Humorous memes, science fiction, religion, philosophy, psychology, astronomy, fantasy, what do they all have in common? I guess you read the title of this article, so you already know the answer.

In a parallel universe, you could have whatever you can’t have here and do stuff you can’t do here. An alternate timeline of your life where you chose a different college, had different parents or probably were born in another country. It is fascinating to think that there may be a world, where the two deadly world wars did not happen. But, is it just a wild fantasy? Or, is there more to it?

What does Physics say about it? Let us begin with the past. In a 1952 lecture, Erwin Schrödinger (the great physicist who gave us the Schrödinger equation, which is the ‘F = ma’ of super-tiny aka Quantum systems) famously said that when his Nobel equations seemed to describe different histories, they were ‘…not alternatives but all really happen[ing] simultaneously’. That was probably the beginning of the idea of parallel universes or multiverses.(To be fair, Isaac Newton wrote some stuff back in the 1700s referring to idea of a Multiverse, but that was more philosophical rather than mathematically concrete).

CLARIFICATION ALERT: The word ‘Universe’ is really a synonym for ‘the whole’, so that makes ‘Multiverse’ seem like a nonsensical word. Whenever Physicists refer to Multiverse, they are talking about multiple ’Observable’ Universes.

In the quantum world, when little particles interact with other little particles (little, as in, really really little, think of an electron or a quark), they do not behave as concrete and determinable particles, rather they exist in a superposition of all possible states in what is called a ‘wavefunction’.When an observation is made, the wave-function collapses and the particle appears at a single spot. The famous Double Slit experiment, Quantum Teleportation and the Stern-Gerlach experiment, amongst many, line up to support this fact.

While all Physicists agree about the collapse of the wave-function, nobody clearly knows how it happens. In other words, it is not clearly known why observation causes a quantum system to end up in a certain state and not the other. This has lead to a group of scientists in speculating many models of the different possible timelines aka Parallel Universes:

1. Quilted Multiverse

This model assumes an infinite Universe. And, infinity is bigger than you think. Consider this: it takes about x =(10 )^(10)⁷⁰ quantum states to completely define 1 m3 of space. In other words, x is the number of different ways that we could put together 1 m³ of space resulting in different arrangements of atoms, one of which can be you or me. That essentially means if the universe was larger than x meters across, then there may be multiple copies of you and me in the Cosmos(Quantum Physics says that, not me). If you didn’t notice, x here is finite but we are not as sure about our Universe. This is the foundation of this model. There could be isolated bubbles of observable universes floating around that are so far away from each other that it will take gazillions of years for light to travel across them(recall that light is only about 3,00,000 km per sec fast, not infinitely fast). Thus, billions of copies of you and me maybe floating around but, alas, we cannot interact with them.

2. Membranes & Multidimensions

This model relies on String theory and M-theory’s idea of spacetime existing in at least 10 dimensions (nostalgia anyone? Interstellar?). Each world lives on a 3D brane(short for membrane)embedded in a higher dimensional multi-brane ‘bulk’. Think of a newspaper, each of its pages is a 2D world embedded into a multi-brane newspaper. These branes can interact with each other, and when they collide, the amount of energy released is enough to produce a big bang. This can be explained mathematically with the help of M-theory. We have got an interpretation that explains the Big Bang better and accounts for Parallel Universes too! Two for one! Unfortunately, it is also an unconfirmed model.

3. Many Worlds’ Interpretation

This is it. This is ‘the’ thing. This is the most popular interpretation of the Multiverse theory. Termed by Hugh Everett III, this is the one that repeatedly pops up in the DC/Marvel Universe, Sci-Fi movies, Fantasy Worlds and so on. This model uses ‘choice’ as the central idea. On a weekday morning, you are faced with two choices: either go to the class or watch your favourite TV-show. You chose the latter(I mean, let’s be honest). But that does not mean the other timeline does not exist. Both the futures are equally real, only you chose the second one(try not to think about GPA now). Imagine a tree, if it helps. An ant starts walking from the trunk to the branches, choosing one branch over the other, at each junction. The ant will experience only one path but the whole tree still exists. Similarly, we are currently living in one possible reality, that we decided to choose. To quote Mark Batterson, “You are one decision away from a totally different life.” It is good to think and explore possibilities but far more important is to stay realistic. How can we accept or reject any of the above-proposed models? Will it ever be possible? There is a continuous heated debate in the scientific community on the idea of Multiverse. How can an experiment rule out a theory if the theory provides for all possible outcomes? Many great physicists speak in favour while others in against of it.

Proponents:

Stephen Hawking

Brian Greene

Michio Kaku

Neil deGrasse Tyson

Max Tegmark

Skeptics:

Steven Weinberg

David Gross

Paul Steinhardt

George Ellis

Neil Turok and more…

But Physics is science, not philosophy. Hopefully, someday, one of us will design, build and conduct an experiment to test these hypotheses. We shall overcome this debate, someday.

Maybe you believe one of the above models, maybe none. Screw it. How does it matter, we are all living inside the Matrix. Or maybe, not living, either.