Schrödinger's Cat

Celia M. Gutiérrez Reyes

Quantum physics is a branch of physics that seeks to explain how small units of energy called quanta behave in our universe.  The fundamental principle of quantum theory states that quantum particles behave as both waves and particles.  Another principle says that it is impossible to know both the position and momentum of a particle at the same time.  These and other principles of quantum theory are sometimes difficult to understand and visualize, yet quantum physics is a very interesting and innovative field in science.  Niels Bohr, one of the most important developers of quantum theory, famously said: " If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet."  A fine example of a shocking part of quantum theory is "Schrödinger's cat," probably one of the most famous thought experiments of all time in this field of physics.  Developed by Erwin Schrödinger in 1935, he sought to respond to the EPR paradox (developed by Albert Einstein and his colleagues Boris Podolsky and Nathan Rosen), which challenges the prediction of quantum mechanics that it is impossible to know both the position and the momentum of a quantum particle.   Schrödinger's experiment consisted in locking a cat in a box, along with a radioactive element (which behaves like a quantum particle), a Geiger counter to measure if the element decays, and a sealed flask of poisonous gas for an hour.  The radioactive element has a 50% chance of decaying in the specified time, and if it does, a small hammer will be released, breaking the flask of gas and killing the cat.  Quantum physics says that the element can simultaneously be decaying and not decaying  (because the radioactive element follows the principle of superposition) during the hour, which would then mean that the cat is both alive and dead.  Because of this, the cat in the experiment is often referred to in popular culture as being some sort of zombie; however, this is not true.  The difference between the cat and the radioactive element is that the cat follows Newtonian physics laws, while the element is a quantum object; this simply means that the cat is either alive or dead, unlike the particle, which can be both decaying and not decaying.  Therein lies the paradox part of the thought experiment: how to know if the cat is alive or dead (or both), if the particle is both decaying and not decaying?  Two different interpretations of the outcome have been proposed: the Copenhagen interpretation, and the Everett many-worlds interpretation.  The first states that the system is in superposition, and as soon as an observer  opens the box, the experiment system will go in one direction or the other; this means that either the element will have decayed and killed the cat, or it will not have decayed and the cat will be alive.  The other interpretation states that when the observer opens the box, two alternate worlds are created, one in which the cat is alive, and one in which the cat is dead; these two worlds exist in a parallel state, and cannot interact with each other.  As the cat is apparently split between two worlds, so is the observer, and one world is not aware of the other.  The procedure and explanation of the thought experiment is complicated to follow, but once it is understood, it is fascinating, and will only serve to make a person want to continue to research the intriguing field of quantum physics.  

References

http://library.thinkquest.org/3487/qp.html

http://www.lassp.cornell.edu/ardlouis/dissipative/Schrcat.html

http://www.youtube.com/watch?v=uWMTOrux0LM

http://www.youtube.com/watch?v=7SjFJImg2Z8