Parallel universes not only exist but they interact with one another, a group of physicists have said.
Researchers at the Griffith University in Queensland, Australia, say the universe we exist in is just one of a "gigantic number of worlds", with some being almost identical to our own and others vastly different.
The idea of parallel universes has been around for over 50 years. In another theory of parallel worlds, Many-Worlds Interpretation, all possible scenarios could be realised – for example, a world where dinosaurs were not killed off by a giant asteroid.
However, in their new theory - Many Interacting Worlds - the researchers say all parallel worlds interact with one another and that their existence could potentially be tested.
Published in the journal Physical Review X, scientists said parallel universes evolve independently and that nearby worlds influence one another by a force of repulsion.
The scientists believe their theory helps to explain quantum mechanics. Howard Wiseman, from the university's Centre for Quantum Dynamics, said: "The idea of parallel universes in quantum mechanics has been around since 1957.
"In the well-known 'Many-Worlds Interpretation', each universe branches into a bunch of new universes every time a quantum measurement is made. All possibilities are therefore realised – in some universes the dinosaur-killing asteroid missed Earth. In others, Australia was colonised by the Portuguese.
"But critics question the reality of these other universes, since they do not influence our universe at all. On this score, our 'Many Interacting Worlds' approach is completely different, as its name implies."
Quantum theory is needed to explain how the universe works on a microscopic scale. However, it is extremely difficult to understand, showing weird phenomena that do not adhere to laws of cause and effect. Theoretical physicist Richard Feynman once said: "I think I can safely say that nobody understands quantum mechanics."
In their interacting parallel words, Wiseman and his colleagues suggest there are countless worlds, some being very similar to our own and others completely different. All worlds are equally real and exist throughout time, and all quantum phenomena come from a universal force of repulsion between nearby worlds.
Co-author Michael Hall said: "The beauty of our approach is that if there is just one world our theory reduces to Newtonian mechanics, while if there is a gigantic number of worlds it reproduces quantum mechanics.
"In between it predicts something new that is neither Newton's theory nor quantum theory. We also believe that, in providing a new mental picture of quantum effects, it will be useful in planning experiments to test and exploit quantum phenomena."