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The quantum theory of the period between Planck 's discovery of the quantum ( 1900 ) and the advent of a mature quantum mechanics ( 1925 ) is often referred to as the old quantum theory .
 Planck
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Planck