The Heisenberg uncertainty principle states that the values of certain pairs of observable quantities cannot be known with arbitrary precision. The most well-known variant states that the uncertainty in a particle's momentum multiplied by the uncertainty in a particle's position must be greater than or equal to Planck's constant divided by 4. This means that if you measure a particle's position to better than Planck's constant divided by 4, you know that there is a larger uncertainty in the particle's momentum. Energy and time are connected by the uncertainty principle in the same way as position and momentum. The uncertainty principle is responsible for numerous physical phenomena, including the size of atoms, the natural linewidth of transitions in atoms, and the amount of time virtual particles can last.