Is there a copy of you reading this article?
This is how Max Tegmark’s article starts:
A person who is not you but who lives on a planet called Earth, with misty mountains, fertile fields, and sprawling cities, in a solar system with eight other planets. The life of this person has been identical to yours in every respect. But perhaps he or she now decides to put down this article without finishing it, while you read on.
These are the mesmerizing words of Tegmark. That article primarily argues that Parallel Universes are not just science fiction. It didn’t provide proof of the existence of Parallel Universes, but it classified the Parallel Universes or Multiverses into 4 levels. It can be falsified.
Tegmark’s classification counts our observable universe, also known as the Hubble volume, as one universe—i.e., the edge of the universe we can observe using electromagnetic waves. Now the size of our observable universe is 93 billion light-years in diameter. This is our cosmic bubble. Let’s dive into the classifications. This part was really interesting when I read it. Let me explain.
The main idea of this theory is that our universe is infinite, and the limit of the universe is our cosmic horizon of the Hubble volume. You should remember this.
Level 1 Multiverse is a very simple and less controversial idea of the multiverse. What do you think about the region beyond our cosmic horizon? I can say there is another set of galaxies, galaxy clusters, and cosmic webs out of intuition. Tegmark said there will be another cosmic bubble with different initial conditions—like different numbers of particles. But the laws of nature are always the same. And we can find an infinite number of cosmic bubbles.
And those cosmic bubbles can have stellar systems with a yellow dwarf star and 8 planets, 6 or 7 dwarf planets, and millions and millions of asteroids orbiting as belts and covered by comet clouds as a small bubble. Maybe in one of the 8 planets there is life exactly like you. In the next Hubble volume, you can find your doppelganger with small differences. This estimate was derived from the assumption that on large scales our universe is homogeneous.
They used probability and counting of possible quantum states a Hubble volume can have if it is no hotter than 108 Kelvin to calculate the distance between your doppelganger and you. They used to consider our possible universe can have 1058 protons. And by calculations he mentions that we can find our doppelganger at the distance of m. And about m away you can find the 100-light-year-wide bubble similar to our 100-light-year-wide neighbourhood. And about m away you can find an exactly identical universe to ours. If you are immortal and can see our observable universe expand, then you can see your identical person after billions of billions of years.
I don’t think this is the right estimate because we may miss other factors affecting the existence of such a doppelganger. Maybe it can be true if we can get out of our limits in our universe. But in some sense, it’s convincing.
Try imagining infinite sets of Level 1 universes, and you find some odd universe with different dimensionality and different axiomatic constants of the universe. This is the Level 2 Multiverse. It has an infinite collection of Level 1 universes with different physics. If you want to reach here, you can’t reach it, because its physics is completely different.
It can form from hyperinflation of the universe. This concept can explain how we get 3 spatial dimensions, why our universe is uniform at higher scales, etc.
Tegmark argues the symmetry-breaking process can give each universe different dimensionality, different particles, and different constants. And he argues that maybe our universe was born with 9 or 10 dimensions but only 3 expanded and the others did not expand and curled up and confined objects into 3D branes.
Whether we can find life here is a big question mark because our universe’s structures, strength of forces, and basic properties of atoms or subatomic particles make life possible. But having a universe with different properties may cause instability in the system, so those universes must be different.
Level 2 is hard to imagine, and I was thinking it can connect a few things from string theory. I am not clear about it. Maybe these universes are possible. We may find worlds with different metrics, and strange-looking people acting differently.
It seems we covered everything possible using science. But there is still space to discuss something even weirder: interpretations of our universe. I was surprised when I read it. There is a theory that says quantum events create branching universes, making multiple coexisting realities.
This is the Level 3 Multiverse. Quantum theory describes the universe not by position and velocity but by the wave function, evolving deterministically in Hilbert space. The strangeness arises only when we ask how this wave function produces our definite experiences. The old solution invoked a mysterious “collapse,” but Hugh Everett showed in 1957 that collapse is unnecessary: the wave function never jumps. Instead, it contains many classical worlds in superposition, which observers perceive as probabilistic branching. This branching is the Level III multiverse.
Understanding Level 3 needs three perspectives: one is the eagle view, which provides the mathematical idea, and another is the observer view from one classical branch. From the eagle view, there is a wave function containing multiple storylines of realities. But from the observer’s view, other realities hide.
Every decision you make or your subatomic particles make creates a branch in reality. If you continue reading this or stop, you create two realities. In Level 1 universes, the doppelganger of you is in a very distant galaxy, but in Level 3, you have your doppelganger in the next reality. The difference is that Level 1 discusses distance separation, and Level 3 discusses quantum separation.
The existence of Level III depends entirely on unitary time evolution, and all experiments so far uphold unitarity. Decoherence theory strengthens the case, and even concerns from quantum gravity, such as information loss in black holes, have been softened by results like the AdS/CFT correspondence.
If physics is unitary, then quantum fluctuations in the early universe did not produce a single set of initial conditions but a superposition of all possible initial conditions, later separated into classical branches by decoherence. Crucially, the distribution of outcomes across branches (Level III) matches the distribution across spatially distant Hubble volumes (Level I). The same logic applies to Level II: symmetry breaking yields a quantum superposition of all possible physical constants before decoherence isolates them.
Thus Level III adds no fundamentally new types of universes; it simply reproduces those already present in Levels I and II in quantum form. What once seemed a dramatic philosophical shift ends in anticlimax: many-worlds is vast, but no larger than the multiverse we already had.
The implications remain profound. Despite the branching, the total number of distinguishable universes is finite—about those cooler than certain Kelvin values. This idea also seems logical, but I feel like we don’t have a great mathematical framework for discussing this.
It seems we have now discussed every physical phenomenon supporting this theory, but what about mathematics? Yes—the Level 4 Universe is every possible mathematical structure. Most people believe our universe is a perfect mathematical framework. I also believe that.
The idea is good and constructed with great intuition, but sometimes it raises the question: is it possible? Even critics ask many questions. Many argue Level 4 cannot exist because mathematics is a tool, not physical reality. And they say this theory is not physics, just metaphysics.
Let’s see how true this is with the development of mathematics.
And I asked a question on social media: if the multiverse is possible and we have infinite universes, can my doppelganger be a superhero like Thor or Superman? On Reddit, I got some replies. Man, those were great replies, filled with passion for physics. For some, my question made them angry for combining childish, superstitious dreams with physics. I am really sorry for that. I liked that comment section, man.
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