What happens in Avengers: Endgame (SPOILER ALERT!!)

The fourth Avengers movie wraps up the events of the previous film Infinity War, in which half of the main characters were turned into dust. In order to remedy that, the remaining superheroes concoct a plan to travel back in time and retrieve the powerful Infinity Stones from various points in history before the villain Thanos destroys them. Though they did their best to do a neat job, their plan falls apart and they get noticed multiple times, talk to strangers in the past and even fight (remember the scene in which Captain America fights against himself?)

However, despite all the drama and setbacks, it looks like participation in this risky project didn’t affect their future at all. Their world remained the same. It is the world they traveled to in order to get the stones that changed forever turning into some kind of an alternate reality. At least, this is how the characters themselves describe it.

In order to find out how much of the action in Endgame is supported by science, Business Insider talked to Sean Carroll, a theoretical physicist from the California Institute of Technology and one of the science advisers on the movie, who also served as a science adviser for the films Thor, Thor: The Dark World, and Avengers.

The grandfather paradox

One of the biggest questions that arise in stories about traveling to the past is the possibility of inadvertently creating inconsistencies. This phenomenon is called the grandfather paradox. First described by René Barjavel in his 1944 novel Future Times Three, this paradox implies that it’s impossible to change something in the past without it affecting the future. Imagine you went back in time and killed your grandfather before he married your grandmother. What would happen to you? Based on that revision to your past, you would never have been born.

Screenwriters and film directors have mixed feelings about this paradox. For example, the creators of Back to the Future don’t allow their characters to interact with themselves in the past as such encounters may “destroy the space-time continuum”. The same rules are applied in The Butterfly Effect, X-Men: Days of Future Past, and the comedy Bill & Ted’s Excellent Adventure. All these movies comply with the same rule: if you change something in the past, it will affect your future.

However, this is not the only way things can be done in the world of cinema. In such films as Terminator and Twelve Monkeys, the past is the past, and whatever you do and no matter how hard you try you cannot possibly change it.

According to the authors of the fourth Avengers together with Sean Carroll, by changing something in the past you don’t change the present but create alternate realities. Just like the creators of Back to the Future, the authors of Avengers: Endgame use time travel to evoke emotions in viewers. That’s why they can often sacrifice scientific accuracy and their own logic for dramatic effect.

Scientific aspects of time travel

According to Einstein’s theory of special relativity, time and space are two parts of the same thing, space-time, and the rate at which time passes is relative to how fast something is moving. Therefore, if a person is moving very fast (for example, at the speed of light), time would pass more slowly than it would for the rest of us on Earth. So time travel is theoretically possible, according to the laws of physics — but only into the future, not the past.

This principle is described in Pierre Boulle’s novel Planet of the Apes. The plot revolves around a group of astronauts who crash-land on a strange planet in the distant future. Although the planet appears desolate at first, the surviving crew members stumble upon a society in which apes have evolved into creatures with human-like intelligence and speech. By the end of the movie, one of the astronauts reveals that the “alien” planet is actually Earth 2,000 years after a nuclear war.

A more experimental approach to physics is used in Interstellar. The movie is mainly based on Caltech physicist and relativity expert Kip Thorne’s scientific theory that wormholes exist and can be used for time travel. A Nobel Prize laureate in Physics, he even wrote a book called The Science of Interstellar, in which he expands about what experimental theories helped to shape the film’s plot.

The most cerebral time travel movie of all time

The 2004 American science fiction film Primer is often referred to as the most complicated movie about time travel. This movie tells the story of two engineers who accidentally invent a time machine. It doesn’t look like a Delorean, but rather a box resembling a fridge. To travel back in time, you get inside this box and wait for a certain period of time. For example, to travel four hours back, you have to spend four hours in the machine. Watch this video if you want to get to the core of the movie’s concept of time travel.