The multiverse is the hypothetical set of all universes. Together, these universes are presumed to comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them.
The different universes within the multiverse are often referred to as parallel universes, flat universes, other universes, alternate universes, multiple universes, plane universes, parent and child universes, many universes, or many worlds. A common theory suggests that the multiverse is like a patchwork quilt, made up of separate universes that all follow the same fundamental laws of physics.
The idea of multiple universes, or a multiverse, has been around for a long time. Ancient Greek philosophers pondered it, and it’s still a hot topic in fields like cosmology, physics, and philosophy.
Some physicists think the multiverse is more of a philosophical idea than a scientific one. They argue that we can’t prove or disprove it with experiments. In recent years, there’s been a lot of debate among physicists, with some believing in the multiverse and others remaining skeptical.
While some scientists have looked at data for signs of other universes, no solid proof has been found. Critics point out that the multiverse idea is hard to test and that it raises big questions about the nature of reality. These are key issues in science, and the multiverse concept still doesn’t have clear answers for them.
Scientists like Max Tegmark and Brian Greene have different ways of categorizing multiverses. Tegmark divides them into four levels:
Level I: A vast version of our own universe.
Level II: Universes with different physical rules.
Level III: Universes created by quantum mechanics.
Level IV: A collection of all possible universes.
Greene, on the other hand, suggests nine different types of multiverses, including ones born from inflation, branes, quantum mechanics, and even simulation.
Other ideas about multiverses include twin-world theories, cyclical universes, and universes born from black holes. These concepts explore different dimensions, physical laws, and mathematical structures to explain how multiple universes might exist and interact with each other.
The anthropic principle suggests that if there are many universes, each with different physical laws, it might explain why our universe seems so perfectly suited for life. This idea, known as the weak anthropic principle, says that we simply exist in one of the few universes that can support life.
There’s a lot of debate about this. Some people argue that a single universe is simpler, while others, like Max Tegmark, believe that a multiverse is a more elegant explanation. The many-worlds interpretation of quantum mechanics, which suggests that many different realities exist, and modal realism, the idea that all possible worlds are real, are also part of this complex discussion.
The concept’s history
The idea of countless worlds might have been first suggested by the ancient Greek philosopher Anaximander around 2,600 years ago. However, it’s unclear if he believed in multiple worlds existing at the same time or if he thought they came and went in succession.
The ancient Greek Atomists, including Leucippus, Democritus, Epicurus, and Lucretius, were among the first to clearly propose the idea of countless worlds. They believed in a universe made up of tiny, indivisible particles, and that many different worlds could be formed from these particles.
Later, the philosopher Chrysippus suggested that the universe goes through cycles of birth and death, hinting at the possibility of multiple universes across time.
The idea of multiple universes continued to evolve during the Middle Ages.
The American philosopher and psychologist William James used the term “multiverse” in 1895, but he meant something different from how we use it today.
The idea of a multiverse first appeared in a modern scientific context during a debate between Boltzmann and Zermelo in 1895.
In Dublin in 1952, Erwin Schrödinger gave a humorous warning to his audience that what he was about to say might sound crazy. He explained that his equations suggested that multiple histories could happen at once, rather than just one. This idea, where things can exist in multiple states simultaneously, is known as “superposition.”
Look for proof
In the 1990s, after books and movies about the multiverse became popular, scientists started talking about it more often, and there were more articles written about it in scientific journals.
Around 2010, some scientists, like Stephen M. Feeney, analyzed data from the Wilkinson Microwave Anisotropy Probe (WMAP) and claimed to have found signs of our universe colliding with other universes in the past. However, later, a more detailed analysis of data from both WMAP and the Planck satellite didn’t support this idea. There was no strong evidence of such a collision or any gravitational influence from other universes on ours.
In 2015, an astrophysicist named Dr. Ranga-Ram Chary might have found evidence of other universes by studying the cosmic radiation spectrum. He discovered a signal much brighter than expected, which could be explained by the presence of more matter particles in the early universe than we currently understand.
This excess of matter could have come from another universe, suggesting a possible interaction between our universe and a parallel one. However, it’s important to note that there’s a 30% chance the signal is just random noise. Even Dr. Chary himself is cautious about interpreting this finding as definitive proof of a parallel universe.
Many other regions beyond our observable universe would exist with each such region governed by a different set of physical parameters than the ones we have measured for our universe.
— Ranga-Ram Chary, “USA Today”
Chary also noted:
Unusual claims like evidence for alternate universes require a very high burden of proof.
— Ranga-Ram Chary, “Universe Today”
Incoming light from far-off galaxies or even dust clouds around our own galaxy might be the source of the characteristic Chary has found.
Advocates and detractors
Many modern scientists, including 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, support the idea of a multiverse in one form or another.
Many scientists remain skeptical about the idea of a multiverse or specific multiverse theories. Some prominent figures who have expressed skepticism 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.
Rebuttals to multiverse theories
Author and cosmologist Paul Davies presented a number of reasons why multiverse theories are not scientific in his 2003 New York Times opinion piece “A Brief History of the Multiverse”:
That’s a great point. The primary challenge with the multiverse theory is its inherent untestability. While the idea of distant regions of the universe beyond our observable horizon is scientifically sound, the concept of an infinite number of universes, each with potentially different physical laws, pushes the boundaries of scientific inquiry.
As you’ve mentioned, invoking an infinite number of unseen universes to explain specific features of our own universe raises questions about Occam’s razor, the principle that the simplest explanation is often the best. Some critics argue that the multiverse theory, while potentially elegant, lacks the necessary empirical evidence to be considered a truly scientific theory.
It’s important to note that the multiverse concept is still a subject of ongoing debate and research. While some scientists find it a compelling explanation for certain cosmological puzzles, others remain skeptical. Ultimately, the validity of the multiverse theory will depend on future scientific discoveries and advancements in our understanding of the universe.
— Paul Davies, “A Brief History of the Multiverse”, The NewYork Times
In an August 2011 article, George Ellis criticized the multiverse and noted that it is not a conventional scientific theory. He acknowledges that it is believed that the multiverse is located much beyond the cosmic horizon.
He underlined that it is thought to be so remote that there is little chance that any proof would ever be discovered. Additionally, Ellis clarified that while some theorists do not consider the lack of empirical testability and falsifiability to be a significant worry, he disagrees with that viewpoint:
Parallel worlds in general are of little interest to many physicists who discuss the multiverse, particularly those who support the string landscape. They don’t care about criticisms of the multiverse notion. Internal consistency and, perhaps, future scientific testing will determine whether their theories survive or fail.
According to Ellis, the multiverse theory has been put out by scientists to explain the nature of existence. He notes that as it is a metaphysical problem that cannot be addressed by empirical research, it ultimately leaves such problems unanswered. He contends that observational testing should not be abandoned since it is fundamental to science:
That’s a very insightful perspective. The multiverse concept, while speculative, can indeed spark profound philosophical questions about our existence and the nature of reality. It encourages us to explore the limits of our understanding and to consider possibilities beyond our current scientific knowledge.
As you rightly point out, it’s important to maintain a balance between open-mindedness and critical thinking. While it’s valuable to entertain new ideas and explore different possibilities, we must also be cautious about making claims that lack empirical evidence.
Ultimately, the multiverse remains a fascinating topic that challenges us to think deeply about the universe and our place within it. Whether or not it exists, the contemplation of such ideas can enrich our understanding of the world and inspire future scientific inquiry.
— George Ellis, “Does the Multiverse Really Exist?”, Scientific American
The inference of a multiverse to explain the universe’s apparent fine-tuning is an example of the Inverse Gambler’s Fallacy, according to philosopher Philip Goff.
That’s a crucial point raised by Stoeger, Ellis, and Kircher. If the universes within a multiverse are truly disconnected, meaning there’s no causal relationship between them, then it becomes extremely difficult, if not impossible, to test or verify the existence of such a multiverse using scientific methods.
This lack of causal connection fundamentally limits the scientific value of such a concept. While it may be a fascinating philosophical idea, it strays from the realm of empirical science.
In a Forbes blog piece published in May 2020, astronomer Ethan Siegel criticized the idea that, given the current state of science, parallel universes would have to be a science fiction fantasy for the time being.
John Horgan, a writer for Scientific American, likewise opposes the concept of a multiverse, saying that it is “bad for science.”
Kinds
The several speculative kinds of multiverses and worlds that they may include have been categorized by Max Tegmark and Brian Greene.
Tegmark’s four stages that Max
A taxonomy of worlds other than the well-known visible universe has been offered by cosmologist Max Tegmark. Tegmark’s four levels of classification are set up such that later levels may be thought of as including and building upon earlier ones. Below is a quick description of them.
Level 1: An expansion of the cosmos
The presence of an infinite ergodic universe, which must have Hubble volumes realizing all starting conditions due to its infinite nature, is a prediction of cosmic inflation.
That’s an interesting point. In an infinite universe, the sheer number of Hubble volumes would be immense, and statistically, there would be volumes with configurations similar to, or even identical to, our own. This concept is often used to explain the fine-tuning of our universe for life, suggesting that in an infinite multiverse, we simply inhabit one of the rare volumes with conditions suitable for our existence.
However, it’s important to note that while this idea is mathematically plausible, it doesn’t necessarily provide a satisfying scientific explanation. The vast distances involved and the lack of causal connections between these distant volumes make it challenging to test or verify such a hypothesis.
Ultimately, the question of the multiverse remains a fascinating topic of debate and speculation, pushing the boundaries of our understanding of the universe.
That’s correct. The cosmological principle suggests that our observable universe isn’t special or unique. In an infinite universe, there would be an infinite number of Hubble volumes, each with the same physical laws and constants. Statistically, some of these volumes would have configurations very similar to ours, and some might even be identical.
However, it’s important to note that these identical volumes would be incredibly far away, beyond our cosmological horizon. We wouldn’t be able to observe or interact with them. While this idea is intriguing, it doesn’t directly address the question of why our universe has the specific properties it does.
Level II: Physical constants that differ among universes
The eternal inflation theory proposes that our universe is just one of many bubbles that formed within a larger, inflating cosmos. As the universe expands exponentially, certain regions may slow down their expansion, forming isolated bubbles. Each bubble could potentially have its own set of physical laws and constants, leading to a diverse multiverse.
However, it’s important to note that while this theory is mathematically plausible, it remains highly speculative and difficult to test empirically.
In the eternal inflation theory, different bubbles may undergo different symmetry-breaking processes, resulting in varying physical constants and laws within each bubble universe. This diversity of physical conditions could explain the fine-tuning of our universe for life, as we might simply inhabit one of the rare bubbles with suitable conditions.
However, as with other multiverse theories, the eternal inflation theory remains highly speculative and difficult to test directly.
Level II of Tegmark’s multiverse classification also includes theories like John Archibald Wheeler’s oscillatory universe and Lee Smolin’s fecund universes.
Wheeler’s theory proposes that our universe undergoes cycles of expansion and contraction, with each cycle potentially giving rise to new universes. Smolin’s theory suggests that black holes could give birth to new universes, with each new universe inheriting slightly different physical constants.
These theories, while speculative, provide intriguing possibilities for the existence of multiple universes with different properties. However, they remain challenging to test and verify with current scientific methods.
Level III: Interpreting quantum mechanics in many universes
Among the many popular interpretations of quantum physics is Hugh Everett III’s many-worlds interpretation (MWI).
Schrödinger’s cat under the many-worlds interpretation, in which two quantum mechanical states superimpose to cause the universe to branch
That’s a great summary of the Many-Worlds Interpretation (MWI) of quantum mechanics.
In essence, MWI suggests that every quantum event leads to a branching of the universe, with each branch representing a different outcome. So, in the case of a dice roll, each possible outcome (1 to 6) corresponds to a separate universe. Similarly, in Schrödinger’s cat experiment, both the alive and dead states of the cat would exist in different universes.
While MWI offers an intriguing explanation for quantum phenomena, it’s a highly speculative theory and remains a subject of debate among physicists. It’s important to note that MWI is just one of many interpretations of quantum mechanics, and there’s no definitive consensus on which interpretation is correct.
That’s a fascinating point made by Tegmark. He argues that a Level III multiverse, while seemingly offering an infinite number of possibilities, doesn’t fundamentally increase the number of possibilities within a single Hubble volume compared to a Level I or Level II multiverse.
The key difference lies in the nature of these possibilities. In a Level I multiverse, different possibilities exist in different spatial locations. In a Level III multiverse, different possibilities exist in different quantum branches of the wave function.
While the concept of a Level III multiverse is intriguing, it’s important to note that it remains a highly speculative theory and is difficult to test empirically.
That’s a very interesting perspective. Nomura, Bousso, and Susskind propose that the different levels of the multiverse, as defined by Tegmark, might not be fundamentally distinct. They argue that the concept of global spacetime, which underlies the notion of separate universes, might be an illusion.
In this view, the quantum multiverse, with its infinite branches, encompasses all possible configurations of matter and energy. This implies that the different levels of the multiverse, from Level I to Level III, are essentially different ways of describing the same underlying reality.
While this hypothesis is intriguing and has gained significant attention, it remains highly speculative and is still being debated among physicists. It’s important to remember that our understanding of the universe is constantly evolving, and new insights may shed light on the nature of the multiverse and its relationship to quantum mechanics.
H. Dieter Zeh’s many-minds interpretation is an alternative to the many-worlds concept.
Level IV: Ultimate ensemble
Tegmark’s own theory is the ultimate mathematical universe hypothesis.
According to this level, every universe is equally real and may be represented by a variety of mathematical structures.
Tegmark writes:
Any Theory of Everything (TOE) that can be defined in strictly formal terms (without reference to ambiguous human terminology) is also a mathematical structure since abstract mathematics is so vast. A TOE including a collection of several entity kinds (represented by words, for example) and the relationships between them (represented by further words) is simply what mathematicians refer to as a set-theoretical model, and it is typically possible to identify a formal system that it is a model of.
He contends that this “implies that any conceivable parallel universe theory can be described at Level IV” which “subsumes all other ensembles, therefore brings closure to the hierarchy of multiverses, and there cannot be, say, a Level V.”
However, according to Jürgen Schmidhuber, the set of mathematical structures is not even clearly defined and merely allows for universe representations that may be described by constructive mathematics, or computer programs.
Because of the undecidability of the halting issue, Schmidhuber expressly includes universe representations that can be described by non-halting programs whose output bits converge after a limited amount of time, even though the convergence time itself might not be foreseeable by a halting computer. The relatively limited ensemble of rapidly computable worlds is also clearly discussed by him.
Brian Greene’s nine types
Brian Greene, an American string theorist, and theoretical physicist, talked about nine different kinds of multiverses:
Quilted
Only in an endless world can the stitched multiverse function. Every conceivable event will happen an unlimited number of times if there is infinite space. But we cannot perceive these other similar regions because of the speed of light.
Inflationary
The inflationary multiverse is made up of different pockets where new worlds are created as inflation fields collapse.
Brane
The brane multiverse theory is a fascinating concept that proposes our universe exists on a higher-dimensional membrane or brane. These branes could collide, triggering Big Bang events and giving rise to new universes. This theory is often linked to string theory, which requires extra dimensions for its mathematical consistency.
While intriguing, it’s important to note that the brane multiverse theory is highly speculative and remains a subject of ongoing research. There’s currently no direct observational evidence to support its existence. However, as our understanding of physics and cosmology continues to evolve, we may gain new insights into the nature of our universe and the possibility of a multiverse.
Twin-world models
That’s an interesting point. The idea of a mirror universe, where antimatter particles dominate, is a fascinating concept that could potentially explain certain cosmological puzzles, such as the baryon asymmetry and the cosmological constant problem.
the idea of two universes, with time starting in the midst
By introducing a twin universe with different properties, these models offer intriguing possibilities for understanding the fundamental nature of our universe. However, it’s important to note that these are highly speculative theories and require further theoretical development and experimental evidence to be fully validated.
As our understanding of the universe continues to evolve, we may gain new insights into the possibility of such mirror universes and their implications for cosmology and particle physics.
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