Almost all mythologies and religions refer to different planes of existence, such as a home to gods, fairies, or the dead. The existence of parallel universes is a hotly debated topic in physics, one that has the potential to redefine our concept of reality. Scientific literature defines three major types of parallel universes: hyperspace (or higher dimensions), the multiverse, and quantum parallel universes.

Ancient Greek geometry was founded on the assumption that we live in three dimensions: length, width, and height. Physicists have long considered time the fourth dimension, but not until the late 19th century was the idea of further higher dimensions taken seriously. Mathematicians calculated the properties of the fifth dimension, and physicist Theodor Kaluza (1885-1954) caused a sensation in scientific circles by showing that these equations could unify Einstein’s theory of special relativity and Maxwell’s theory of light, meaning that three of the four fundamental forces of the universe could be unified in five-dimensional space-time. His theories posited that light is ripples in the fifth dimension, and that the fifth dimension itself is curled up too small to be observed. This work inspired artistic and literary movements across the world, such as cubism and surrealism, and was vital to the development of superstring theory.

Superstring theory explained experimental data on subatomic particles and offered a means to unite the two major physics theories of the last century: quantum theory and general relativity. Superstring theory proposes that subatomic particles are different vibrations of strings and that gravity is one of the lowest vibrations. The strings can vibrate only in specific dimensions of space-time, however, and scientists are attempting to find proof of the existence of these additional higher dimensions by searching for deviations from Newton’s inverse square law of gravity over microscopic distances. The most promising branch of string theory proposes that there are 11 dimensions: the four familiar dimensions in which we exist (length, width, height, and time); six additional dimensions collapsed or curled in on themselves; and an 11th dimension called the membrane that exists like the surface of a sphere. The multiverse theory proposes that our universe exists as one of trillions of universes floating like soap bubbles. Some physicists hypothesize that dark energy and dark matter are the effects of gravity from undetectable ordinary matter in another universe. These universes could hypothetically merge, split, or collide, and a collision is the basis of the “Big Splat” hypothesis for the end of the universe.

Quantum theory has no underlying logical structure and depends on probabilities. Schrodinger’s cat is the best-known example of this: A person cannot know whether a cat locked in a box is alive or not, so it exists in a state of uncertainty as a sum of the probability of its being alive and being dead. The absurdity of such a paradox vexes many physicists. One proposed solution is that a fundamental difference exists between the probabilistic quantum world and the definite macroscopic world. The Copenhagen school envisions an invisible wall between the atomic universe and the measurable world of large objects, where waves of probability collapse into definite states of existence. This theory is undermined, however, by the invention of nanotechnology and atomic microscopes, which allow physicists to make measurements on an atomic scale, effectively removing any wall. There’s no consensus on a resolution. Some believe that a cosmic consciousness pervades the universe and that its observation determines states of existence. Others see a solution in the proposed existence of quantum parallel universes. The many-worlds hypothesis posits that any possible universe exists, as an infinite number of branching, parallel universes spring into existence and diverge each time a quantum event occurs. No interaction is possible between these universes, which are each defined by the sum wave function of the universe as a whole, because they decohere as their respective wave functions diverge and fall out of tune with each other.

This many-worlds hypothesis has become more popular as physicists apply quantum theory to the universe itself; although quantum cosmology initially appears to be an oxymoron, the fact that the universe must have been smaller than an electron at the moment of the Big Bang means that it must have existed in parallel states. Hawking spent many years advocating for this theory and for the idea that our universe began as a quantum fluctuation in the vacuum, one bubble in the foam of space-time that happened to expand rather than immediately contract into nothingness. Although seemingly untestable, this theory is consistent with several observations: that our universe has no spin, that the total amount of matter in the universe is very small (perhaps zero in total), and that positive and negative charges in the universe cancel one another out. In a closed universe coming from nothing, the net amount of matter and energy it contains should be zero.

An extremely advanced civilization might develop the capability to contact another universe in the multiverse. One theory is that gravity would be central to the contact because it warps space-time. Another possibility in the distant future is that physicists might be able to create a new universe in a laboratory by pumping energy around the level of Planck’s constant into one of the innumerable baby universes that exist as tiny bubbles in space-time, and then travel into that universe through a wormhole. Theories that our current universe may be suited to host life because it evolved through changes wrought over generations by inhabitants of successive older universes are untestable and therefore exist primarily in the realm of science fiction rather than academia. Our current technological capabilities are far too primitive to detect the existence of any kind of parallel universe, but because parallel universes might be discoverable for a far more advanced civilization, they are a Class 2 impossibility.

Almost all mythologies and religions refer to different planes of existence, such as a home to gods, fairies, or the dead. The existence of parallel universes is a hotly debated topic in physics, one that has the potential to redefine our concept of reality. Scientific literature defines three major types of parallel universes: hyperspace (or higher dimensions), the multiverse, and quantum parallel universes.

Ancient Greek geometry was founded on the assumption that we live in three dimensions: length, width, and height. Physicists have long considered time the fourth dimension, but not until the late 19th century was the idea of further higher dimensions taken seriously. Mathematicians calculated the properties of the fifth dimension, and physicist Theodor Kaluza (1885-1954) caused a sensation in scientific circles by showing that these equations could unify Einstein’s theory of special relativity and Maxwell’s theory of light, meaning that three of the four fundamental forces of the universe could be unified in five-dimensional space-time. His theories posited that light is ripples in the fifth dimension, and that the fifth dimension itself is curled up too small to be observed. This work inspired artistic and literary movements across the world, such as cubism and surrealism, and was vital to the development of superstring theory.

Superstring theory explained experimental data on subatomic particles and offered a means to unite the two major physics theories of the last century: quantum theory and general relativity. Superstring theory proposes that subatomic particles are different vibrations of strings and that gravity is one of the lowest vibrations. The strings can vibrate only in specific dimensions of space-time, however, and scientists are attempting to find proof of the existence of these additional higher dimensions by searching for deviations from Newton’s inverse square law of gravity over microscopic distances. The most promising branch of string theory proposes that there are 11 dimensions: the four familiar dimensions in which we exist (length, width, height, and time); six additional dimensions collapsed or curled in on themselves; and an 11th dimension called the membrane that exists like the surface of a sphere. The multiverse theory proposes that our universe exists as one of trillions of universes floating like soap bubbles. Some physicists hypothesize that dark energy and dark matter are the effects of gravity from undetectable ordinary matter in another universe. These universes could hypothetically merge, split, or collide, and a collision is the basis of the “Big Splat” hypothesis for the end of the universe.

Quantum theory has no underlying logical structure and depends on probabilities. Schrodinger’s cat is the best-known example of this: A person cannot know whether a cat locked in a box is alive or not, so it exists in a state of uncertainty as a sum of the probability of its being alive and being dead. The absurdity of such a paradox vexes many physicists. One proposed solution is that a fundamental difference exists between the probabilistic quantum world and the definite macroscopic world. The Copenhagen school envisions an invisible wall between the atomic universe and the measurable world of large objects, where waves of probability collapse into definite states of existence. This theory is undermined, however, by the invention of nanotechnology and atomic microscopes, which allow physicists to make measurements on an atomic scale, effectively removing any wall. There’s no consensus on a resolution. Some believe that a cosmic consciousness pervades the universe and that its observation determines states of existence. Others see a solution in the proposed existence of quantum parallel universes. The many-worlds hypothesis posits that any possible universe exists, as an infinite number of branching, parallel universes spring into existence and diverge each time a quantum event occurs. No interaction is possible between these universes, which are each defined by the sum wave function of the universe as a whole, because they decohere as their respective wave functions diverge and fall out of tune with each other.

This many-worlds hypothesis has become more popular as physicists apply quantum theory to the universe itself; although quantum cosmology initially appears to be an oxymoron, the fact that the universe must have been smaller than an electron at the moment of the Big Bang means that it must have existed in parallel states. Hawking spent many years advocating for this theory and for the idea that our universe began as a quantum fluctuation in the vacuum, one bubble in the foam of space-time that happened to expand rather than immediately contract into nothingness. Although seemingly untestable, this theory is consistent with several observations: that our universe has no spin, that the total amount of matter in the universe is very small (perhaps zero in total), and that positive and negative charges in the universe cancel one another out. In a closed universe coming from nothing, the net amount of matter and energy it contains should be zero.

An extremely advanced civilization might develop the capability to contact another universe in the multiverse. One theory is that gravity would be central to the contact because it warps space-time. Another possibility in the distant future is that physicists might be able to create a new universe in a laboratory by pumping energy around the level of Planck’s constant into one of the innumerable baby universes that exist as tiny bubbles in space-time, and then travel into that universe through a wormhole. Theories that our current universe may be suited to host life because it evolved through changes wrought over generations by inhabitants of successive older universes are untestable and therefore exist primarily in the realm of science fiction rather than academia. Our current technological capabilities are far too primitive to detect the existence of any kind of parallel universe, but because parallel universes might be discoverable for a far more advanced civilization, they are a Class 2 impossibility.