The existence of parallel universes may seem invented by science fiction writers, with little relevance to modern theoretical physics. But the idea that we live in a ‘multiverse’ consists of an infinite number of parallel universes has been considered a scientific possibility – although it is still a matter of vigorous debate among physicists. The race now is to find a way to test the theory, including the search for signs of collisions with other universes.
It is important to bear in mind that the multiverse view is not really a theory, it is rather a consequence of our current understanding of theoretical physics. This distinction is crucial. We have not waved our hands and saying, “Let there be a multiverse.” Instead, the idea that the universe is perhaps one of infinitely many is derived from current theories, such as quantum mechanics and string theory.
String theory is one of our most, if not the most, promising theory that is able to unify quantum mechanics and gravity. This is notoriously difficult because the gravitational force is so difficult to describe in small scales as the atoms and subatomic particles – which is only explained by the science of quantum mechanics. But string theory, which states that all fundamental particles are made of one-dimensional strings, can describe all the known forces of nature at once: gravity, electromagnetism and the nuclear forces.
However, for string theory work mathematically, it requires at least ten physical dimensions. Since we can only observe four dimensions: height, width, depth (space) and time (time), the extra dimensions of string theory should therefore be hidden in any way, if it is correct. To be able to use the theory to explain a physical phenomenon, these additional dimensions must be “compressed” so that they are too small to be seen.
During the early universe shortly after the Big Bang, the universe underwent a period of rapid expansion called inflation. Although the exact details of the theory are still being discussed, inflation is widely accepted by physicists. However, a consequence of this theory is that there must be other parts of the Universe that are still accelerating. By combining this scenario string theory, there is a possibility that each of these having a different universes compactification of other dimensions and, consequently, have different physical laws.