A physicist is stopped for speeding. ``No, Officer, I don't know how fast I was going. But I do know exactly where I am.'' The joke is built on the implicit reference to Heisenberg's uncertainty principle, a bedrock of modern Physics: the position and the momentum of an object cannot both be measured exactly, at the same time. Quantitatively, Δ x Δ p ≥ h ⁄ 4π, where Δ x is the uncertainty in the measurement of position x, Δ p the uncertainty in the measurement of momentum p, and h is Planck's constant. In MKS units, h ⁄ 4π works out to be approximately 0.5 10-34kg m2⁄s.
For example, the mass m of the electron is close to 10-30 kg. So any simultaneous measurements of its positon x and velocity v are constrained by Δ x Δ v ≥ 0.5 10-4 m2⁄s, using p = mv. If in some experiment we can measure the position of an electron to within one micron, or 10-6 m, then the uncertainty Δ v must be greater than 0.5 10-28/10-30 = 50 m/s: we can only know its velocity to within plus or minus 50 meters/sec. (I checked with our Physics Department and was told not to worry: an electron in an orbit of that diameter typically travels at around 30,000 m/s.)