|MadSci Network: Physics|
The short answer to your question is that you can't change gravity. It's one of the fundamental constants of the universe (As far as we can tell). However, you can change the *mass* of a nucleus by bombarding it with neutrons. This is done every day in experiments done at cyclotrons by nuclear physicists. They do this to search for new nuclei and to study the properties of known nuclei. This is also done at medical facilities to produce radio-isotopes for use in diagnoses.
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