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What Holds it Together? Gravity 
What about gravity?
Gravity is weird. It is clearly one of the fundamental interactions, but the Standard Model cannot satisfactorily explain it. This is one of those major unanswered problems in physics today. In addition, the gravity force carrier particle has not been found. Such a particle, however, is predicted to exist and may someday be found: the graviton. Fortunately, the effects of gravity are extremely tiny in most particle physics situations compared to the other three interactions, so theory and experiment can be compared without including gravity in the calculations. Thus, the Standard Model works without explaining gravity. |