Uncertainty and Virtual Particles

1927- Heisenberg formulates a fundamental
       property of quantum mechanics:

• it is impossible to measure both a particle's position and its momentum exactly
• the more precisely we determine one
• 
  the less we know about the other

      in terms of energy and time

uncertainty in energy of particle multiplied by uncertainty of time is greater than Planck's constant (h-bar / 2)
uncertainty in energy can be large