Albert Einstein (1879-1955) built further on this pivotal idea and it led him to the discovery of the photo-electric effect in 1906. In fact this work got Einstein his Nobel Prize. He showed that the reverse of Plānck’s hypothesis of energy quanta is also true. Light shining on certain sensitive metals generates a small electric current in them. This current varies not with light intensity but with changing light frequency. Mathematically, Einstein showed that light is absorbed too in discrete amounts or quantum packets of energy, and these were the same *photons* of Plānck’s Black-body radiation.

As the work progressed in early 1900s, light was established to have a *dual* nature. This came to be known as the *wave-particle duality* and initially was a very confusing concept. Neils Bohr, one of the co-founders of the atomic theory, accepted this duality and extended it to the ‘electron’**[16]**. He considered the Hydrogen atom and its light-emission spectrum. The discrete lines of different light frequencies in this spectrum pointed towards discrete energy levels for the planetary electron orbiting the hydrogen nucleus. This was like saying that *a planet can orbit the sun in a specific orbit only with all other possibilities being forbidden or out of bounds. *Bohr further showed that the ‘electron’ is not only *particle–like* in its behavior but it has *wave-like* characteristics too under certain conditions. In the case of the Hydrogen atom the electron assumes only those orbits which are integral multiples of its natural ‘wave-length’, λ_{e}. That is it accepts only those orbits which have circumferences of ‘n’ times λ_{e} where n= 1,2,3 and so on. This theory gave a brilliant *first * correspondence to the energy levels experimentally observed in the hydrogen spectrum and this hypothesis gave rise to atomic Quantum Theory, rest is history. Schrödinger, Dirac, Pauli, Heisenberg and many other minds contributed in quick succession to cement this theory into one of the major scientific breakthroughs of 20^{th} century.

Werner Heisenberg (1901-1976) gave his famous *Uncertainty Principle* based on this overlapping concept of *wave-particle duality*. He showed that all matter behaves sometimes as a *wave * and at other times as a *particle*. It is the very process of observation which determines which aspect of matter is revealed to us the *Observer*. As long as the *entity* is not disturbed it behaves like a wave. The moment we try and pin-point it the particle nature pops up. This switching of nature is significant at the atomic level and lends itself to an *Uncertainty* in measurement during experiments. It limits our capability of knowing exactly either the position or speed of the particle. The precision in trying to locate the *spot* at which the atomic entity is results in indeterminacy in knowing *how fast * it is moving – we cannot measure both aspects with infinite precision. The everyday *billiard-ball* concept of the particle literally goes out of the window. It has to be replaced by a fuzzy cloud of mathematical probability.

The spatial picture is therefore diffused and dual. Heisenberg’s principle in essence shows that the *Observer (and his consciousness) * is an integral part of any quantum level experiment. Truly speaking *our consciousness* cannot ‘know’ the ‘Unknown’ in its entirety. Our experiments, when repeated umpteen times, can only give us probabilistic, time-tested answers.

This is where the modern day Metaphysics began…