A man sits down before a gun, which is pointed at his head. This is no ordinary gun; it's rigged to a machine that measures the spin of a quantum particle. Each time the trigger is pulled, the spin of the quantum particle -- or quark -- is measured. Depending on the measurement, the gun will either fire, or it won't. If the quantum particle is measured as spinning in a clockwise motion, the gun will fire. If the quark is spinning counterclockwise, the gun won't go off. There'll only be a click.

Nervously, the man takes a breath and pulls the trigger. The gun clicks. He pulls the trigger again. Click. And again: click. The man will continue to pull the trigger again and again with the same result: The gun won't fire. Although it's functioning properly and loaded with bullets, no matter how many times he pulls the trigger, the gun will never fire. He'll continue this process for eternity, becoming immortal.

Go back in time to the beginning of the experiment. The man pulls the trigger for the very first time, and the quark is measured as spinning clockwise. The gun fires. The man is dead.

But, wait. The man already pulled the trigger the first time -- and an infinite amount of times following that -- and we already know the gun didn't fire. How can the man be dead? The man is unaware, but he's both alive and dead. Each time he pulls the trigger, the universe is split in two. It will continue to split, again and again, each time the trigger is pulled [source: Tegmark].

This thought experiment is called quantum suicide. It was first posed by then-Princeton University theorist Max Tegmark in 1997 (now on faculty at MIT). A thought experiment is an experiment that takes place only in the mind. The quantum level is the smallest level of matter we've detected so far in the universe. Matter on this level is infinitesimal, and it's virtually impossible for scientists to research it in a practical manner using traditional methods of scientific inquiry.

Video Gallery: Multiverse TheoryDavid Deutsch is author of "The Fabric of Reality: The Science of Parallel Universes and Its Implications" and the leading proponent of the multiverse theory, the astounding idea that our universe is constantly spawning parallel worlds. See how the multiverse theory works in this video from TED.

Instead of using the scientific method -- investigating empirical evidence -- to study the quantum level, physicists must use thought experiments. Although these experiments are only carried out hypothetically, they're rooted in the data observed in quantum physics.

What science has observed at the quantum level has raised more questions than it has answered. The behavior of quantum particles is erratic, and our understanding of probability becomes questionable. For example, photons -- the smallest measure of light -- have been shown to exist in both particle and wave states. And the direction of particles is thought not to travel in one way or the other, but in both directions at the same time. So when we examine the quantum world, we are outsiders to the knowledge it holds. As a result, our understanding of the universe as we know it is challenged.

This has led some to believe that our grasp of quantum physics is as basic as the understanding of ancient Egyptian astronomers centuries ago, who claimed that the sun was a god. A few scientists believe further investigation into quantum systems will reveal order and predictability within what we currently see as chaos. But is it possible that quantum systems can't be understood within the traditional models of science?

In this article, we'll look at what quantum suicide reveals about our universe, as well as other theories that either support or contradict it.

But first, why can't a physicist simply measure the particles he's attempting to study? In the next section, we'll learn about this fundamental flaw of quantum observation, as explained by Heisenberg's Uncertainty Principle.