The force of gravity, explained using Einstein’s general theory of relativity, can be associated with a particle called the graviton that is thought to have zero mass. One way to check if the particle is really massless is to calculate an upper limit on its mass from detailed observations of planetary motions in the Solar System. Now a team has improved upon a previous effort to do this by avoiding a hidden assumption in the earlier work. Any sign that the mass might not be zero could suggest that new theories beyond general relativity are required.

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