Final answer:
As a particle with mass approaches the Planck energy, classical general relativity and quantum mechanics predictions may fail, possibly leading to new phenomena like micro black holes or unknown quantum gravitational effects; the specifics of these effects are yet to be understood without a theory of quantum gravity.
Step-by-step explanation:
The question concerns the behavior of a massive elementary particle as it approaches the Planck energy, within the framework of quantum gravity and special relativity. According to special relativity, as a particle with mass accelerates closer to the speed of light, its energy increases without limit, but it can never reach or exceed the speed of light.
This is due to the relativistic effect of mass increasing with velocity, resulting in an infinite amount of energy needed to reach the speed of light. Regarding quantum gravity, while we lack a complete theory, it suggests that there are also fundamental limits related to the Planck scale, which is an incredibly small length and energy scale at which gravity is expected to become a strong force and quantum effects cannot be ignored.
As such, when a particle's energy approaches the Planck energy, the predictions of classical general relativity and non-gravitational quantum mechanics may no longer be valid, and new physics may dominate, potentially involving phenomena such as the formation of micro black holes or the emergence of new quantum gravitational effects.
However, without a completed theory of quantum gravity, the precise details of what happens as we approach this scale remain speculative.