Subject: what determines virtual particles energy?

i have read that when a virtual particle is produced the lenth of time it is in 
exsistence is directly related to the amount of energy it has.  what determines 
the amount of energy a virtual particle has and is there a formula that shows 
energy compared to time?  also i have learned that the lamb shift is due to 
virtual particles interacting between atoms.  does the shifts energy change 
depending on the energy of the virtual particle?  i would appreciate any 
information you can give me.

Re: what determines virtual particles energy?