Mathematically, the Heisenberg uncertainty principle is a lower bound on the product of uncertainties of a pair of conjugate variables. The most well-known expression takes the position and momentum to be the conjugate variables:
Another uncertainty relation which is often referenced in discussion of quantum mechanics is the energy-time uncertainty principle.
It is tempting to interpret this equation as the statement that a system may fluctuate in energy by an arbitrarily large amount over a sufficiently short time scale. The principle is at the heart of many things that we observe but cannot explain using classical physics.
Take atoms, for example, where negatively-charged electrons orbit a positively-charged nucleus. We might expect, by classical logic, the two opposite charges to attract each other, leading everything to collapse into a ball of particles.
The uncertainty principle explains why this doesn’t happen: if an electron got too close to the nucleus, then its position in space would be precisely known and, therefore, the error in measuring its position would be minuscule.
This means that the error in measuring its momentum (and, by inference, its velocity) would be enormous. In that case, the electron could be moving fast enough to fly out of the atom altogether.
Heisenberg’s idea can also explain a type of nuclear radiation called alpha decay. Perhaps the strangest result of the uncertainty principle is what it explains about the absence of particles in vacuum.
Uncertainty, then, is nothing to worry about in quantum physics and, in fact, we wouldn’t be here if this principle didn’t exist. It has wide range of implications in several quantum processes and thus in several physical phenomena of the universe.