In 1995, Matt Visser suggested that there are many wormholes in the universe if cosmic strings with negative mass are generated in the early universe. On the other hand, some physicists such as Frank Tipler and Kip Thorne  suggested how to make artificial wormholes .
In 1995, Matt Visser suggested that there may be many wormholes in the universe if cosmic strings with negative mass were generated in the early universe. Some physicists, such as Frank Tipler and Kip Thorne , have suggested how to make wormholes artificially .
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are
may be
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are
were
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On the other hand, some physicists
Some physicists,
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, have
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artificial wormholes
wormholes artificially