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In 1993 , Matt Visser argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other , or otherwise prevent information from passing through the wormhole .
 person: Matt Visser
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
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Answer:
 person: Matt Visser