The uncertainty principle states that the product of the uncertainty in measurement of one variable, say momentum p, multiplied by the uncertainty of measurement of another variable, say position x, can never be smaller than Planck's constant h. Then Δp Δx = h. So, if we know the position of a particle very accurately we cannot determine its momentum with great precision. The same relationship occurs between other pairs of variables such as energy and time.
The uncertainty principle was postulated by the German physicist Werner Heisenberg, who asserted that certain pairs of physical properties, like position and momentum, cannot both be known to arbitrary precision. That is, the more precisely one property is known, the less precisely the other can be known. It is impossible to measure simultaneously both position and velocity of a microscopic particle with any degree of accuracy or certainty. Heisenberg formulated the uncertainty principle in Niels Bohr's institute at Copenhagen, while working on the mathematical foundations of quantum mechanics.