**This article will focus on two theories that consider the feasibility of parallel universes and will attempt to review the practicality of including such universes in these physical models. This article will first discuss the many-worlds interpretation in quantum theory and then touch on the concept of parallel universes in the theory of braneworld cosmology. Both these methods allow for the investigation of additional universes. There may be other theories that can manifest parallel realities, but these will not be explored in this article.**

*Image Credit: GaudiLab/Shutterstock.com*

## Many-Worlds Interpretation

### A Quick Word on Quantum Mechanics

All quantum systems are described by a wave function. A wavefunction can be thought of as the addition of all possible outcomes of the system (known as a superposition of states), and it contains information about the probability of what state the system is in.

When a system is unobserved, the wavefunction is large as there are many possible states that the system may be in, but observing the system causes the wavefunction to collapse and the true state of the system is determined. Until the system is observed, the wavefunction precisely describes the system, therefore the system is in all possible states at once, with varying degrees of probability.

### The Copenhagen Interpretation

The founders of quantum theory devised a set of postulates that must hold for the theory to have relevance to the observable world. Ensuring these postulates are upheld, scientists are free to interpret the consequences of the quantum realm, attempting to explain phenomena using minimal assumptions.

The most commonly taught (and used) interpretation is the Copenhagen interpretation, which describes the quantum theory as intrinsically indeterministic, with a clear boundary between the observer and the observed system.

### Schrödinger’s Cat Thought Experiment

The renowned thought experiment, devised by Schrödinger and involving an unfortunate cat, successfully devalued the Copenhagen interpretation by linking the macroscopic world with the indeterministic quantum world.

Using quantum logic, the experiment, involving a radioactive substance, a poison gas, and a feline, shows that the cat is in a superposition of being both dead and alive until being directly observed.

**More Physics Questions: Has Physics Solved the Three-Body Problem Yet?**

Schrödinger’s thought experiment illustrates a quantum object in a superposition of states, in this case, the radioactive substance, becoming entangled with the environment, in this case, the poison gas and the cat. This is known as environmental decoherence. The Copenhagen interpretation suggests that the cat is both dead and alive until we observe the cat to be one or the other.

### Introducing the Many-Worlds Interpretation

However, a more advanced interpretation may be equally plausible. Interpreting the experiment differently, as the observer opens the box to reveal the state of the cat, thus collapsing the wavefunction of the system, they too become entangled with the system.

The many-worlds interpretation suggests that the cat is both dead and alive and both a dead cat and an alive cat is observed. The collapsing of the wavefunction causes the branching of the Universe into two distinct universes, one in which the cat survived and one in which the cat died. Practically, the consciousness of the observer only registers one outcome, but both outcomes happen in two different, parallel universes.

The many-worlds interpretation suggests that when the wavefunction of a system involving environmental decoherence collapses, a different universe for each different outcome is created. Using this philosophy, every possible outcome of a quantum system occurs, but each one happens in a different universe.

Quantum theory has many different schools of thought, each with its own assumptions that attempt to explain the link between reality and the abstract mathematics of quantum mechanics. The many-worlds interpretation has gained popularity due to its absurd implications.

If the many-worlds interpretation is in fact the correct interpretation, that would imply there is a version of you elsewhere in spacetime that is a lottery winner, or the prime minister, or even a three-time lottery-winning prime minister (or any other possibility you can imagine).

## Braneworld Cosmology

Another theory that allows for the introduction of parallel universes is string theory. String theory is a ten-dimensional theory that is consistent with quantum theory and seems to predict gravity. With this, it is, therefore, the leading contender for marrying quantum field theory with general relativity. ‘Branes’ arise naturally in string theory. All standard model matter fields and gauge interactions take place on this brane, conceptually described as a slice of spacetime.

Sean Carroll: The many worlds of quantum mechanics

*Video Credit: New Scientist/Youtube.com*

In other words, a brane corresponds to what we would regard as a ‘universe’, and so multiple branes would correspond to multiple universes. The braneworld model hypothesizes that the observable Universe is an infinitesimally thin three-dimensional brane contained within a five-dimensional spacetime.

In the braneworld model, it is perfectly consistent to place multiple branes in the larger five-dimensional spacetime to theoretically observe the effects of interacting branes. Physicists can investigate the cosmological signature that interacting branes would leave in the cosmic microwave background, and look for such a signature using state-of-the-art telescopes such as the ESA’s Planck.

Braneworld theory is currently being studied in the physical community, and it is considered to be one of the main theoretical pursuits of modern physics. It may be a number of years before braneworld theory is advanced to a degree that allows scientists to test its relevance to our Universe.

## The Feasibility of Parallel Universes

The idea of ‘parallel universes’ draws attention due to the intriguing title and the amusing implications associated with such universes. While it may be interesting to discuss the many possibilities that lie out in the cosmos, science aims to be fact-based, and for that reason: apathetic.

The many-worlds interpretation is just one of many equally-feasible interpretations of quantum theory, and multiple branes in braneworld theory is (for now) a mathematical fishing exercise. Mirroring sciences apathy, this article must conclude that parallel universes are not likely to be proven or disproven in the foreseeable future.

With this, they must be regarded as purely mathematical endeavors with no tether to the observable world until further theoretical and experimental work is dedicated to their study.

## References and Further Reading

J. Fay, Copenhagen Interpretation of Quantum Mechanics, Stanford Encyclopedia of Philosophy, https://plato.stanford.edu/entries/qm-copenhagen/, 2002

E. Schrödinger, Die gegenwärtige Situation in der Quantenmechanik (The present situation in quantum mechanics), Naturwissenschaften 23, 807-812, 1935 https://doi.org/10.1007/BF01491891

H Everett, The Theory of the Universal Wave Function, Thesis at Princeton University, 1956, https://www.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf

M. Schlosshauer et. al., A snapshot of foundational attitudes toward quantum mechanics, https://doi.org/10.1016/j.shpsb.2013.04.004, 2013

L. Randall, R. Sundrum, A Large Mass Hierarchy from a Small Extra Dimension, arXiv:hep-ph/9905221, Phys. Rev. Lett. 83, 3370, 1999, https://arxiv.org/pdf/hep-ph/9905221.pdf

A. Padilla, Braneworld Cosmology and Holography, Thesis at Durham University, 2002, https://arxiv.org/pdf/hep-th/0210217.pdf

ESA - Planck and the cosmic microwave background, https://www.esa.int/Science_Exploration/Space_Science/Planck/Planck_and_the_cosmic_microwave_background, last accessed 17/11/21

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