Quantum mechanics took a giant leap with the principle of uncertainty. This principle was developed by Werner Heisenberg . According to uncertainty principle no one can measure velocity and precise position of an particle simultaneously at any given moment of time. Our very act of measurement of one quantity causes high degree of error in value of other quantity. This means that if not measured, a particle does not have a definite velocity and momentum before we perform an act of measurement. Einstein along with other fundamentalist were not convinced. According to him closing our eyes and not watching the moon doesn’t mean moon is not there. Moon will be part of the reality whether we are looking at it or not. So three scientist Einstein, Podolsky and Rosen (EPR) found out a way to a way to derive a way to measure particle’s both speed and position without actually disturbing the particle.
In physics we encounter many instances where two particle (of same mass, density)collide with one another and travel at same speed after collision. This happen in accordance to newtons third law of motion. So by measuring position and velocity of one particle we can determine velocity and position of the other particle without actually disturbing the other particle.
This proved that a particle can have definite position and velocity. But EPR experiment did not put an end to this debate . Quantum physicist shot back with Entanglement Principle. At the end EPR had to admit that we cannot measure position and velocity simultaneously and uncertainty in it was valid. But it was Einstein’s frequent arguments against Quantum mechanics which led to its developement