# What is the difference between a parallel universe and an alternate timeline?

"Star Trek" fans have labeled the new J.J. Abrams movie as being set in an alternate timeline. Alternate because it is different from the established history of the Star Trek universe.

The word alternate here also seems to imply that the original timeline must be continuing as it was, and the new timeline is a branch of the original.

Assuming the two timelines continue to move forward does this mean they are in fact parallel universes? Is that a reasonable wording?

Are parallel universes and alternate timelines the same thing or are they completely different concepts?

A parallel universe is a different universe that occupies the same timeline. However, the events that transpire in this universe are (usually) independent of all other universes. A different timeline is when something happens in a universe in different ways, at the same time. You can have multiple timelines in 1 universe, but not multiple universes in 1 timeline. That said, there is the (real) theory that every possible action has happened, and that a separate universe is created for each one: While walking, I choose to go left, and another universe is created where I choose to go right. This could be interpreted as both a multiverse and multi-timeline scenario.

• Provided the other universe supports timeline feature.. I mean, its not necessary that other parallel universe is similar to ours with 4 dimensional cosmos.. Commented Jan 28, 2012 at 3:45
• @Teknophilia: "You can have multiple timelines in 1 universe, but not multiple universes in 1 timeline" .... um... this is exactly what parallel universes are... Commented Jan 22, 2015 at 16:14
• This answer contradicts itself. You lead with "a parallel universe...occupies the same timeline." Then a few sentences later you say "you [cannot] have multiple universes in 1 timeline". Commented May 29, 2015 at 12:34
• Remember the TNG episode "Parallels"? Persons or things from parallel universes could be detected by looking at their "quantum signature". I suspect this would not be the case for alternate timelines. Parallel universes develop organically as different decisions are made in different manners with different results. Alternate timelines develop from temporal interference. Commented Jun 26, 2021 at 15:15

An alternate timeline is a type of parallel universe. You can have parallel universes that have nothing in common with each other; "alternate timeline" implies that they had the same history up to a point of divergence.

• Very simple, easy to understand answer. Short and to the point. Thanks Commented May 11, 2011 at 20:12

A parallel universe runs concurrent to another and shares the same timestream. That is, they develop things in the same pace according to the elements of existence but not necessarily with the same rate of technological or intelligence based advances.

An alternate timeline is a different take on a universe and is caused by a split or fork at one juncture or event to create this new existence.

You can travel between parallel universes because they share the same string of time but access it through a different plane of dimension.

You can't do that with alternate timelines since there is a point early on where they diverged and split apart. To get to the alternate timeline you need to travel back in time to reach that point and take the other route.

• this theory of not able to switch between parallel universes when they are alternate timelines won't make sense to me. do you have any reference or book which you are basing your theory on? Commented Jan 29, 2011 at 18:08
• I would add the parallel U's have many differences with no apparent single cause. They just coexist. Think Fringe. Bearded Spock. In contrast,alternate U's fork at one specific event. They may diverge wildly via chaos theory, but still it's that one event. George Mcfly punches out Biff, or not. Mcoy saves Joan Collins or not (TOS-City-Edge-Forever). In the JJ Abrams Star Trek, the destruction of Vulcan is that event. There is no jumping back and forth, I agree. Unless your are Q (TNG Tapestry). Commented Sep 26, 2016 at 2:16
• @Steve: "many differences with no apparent single cause. They just coexist. Think Fringe. Bearded Spock." - I see no basis for the assumption that the parallel universes on Fringe or on Trek's mirror universe episodes did not fork off from "our" universe at a single event. Commented Jun 26, 2021 at 5:38

Taking a more meta point of view: from within a story, whenever a "parallel universe" or "alternate timeline" are mentioned, it's usually something that will be woven into the plot to demonstrate alternative scenarios. In "Stargate" for example, they have cited both and there isn't really a specific rule they follow to differentiate one from the other; though it is fairly soft.

From the point of view of the reader/viewer, those concepts are more of a hand-wave to tie things together when they're similar, but have irreconcilable differences.

It's a difficult thing to state authoritatively because writers do sometimes use the terms interchangeably, as another answer mentioned with Stargate.

It seems like the difference is that parallel universes happen from natural divergences from things that belong in the timeline, while alternate timelines happen because of influence from a time traveler.

Sliders explored parallel universes, Flashpoint (DC), Avengers: Endgame, and the reboot of Star Trek explore alternate timelines.

Spoilers for a current show, Loki (Marvel), Episode 1:

The titular character is an organic piece of his timeline, but his actions were caused by outside-influence (The Sacred Timeline's Avengers, who traveled about 10 years into their past.)

That same "Time Heist" by the Avengers, also apparently caused a Thanos to leave his timeline and proceed to the Sacred Timeline, altering forthcoming history greatly.

In shows like Loki and Legends of tomorrow, Time-Police tend to exist outside of time, and can access the end of time. So by the time any time-travel story starts, all parallel universes created by organic actions have already been created.

There is no real difference between the two concepts.

I'll keep this in simple terms.

Parallel universes are the result of every possible decision realized. Alternate timelines are essentially the same thing.

Multiple realities can have multiple timelines. The most likely outcome is that there would be a very strong likelihood of convergence or overlapping/identical realities and timelines.

Using Star Trek as the paradigm, what would happen in the main universe if someone in the mirror universe went back in time (mirror timeline)? How did Nero's time displacement effect the mirror universe?

If there are quantum realities then it must be that there are quantum timelines within those realities. The two concepts are not mutually exclusive of each other.

• you should post link to where you get your facts ;) Commented Jan 22, 2015 at 6:51