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Phase, phase, phase

University of Tulsa, Oklahoma, U.S.

Editor's note: Chris Liner is a faculty member in the Department of Geosciences at the University of Tulsa and author of the books Elements of 3-D Seismology (Pennwell Publishing Co., 1999), and Greek Seismology available free from Samizdat Press. When not engaged in the serious business of teaching and research, he can often be caught playing with computers and reading really old books.

The first 20% of the full text of this article appears below.

The jury all wrote down on their slates, "She doesn't believe there's an atom of meaning in it."

—Lewis Carroll, 1832–1898, Alice's Adventures in Wonderland (ch. 11)

I have been trying for some time now to write about phase. Sounds easy enough, but the more I think about it, the less easy it gets. I think I know why.

Phase is one of those technical terms that has way too many meanings attached to it. Sheriff's dictionary gives four primary definitions of the word, and no less than 20 more derivative definitions.

The early history of this word is not so early. Unlike a term such as elastic which dates from 1674, the Oxford English Dictionary (OED) gives the first occurrence of phase as 1812, but this is in an astronomical setting applied to the phases of the moon. Our multitude of phase terms have their origins with the OED third definition related to physics: "A particular stage or point in a recurring sequence of movements or changes, e.g., a vibration or undulation." This kind of phase occurs in the 1864 edition of Webster's Dictionary (first published in 1806) and by 1874 was becoming common. The late date is surprising considering complex numbers were well understood by the early 1800s and Maxwell published his electromagnetic theory of light about 1861.

So let us delve into phase, specifically the top four senses of how seismic people use that word.

	Phase I

 
Complex numbers are at the heart of all our phase definitions. Recall that a complex number, z, can be written as z = x + iy where x and y . . . [Full Text of this Article]