Reading Steiner

Reading Steiner is the name that Okabe gives to his rare ability to remember the events of other world lines. In theory, when a world line shift takes place, everything is reconstructed to match the new active world line, including people’s memories. However, Okabe’s mind is seemingly an exception to this rule, allowing him to retain knowledge of the world’s genuine history.

This quality is also not entirely unique to Okabe. Many more people, perhaps everyone, can remember other world lines to some extent. These memories manifest in dreams, or feelings of deja vu.

Full Reading Steiner
Strictly speaking, what Okabe calls Reading Steiner is the particular sensation he feels when a measurable divergence change takes place. According to his narration, this is:

'Slight vertigo. As if my feet are off the ground. My vision blurs as if I had been thrown into the ocean. Everything in sight turned monochrome. Did the ground shake? Physiological feelings of fear. Instinctive feelings of fear.'

''Mayuri: "Okarin? What’s wrong?" '''Mayuri’s calling voice snaps me to my senses. I notice that color has returned to the world.' — Chapter 3, Steins;Gate

There are other points where Okabe retains his memories, but does not feel this sensation, such as when a change to the past is too small to alter the divergence meter reading.

The Reading Steiner sensation takes place when the world line changes in the Present, as a result of a D-mail or present day change, so coincides with the moment of world line reconstruction. Its effect is to maintain Okabe’s present memories, effectively copying them onto the new world line.

In terms of the larger world line structure, Reading Steiner also makes changes to inactive world lines. Just as a D-mail or present day change arrives on all inactive world lines that are consistent with it, so too do Okabe’s memories from the moment of making that change. The D-mail arrival and Reading Steiner arrival are inseparable. Those modified world lines will evolve as time progresses, although being inactive, no divergence-changing events will take place.

This explains how characters on a certain world line can recognise Reading Steiner as something that Okabe has displayed previously, even though that event happened prior to the world line ever becoming active.

Déjà vu
Everyone has the ability to remember other world lines to some extent, which manifests as dreams or feelings of deja vu. "Okabe: 'I thought memories couldn’t be shared between world lines. I thought my Reading Steiner was special. But maybe that isn’t true. Maybe everybody has the same power as me, but to varying degrees. It might even be to the point where it can’t be called a power.' — Chapter 10, Steins;Gate"Precisely what triggers this is not specified. Often, deja vu memories are of events that a person experienced on a previously active world line, but forgot due to reconstruction. (For example, female Luka remembering being male.) It is also possible for people to recall events that happened to other iterations of themselves, from before time travel moved the present back and led to different events. (For example, the Okabe of the Steins;Gate 0 anime remembering the USSR world line experienced by an earlier version of himself.)

Other Manifestations
As mentioned previously, there are other cases of memory retention between world lines that do not cause particular sensations. One instance happens when a world line shift takes place, but its divergence change is too small to register on the meter. Here, Okabe retains his memories of the previous world line, but does not feel the shift taking place.

Physical time travellers are another such case where memories are retained, but the specific Reading Steiner sensation is not felt. This is because objects moving between world lines, like D-mails or time machines, are not subject to the reconstruction that their arrival triggers. "Titor: In the case of physical time travel, memories are conserved even if the world line changes. That’s been proven by my very own experience. — Chapter 3, Steins;Gate"It is also notable that Okabe does not experience Reading Steiner when time leaping, or when he returns to the moment that a time leap was sent from. If time leaping acted identically to D-mails, then we would expect the current Okabe’s memories to be overwritten by the Okabe who pressed "send" on the time leap. But this does not happen. The universe recognises the Okabe who arrives in the past as the genuine one, supporting the idea that unlike D-mails, time leaping does move the Present.

A unique type of memory retention occurs in Luka’s ending. Here, Luka uses the time leap machine to change the world line by a small amount, causing a divergence shift in the past from our Okabe’s perspective. Instead of experiencing the usual Reading Steiner sensation (or nothing), he experiences a sensation like a time leap.

Okabe: My entire body goes through shock and chills, just like it always does right after time leaping.

“...This is...”

''It’s not like I time leaped myself. Nevertheless, I teleported from the lab to Yanagibayashi Shrine. Did the world line change...? No, Reading Steiner didn’t activate, so there’s no way the world line changed. —'' Chapter 8, Steins;Gate

Luka’s time leap must move the location of the Present back, so this is the only example in the story where our Okabe is not around at the moment of world line reconstruction. Other instances of Reading Steiner could be described as Okabe’s experience of his surroundings being reconstructed. But in this case the reconstruction took place some time earlier, and all that happens from Okabe’s perspective is that his memories of the previous world line overwrite his memories on the new one.

Trivia

 * The movie Steins;Gate: The Movie - Load Region of Déjà vu features Reading Steiner as an important plot element, as Kurisu and Mayuri use recollection of the pre-Steins Gate world lines to piece together information about Okabe.
 * In popular culture, the Mandela effect is similar to Reading Steiner.