Understand ‘Loki’ time travel through the real science of branching universes

Marvel’s infamous anti-hero has wriggled his way out of many perilous situations before, but in the new Disney+ show Loki, the God of Mischief may have finally become entangled in a fight that he can’t win — one against time.

Loki picks up during Avengers: Endgame before branching into its own adventure. Tom Hiddleston’s would-be-supervillain steals the tesseract containing the Infinity Space Stone, escapes to an alternate timeline “branched” off our own universe, and is promptly arrested by the Time Variance Authority for “violating the sacred timeline.”

Multiverses and branching timelines are central to Loki, but they stem from the actual realms of academic research. In the real world, they’re understood with a twist: While Loki presents a swashbuckling world where branches of time can either be reset or spiral into chaos, scientists use these concepts to explain branching universes — a way to understand many possible outcomes caused by chance.

The science of branching timelines can be difficult to wrap your mind around, but once you can understand it you might just get Loki on a deeper level too.

What is the branching universe?

While Loki is awaiting trial for violating the sacred timeline, the guards play an educational video featuring “Miss Minutes” — an animated clock that explains that long ago there was a war where multiple timelines battled, leading to the destruction of everything.

Eventually, the Time Keepers emerged and reorganized the multiverse into a single timeline — a single universe.

“Now, Time Keepers protect and preserve the timeline for everyone,” Miss Minutes explains, “but sometimes, people like you veer off the path the Time Keepers created.”

Picture a single timeline proceeding from left to right. Now, imagine a new branch forming off the timeline — in Loki these branches are caused by “variants,” or individuals who mess with the way time works.

The show describes this concept as time branching off and forming an alternate timeline, but scientists see it another way.

“It's not really time that branches, it's the whole universe that branches at certain times,” Sean Carroll, a research professor in physics at the California Institute of Technology, tells Inverse. Carroll is an expert in quantum mechanics and spacetime.

The idea of branching is well outlined in theoretical physics, which uses quantum mechanics — the study of subatomic quantum particles like electrons — to explain how it works.

The movement of these quantum particles is highly uncertain, leading to many different possible states or “superpositions” in which the particle can exist.

“The mystery of quantum mechanics is that objects exist in ‘superpositions’ of different possible measurement outcomes,” Carroll says. Under quantum mechanics, a particle could spin both counterclockwise or clockwise.

“There are branches that correspond to every different quantum-level chance event.”

But when humans measure these particles, we only see one way the particle spins or one “outcome” — the outcome that exists in our slice of the universe.

“They just happen on different ‘branches.’ But you and I only ever see a single branch — the one we are on,” Kristie Miller, a professor in the department of philosophy and joint director of the Center for Time, tells Inverse.

However, under the “many worlds” interpretation, which Hugh Everett came up with in his 1957 Princeton doctoral dissertation, “every outcome comes true, but in a different universe,” Carroll explains. In essence, a different future rests on each branch.

“Each universe — or ‘branch’ or ‘world’ — is then a bit different,” Carrolls says. “They all evolve independently of each other.”

These different outcomes occur as a result of branching after a quantum event, which leads to different universes or worlds. In Loki, Miss Minutes explains how Loki’s actions caused the universe to branch:

“Stepping off your path created a nexus event that, left unchecked, could branch off into madness, leading to another multiverse war.”

That portrayal isn’t quite accurate, experts say. Our individual decisions don’t create branching — rather, every single quantum event that occurs creates its own branch, spawning multiple universes with every new outcome.

“Sometimes in popular accounts, it’s made to sound as though it’s our making a decision which ‘causes’ branching,” Miller says. “But in fact, there are branches that correspond to every different quantum-level chance event.”

Could we use branching universes to time travel?

According to Marvel, the answer is yes. Back in Avengers: Endgame, the Avengers used an Infinity Stone to travel back in time by creating branched universes — or branched timelines, as they’re portrayed in the movie.

Loki’s actions fall in line with one theoretical view of branching universes, which permits the possibility of traveling from one branch to another.

But the actual science behind branching universes and time travel is less clear. While time travel to the future is possible under general relativity, the idea of time traveling between different universes or timelines is not likely, according to physicists.

“[If] I can travel to other branches: then I can, as it were, see things that did not happen on the branch I came from,” Miller says.

“In any event, we don't think it's ever possible to travel between different worlds (timelines),” Carroll says.

Can you bring the branching universe back to a single timeline?

On Loki, the Time Keepers enforce a singular “sacred” timeline. The TVA exists to prevent multiple branches or worlds from emerging.

On its face, this may seem more fiction than science — but there is a hypothetical way to “prune” a branch within the current laws of physics, using something called a wave function, explains Nicole Yunger Halpern, an expert in theoretical physics and a postdoctoral fellow at Harvard University.

“We do know of ways to, in certain situations, partially prune some wave-function branches that we dislike,” Yunger Halpern says. “Quantum-computing scientists develop quantum algorithms, which end in measurements of the quantum system, to perform this pruning.”

“But we typically can't prune the wave function perfectly, even if we're able to measure the system,” she explains.

“You can try to change the laws of physics to get rid of the other worlds ...”

If you could prune away all the undesirable branches, as the TVA attempts to do, “we'd be doing something impossible,” Yunger Halpern says.

Specifically, Yunger Halpern thinks the characters in the show might be doing something actual scientists call “impossible postselection,” meaning they prune away “all the undesirable positions” without measuring the quantum system. This is something that is currently impossible under the known laws of physics.

Carroll agrees, suggesting that the idea of removing other worlds or branches is not testable within our current knowledge of physics.

“You can try to change the laws of physics to get rid of the other worlds, but there's no experimental evidence for that,” Carroll says.

Is there scientific evidence to support the multiverse?

Marvel uses the term “multiverse” as a shorthand for the many parallel worlds and branching timelines that emerge throughout the MCU.

But what does the scientific community think about multiple universes or “many worlds?” It depends on whom you ask. The multiverse is a disputed idea in the realm of theoretical physics, with fierce proponents on either side.

“Some people say that all the tests that have ever been done of quantum theory, support the many-worlds concept,” Yunger Halpern says. “And some people say the many worlds concepts can't really be tested.”

Carroll falls into the first category. He believes the many-worlds idea is in line with the existing laws of physics.

“The existence of the many-worlds is not postulated as a new ingredient to quantum mechanics — it's a natural prediction of the existing formalism,” Carroll says.

But for now, the idea of branching universes still remains somewhat in dispute. It’s not that it isn’t theoretically possible — we just don’t have the hard evidence to support this theory.

“In the meantime, we have several different ways of understanding [the universe], some of which have branching, and some of which don’t,” Miller says.

In any case, there probably aren’t stealthy Time Keepers secretly preventing humans from branching off into multiple timelines — but it’s a fun idea to contemplate as we watch Loki race against the clock.

Loki is now streaming on Disney+.