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It may sound like the realms of science of fiction, but many areas of physics lead to the possible conclusion that there isn't just one universe. Our universe could in fact be one of many; we may live in a multiverse. This has mind-boggling implications, as there could be many parallel worlds, similar to our own where replicas of us lead different lives. There could be a world where you've won a Nobel Prize, or even got accepted into Hogwarts. There are three main areas of physics that all have independently come to this remarkable conclusion: astrophysics, string theory and quantum mechanics.

**Astrophysics**

One of the biggest goals of astrophysics is to determine whether or not our universe is finite, this question causes disagreement between the world's greatest minds.

If the universe is finite, astrophysicists believe it folds back round onto itself to make a four-dimensional shape, perhaps a four-dimensional sphere. The concept of our three-dimensional space folding round to make a four-dimensional sphere is challenging, but it can be more easily understood when you consider the similar example of a two-dimensional map folding back onto itself to make a three-dimensional globe. It is the exact same process as our three-dimensional space folding back round on itself to make a four-dimensional sphere.

The main way astrophysicists are trying to establish whether or not the universe is finite is by looking for data from cosmic microwave background radiation which indicates that one side of the night sky is connected to the other; analogous to how on a world map Alaska and Eastern Russia appear far apart, but when we look at a globe they are right next to each other. So if our three-dimensional universe really does fold round to make a four-dimensional sphere, stars which appear to lie at opposite ends of the night sky, may in fact lie next to each other on the four-dimensional sphere. However, despite great efforts, scientists have not yet found data to indicate that one side of our night sky is connected to the other.

Suppose the universe is not finite, then it has to be infinite, which would mean that there are an infinite number of particles in all directions. Therefore, if you were to travel far enough in any direction, you would come across exact replicas of the configuration of the particles in our world. Perhaps more excitingly, there would also be configurations of particles, which are very close to the configuration to our world, but slightly different. For example, in one configuration you may have a twin or the world may be ruled by artificial intelligence. In fact, every possible variation of our world would exist. Thus according to astrophysics in an infinite universe there will be an infinite number of other worlds; we would exist in a multiverse.

**String Theory**

String theory was born as a means to combine quantum mechanics with Einstein's relativity, as the two disagree about gravity. Scientists are working on string theory in an attempt to unify the two. String theory describes everything as made up of one-dimensional strings (strings with no width, only length), and the mathematics of this theory require there to be at least ten dimensions. A lot more than the three space and one time dimension we are familiar with! Extra dimensions may sound like science fiction but the idea of extra dimensions has been around for a century; Einstein published mathematician Kaluza's paper in 1921, in which the existence of a fifth dimension was proposed.

We cannot detect or communicate with these additional six spatial dimensions proposed by string theory; they are hidden from us, floating above us in higher dimensional space.

The existence of matter in these other dimensions could be revealed by the presence of dark matter. Dark matter is invisible to us and it only interacts with our universe via the gravitational pull it exerts on matter in our universe, similar to the gravitational pull the sun exerts on Earth. In fact, dark matter plays an essential role in holding our galaxy together. Perhaps this dark matter is matter in other universes, in other dimensions, which we cannot see. This suggests that we are not living in a universe, but a multiverse, with universes existing in higher dimensional space than our own, and where a gravitational pull is the only thing shared between these universes.

**Quantum Mechanics**

Quantum mechanics describes how sub atomic particles, such as electrons, behave. It describes how the act of observing an object forces it to collapse from a wave into an object in a given state. It might be an electron, which is observed and forced to be in a spin up or spin down state, or a cat, which is observed and forced to be in a dead or alive state. For example, when we fire a tiny particle of light, called a photon, at two slits, we observe the photon to pass through one of the two slits. There is no way to predict which slit the photon will pass through, it appears to 'randomly decide'. But why should the photon decide? Can't both outcomes happen? Some quantum physicists believe that the photon doesn't decide and that the universe actually splits when the photon makes a 'decision'; in one universe the photon passes through the left slit, and it the other universe, the right slit. If our universe splits at every decision, a vast number of parallel universes are created, where every possibility exists. So every time you make a decision, there is a parallel universe where you did not make that decision. If you toss a coin three times, our universe splits each time so both outcomes occur.

**Conclusion**

All three of these theories may point to other universes, suggesting we live in a multiverse. If we live in an infinite universe, there are exact replicas of ourselves and every possible scenario is played out somewhere in space. If string theory is correct and we live in more than three spatial dimensions, there could be other universes out there, perhaps similar to ours or perhaps entirely different. And finally, according to quantum physics, every time a decision is made, every outcome happens as the universe splits into parallel universes, and we are just one of many similar universes. If any of these theories are correct the implications are complicated. What determines which universe we live in? What is the significance of our own existence if there are other universes?

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