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Later , other types of traversable wormholes were discovered as allowable solutions to the equations of general relativity , including a variety analyzed in a 1989 paper by Matt Visser , in which a path through the wormhole can be made where the traversing path does not pass through a region of exotic matter .
 Matt Visser
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Matt Visser