Multiverse
In the sixth century BCE, the pre-Socratic Greek philosopher Anaximander first suggested that infinite worlds might exist. Historians debate whether he believed these worlds co-existed or appeared in succession. The Ancient Greek Atomists Leucippus and Democritus definitively attributed the concept of innumerable worlds to their philosophy in the 5th century BCE. Epicurus followed them from 341 to 270 BCE, while Lucretius expanded the idea in the 1st century BCE as a Roman Epicurean. Chrysippus offered a different view in the third century BCE by suggesting the world eternally expired and regenerated. This effectively proposed multiple universes across time rather than space. Giordano Bruno expressed the concept of infinite worlds during the Renaissance between 1548 and 1600. William James used the term multiverse in 1895 within his work The Will to Believe, though he applied it differently than modern science does. The scientific context emerged later during a 1895 debate between Boltzmann and Zermelo. Erwin Schrödinger gave a lecture in Dublin in 1952 where he warned his audience that his ideas might seem lunatic. He stated that when equations described several histories, they were not alternatives but all happened simultaneously.
Cosmologist Max Tegmark organized universes into four distinct levels in his classification scheme. Level I describes an extension of our universe containing infinite Hubble volumes with identical physical laws. Tegmark estimates an identical volume to ours should be about 10 to the power of 10 to the 115th meters away from us. Level II involves universes with different physical constants formed as bubbles in eternal inflation theory. John Archibald Wheeler's oscillatory universe theory fits here alongside Lee Smolin's fecund universes theory. Level III represents Hugh Everett III's many-worlds interpretation of quantum mechanics where every possible observation creates a real world. Tegmark argues this level contains no more possibilities than Level I or Level II because doppelgängers reside elsewhere in three-dimensional space or on another quantum branch. Level IV is the ultimate ensemble considering all universes equally real if describable by mathematical structures. Jürgen Schmidhuber disputes this set is well-defined and limits it to constructive mathematics. Brian Greene proposed nine types including quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated, and ultimate multiverses. The brane version postulates our universe exists on a membrane floating in a higher dimension called bulk.
Scientists analyzed data from the Wilkinson Microwave Anisotropy Probe around 2010 looking for signs of parallel universes. Stephen M. Feeney claimed evidence suggested this universe collided with other universes in the distant past. A more thorough analysis using Planck satellite data did not reveal any statistically significant evidence of such collisions. The Planck satellite has a resolution three times higher than WMAP yet found no gravitational pull from other universes. In 2015 astrophysicist Ranga-Ram Chary analyzed cosmic radiation spectrum data and found a signal 4,500 times brighter than expected. This emission line arose during the era of recombination when atoms formed after the Big Bang. The signal suggests a ratio of matter particles to photons about 65 times greater than our own universe. There remains a 30% chance that this signal is noise rather than real evidence. Chary noted the signature might be incoming light from distant galaxies or clouds of dust surrounding our galaxy. No conclusive proof exists despite these attempts to detect bubble universe collisions through cosmic background data.
Modern proponents include Lee Smolin, Don Page, Brian Greene, Max Tegmark, Alan Guth, Andrei Linde, Michio Kaku, David Deutsch, Leonard Susskind, Alexander Vilenkin, Yasunori Nomura, Raj Pathria, Laura Mersini-Houghton, Neil deGrasse Tyson, Sean Carroll, and Stephen Hawking. Skeptics generally include Sabine Hossenfelder, David Gross, Paul Steinhardt, Anna Ijjas, Abraham Loeb, David Spergel, Neil Turok, Viatcheslav Mukhanov, Michael S. Turner, Roger Penrose, George Ellis, Joe Silk, Carlo Rovelli, Adam Frank, Marcelo Gleiser, Jim Baggott, and Paul Davies. Paul Davies wrote an opinion piece in the New York Times in 2003 arguing multiverse hypotheses are non-scientific. George Ellis criticized the concept in August 2011 stating it is not a traditional scientific theory. He emphasized that evidence will likely never be found because the multiverse lies far beyond the cosmological horizon. Philip Goff argues inferring a multiverse to explain fine-tuning is an example of Inverse Gambler's Fallacy. Stoeger, Ellis, and Kircher note universes in true multiverse theories are completely disjoint with no causal connection. Ethan Siegel expressed criticism in a Forbes blog post in May 2020 saying parallel universes remain science fiction dreams for now. John Horgan claims they are bad for science.
The weak anthropic principle posits we exist in one of the few universes that support life. If many universes exist with different physical laws, some would naturally have parameters suitable for conscious life. This removes the need for intelligent design as an explanation for conditions promoting our existence. Arthur Schopenhauer presented early reasoning in his 1844 work Von der Nichtigkeit und dem Leiden des Lebens. He argued our world must be the worst possible because if significantly worse it could not continue to exist. Critics argue postulating infinite unobservable universes violates Occam's razor by adding unnecessary complexity. Proponents like Max Tegmark counter that Kolmogorov complexity makes the proposed multiverse simpler than a single idiosyncratic universe. The probability might be extremely small for any particular universe to have requisite conditions for life. Those conditions do not require intelligent design but rather statistical inevitability across a vast ensemble. Some scientists suggest humanity exists in only one unique history while others conceive universes so similar humanity appears in many equally real separate worlds.
David Lewis posited that all possible worlds exist and are just as real as the world we live in. This position is known as modal realism within philosophical debates about reality. Possible worlds explain probability and hypothetical statements regarding histories or variables of nature. Not all potential universes may ever be realized even over infinite time. A universe containing life on Earth is radically non-ergodic since most organisms will never be realized. Quantum Darwinism allows one to avoid adopting a many-worlds interpretation where all branches are equally real. The debate continues whether other worlds in quantum mechanics are truly real or merely mathematical constructs. Philosophers distinguish between possible versus real worlds when analyzing the existence of multiple universes. Some theories propose universes form following logical order without existing simultaneously. Key natural constituents potentially vary between these sequential universes in cyclic theories. The distinction remains central to understanding whether parallel universes are physical realities or abstract possibilities.
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Common questions
When did Anaximander first suggest infinite worlds might exist?
Anaximander first suggested that infinite worlds might exist in the sixth century BCE. Historians debate whether he believed these worlds co-existed or appeared in succession.
Who organized universes into four distinct levels in his classification scheme?
Cosmologist Max Tegmark organized universes into four distinct levels in his classification scheme. Level I describes an extension of our universe containing infinite Hubble volumes with identical physical laws, while Level IV is the ultimate ensemble considering all universes equally real if describable by mathematical structures.
What evidence did scientists find from the Planck satellite regarding parallel universes?
A more thorough analysis using Planck satellite data did not reveal any statistically significant evidence of such collisions. The Planck satellite has a resolution three times higher than WMAP yet found no gravitational pull from other universes.
Why do skeptics like George Ellis criticize multiverse hypotheses?
George Ellis criticized the concept in August 2011 stating it is not a traditional scientific theory. He emphasized that evidence will likely never be found because the multiverse lies far beyond the cosmological horizon.
How does the weak anthropic principle explain conditions promoting life without intelligent design?
The weak anthropic principle posits we exist in one of the few universes that support life. If many universes exist with different physical laws, some would naturally have parameters suitable for conscious life.