Imagine a physicist sitting in a chamber with a gun pointed directly at her head.

Every few seconds, the spin direction of a random particle in the room is measured. If the particle is spinning one direction then the gun goes off and the physicist dies. If the particle is spinning in the opposite direction, there's just a clicking sound and the physicist survives.

She has a 50/50 chance of surviving, right?

It might not be that simple if we live in a multiverse — the idea that multiple universes, apart from the one we call home, exist.

This scenario with the physicist and the gun is the start of a famous thought experiment called "quantum suicide," and it's one way for physicists to consider if we really are living in just one of many (and potentially infinitely many) universes.

This thought experiment relies on quantum mechanics and the idea that there is no single objective reality. Everything that we see around us is just one possible configuration of all the probabilities of any one thing happening. One interpretation of quantum mechanics is that all the other arrangements of probabilities could exist in their own separate universe. So if you follow the thought experiment all the way through with this idea in mind, then the second that first particle is measured, the universe splits into two universes, based on two possible outcomes: one in which the physicist lives and one in which she dies.

Her survival is now tied to a quantum probability, so she'll be both dead and alive at once — just in different universes. If a new universe splits off every time a particle is measured and the gun either fires or doesn't, then in one of those universes, the physicist will end up surviving, say 50 particle measurements. You can think of this as flipping a coin 50 times in a row. You have an extremely low likelihood of getting heads each of those 50 times — less than a 1 quadrillion chance, but it is possible.

And if it happens, that's enough for the physicist to conclude that the multiverse is real, and effectively she becomes immortal in the universe in which the gun never goes off. But she also becomes the only person who knows that parallel universes exist.

This probably sounds like the plot of a sci-fi movie, but there are other, more reasonable-sounding versions of the multiverse that are backed up by math and are potentially testable.

"Some people conflate parallel universes with jumping through a portal into another world, or something like that," Matthew Johnston, a physicist at the Perimeter Institute, told Business Insider. "But it's not really like that at all."

Actual observational evidence of a multiverse will be tricky to find, but it is possible. Here's how physicists will do it:

View photos big bang More

Multiverse versions

There are actually many different multiverse theories, and the multiverse from the quantum suicide thought experiment, where every possibility becomes reality, is one of the most radical.

A convenient way to think about different multiverse theories is MIT physicist Max Tegmark's multiverse hierarchy where he organizes them into four different levels.

We will focus just on level one multiverses — the version with the most traction in physics, and thankfully the easiest to wrap the mind around. Level one is also where we have the best chance of actually finding evidence that proves the multiverse is real.

Multiple universes are predictions of the math behind existing theories, and the level one multiverse idea is predicted by a very well-respected and central concept in physics: inflation.

So it's an idea that physicists have to take seriously, Johnson said.

What we mean by 'universe'

Story continues