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Quantum Physics in its simplest possible definition is the physics of phenomena for which no specific history has been written. In the absence of such an "observed" history, all of the conceivable histories for how that phenomenon could unfold over time combine in a precise, mathematical fashion to form a wave-like entity called the quantum wave function. In this wave function the clusters of histories that are most similar combine positively to create higher probabilities for one of those histories to become real when the wave function is examined closely, e.g. by shining a laser or other energy source on it.
Frequently Asked Questionsedit
Why does quantum mechanics use probabilities?edit
No one really knows why reality at its simplest levels is probabilistic instead of deterministic. It is simply what is observed with absolute consistency whenever experiments attempt to examine matter and energy at extremely small or extremely low energy levels. One important factor, however, is that (to paraphrase a famous older assertion) "nature abhors infinite information." More specifically, for every entity in the universe, as its total mass or energy decreases, so does the amount of information that it can contain. This reduction in available information continues until physics is forced to resort to probabilities rather than certainties to describe even where the particle is located or how fast it is moving (momentum).
Can quantum mechanics ever be seen in everyday life?edit
Contrary to what you have likely heard in science classes, there are a number of large-scale, everyday physics phenomena that are deeply quantum in nature. The very fact that you can read this text requires light to behave in a quantum fashion, since every photon that enters your eye must "decide" where to land on your retina based on its its individual wave function. Photon wave functions are very robust and fully capable of operating at the ordinary temperatures and large scales of your cornea and eye lens. If this were not the case, the photons would behave like particles and get lost in the proteins of your eye the instant they hit your cornea, leaving you completely blind! Another large-scale example of quantum mechanics is volume, which depends on the famous quantum uncertainty factor known as Planck's constant. This constant would have been more aptly named the "size constant", since if you could double its size you would also double the diameter of every object in the universe, including yourself. Finally, mirrors are extremely quantum, since light reflects only from extremely hot, mirror-spanning electron wave functions that exist only because metals "forget" where their conduction electrons are located.