, our universe.
A possible interpretation of the code: The red code waits for the death of U1 (“~ATH(U1)”), then waits for the end of the blue code (“~ATH(”“THIS”“)”). The blue code waits to until the death of "!U2." In programming, "!" indicates negation, or "the opposite of," which could be interpreted to mean "the death of U2's nonexistence", i.e. "the creation of U2." The blue code then waits for the red code to end (“~ATH(”“THIS”“)”).
However, since the first blue loop begins within the loop of the red code, but ends outside it, and also contains within its first loop the closing line of the red loop, the lines are twisted in such a way that the red code cannot finish executing until the blue code is over, which can only end once the red code has ended, apparently defying logic. The only thing that is clear is that the program cannot finish running until U1 dies and U2 is created.
The fact that each universe contains the other in its loop illustrates to the interconnection of both universes - and how that interconnection leads to their ultimate simultaneous destruction during Cascade. In reference to this, the map's subheading of Cascade is, indeed “[”“THIS”“, ”“THIS”“].DIE();” Therefore, a related interpretation of the code is that the intertwined program blocks are only "setup"; the final line forces the code to run, which would require a paradox. Other instances of such paradoxes in Homestuck are known to result in doom for all involved.
In a fit of stupidity, Karkat runs the code, blowing up his computer. “Karkat and his friends and everyone they would ever meet thereafter would experience great misfortune on account of the curse unwittingly implemented through Sollux's esoteric MOBIUS DOUBLE REACHAROUND VIRUS.” It is also implied that this curse led to the subsequent death of all of the trolls' lusii.
Kanaya and Aradia believe that the virus may not actually do anything other than blowing up Karkat's computer, and that their misfortune is a consequence of playing Sgrub, as well as the fulfillment of stable time loops.
Additional significance[]
In an alternate timeline, in which Karkat's computer had never exploded, Gamzee goes murderous much earlier (before the completion of Sgrub), and proceeds to kill all of the other trolls, with the exception of Aradiabot, who is technically already dead. She goes back in time to the alpha timeline to “influence events quietly to make sure [the explosion] happened” and prevent her offshoot timeline. Thus, this Aradiabot had a hand in the virus being run and/or the subsequent explosion. It is currently unknown in what manner she accomplished this, as she was not seen at that point in Hivebent.