The Weirdness of Being Small

What is the smallest particle that makes up everything around you, such as the screen on which you are reading this article? If your answer is atoms, you are correct—but what makes up these tiny, indivisible particles? This question led to the discovery of even smaller sub-atomic particles: protons, electrons, and neutrons, which are themselves made of even smaller quarks, neutrinos, and gluons. All these particles together, along with some fundamental forces of nature, form what is called the standard model of particle physics.

These tiny particles do not follow the same rules we do. Welcome to the quantum world, which is a place so strange, so bizarre, and so different from our everyday lives that the rules we are used to do not apply. This is the realm of quantum physics, the science that seeks to understand the remarkable behavior of the universe at its smallest possible scale.

Quantum physics comes with a puzzle so deep that it has split physicists into different groups, each with its own incredible explanation of reality.

Quantum World: A World of Maybes

In our big, everyday world, things are defined and predictable. For example, when you kick a soccer ball, it either goes into the goal or it does not. There is no in between! But in the tiny quantum world, things work very differently. A small particle, like an electron, can be in many places at once before we look at it. It is somewhat “fuzzy” and not stuck in one spot. Instead, it is in a mix of all its possible positions. Scientists call this strange state superposition [1], which means something can be in several places at the same time.

To help explain this weird idea, we must introduce one of the most eccentric pets in the world—Schrödinger’s cat (Figure 1) [2]. Erwin Schrödinger was a leading quantum physicist in the 1900s, and he proposed one of the most bizarre and perplexing thought experiments in quantum physics (also discussed in this Frontiers for Young Minds article). In his story, a cat is locked in a box with a special type of poison that might or might not be released. Because the release is random, no one knows what happened to the cat until the box is opened. So, before opening the box, the cat is in a superposition state of being both dead and alive—just like the fuzzy electron is in many places at the same time. The moment someone opens the box, the cat’s maybe-alive-or-maybe-dead state disappears, and one single reality is instantly true. Similarly, the electron, which was previously in many places, is suddenly in a defined spot as soon as we look at it.

This sudden “collapse” into a single reality is the core puzzle of quantum mechanics. Why does it happen? And how does the electron or the cat “choose” which outcome to show us? The most common answer to this question has been “we do not know”. In the following sections, we will look at two of the most popular interpretations that try to explain this mystery, and you can decide which one sounds more fascinating to you.

Interpretation #1: The Copenhagen Interpretation

The first major attempt to explain this puzzle is called the Copenhagen interpretation [2–6]. As the name suggests, this interpretation of quantum physics was developed in Copenhagen, Denmark, by two of the 20th century’s greatest scientists: Niels Bohr and Werner Heisenberg. This explanation says that the act of measurement itself forces the universe to make a choice. The fuzzy cloud of possibilities (of being in different states at the same time), which scientists call the wave function, is said to “collapse” into a single definitive state, as seen in the case of the electron and Schrödinger’s cat. This is the most widely accepted interpretation of quantum physics in the scientific community due to its simplicity and practicality. However, this explanation does not explain how or why this collapse occurs, and for that reason, many physicists could not accept this argument. Science has no room for “miracles”, and this “wave function collapse” felt a little too magical. This dissatisfaction led some scientists to search for a more complete explanation.

Interpretation #2: The Many-Worlds Solution

In 1957, a young physicist named Hugh Everett III proposed [7] a radical and brilliant solution to the collapse problem [2–6]. He asked a simple question: What if the wave function never collapses at all? What if, instead of the universe picking one reality, all possible realities happen simultaneously? Say you have to choose between pizza and pasta for lunch, as the boy showed in Figure 2. According to the many-worlds view, if something is possible, then it happens in some branch of this vast multiverse. Therefore, there is a universe where you are happily munching on pizza. However, in a parallel universe, a different you is twirling spaghetti! Each choice creates a new “branch”, like a growing tree, where every possibility becomes real somewhere in the multiverse.

This idea elegantly addresses the “miracle” problem. There is no mysterious collapse anymore. The wave function creates more and more branches over time. In fact, by choosing pizza over spaghetti, you have created your own branch of the universe! So, have we solved the quantum mystery? No, we have given one possible solution, but raised many other questions. Many scientists have trouble accepting this interpretation, as trillions of branches are created every instant. It also raises the philosophical question of which “you” is actually you and why you are here in this exact universe and not elsewhere. If these other universes in the multiverse are real, where are they, and why have you not met another version of you?

The Great Divide: What is Decoherence?

Decoherence is the process that seals these parallel universes off from one another, making them completely inaccessible to each other. Imagine two identical dogs starting a race around a huge circular park [1]. They both begin at the same place and are supposed to reach the same finish line. The first dog runs straight to the end without stopping. But the second dog gets distracted in chasing squirrels, sniffing food, or stopping to look at things. Because of all these interruptions, the second dog slows down and changes its path. In the end, the two dogs no longer arrive together.

Decoherence works similarly. At the tiny quantum level, different possibilities (or “branches” of reality) can start out connected and in sync. If nothing disturbed them, they could come back together and interact, showing they are part of the same system. But in the real world, there are always distractions—constant interactions with the environment, like collisions with air molecules or light particles. These interactions quickly push the different possibilities out of sync, just like the distracted dog. After that, they can no longer come back together, and they continue to branch out, getting further and further apart.

That is why we only see one version of reality. Decoherence explains how the messy, busy world around us makes quantum possibilities behave like a single, classical reality. It also provides a possible answer to the collapse problem [2] from earlier. The interactions with the observer and the environment upon measurement force the electron or the cat into one single reality. However, the question remains open as to why we see that particular outcome.

So, Which Idea is Right and Why Do We Care?

The incredible thing is that we have no way of knowing which interpretation is correct. No known experiment can distinguish between a universe that collapses (Copenhagen) and one that splits (many-worlds). Both explanations predict the same outcomes for all the experiments we can actually perform.

It is your personal “taste” whether you prefer a single universe where reality makes a mysterious leap every time we look at it, or a universe that follows smooth, unbroken mathematical rules, but at the cost of creating an infinite number of parallel worlds with every quantum flicker!

Remember, the next time you are faced with a choice, somewhere out there, in a parallel universe, another you might have picked the other option! The universe is a lot stranger and more wonderful than we often imagine. Keep exploring, keep asking questions, and maybe one day you will be the one to unlock the next big secret of the multiverse!

Glossary

Standard Model: ↑ The theory that describes the three fundamental forces (electromagnetic, weak, and strong interactions, excluding the gravitational force) in the universe and classifies elementary particles.

Quantum Physics: ↑ The branch of physics that studies the behavior of matter and energy at the smallest atomic and subatomic levels.

Superposition: ↑ The principle that a physical system can exist in multiple states at once.

Thought Experiment: ↑ An imaginary scenario used to explore and test a theory/concept.

Wave Function: ↑ The mathematical representation of all the information about a quantum system.

Multiverse: ↑ A hypothetical collection of different universes, including our own, containing all matter, space, and time.

Decoherence: ↑ The process by which a quantum system loses its quantum properties due to unavoidable interactions with its surrounding environment, effectively forcing it to behave classically.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank Harjasnoor Kakkar for insightful discussions. AS acknowledges support from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Sklodowska- Curie Grant Agreement No. 847517. GM-R acknowledges financial support from the European Union under ERC Advanced Grant TAtypic, Project No. 101142236. The ICFO group acknowledges support from MCIN/AEI [PGC2018–0910.13039/501100011033, CEX2019-000910 -S/10.13039/501100011033, Plan National STAMEENA PID2022-1390 99NB, project funded by MCIN and by the “European Union NextGeneration EU/PRTR” (PRTR-C17.I1), FPI]; Ministry for Digital Transformation and of Civil Service of the Spanish Government through the QUANTUM ENIA project call—Quantum Spain project, and by the European Union through the Recovery, Transformation and Resilience Plan—NextGeneration EU within the framework of the Digital Spain 2026 Agenda; CEX2024-001490-S [MICIU/AEI/10.13039/ 5011000110 33]; Fundació Cellex; Fundació Mir-Puig; Generalitat de Catalunya (European Social Fund FEDER and CERCA program); Barcelona Supercomputing Center MareNostrum (FI-2023-3-0024); European Union HORIZON-CL4-2022-QUANTUM-02-SGA—PAS QuanS2.1, 1011 13690; EU Horizon 2020 FET-OPEN OPTOlogic, Grant No 899794; QU-ATTO, 101168628; EU Horizon Europe research and innovation program under grant agreement No. 101080086 NeQST.

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