D-Mail

D-Mails (short for DeLorean Mail) are messages that are sent to the past. They can be used to cause the active world line to change by contradicting historical events. To quickly summarise the technology: D-Mails are sent using the Phonewave (name subject to change), an experimental combination of a microwave and a phone that happens to produce mini black holes. A large CRT donates electrons to the black holes, increasing their spin enough for them to become naked singularities. A message passed through the singularity travels to the past, with the microwave timer controlling how far it travels. However, the size of the singularities produced restricts the length of the message that can successfully be sent to 36 bytes, plus metadata.

Most noticeably, D-Mails change which world line the active one is, to one consistent with the arrival of the D-Mail. In practical terms, this can be achieved by selecting an inactive world line which already contains a historical event with similar effects to the D-Mail’s arrival, then overwriting that one cause with the D-Mail’s arrival.

Explanation With an Example
Consider the D-Mail Okabe sent himself with the winning LOTO SIX numbers. The initial world line is one where Okabe did not feel like entering LOTO SIX at the time, but there should also exist inactive world lines where he did decide to enter. The D-Mail forces one of these other world lines to become active, but with the inspiration for entering LOTO SIX on that world line changed from being Okabe’s own initiative to a mysterious mail arriving. "Titor: 'Just judging by the phenomenon you described, I would say what you accomplished is indeed changing the past. I suspect that the mail you sent into the past changed the world line’s divergence, if just a little. BRAVO! The instant your mail arrived in the past, I think you were moved from your original world line to an ever-so-slightly different one. In this world line, the world’s supposed past became this: You learned the Loto6 winning numbers from a mail you got from your future self, told your friend the numbers, and your friend mistook a number upon buying the ticket. So the mail probably disappeared from your sent history when the world line changed.' — Chapter 3, Steins;Gate"We find ourselves proposing the idea that time travel changes the events on a world line, but only in such a way that the following events are unchanged. This agrees nicely with the TIPS definition of a world line, which states that "no matter how much you change the past along a single world line, the result will converge to the same outcome". In fact, D-Mails do more than alter the one world line. After two D-Mails are sent, it is generally found that both have arrived on the most recent world line. The implication is that the first D-Mail, when sent, arrived on both the new active world line and at least one inactive world line, which later became active as the result of a different D-Mail. The simplest rule to explain the behaviour is this: Each D-Mail arrives on every inactive world line on which it has no new effect. The one of these world lines most similar to the sender’s world line becomes the new active world line. Inactive world lines are initially independent, but the overwriting of ordinary events with time travel arrivals creates apparent interconnections across the world line structure. In this way, evidence of earlier time travel can accumulate even on inactive world lines, so it is possible for a newly activated world line to already contain time travel events in its past or future.

No Need to Move the Position of the Present
Following the above mechanics, there is also no need for a D-Mail to move the position of the Present. The events of the new active world line are already fully determined, without having to repeat a period of history. Multiple lines of evidence support this conclusion:


 * First, Suzuha refers to the world’s "supposed past" changing, rather than a new past genuinely being experienced.
 * Second, at no point do D-Mails cause new time travel to take place prior to their sending time. If the period of time following their arrival became active, we would expect new D-Mails to sometimes be sent within that period, appearing like a chaining effect to Okabe. This never happens, so it is reasonable to say that this period of time remains inactive.
 * Third, Okabe experiences Reading Steiner at the moment of world line reconstruction, and that takes place in the Present, when the D-Mail is sent. There is no record of Reading Steiner ever occurring at the time of a D-Mail being received.

D-RINE
On many world lines in the Beta Attractor Field, Itaru Hashida would eventually develop D-RINE, which function similarly to D-Mails but are instead sent via the text messaging app RINE. This would prevent SERN from identifying the messages sent to the past with ECHELON, which would in turn prevent the Attractor Field from changing into an Alpha Attractor Field.

Trivia

 * Itaru Hashida was the one who coined the term DeLorean Mail (while in the anime adaptation, it was Mayuri Shiina), naming it after the model of the car equipped with the time traveling machine in the movie trilogy Back to the Future. Rintaro Okabe suggested a number of names for the mail, but it was Kurisu Makise who shortened DeLorean Mail to D-Mail.
 * As well as being a reference to Back to the Future, the term D-Mail is also likely a reference to DI-Swords from Chaos;Head.