ELI5 : What is this concept of a parallel timeline ??

It is possible to think of time as a ‘line’ with events strung on it like flags/tags.

Some people believe there are points at which events seem like they could result in different outcomes, like flipping a coin. They flip a coin and it comes up ‘heads’ but they can imagine a world where the flip came up ‘tails.’

Some people even think both the ‘heads’ world and the ‘tails’ world are equally real, just that people in one cannot interact with the other. Both worlds continue forever.

These are ‘parallel’ or ‘alternate’ timelines.

Since it’s completely theoretical at this point, anyone can say anything.

To put it another way, last century there were all kinds of books about life on Mars. Today we can talk about ‘the multiverse’ or ‘different dimensions’ the same way.

The generally accepted concept is that there are an infinite number of universes, and that every time someone makes a decision a new ‘timeline’ emerges. In one world you ate strawberry ice cream, in the next chocolate, in the next you skipped dessert…

It’s a fun idea for science fiction authors and theoretical physicists, but no one has proven it’s real.

It comes from quantum mechanics where particles travel all possible routes and when making calculations you need to account for all possible variations, because they affect each other.

When particle is measured, all variations disappear except one and that one continues thereafter without the affects of others.

Multiword model dictates that the other variations don’t disappear. They just disconnect from our reality and continue elsewhere.

Currently multiworld model is popular. Imho it 's the only one that properly explains all quantum phenomena.

Although there is a good argument wether multiworld is proper science.