Temporal energy
From Tardis Wiki, the free Doctor Who reference
Temporal energy was a type of energy sometimes released from the contact of two version of the same item or individual coming from different time ages.
During the total event collapse created by the explosion of the TARDIS, the Eleventh Doctor's sonic screwdriver sparked when touched to a later version of itself. (TV: The Big Bang)
When the Fourth Doctor met the Eighth Doctor and attempted to enter The Master's TARDIS, they touched their sonic screwdrivers together creating enough temporal energy to get inside. (AUDIO: The Light at the End)
