The quantum concept that came into existence precisely in the year 1900 was both revolutionary in outlook and spectacular in outcome. This very concept which was put forward by Max Planck in 1900 when he tried to explain black body radiation was subsequently taken up by a luminary like Albert Einstein (as yet unknown to the world) in 1905 and gave a rational explanation to the hitherto difficult scientific problem.
The classical physics (also known as Newtonian physics) was ruling the day until about 1900 when all day-to-day physical problems could be explained by this discipline. But gradually it was running out of steam as new technically challenging phenomena came up due to invention of new instruments and reliable measurements were made.
The intractable physical processes like the black body radiation, interactions of light with particles, the puzzling behaviour of light and many more physical processes could not be explained by traditional classical mechanics. So, a new method, a new mode of thinking, a new science had to be invented that would explain all these inexplicable things.
Although Max Planck was first to venture outside the conventional concept of light being wave in nature to explain ‘black body radiation’ in 1900, it was Albert Einstein who gave scientific explanation by proposing in 1905 the ‘quantisation’ of light – a phenomenon where light was assumed to consist of discreet packets of energy – which he called quantum of light or photon. This quantum of light was advanced in order to explain the hitherto inexplicable photoelectric process, where light was allowed to fall on the surface of a metal and electrons were detected to have emitted. No matter how long or how intense one type of light was, electrons would not be emitted. Only when light of higher frequencies was allowed, electrons were emitted. Einstein showed that photons (quantum of energy in a bundle) of higher frequencies have higher energies and those higher energy photons could emit electrons. (It was like, no matter how long or how heavy the rain is, the roof would not be dented. Only when hailstorm of sufficient big sizes falls on the roof, does the roof cave in). For this quantisation theory, Einstein was awarded Nobel prize in 1921.
Thus, light came to be viewed as both wave and particle, depending on experimental circumstances, and hence the nomenclature ‘wave-particle duality’ came into common vocabulary. If hitherto electromagnetic light can be viewed both as wave and particle, can particles (like electrons) behave like waves? Indeed, so. If electrons are allowed to go through two slits, they interfere and produce alternate bright and dark spectral lines on a screen, exactly like light waves do. The microscopic world does not distinguish between waves and particles, they are blurred into indistinguishable entities. That is the nature that quantum mechanics has produced.
Although Einstein was the pioneer of quantisation of light, he was not at ease with the way this new concept had been taken up by ‘new lions’ under the stewardship of physicists like Niels Bohr, Wolfgang Pauli, Werner Heisenberg, Erwin Schrodinger, Max Born and many more in the early part of the last century. They collectively produced the full-blown quantum mechanics, which Einstein had difficulty in recognising.
In quantum theory, particles like electrons revolving round the nucleus of an atom do not exist as particles. They are like strata of waves smeared round the nucleus. However, they exist, behaving like particles, when some energy is imparted to the atom or some energy is taken away from the atom resulting in those electrons moving up or down in energy levels. In other words, electrons exist only when there is an interaction or transition. Without such transitions, electrons just do not show up. However, electrons (with negative charge) are there around the nucleus, but there is no way of telling where the electrons are – only probability of their presence (wave function) can be described! No wonder, Einstein was not happy with such description, which he called incomplete.
Heisenberg produced what came to be known as ‘Heisenberg uncertainty principle’. The elementary particle like an electron cannot be measured with absolute accuracy both its position and momentum at the same time. The act of measuring the position of an electron disturbs the complementary parameter like velocity and so certain amount of uncertainty in momentum creeps in – that is the uncertainty principle. Similar uncertainty exists when measuring time and energy of the particle at the same time.
Niels Bohr, the high priest of quantum mechanics, produced from his Advanced Institute of Physics in Copenhagen, what came to be known as ‘Copenhagen Interpretation’ of quantum mechanics. This interpretation advanced the idea that elementary particles like electrons do not exist in stable or stationary conditions; they only exist in transitions and in interactions.
The ‘Copenhagen Interpretation’ further emphasised that a quantum particle can only be said to exist when it is observed, if it is not observed it does not exist. This was a revolutionary concept. Einstein could not reconcile with that idea. He retorted, “When the Moon is there in the sky, it is real; whether one observes it or not”. Thus, the great intellectual battle on the nature of reality ensued between Einstein and Bohr. Einstein firmly believed that the quantum mechanics as it existed in his life time was inconsistent and incomplete (although he withdrew the ‘inconsistent’ branding, as quantum mechanics kept explaining modern technical processes with consistency). To prove that ‘incompleteness’, he produced various ‘thought experiments’ at various times to challenge Bohr’s ‘Copenhagen Interpretation’. Bohr countered those challenges with technical explanations, but Einstein was not fully convinced.
Einstein did not like the abstract nature of quantum mechanics. He always demanded that theory must correspond to the reality, if not, it becomes a ‘voodoo’ science.
For his criticism, he was not very popular with the advocates of ‘Copenhagen Interpretation’. They even lamented that ‘how is it possible that Einstein who was the pioneer of quantum theory and who revolutionised gravitational concept by saying that space is warped by gravity and the gravitational field is indeed the space, now he is reluctant to accept ideas of quantum mechanics’?
Quantum mechanics had solved many intractable problems and predicted many physical aspects which subsequently came to be true. But at the same time, it is incomprehensible, extremely abstract and devoid of ‘elements of reality’. Anybody hoping to see theory mirroring reality would be totally disappointed. Even Richard Feynman, American Nobel laureate, who contributed significantly to the development of quantum physics once retorted, “I think I can safely say that nobody understands quantum mechanics”! Nonetheless, quantum mechanics is the most advanced scientific discipline of today.
– Dr A Rahman is an author and a columnist.