In fiction and in films such as Spiderman: Into the Spider-Verse, parallel universes–also called the multiverse—exist alongside our own, with anything from small differences in events to entirely different rules of physics. It is an exciting and fascinating idea.
Physicists have given the question of whether parallel universes could exist a lot of thought—and have come up with quite a few theories.
Infinite expanse
One popular theory relies on something physicists already know about. The universe is expanding. This means galaxies far from Earth are moving away from us. This means there is a point in the universe that we cannot see past. This process is called cosmic inflation.
That doesn’t mean there is nothing there, though. There are still more galaxies beyond this edge—but we will never be able to see them. Some physicists describe the parts of the universe beyond this edge as a separate, parallel universe. This theory is popular, as it doesn’t require any special physics or changes in our current understanding of the universe.
Many worlds
Another theory requires us to think about a type of science called quantum mechanics. Quantum mechanics is all about the probability of tiny particles doing something. If a tiny particle is fired at a wall, it might bounce off it—or travel right through it. Quantum mechanics tells us which event is more likely. However, there is nothing in the maths that says that only one of the events must occur. Both of these things might happen at once. But we would only see one of these things happen. So, if saw the particle bounce off the wall, it might have also, at the same time, tunnelled through the wall—just in another universe, not our one.
In this parallel universe, everything would be exactly the same as in our universe, except for one tiny change. In that universe, the particle traveleed through the wall, instead of bouncing off it. The many worlds theory suggests that this happens every time a quantum mechanical reaction occurs, creating a separate parallel universe each time.
Once the parallel universe is created, however, we have no way of interacting with it. While this theory is exciting, we have no way to test it.
So is there another Earth?
Unfortunately, we don’t know if these parallel universes exist. Or at least, we don’t at the moment. Physicists are trying to find ways to test these theories, but it is very difficult. For now, they remain just theories. But, who knows? Perhaps scientists in another universe have already figured it out.