The Many worlds theorem
- Lauren Hughes
- Jan 20
- 3 min read
Updated: Feb 27
Potentially the most bizarre mainstream interpretation, the many worlds theorem explores the potential that parallel universes exist as a result of Quantum Mechanics.
When a quantum system is measured, the wave function (the mathematical description of a system as a combination of many possible states) does not collapse into a single outcome. Instead, every possible outcome continues to exist, each in a separate parallel universe.
During measurement, the observer becomes entangled with the measured state of the quantum system. As a result, the observer becomes part of one particular branch of reality, which is why we experience only a single outcome.
In the Many Worlds Interpretation, all branches of the wave function coexist, but they don't interfere with each other. Decoherence explains why: when a quantum measurement occurs, the system rapidly becomes entangled with its vast environment. Each outcome gets correlated with a distinct environmental state, and because these environmental states are so different from one another (mathematically orthogonal), the interference terms between branches vanish. Each branch then evolves independently, which is why we experience a single definite outcome.
Benefits of this interpretation
Parsimony of laws
This interpretation introduces no new physical laws. There is no need for wavefunction collapse, hidden variables, or special rules distinguishing measurement from non-measurement. The theory relies only on Schrödinger’s equation.
Determinism
The universal wavefunction evolves deterministically: every event follows from prior states encoded in the wavefunction, and nothing fundamentally random occurs. Perceived randomness arises only from uncertainty about which branch you are in.
Consistency with decoherence
Modern decoherence theory naturally explains why branches do not interfere, providing a mechanism for the emergence of classical-looking outcomes within MWI.
Critiques
The probability problem
If every outcome happens, why do we observe some outcomes being more likely than others i.e. when we repeat these experiments. We have even developed mathematics to represent this, Born's rule tells us the probably of seeing a certain outcome. Why does a 90/10 superposition feel different from a 50/50 one if both branches exist?
Ontological extravagance
Although MWI is simple in its laws, it requires accepting a potentially infinite number of parallel worlds. This raises philosophical questions about identity and existence.
Although the many worlds interpretation is simple in its laws, it still asks us to accept an extreme and bizarre concept: that there exists possibly infinite parallel worlds. This also brings into question the existence of identity.
MWI raises a deeper issue: what should we value most in a scientific interpretation? It makes the same mathematical predictions as the Copenhagen interpretation and offers theoretical simplicity, yet it is ontologically difficult to accept. Which of these qualities, simplicity of laws or intuitive plausibility, should matter most?
Could you accept parallel universes exist?
Yes
no
References
This is Hugh Everett's original paper if you are interested.
-Everett Hugh, "Relative State" Formulation of Quantum Mechanics, Rev. Mod. Phys, volume 29, issue 3, pages 454-462, 1957, https://link.aps.org/doi/10.1103/RevModPhys.29.454
-Barrett, Jeffrey A.. “Many Worlds Interpretation of Quantum Mechanics.” Compendium of Quantum Physics (2009)
-Barrett, Jeffrey A.. “The Everett interpretation of quantum mechanics : collected works 1955-1980 with commentary.” 2012, https://api.semanticscholar.org/CorpusID:169917794
-D Wallace, ‘Decoherence and Ontology, or: How I Learned To Stop Worrying And Love FAPP’, 2011, https://doi.org/10.1093/acprof:oso/9780199560561.003.0002
-B. Dewitt, Neill Graham, The Many-worlds interpretation of quantum mechanics, MDPI, 2015, https://doi.org/10.1515/9781400868056
Comments Section
" Can you ever prove this though?"
"Pointless, the math's works, leave it at that"
" I like that this conserves locality "
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