Could another version of you be out there? What scientists say about the multiverse

What if, somewhere out there, there's another version of you who is living a completely different life? One where the other 'you' exists because a different outcome played out? It's a strange thought experiment that sounds like the plot of a science fiction movie, but it's actually based on an idea in physics called the multiverse theory. 

Here's a closer look at what this theory actually says and how far scientists have come toward proving it.

At the smallest scales, quantum physics doesn't settle on a single outcome. This means that a radioactive particle can be both decayed and not decayed at the same time, until something measures it. In the mid-1950s, Princeton graduate student Hugh Everett III proposed that this doesn't have to come down to just one result. 

Instead, every possible outcome is actually happening, but in its own separate universe. This idea is known as the Many-Worlds Interpretation. It says that every quantum event that could go more than one way causes the universe to split, with a version of you present for each outcome.

While Hugh Everett III is usually credited with the theory, physicist Erwin Schrödinger also reached a similar conclusion five years earlier. In a 1952 paper, he argued that nothing in his own equations called for a single outcome to be picked out. He wrote that it was "patently absurd" that the wave function should "be controlled in two entirely different ways, at times by the wave equation, but occasionally by direct interference of the observer, not controlled by the wave equation."

In his view, nature should not switch between two different sets of rules depending on whether someone is observing it. Physicist Bryce DeWitt of the University of North Carolina later expanded on the idea, writing that "every quantum transition taking place in every star, in every galaxy, in every remote corner of the universe is splitting our local world on Earth into myriad copies of itself."

Surprisingly, physicists actually have a way to catch a glimpse of these extra possibilities with the help of a device called an interferometer. It works by splitting a beam of light into two separate paths and then bringing the beams back together. Under the right conditions, the two paths interfere with each other in a way that shows both routes contributed to the outcome. 

But scaling that up to something as complex as a cat or a person is essentially impossible because countless outside factors constantly disturb the system. That's why the world appears to have a single, definite outcome to us, even if it doesn't.

Physicist David Deutsch took the theory further. He suggested that the multiverse could contain an infinite number of universes, each representing a different outcome allowed by the laws of quantum mechanics. Some of these universes might look almost identical to our own, while others might not support anything like stars or people at all.

Physicist John Bell summed up what that means for us personally, noting in this paper that "persons of course multiply with the world, and those in any particular branch would experience only what happens in that branch." In other words, if the theory holds, there isn't just one version of you. There are countless versions, each living out a different branch from the same starting point.

There's an even stranger twist. Under Everett's framework, the universe doesn't move through time the way it feels like it does to us. Instead, every possible moment exists as its own separate state, each with its own memories. For example, a version of "you" one second, one hour, or even one year into the future exists too, complete with all the extra memories that come with it. But nothing actually flows from one state to the next. Each moment simply exists.

Even physicist John Wheeler, who once supported the theory, later stepped back from it, saying he believed it carried "too great a load of metaphysical baggage." Even today, the many-worlds interpretation remains contested, including among physicists who take it seriously.