Multiverse

The multiverse is a hypothetical collection containing every universe that exists. Together, these universes are thought to make up all of reality: every bit of space, time, matter, energy, information, and the physical laws and constants that govern them. These different universes go by many names, such as parallel universes, alternate universes, many worlds, or parent and child universes. A common view is that the multiverse resembles a patchwork quilt of separate universes, all following the same physical laws.

The idea of multiple universes has been around for centuries, debated across cosmology, physics, and philosophy. Some physicists argue that the multiverse is more a philosophical concept than a scientific one, since it cannot be empirically falsified. In recent years, both supporters and skeptics have emerged within the physics community. While some scientists have searched for evidence of other universes, no statistically significant proof has been found. Critics point out that the multiverse lacks testability and falsifiability, key requirements for scientific inquiry, and raises unresolved metaphysical issues.

Max Tegmark and Brian Greene have each created classification systems for multiverses and universes. Tegmark’s four levels include: Level I, an extension of our own universe; Level II, universes with different physical constants; Level III, the many-worlds interpretation of quantum mechanics; and Level IV, the ultimate ensemble. Greene’s nine types include quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated, and ultimate. These ideas explore different dimensions, physical laws, and mathematical structures to explain how multiple universes might exist and interact. Other concepts include twin-world models, cyclic theories, M-theory, and black-hole cosmology.

The anthropic principle suggests that a multitude of universes, each with different physical laws, could explain why our universe appears fine-tuned for conscious life. The weak anthropic principle states that we exist in one of the few universes that can support life. Debates over Occam’s razor—whether the multiverse or a single universe is simpler—arise, with proponents like Max Tegmark arguing the multiverse is more elegant. The many-worlds interpretation of quantum mechanics and modal realism, the belief that all possible worlds are equally real, are also discussed in relation to the anthropic principle.

Historically, the idea of infinite worlds may trace back to the pre-Socratic Greek philosopher Anaximander in the sixth century BCE, though it is unclear if he believed in multiple coexistent or successive worlds. Ancient Indian Hindu and Buddhist thinkers also proposed multiple worlds or realms. The first clear attribution of innumerable worlds comes from the Greek Atomists Leucippus and Democritus in the fifth century BCE, followed by Epicurus and the Roman Epicurean Lucretius. In the third century BCE, Chrysippus suggested the world eternally expired and regenerated, implying multiple universes across time. The concept became more defined in the Middle Ages, and during the Renaissance, Giordano Bruno expressed the idea of infinite worlds.

In modern science, the American philosopher and psychologist William James used the term “multiverse” in 1895, but in a different context. The concept first appeared in a modern scientific setting during the 1895 debate between Boltzmann and Zermelo. In 1952, Erwin Schrödinger gave a lecture in Dublin, jokingly warning his audience that what he was about to say might seem lunatic. He explained that when his equations described several different histories, these were not alternatives but all happened simultaneously—a duality called superposition.

In the 1990s, after fictional works about the multiverse gained popularity, scientific discussions and journal articles on the topic increased. Around 2010, scientists like Stephen M. Feeney analyzed data from the Wilkinson Microwave Anisotropy Probe (WMAP) and claimed to find evidence that our universe had collided with other parallel universes in the distant past. However, a more thorough analysis of data from WMAP and the Planck satellite, which has three times higher resolution, found no statistically significant evidence of such a bubble universe collision. No evidence of any gravitational pull from other universes on ours was found either.

In 2015, an astrophysicist may have found evidence of alternate or parallel universes by looking back to a time just after the Big Bang, though this remains debated. Dr. Ranga-Ram Chary, analyzing the cosmic radiation spectrum, detected a signal 4,500 times brighter than expected based on the number of protons and electrons believed to exist in the early universe. This signal—an emission line from atom formation during the era of recombination—is more consistent with a universe whose ratio of matter particles to photons is about 65 times greater than our own. There is a 30% chance that this signal is simply noise.