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September 28, 2008

Quantum Suicide and The Large Hadron Collider

Stern-Gerlach Experiment
Stern-Gerlach experiment. Source: Wikipedia Commons. Licensed under GNU Free Documentation License version 1.2.

Quantum mechanics is a theory that describes the behavior of objects at the atomic scale. The effects of quantum mechanics are typically observable only at this small scale, and not at larger ones, except in unusual or contrived situations.

Electron Spin

Electrons have a property called spin that may be measured in relation to an arbitrary axis. The name is somewhat misleading. It's not quite the same concept as a ball rotating around an axis but there are some useful similarities. Since an electron has an electric charge, its spin causes it to interact with a magnetic field, deflecting the electron's path in a manner similar to the way a charged sphere's course would be altered. An electron can have its spin measured by passing it through a magnetic field. If electrons were truly spinning spheres, a beam of electrons would spread out smoothly when passed through a shaped magnetic field since each rotating sphere would take on an arbitrary spin alignment.

However, what is actually observed is amazing and counter-intuitive. The 1922 Stern-Gerlach experiment showed that spin is quantized and only two values are observed - denoted up and down.

Standard Interpretation

In the standard Copenhagen interpretation of Quantum Mechanics, the electron does not have a definite spin until a measurement is made, and the quantum wave function collapses to a definite value. Schrödinger's Cat is a famous thought experiment which was originally conceived by Austrian physicist Erwin Schrödinger as a critique of the Copenhagen interpretation. In a variation of this thought experiment, one imagines that a cat is placed in a box with a flask of poison and a device that can measure electron spin.

If a single electron that is passed through the device is measured with spin up, the flask of poison is released and the cat expires. If the spin is down, the cat survives. There is a 50 percent chance of either outcome. If the box is sealed so that it is impossible to determine the state of the experiment from outside, the cat will exist in a superposition of states to the outside world with equal probability of it being alive and dead. It's not that the cat actually exists in one state or another according to the Copenhagen interpretation. The cat has become entangled in the quantum wave function describing the contents of the box and truly exists in a superposition of both states.

Quantum Suicide; Many Worlds
iStockphoto / Sirin Buse.

Quantum Suicide

However, in the Many-Worlds interpretation of Quantum Mechanics, two different worlds exist - one in which the cat remains alive, and another in which the cat has perished.

A thought experiment called Quantum Suicide has been crafted as a hypothetical test of the Many-Worlds interpretation. In this experiment, an observer takes the place of the cat and the experiment is performed many times. In some worlds, the observer perishes, but his conscious experience continues in the worlds in which he survives. He will never observe his own death. The observer perishes in half of the worlds, but it does not appear that way from his point of view. After repeating the experiment as many times as necessary to satisfy his curiosity, the observer concludes that the Many-Worlds interpretation is correct.

With the Large Hadron Collider shut down for two months due to a malfunction, some have suggested with tongue-in-cheek that the Quantum Suicide experiment is being conducted in real time with our own world. In some parallel universes, the LHC creates stable black holes which destroy the Earth. We only remain conscious to observe this in universes where that doesn't happen. In those universes, events happen that prevent the LHC from creating those kinds of black holes.

While the LHC's troubles are more likely explained by mundane problems, the idea behind the Quantum Suicide thought experiment is still an intriguing one.

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