[Parallel Universes] A Movie and a Question

This post has been sitting in my drafts for the past few years. Over the past few months I kept going back to it, and somewhere along the way it turned into a series.

Do you believe in parallel universes?

In 2014 I watched You Are the Apple of My Eye, a Taiwanese film from 2011. Late in the film, the main character asks the girl he liked in school almost exactly that question. In the English wording fans usually quote, the line goes: "Do you believe in parallel universe? Maybe in that parallel universe we are together."

That line made me wonder if physics had anything to say about parallel universes, or if they only exist in movies.

Physics has a lot to say. Physicists describe several kinds of parallel universes, each coming out of a different theory, and they argue about all of them.

Why I'm writing this

I'm writing this out of pure interest in the theory. I'm a software engineer, and I work on payment systems in banking, which has nothing to do with cosmology.

I think parallel universes exist, and I hope they do.

The movie

You Are the Apple of My Eye was written and directed by Giddens Ko, and it was his first feature film. It's based on his own semi autobiographical novel of the same name. Ko Chen-tung plays Ko Ching-teng, a troublemaker in high school, and Michelle Chen plays Shen Chia-yi, the top student in his class. It opened in Taiwan on 19 August 2011, the day the Taipei Times reviewed it.

The parallel universe line comes in a phone call after the earthquake that hit Taiwan on 21 September 1999, when Ching-teng calls to check that Chia-yi is safe. The original line is in Mandarin, where the phrase is literally "parallel time and space". English subtitles word it in slightly different ways, so treat the quote above as the common fan version, not an official subtitle.

I won't spoil the rest. The movie never says whether that other universe exists, and twelve years later I still want to know.

Interstellar and X-Men: Days of Future Past

Two other films shaped how I picture the idea, and both came out in 2014.

X-Men: Days of Future Past, directed by Bryan Singer, opened in the US on 23 May 2014. In the dark future of 2023, Kitty Pryde sends Logan's mind back into his body in 1973. His job is to stop Mystique from killing a scientist named Bolivar Trask, the event that leads to that future. When Logan wakes up in 2023 again, the world has changed. The old future is gone, replaced by a better one. As ScreenCrush's breakdown of the ending points out, the film never shows the old future continuing anywhere. There's one timeline, and the film rewrites it.

Interstellar, directed by Christopher Nolan, went into wide release in the US on 7 November 2014. Its science advisor and executive producer was Kip Thorne, a physicist at Caltech. In the film, our universe is a sheet, called a brane, floating in a bigger space with one more dimension, called the bulk. Beings who live in the bulk build a structure called a tesseract, where Cooper can see moments of his daughter's room from the outside and send her a message using gravity. Thorne wrote a whole book about the physics behind it, The Science of Interstellar.

Put the movies next to each other and you get three different ideas:

Comparison: think of a video game with save files. Days of Future Past is a game with one save slot: load an old save, play differently, and the old progress is gone. Most parallel universe ideas in physics are a game that keeps every save file running, and you can only ever open your own.

No physics theory of parallel universes lets you travel to another universe, send a message to one, or change the past. If the other universes exist, they don't replace ours and they never touch it.

What "universe" means here

The word universe usually means everything that exists, so "another universe" sounds like a contradiction.

Physicists fix this by using the word in a smaller way. Most of the time, "our universe" means the part we can see, or a region of space with one shared history. The word for the whole collection, all the universes together, is multiverse.

To talk about the size of the part we can see, you need the light year: the distance light travels in one year. Light moves at about 300,000 kilometers each second, so a light year is about 9.46 trillion kilometers. The Sun is about 8 light minutes away. The nearest star after the Sun is a bit more than 4 light years away.

The universe is about 13.8 billion years old. So how far away is the most distant place you can see?

The obvious answer is 13.8 billion light years, because light has had 13.8 billion years to travel. The real answer is about 46 billion light years.

The difference comes from the expansion of the universe. Light from the most distant regions has been traveling toward us for billions of years, and space has kept expanding the whole time. So what we see is an ancient image of those regions, while the regions themselves have since moved much farther away, to about 46 billion light years from us.

This sphere around us is called the observable universe: everything whose light has had enough time to reach us since the universe began. Its boundary is known as the cosmic horizon.

Think of it like the horizon at sea. A ship that disappears beyond the horizon hasn't vanished. You just can't see it from where you are.

The simplest kind of parallel universe is the region past this horizon. Space keeps going out there, and it may look just like ours, with stars and galaxies and planets. We can't see it and it can't see us. From where we stand, it's another universe.

Tegmark's four levels

In 2003, the physicist Max Tegmark wrote an article for Scientific American called Parallel Universes. A longer, more technical version is free on arXiv. He sorted the ideas into four levels. Each one allows more variety than the one before and comes from a different part of physics.

graph TD
    M["The multiverse: all universes together"] --> L1["Level I: regions beyond our cosmic horizon"]
    M --> L2["Level II: other bubbles made by inflation"]
    M --> L3["Level III: branches of quantum physics"]
    M --> L4["Level IV: other mathematical structures"]
    L1 --> A1["Same laws of physics, different arrangement of matter"]
    L2 --> A2["Same deep laws, different constants and particles"]
    L3 --> A3["Same space, every possible outcome happens"]
    L4 --> A4["Different laws entirely"]

Level I is the horizon idea from the last section. If space goes on far enough, our observable universe is one sphere among a huge number of spheres. Each sphere follows the same laws of physics as ours. Only the arrangement of matter differs. Tegmark estimated how far you would have to go to find an exact copy of yourself: about 10¹⁰²⁹ meters.

Level II comes from a theory about the first fraction of a second of the universe, called inflation. In many versions of that theory, inflation never stops everywhere at once. It keeps making new regions, like bubbles, and each bubble can end up with its own values for some constants of nature. A bubble next door might have different particles, or a different number of dimensions you can move in.

Level III comes from quantum physics, the physics of very small things like electrons and atoms. In one reading of quantum physics, called the many worlds interpretation, every possible result of a quantum event happens, each in its own branch of reality. These branches don't sit far away in space. They overlap with ours, in the same space, and they can't affect each other.

Level IV is the strangest. Tegmark suggests that every mathematical structure exists as a universe of its own, including ones with laws of physics nothing like ours. I leave Level IV out, because it's the hardest of the four to test.

Tegmark's map isn't the only one. Brian Greene, another physicist, lists nine kinds of parallel universes in his 2011 book The Hidden Reality: the quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated, and ultimate multiverse. One of them is the string theory landscape: string theory, a candidate for a deeper theory of physics, seems to allow a huge number of possible universes with different laws. Another is the braneworld: our whole universe could be a three dimensional sheet floating in a space with more dimensions, with other sheets nearby. That's the idea behind the bulk in Interstellar.

Is this science or science fiction?

The obvious objection is that you can't see another universe. Light from it never reaches us. So how can physics say anything about it?

The common answer is that you never test the other universe. You test the theory that predicts it.

No one has seen the inside of a black hole. Nothing that crosses its edge, the event horizon, can come back out to report. Physicists still talk with confidence about what happens inside, because Einstein's theory of gravity, general relativity, predicts it. That theory has passed many tests we can see. In 2015, the LIGO detectors measured gravitational waves from two black holes merging, a signal that matched the prediction of general relativity. That signal arrived on 14 September 2015, and the LIGO and Virgo teams published it as Observation of Gravitational Waves from a Binary Black Hole Merger. It won the 2017 Nobel Prize in Physics. When a theory passes that many tests, you take its other predictions seriously, even the ones you can't check.

Parallel universes work the same way. Inflation explains features of the oldest light in the sky that we can measure, and no experiment so far contradicts quantum physics. Many versions of inflation predict other universes, and so does quantum physics when you read it as many worlds. That's why some physicists take parallel universes seriously, even though nobody can prove they exist.

Not every physicist accepts this. In December 2014, the cosmologists George Ellis and Joe Silk wrote a comment in Nature called Scientific method: Defend the integrity of physics. They argued that a theory which can't be tested by experiment shouldn't be treated as established physics, and the multiverse was one of their main examples. In 2018, Sean Carroll answered in Beyond Falsifiability that the theories which predict parallel universes still get tested, through everything else they predict.

Where I stand

Tegmark calls Level I "rather uncontroversial", because it only assumes that space is infinite, and he argues that Level III adds nothing new beyond Levels I and II. Both come out of physics that already passes tests. That's a big part of why I think parallel universes exist.

There's also a less serious reason to hope. If Level I is right, then somewhere very far away a copy of Chelsea plays a copy of this season with copies of these exact players. Somewhere even further away, the copies win every match. As far as I'm concerned, that's the most important prediction in cosmology haha.

The parts of this series

1. A Movie and a Question (this post): what does "parallel universe" mean, and which kinds exist?
2. Another You, Very Far Away: if space goes on forever, does a copy of you exist somewhere?
3. Bubble Universes from Inflation: how could the first fraction of a second make more than one universe?
4. String Landscape and Branes: why might other universes have different laws of physics, and what is the bulk in Interstellar?
5. Many Worlds, Every Outcome: does quantum physics split reality into branches?
6. The Other Me: is another version of me living a different life, and does my choice matter?
7. What I Believe, and Why: why I believe parallel universes exist, and what I hope.