Quantum Mechanics for Dummies: Schrödinger


In the final instalment of the enlightening Quantum Mechanics for Dummies series, Georgie McDonald talks to us about a certain kitty who meows close to every Physicist’s heart.

As seen in Pegasus Pages, March 2013.

Schrodinger’s cat is the celebrity cat of the Physics world, whether you are a theoretical physicist or not. Although most of us have probably heard of this experiment, you may not know what it actually means, and how it makes the quantum world even crazier.

Schrödinger came up with this famous thought experiment (i.e. an experiment that isn’t actually carried out) and it poses the question: When does a quantum system stop existing as a superposition of states and become one or the other? It serves to demonstrate the apparent conflict between what quantum theory tells us is true about the nature and behaviour of matter on the microscopic level, and what we observe to be true about the nature and behaviour of matter on the macroscopic level – everything visible to the unaided human eye.

Schrödinger said that if you placed a living cat into a steel chamber, along with a device containing a vial of a radioactive substance and a vial of poisonous gas, there is exactly a 50:50 chance that a single atom of the radioactive substance will decay during the test period to emit an electron, a relay mechanism which will trip a hammer which will, in turn, break the vial of poisonous gas and kill the cat.  If you were to theoretically do this experiment many times, the cat would be dead 50% of the time and alive for the other 50%, when you looked inside the chamber. The cat can’t be both dead and alive once you’ve seen it.

But according to quantum law, before you look, the cat is in superposition, both dead and alive and the act of looking makes nature’s decision. When the box is broken open and the condition of the cat is learnt, the superposition is lost, and the cat becomes either one or the other. Our curiosity could have killed the cat. Quantum theory shows us that the observation or measurement itself affects the outcome, so that the outcome does not exist unless the measurement is made. This means that there is no outcome at all unless it is observed.

I hope that I have given you a teasing taste of quantum mechanics and proved to you that we really do live in mad and crazy world.

Read the previous articles in the series here and here.

Photography: wubomei on Flickr (CC BY-NC 2.0).
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