In what tense (present/past) should papers be written?
The rules of thumb are:


Established facts are reported in the present tense (“The path of light follows Fermat's principle of least time”). However, you should use the past tense when you refer to previous work in the field (“Maxwell et al. demonstrated clearly in a laser cavity experiment that no mirror is perfect”).
The experiments, simulations or calculations you performed are narrated in the past tense (“We dissolved the remaining solid in a 5:1 solution of acetone and benzonitrile, and heated to 200°C for three hours.”)
Discussion of the data presented in the paper uses the present tense (“The results obtained, shown in Fig. 3, clearly emphasize that the cell colonies grew faster on pink toothbrushes than green ones. We attribute this to the color-sensitivity, or kawai factor.”)
Mathematical proofs are written using the present tense, because going through the proof occurs at the time of reading (“From Eqn. 1, we derive the following system of inequalities”).


Overall, the choice of tenses is actually pretty logical.