pair production and perfect vacuum I was told that perfect vacuum is impossible as virtual photons, permitted by the uncertainty principle, and undergo pair production to produce a particle-antiparticle pair, and they annihilate before the time limit set by the uncertainty principle.


*

*I was also told that some sort of particle must be present near a photon in order for it to undergo pair production (normally a nucleus) due to momentum conservation. a) what is this particle in a 'vacuum'?
b) let's say there is a particle nearby when the photon undergoes pair production. Then the particle would have gained some momentum during the pair production process. When the particle and antiparticle annihilate, the particle would still carry some momentum; surely this is not allowed because the energy that the universe 'loaned' must all be 'paid for' at the end as photons, isn't it?

*due to momentum conservation, when a particle and its antiparticle annihilate they produce more than one photon - is this okay in terms of 'borrowing' and 'paying back'?

*when a virtual photon first appears, wouldn't it violate momentum conservation? from zero to whatever the momentum the photon carries
 A: 
perfect vacuum is impossible as virtual photons, permitted by the uncertainty principle, and undergo pair production to produce a particle-antiparticle pair, and they annihilate before the time limit set by the uncertainty principle.

This is a physics narrative that is being stretched beyond its limits of validity. In my opinion it should be stating:
"It is impossible to measure a perfect vacuum since interactions are needed for measurement and the virtual loops in the calculation of the probabiliy of the specific measurement populate the vacuum with virtual pairs."
Any energy is supplied by the particles that travel in the vacuum. To posit virtual pairs when no energy is inputted  has no meaning . The universe is not a bank.

This shows the definition of a virtual particle, which is always tied up with Feynman diagrams that will be used to calculate measurable quantities, crossections, lifetimes ...
Virtual particles are mathematical constructs that carry the quantum numbers of the named particle but not its mass. They are off mass shell, and they do not appear, because they are not real particles, they are a function that is integrated over the limits of the integrals in the problem. The only meaning in budgeting energy and momentum comes from the real incoming and out going particles, and the total interaction always conserves energy/ momentum.
for vacuum loops 

This is the function of the vacuum loops, they appear when at least one real particle is running along, and the energy/momentum are supplied by the particle. If no real particle is around there are no virtual loops 
