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Introduction to How Quantum Suicide Works

­­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 now 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 at this level is infinitesimal, and it's virtually impossible for scientists to research it in a practical manner using traditional methods of scientific inquiry.­

A woman stands near a superconducting solenoid magnet used to measure quantum particles.

Fabrice Coffrini/AFP/Getty Images

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Quantum Physics

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 to travel in both directions at the same time, rather than in only one direction at different times. 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.

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Heisenberg's Uncertainty Principle

­One of the biggest problems with quantum experiments is the seemingly unavoidable tendency of humans to influence the situati­on and velocity of small particles. This happens just by our observing the particles, and it has quantum physicists frustrated. To combat this, physicists have created enormous, elaborate machines like particle accelerators that remove any physical human influence from the process of accelerating a particle's energy of motion.

Still, the mixed results quantum physicists find when examining the same particle indicate that we just can't help but affect the behavior of quanta -- or quantum particles. Even the light physicists use to help them better see the objects they're observing can influence the behavior of quanta. Photons, for example -- the smallest measure of light, which have no mass or electrical charge -- can still bounce a particle around, changing its velocity and speed.

This is called Heisenberg's Uncertainty Principle. Werner Heisenberg, a German physicist, determined that our observations have an effect on the behavior of quanta. Heisenberg's Uncertainty Principle sounds difficult to understand -- even the name is kind of intimidating. But it's actually easy to comprehend, and once you do, you'll understand the fundamental principle of quantum mechanics.

Imagine that you're blind and over time you've developed a technique for determining how far away an object is by throwing a medicine ball at it. If you throw your medicine ball at a nearby stool, the ball will return quickly, and you'll know that it's close. If you throw the ball at something across the street from you, it'll take longer to return, and you'll know that the object is far away.

The problem i­s that when you throw a ball -- especially a heavy one like a medicine ball -- at something like a stool, the ball will knock the stool across the room and may even have enough momentum to bounce back. You can say where the stool was, but not where it is now. What's more, you could calculate the velocity of the stool after you hit it with the ball, but you have no idea what its velocity was before you hit it.

­This is the problem revealed by Heisenberg's Uncertainty Principle. To know the velocity of a quark we must measure it, and to measure it, we are forced to affect it. The same goes for observing an object's position. Uncertainty about an object's position and velocity makes it difficult for a physicist to determine much about the object.

Of course, physicists aren't exactly throwing medicine balls at quanta to measure them, but even the slightest interference can cause the incredibly small particles to behave differently.

This is why quantum physicists are forced to create thought experiments based on the observations from the real experiments conducted at the quantum level. These thought experiments are meant to prove or disprove interpretations -- explanations for the whole of quantum theory.

In the next section, we'll look at the basis for quantum suicide -- the Many-Worlds interpretation of quantum mechanics.

The Many-Worlds Theory

The quantum suicide thought experiment is based on and seeks to prove what has bec­ome an increasingly accepted interpretation of quantum physics, the Many-Worlds theory. This theory was first proposed in 1957 by a doctoral student at Princeton University named Hugh Everett III. The theory was scorned for decades until fellow Princetonian Max Tegmark created the quantum suicide experiment, which lends support to the interpretation [source: The Guardian].

According to the Many-Worlds theory, for each possible outcome to an action, the world splits into a copy of itself. This is an instantaneous process Everett called decohesion. It's kind of like a choose-your-own-adventure book, but rather than choosing between either exploring the cave or making off with the treasure, the universe splits in two so that each action is taken.

One vital aspect of the Many-Worlds theory is that when the universe splits, the person is unaware of himself in the other version of the universe. This means that the boy who made off with the treasure and ends up living happily ever after is completely unaware of the version of himself who entered the cave and now faces great peril, and vice versa.

This is the same case with quantum suicide. When the man pulls the trigger, there are two possible outcomes: the gun either fires or it doesn't. In this case, the man either lives or he dies. Each time the trigger is pulled, the universe splits to accommodate each possible outcome. When the man dies, the universe is no longer able to split based on the pulling of the trigger. The possible outcome for death is reduced to one: continued death. But with life there are still two chances that remain: The man continues living or the man dies.

A guidebook to the quantum world?

Image courtesy Amazon.com

­When the man pulls the trigger and the universe is split in two, however, the version of the man who lived will be unaware that in the other version of the split universe, he has died. Instead he will continue to live and will again have the chance to pull the trigger. And each time he does pull the trigger, the universe will again split, with the version of the man who lives continuing on, and being unaware of all of his deaths in parallel universes. In this sense, he will be able to exist indefinitely. This is called quantum immortality.

So why aren't all of the people who have ever attempted to kill themselves immortal? What's interesting about the Many-Worlds interpretation is that according to the theory, in some parallel universe, they are. This doesn't appear to be the case to us, because the splitting of the universe isn't dependent on our own life or death. We are bystanders or observers in the case of another person's suicide, and as observers we're subject to probability. When the gun finally went off in the universe -- or version -- we inhabit, we were stuck with that result. Even if we pick up the gun and continue shooting the man, the universe will remain in a single state. After all, once a person is dead, the number of possible outcomes for shooting a dead person is reduced to one.

But the Many-Worlds theory stands in contradiction to another quantum theory, the Copenhagen interpretation. In the next section, we'll look at this theory and see why it changes the rules of quantum suicide.

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The Copenhagen Interpretation

The Many-Worlds theory of quantum mechanics supposes that for each pos­sible outcome of any given action, the universe splits to accommodate each on­e. This theory takes the observer out of the equation. No longer are we able to influence the outcome of an event simply by observing it, as is stated by the Heisenberg Uncertainty Principle.

But the Many-Worlds theory turns a widely accepted theory of quantum mechanics on its ear. And in the unpredictable quantum universe, this is really saying something.

For the better part of the last century, the most accepted explanation for why the same quantum particle may behave in different ways was the Copenhagen interpretation. Although it's getting a run for its money from the Many-Worlds interpretation lately, many quantum physicists still assume the Copenhagen interpretation is correct. The Copenhagen interpretation was first posed by physicist Niels Bohr in 1920. It says that a quantum particle doesn't exist in one state or another, but in all of its possible states at once. It's only when we observe its state that a quantum particle is essentially forced to choose one probability, and that's the state that we observe. Since it may be forced into a different observable state each time, this explains why a quantum particle behaves erratically.

This state of existing in all possible states at once is called an object's coherent superposition. The total of all possible states in which an object can exist -- for example, in a wave or particle form for photons that travel in both directions at once -- makes up the object's wave function. When we observe an object, the superposition collapses and the object is forced into one of the states of its wave function.

Bohr's Copenhagen interpretation of quantum mechanics was theoretically proven by what has become a famous thought experiment involving a cat and a box. It's called Schrödinger's cat, and it was first introduced by the Viennese physicist Erwin Schrödinger in 1935.

In his theoretical experiment, ­Schrödinger put his cat in a box, along with a bit of radioactive material and a Geiger counter -- a device for detecting radiation. The Geiger counter was designed so that when it sensed the decay of the radioactive material, it triggered a hammer which was poised to break a flask containing hydrocyanic acid, which, when released, would kill the cat.

To eliminate any certainty regarding the cat's fate, the experiment was to take place within an hour, long enough so that some of the radioactive material could possibly decay, but short enough so that it was also possible none would.

In Schrödinger's experiment, the cat was sealed in the box. During its stay there, the cat came to exist in an unknowable state. Since it could not be observed, it could not be said whether the cat was alive or dead. It existed instead in the state of both life and death. It's sort of like quantum physics' answer to the old Zen question: If a tree falls in the woods and no one is around to hear it, does it make a sound?

Since the Copenhagen interpretation says that, when observed, an object is forced to take one state or another, the quantum suicide experiment doesn't work according to this theory. Since the direction of the quark measured by the trigger can be observed, eventually the quark will be forced to take the clockwise direction that will fire the gun and kill the man.

But isn't all of this just silly? Do these thought experiments and quantum interpretations really teach us anything? In the next section, we'll look at some of the possible implications of these ideas.

As our understanding of quantum physics deepens, how will it change our perception of the physical world?

Photo courtesy Daniel Johansson

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The Implications of Quantum Physics

When compared to classical science and Newtonian physics, the theories pr­oposed to explain quantum physics seem insane. Erwin Schrödinger himself called his cat experiment "quite ridiculous" [source: Goldstein, Sheldon]. But from what science has been able to observe, the laws that govern the world we see every day don't hold true on the quantum level.

Quantum physics is a relatively new discipline, dating back only to 1900. The theories that have been posed on the subject are all just theories. What's more, there are competing theories that give different explanations for the peculiar happenings that take place on the quantum level. Which one will history show is the correct one? Perhaps the theory that proves to be the true explanation for quantum physics hasn't been posed yet. The person who poses it may not have even been born yet. But given the logic that this field of study has established, is it possible that all theories explaining quantum physics are all equally true at the same time -- even the ones that contradict each other?

Niels Bohr's Copenhagen interpretation of quantum physics is perhaps the most comforting theory put forth. By explaining that particles exist in all states at once -- in coherent superposition -- our understanding of the universe is put slightly askew, but still remains somewhat comprehensible. Bohr's theory is additionally comforting because it makes us humans the cause for an object to take a determined shape. Although scientists find a particle's ability to exist in more than one state frustrating, our observations affect the particle. At least it doesn't continue to exist in all states while we're looking at it.

Much less comforting is Everett's Many-Worlds interpretation. This theory takes out of our hands any power over the quantum universe. Instead, we are merely passengers of the splits that take place with each possible outcome. In essence, under the Many-Worlds theory, our idea of cause and effect goes out the window.

This makes the Many-Worlds interpretation somewhat disturbing. If it's true, then in some universe parallel to the one we currently inhabit, Adolf Hitler was successful in his campaign to conquer the world. But in the same token, in another universe, the United States never dropped atomic bombs on Hiroshima and Nagasaki.

The Many-Worlds theory also certainly contradicts the idea of Occam's razor, that the simplest explanation is usually the correct one. Even stranger is the implication by the Many-Worlds theory that time doesn't exist in a coherent, linear motion. Instead, it moves in jumps and starts, existing not as a line, but as branches. These branches are as numerous as the number of consequences to all of the actions that have ever been taken.

It's tough not to imagine what our understanding of the quantum world will prove to be. The theoretical field has already progressed tremendously since its inception more than a century ago. Although he had his own interpretation of the quantum world, Bohr may have accepted the later theory that Hugh Everett introduced concerning the Many Worlds. After all, it was Bohr who said, "Anyone who is not shocked by quantum theory has not understood it."

For more information on quantum suicide, including related HowStuffWorks articles, visit the next page.

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