**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