Classical And Modern Physics
The field of physics is commonly sub-divided into two large categories: classical and modern physics. The dividing line between these two sub-divisions can be drawn in the early 1900s, when a number of revolutionary new concepts about the nature of matter were proposed. Included among these were Einstein's theories of general and special relativity, Planck's concept of the quantum, Heisenberg's principle of indeterminacy, and the concept of the equivalence of matter and energy.
In general, classical physics can be said to deal with topics on the macroscopic scale, that is on a scale that can be studied with the largely unaided five human senses. Modern physics, in contrast, concerns the nature and behavior of particles and energy at the sub-microscopic level. As it happens, the laws of classical physics are generally inapplicable or applicable only as approximations to the laws of modern physics.
The discoveries made during the first two decades of the twentieth century required a profound re-thinking of the nature of physics. Some broadly-accepted laws had to be completely re-formulated. For example, many classical laws of physics are entirely deterministic. That is, one can say that if A occurs, B is certain to follow. This cause-and-effect relationship was long regarded as one of the major pillars of physics.
The discoveries of modern physics have demanded that this relationship be re-evaluated. With the formulation of quantum mechanics, physical phenomena could no longer be explained in terms of deterministic causality, that is, as a result of at least a theoretically measurable chain causes and effects. Instead, physical phenomena were described as the result of fundamentally statistical, unreadable, indeterminist (unpredictable) processes. Physicists are now more inclined to say that if A occurs, there is an X percent chance that B will follow. Determinism in physics has been replaced by probability.
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