The graviton is a particle whose existence physicists have predicted but not proven. If real, it would tell spacetime how to bend. Quantum mechanics as applied to gravity (a field called superstring theory) does not make sense without it. Shamit Kachru explains why the graviton is so necessary. Lawrence Krauss reminds us that superstring theory could be entirely wrong, and Brian Greene counters with the suggestion that it is useful to physicists regardless of whether or not it is right.
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