At the end of the nineteenth century, scientists were convinced that we knew whatever there was to know about the mysteries of Nature. This conviction was based on our unflinching faith on Newton's laws of motion and his universal theory of gravitation. In the words of British poet Alexander Pope, "Nature and Nature's law lay hid in night; God said let Newton be! And all was light." These laws together with Maxwell's theory of electro-magnetism, laws of thermodynamics, and kinetic theory of gases provided causal interpretations for the behavior of all objects in the Universe. They form the basis for what we call "classical physics."
The bedrock of classical physics is the aphorism of a deterministic Universe, a Universe where every observable can be simultaneously measured to any level of accuracy. Our dogmatic belief in the deterministic nature of classical physics remained unchallenged for more than 200 years. Things, however, changed at the turn of the twentieth century when discoveries of electrons, protons, neutrons, X-rays, radioactivity, etc. gave us the first glimpse of the denizens of subatomic world.
The path to quantum mechanics began when experiments to study the spectrum of thermal radiation, emission of electrons from surfaces illuminated with light, and scattering of X-rays by electrons presented us with new and puzzling results concerning the properties of matter and radiation. These experiments not only exposed the inadequacy of classical physics to explain phenomena at the microscopic level, they also profoundly influenced our perspective of the Universe.
Within a short span of time, two decades to be precise, quantum mechanics was developed to study the microscopic world. The basic tenet of quantum mechanics was provided in 1900 by the "reluctant revolutionary" Max Planck. He postulated that electromagnetic radiation is quantized and occurs in finite packets of energy called photons which are massless particles travelling at the speed of light. Likewise, Louis deBroglie hypothesized that particles at the microscopic level should exhibit wave-like properties.
The bold theories that were advanced and the revolutionary notion of dual property of matter and radiation as waves and particles, respectively, rendered Newton's concept of deterministic nature of particle, space, and time redundant in the microscopic domain. Instead, it was replaced by quantum theories based on the probabilistic nature of measurements, implying certain amount of fuzziness or uncertainty in the description of reality. The excitement of the era was described by Einstein "as a marvelous time to be alive."
In 1927, the probabilistic precept of quantum mechanics introduced a new level of reality in physics - Heisenberg's Uncertainty Principle. According to the principle, "Events at the microscopic level occur randomly, by pure chance meaning that they aren't determined by any cause whatsoever." Simply stated, at the incredibly small dimensions of the quantum world, there is a reciprocal uncertainty between position and velocity. If an electron's position, for example, is measured with high precision, measurement of its velocity would be imprecise, and vice-versa. Furthermore, we won't be able to predict with certainty where the electron would be when a next attempt is made to measure its position. These restrictions are not a result of errors in making measurements; they are fundamental limitations imposed by the idiosyncrasies of the Universe
The view that Nature is governed by probability rather than certainty flustered many scientists, including Einstein. According to him, the Universe was deterministic in the sense that every event that occurs is caused by other events in such a way that the causing events bring about their effects. He presumed that "God created Newton's laws of motion and the necessary masses and forces. That is all; everything else follows by deduction upon development of suitable mathematical methods."
Although he was well aware that quantum mechanics had survived stringent experimental tests, Einstein believed that if it "were correct then the world would be crazy." Flamboyant Nobel physicist Richard Feynman was frank enough to admit, "I think I can safely say that nobody understands quantum mechanics." True, we don't understand it; but we got inured to it.
The advocates of quantum indeterminacy deemed that the probabilistic mathematics of quantum mechanics is due to the failure of causality in reality. They analogized that behavior of the Universe, particularly at the atomic level, is similar to God playing dice with it. Einstein retorted, "God does not play dice with the Universe." Danish physicist Niels Bohr argued instead that Nature is apathetic to our preconceptions. His advice to Einstein was: "Stop telling God what to do." Cosmologist Stephen Hawking thinks, "Einstein was wrong when he said 'God does not play dice.' Consideration of black holes suggests, not only that God does play dice, but that he sometimes confuses us by throwing them where they can't be seen."

The writer is a Professor of Department of Physics & Engineering Physics, Fordham University, New York.