Problem I get while study physics

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1, After the relative theory, the mass and energe has been united to one
physical paremeter? But another thing has similar property with
them-Information. The maximum trasforming velocity of information and
mass/energe all are c. So can information be united with them (mass and
parameters)?

2, Why electrons can only exist in one energe eigenstate.

It can be different eigenstate of position or other Hermitian
operaters. But they cannot be different eigenstate of energe. Maybe in
some problems on the text book, electrons can exist in some mixed
energe state. But in the actual system, is these mixed energe system
exist? so energe eigenstate state is something different than other
Hermitian operator's eigenstates?

In factly, it is very simple,because the uncerteinty of the energe and time. If a states wants to exist longer, its energe should be determined.