Before we get started, a disclaimer: I don’t necessarily believe what I’m about to relate, but there are a group of physicists who do believe just this. Whatever you do, don’t try to test it…
Imagine you are playing a game of Russian roulette. The game is played by placing a gun to one’s head and pulling the trigger. Only one chamber of the gun is loaded and if you happen to have that chamber to your head then you will die. You will cease to exist. Or will you?
At this point a reader may be wondering why this is in the science section, rather than as a piece of creative writing. After all, the assertion that one won’t die when shot is ludicrous at best, dangerous at worst; but then so is the non-locality of particles and in modern physics separating the fact from the fiction can be tricky. I should state here and now, I won’t be placing a gun to my head, and whilst all I say is published work, I remain remarkably skeptical about whether this is true.
About a century ago, physics began to realise that the world was a remarkably strange place. A group of revolutionaries proposed a new theory of the smallest things in the world, a theory which has become known as quantum mechanics. Coincidentally, this is one of the most powerful theories we have ever had, and arguably has done more for technology than any other set of ideas, so we know it’s true. Right? Well, the predictions are astoundingly accurate but this does not imply we understand quantum theory.
The entirety of quantum theory is based on an entity known as a wave function - this wave function can be written for any system, including our own universe (well, not written, but one exists… another story…(probably…;)). The reason we need this wave function is that we cannot know both position and momentum (I’m simplifying, I know). These pieces of information are what allow us to do anything with classical mechanics - given them, we can define a system. Since these don’t even “exist” (in the sense a layman might know the word) in quantum theory, we need to use what sums up to essentially a probability distribution, which is just a property of the wave function.
But this is where it gets interesting: when you aren’t observing a particle it doesn’t have a definite position and momentum (which is what I mean by “these properties don’t exist”;), it exists in a superposition of position and momentum. This superposition, in normal quantum mechanics (Copenhagen interpretation), is said to collapse to a single position or momentum when observed. So what?
Hugo Everett, a physicist who was around in the second half of the twentieth century didn’t like this idea so he thought of a way around it. What if every time we look at something and collapse a superposition we create an alternative “quantum state” or what you might think of as parallel universe? Then we wouldn’t need a superposition to explain the universe at all!
I must admit, my (and the entire physics community’s) initial reaction was to scoff at this idea; surely it isn’t a serious suggestion? Everett left physics and was largely forgotten about, until now. A recent paper (2014) published in Physical Review X  argues that Everett’s argument fits the facts (a real and peer reviewed journal, believe it or not!). A series of computations convincingly show that some simple results may be explained by Everett’s hypothesis. Of course I still think this hypothesis is unlikely and remain highly skeptical. Simply because something works in physics does not mean that it is true! I could dream up any number of models which have no bearing on reality, and only evidence can prove them.
But what if Everett were right? As an observer in quantum mechanics, you wouldn’t be able to die in Russian roulette: although it would be somewhat tricky to observe from the grave. Since your gun’s state is determined ultimately by some quantum events, each trigger pull will create a “branch” in what Everett might have called the multiverse. “You” could only experience the universe in which you survived (although you’d get a load of upset relatives).
Thus, you could sit there all day and pull the trigger: we’re all immortal, hooray! One proponent of this theory was challenged to do just this (for a million pounds) but declined due to the sheer number of upset wives that he would be creating.
Yet this poses a couple of problems. Mainly, there is no hope. You nearly crashed your car? A few billion of you just died in a parallel universe. Failed your exam? Ah well, a good number of your alter egos probably passed. In a world without hope or luck, just where is there room for humanity?
 Hall, Michael J. W., Dirk-Andre Deckert, and Howard M. Wiseman. "Quantum Phenomena Modelled by Interactions between Many Classical Worlds." Physical Review X 4.4 (2014): n. pag. Web.
 Hooper, Rowan. "Life in the Multiverse." New Scientist 223.2988 (2014): 6. Web