"Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying."
~Arthur C. Clarke
~Arthur C. Clarke
If you lived in the medieval ages and someone told you that there were not one but many universes, you would probably laugh it off, or maybe get the guy burnt on a stake. Till recently, the concept of parallel universes or multiverse was the stuff of science fiction.
The universe was considered to be one big entity that contained everything that there is. The notion of more than one universe where our wars have a different result or the extinct species are still alive, was laughable and mind-boggling. So do these parallel universes really exist? Could there not only be parallel Earths, but also parallel you and me?
According to many physicists, there actually might be more than one universe and in these universes we could have a copy of ourselves. As research in the field continues to unveil new data, many credible proponents of this theory are trying to find ways to scientifically decode the data.
There are a number of physics and mathematics theories to back the parallel universe theory. Here are some of the popular theories of parallel universes.
Quantum Theory and Many-Worlds Interpretation
In this theory the entire universe is treated as a quantum-mechanical, an indeterministic microscopic world, where elementary particles are in constant erratic motion. Everett backed this theory with mathematical tools like interpretation.
Instead of just saying that doing A will give you the conclusion B, Everett's work showed why in the quantum world doing A would lead to B.
He came up with the universal wave function to describe how elementary particles move around. This function describes how elementary particles like an electron, can take up different positions.
A position that is assumed by an electron is as real as the other position that it takes. When you measure it, it obviously takes one position, which in quantum mechanics is just one probability. This might vary as you measure it again in the same way.
This idea is further supported by the Copenhagen interpretation of quantum mechanics which states that all quantum particles do not exist in one state or other, but rather in all of its possible states at once. This is known as superimposition. The only difference is our observation which allows the object to choose just one state.
If we use this universal wave function to describe the universe, we could say that the universe is duplicated or rather split, with each possible outcome of the measurement. This theory thus, helps in describing the universe in terms of probabilities.
What it Implies
What it means is that there are different and equally real set of circumstances in separate universes. It is our choice that splits the universe. So, if you have a near fatal accident in your current universe, in another parallel universe you would probably be dead. The probabilities are endless.
String Theory and "Bubble Universes"
According to this theory, the universe can be explained in form of ten to eleven strings that vibrate in not one, but eleven dimensions. Instead of the point-like particles of particle physics, they are replaced by one-dimensional strings which oscillate in many ways.
Michio Kaku, who co-founded the field theory of strings in the late 1960s' likens universe with soap bubbles that are constantly expanding and dying out. Our universe is just one bubble among them. Brian Greene refers to this as "Inflationary multiverse."
What it Implies
According to string theorists, the parallel universes or the soap bubbles, can come into contact with each other. When this happens the gravity can flow between these two bubbles leading to a Big Bang, similar to the one in which our universe was created. They believe that the laws in these universes can be vastly different from the one which we inhabit.
String Theory and "Brane multiverse"
There are many other theories of parallel universes that stem from the string theory. Brian Greene, a string theorist, talks of the "brane multiverse" in his book The Hidden Reality.
According to this theory, our universe is a three-dimensional brane. Brane (short for membrane) refers to objects that can have any number of dimensions. Similar branes with different dimensions could have other parallel universes in them.
One of the best ways to understand the concept of "braneworlds" is provided by Brian Greene. He says that "our universe is one of potentially numerous 'slabs' floating in a higher-dimensional space, much like a slice of bread within a grander cosmic loaf."
What it Implies
These branes are not necessarily parallel to each other, and may at some point slam into each other, thus causing the Big Bang. This cyclic multiverse resets the universe multiple times.
Like the "brane multiverse" and bubble universes, string theory leads to other classifications of parallel universe like the "quilted multiverse" which says that, since space is infinite, there are other regions that could be exactly like our own.
"Holographic multiverse" that stems from string theory and the holographic principle, explains that there is a distant bounding surface, possibly the edge of the universe, in which everything from our universe is precisely mirrored.
Although the proponents of these conjectures would like to believe that these theories prove the existence of parallel universe, many would disagree. Lack of any scientific and experimental evidence is a major reason why the existence of parallel universe is viewed with skepticism.
However, with advances in string theory and sciences, multiverse proposals may be testable in the long run. Moreover, if they are tested, concepts like science fiction concepts like time travel can also be explored.