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A century after its conception, quantum mechanics continues to be a disturbing theory. It tells us to think of all matter as waves, and yet in all objects that surround us these matter waves are far too small to be seen. Although the quantum laws are thought to be valid for objects of all sizes--from elementary particles to the universe as a whole--we do not usually see matter waves or any other quantum behavior in our everyday world.
In some subtle way, which physicists still do not completely understand, quantum mechanics conceals its strange effects when many particles interact in a disordered manner or when temperature rises much above absolute zero--that is, whenever things get a little bit complicated, as they usually do in the macroscopic world. As a result, quantum phenomena tend to be associated only with the world of elementary particles and with abstract thought experiments, such as the famous but mysterious Schrödinger's cat, which exists in a quantum state that is simultaneously alive and dead.