It’s a bit of a controversy this one. The thought that there are parallel universes may seem a little far fetched and something that belongs in a science fiction movie. But, the idea that we all live in a multiverse that’s made up of a number of parallel universes is still a view nonetheless and it’s one that’s taken by a lot of people. The hard part now is in trying to prove they exist.
On famous experiment that set out to do this was the thought experiment of Schrodinger’s cat. This animal was a spooky animal that lived in a closed box. The idea was that by opening the box you’re opening the one of the possible future’s of the cat, meaning it could be either dead or alive, or perhaps both. At first, this may seem impossible due to what us humans believe to be true about life and death, but according to the laws of quantum mechanics, it’s perfectly possible. A quantum mechanical state is one that is possible of all states and in the case of Schrodinger’s cat, the cat is the superposition of dead and alive states. For one to accept that all these possibilities could be true, one must accept the idea of the multiverse.
String theory is a popular theory among physicists and one that seems the most promising in terms of unifying quantum mechanics and gravity. But this in itself is difficult because gravitational force is so hard to prove on a small scale. However, when it comes to string theory, it states that all fundamental particles are made up of 1D strings that can describe all four known forces of nature at once. They are gravity, electromagnetism, and the nuclear forces. But in order for string theory to work, a minimum of ten dimensions are needed. With us only being able to see four (height, width, depth (all spatial) and time (temporal)), we must assume that the others remain hidden if string theory is to be correct. If we’re to try and use the theory to explain what we see, the extra dimensions must be “compactified” in such a way that they’re too minute for anyone to see.
The problem with this is that there are various ways of doing this compactification and each would result in its own universe with its own laws. However, there are still many objections to this methodology and as of yet, the issue is still unsolved. String theory won’t provide any mechanism to predict which landscape of possibility we live in either, which makes it pretty useless. Luckily, an idea that stems from an early universe study seeks to work around this. Very shortly after the Big Bang occurred the universe went through a period of accelerated expansion called inflation. The theory predicted there was a spectrum of temperature fluctuations that was later confirmed by the Planck spacecraft, Cosmic Background Explorer, and the Wilkinson Microwave Anisotropy Probe.
Most physicists have come to accept the inflation theory, although the exact details are still largely debated. But, the problem with the theory lies in that there must still be parts of the universe that are accelerating, and due to quantum fluctuations of space-time, there will be certain parts of the universe that will never stop inflating. Therefore some of these expanding parts will become other universes, and so would they and so on and so on. This will keep happening an infinite number of times. Add this to the string theory and you get an infinite number of universes that each have a different compactification of the extra dimensions and therefore different physical laws too. These predicted universes live in the same physical space and can even overlap or collide with one another. If collisions do occur, possible signatures could be left if the cosmic sky which gives us something to search for. While scientists are actively seeking these signatures, none yet have been found. But scientists will not give up and are determined to find proof of these parallel universes and so the search must go on.
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