Why are parallel universes so exciting? Underpinning all of them is the idea that our universe isn’t the only one in existence, but rather just one island in a vast multiverse. The possibilities raised by this are enough to send sci-fi fans into overdrive. There’s just one problem: it’s not clear which theories should properly fall under the umbrella of “parallel universes.” More importantly, none have been discovered or proven to exist.
The two concepts that seem to inspire most excitement here are the metaverse and the multiverse. They both sound similar, but they involve radically different ideas about how everything works…
Technically speaking, there are no global rules for what defines a multiverse. Instead, it’s more of an idea that can be attached to several different concepts.
The main distinction made by physicists about the multiverse is whether it involves separate spacetimes or universes that are somehow "parallel" to our own, but still embedded in a single space.
The term “multiverse” was coined by Max Tegmark, a physicist at the Massachusetts Institute of Technology (MIT). His model states there are four levels of parallel universes: Level I, Level II, Level III and Level IV.
Level I contains other regions of spacetime that are forever beyond the reach of Earthly experiments because light signals from them will never reach us. This level might contain other universes that are actually separate from our own.
Level II, however, is what most people think of when it comes to parallel universes: basically just copies of ours that contain Earth-like planets and potentially intelligent life forms.
Level III and IV parallel universes go beyond the same old "alternate reality" idea we see in science fiction stories. Level III involves parallel worlds that might not even be connected to our own through any physical laws, but rather exist outside of physics as we know it by containing different mathematical structures. Level IV parallel universes repeat all possible configurations over and over again, so every single possible choice every imaginable person could have made throughout history happens somewhere. The problem with this theory is there’s no plausible way to test its validity.
The metaverse is a theoretical framework that doesn’t depend on the idea of multiple universes, but rather builds upon it by proposing all universes are somehow connected. While spacetime itself may be a single continuum, there's no reason why the broader fabric of space-time should also be one continuous whole. Theories involving wormholes or other extra dimensions predict such connections might exist without us fully understanding them yet...
