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Tagore’s philosophical views and Quantum Mechanics

Rabindranath Tagore (actual Bengali name: Rabindranath Thakur) (1861–1941), the great Indian philosopher, a Bengali poet and a polymath, who received Nobel Prize for Literature in 1913, lived during a transition period of Indian history in general and the Bengali culture in particular, when physics also went through revolutionary changes. Albert Einstein (1879–1955), the most prominent physicist of the 20th century, was the pioneer of modern physics, who produced theories which advanced physics to unprecedented levels. Although Einstein produced the ‘the principle of photoelectric effect’ for which he received the Nobel Prize for physics in 1921 and which was pivotal to the advent of quantum mechanics, he could not fully reconcile with the multifarious implications of quantum mechanics. The two stalwarts of the first half of the 20th century met a number of times from 1926 onwards. When Tagore visited continental Europe and then America in 1930, they met at least four times in Berlin and New York. The meeting at Einstein’s summer villa outside Berlin was of particular interest when they exchanged views and philosophical ideas extensively. During that meeting – poignantly described by Dmitri Marianoff, a journalist in the New York Times, as being between ‘Tagore, the poet with the head of a thinker, and Einstein, the thinker with the head of a poet’ – the two exchanged views on the reality of nature.

Einstein held the view that the world and, for that matter, the whole universe, is there independent of humanity. Tagore held the view that the world is a human world and hence without humanity, the world is irrelevant and non-existent. Einstein persisted and queried that aren’t beauty and truth absolute and independent of man? Tagore disagreed and said that truth is realised through man and without man it does not exist. The whole conversation between these two giants was absolutely fascinating – it brought out the mindset of a scientist seeking out nature as it exists and that of a poet observing nature through the eyes and minds of human beings.

Einstein’s commitment to the reality of nature was absolute, and that absolutism brought him into conflict with the quantum reality proposed by Niels Bohr, Werner Heisenberg and others. Einstein believed in the existence of causal, observer-independent reality; whereas quantum mechanics considers reality dependent on the act of observation. Bohr/Heisenberg proposed that an atomic particle like an electron is there only when it is observed. If it is not observed, it is not there; it could be anywhere only to be described by quantum functional description. But Einstein would not accept that. He retorted by saying that the moon is there in the sky whether you observe it or not. Quantum mechanics states that an entity having unobserved presence cannot be claimed to be present with absolute certainty (with the probability of 1). Quantum mechanics tell us that the observer and the observed are entwined. The reality is not pre-ordained; reality is what is observed.

In 1928, Tagore received Arnold Sommerfeld, professor of theoretical physics at the university of Munich and a pioneer of atomic spectra, at Shantiniketan, West Bengal. Sommerfeld stated, “Tagore is to India what Goethe (pronounced as Görta) is to Germany”. Sommerfeld’s student Werner Heisenberg visited India the following year.

Heisenberg was one of the principal architects of quantum mechanics and his ‘uncertainty principle’ is the cornerstone of quantum mechanics. During the 1920s, he, along with Niels Bohr and others, produced what is now known as the ‘Copenhagen Interpretation of quantum mechanics’, where multiple existence of an atomic particle at different locations with superposition of quantum states was considered to be the reality of nature.

Although quantum mechanics had enormous success and explained various physical phenomena, which classical physics was incapable of explaining, the conflict with Einstein on quantum mechanical fundamental assumptions of probabilistic description was deep rooted. Einstein considered quantum mechanics as an incomplete description of nature.

In 1929, when Heisenberg undertook a lecture tour around the world, he came to India. On 4 October 1929, he visited the University of Calcutta and in the afternoon, he visited Tagore. In fact, he was taken to Tagore’s house at Jorasanko by the scientist Debendra Mohan Bose, a nephew of Jagadish Chandra Bose, and they had a number of conversations over the next few days. Heisenberg was very much impressed by Tagore’s philosophical views. Fritjof Capra in his book Uncommon Wisdom wrote,:

“In 1929 Heisenberg spent some time in India as the guest of the celebrated Indian poet Rabindranath Tagore, with whom he had long conversations about science and Indian philosophy. The introduction to Indian thought brought Heisenberg great comfort. He began to see that the recognition of relativity, interconnectedness and impermanence as fundamental aspects of physical reality, which had been so difficult for himself and his fellow physicists, was the very basis of the Indian spiritual traditions.”

Heisenberg said, “After these conversations, some of the ideas that had seemed so crazy suddenly made much more sense. That was great help for me.”

Heisenberg’s comfort was to be seen in the context of a great intellectual battle that had been raging at that time between Einstein and Bohr/Heisenberg on the reality of nature. Indian mysticism or more accurately, Tagore’s interpretation of Oriental (Brahma) philosophy, giving a support to modern physics and quantum theory, was undoubtedly a great comfort to Heisenberg. No wonder, Heisenberg even said after their conversations that Tagore reminded him of a prophet of the old days!

Tagore’s philosophy of viewing the world with human eyes may seem to conflict with Einstein’s observer-independent reality, but these are two perspectives of the reality. Tagore’s view of reality resonates very well with the quantum philosophy of observer-dependent reality.

Dr A Rahman is an author and a columnist

Advanced science, Life as it is, Technical

Quantum Formalism

Quantum mechanics came into existence at the turn of the twentieth century when many newly discovered experimental evidences could not be explained with classical mechanics. Max Planck initiated the concept of quantisation of light in 1900 to give a rational explanation of the black body radiation. Albert Einstein laid the concept of quantisation on a firm foundation in 1905 when he produced the theory of photoelectric effects and established photons as the entity of light quanta.

Since then quantum mechanics had gone from strength to strength and produced many laws, principles and theories to explain successfully the newly emerging scientific and technical problems that came up with advanced technologies. But at the same time there were some most bizarre and mind-boggling phenomena that defied intuitive logical explanation and challenged quantum principles right up to the limits. This write-up presents some of those bizarre inexplicable phenomena.

But, first of all, we need to define specifically the broad areas of quantum mechanics and differentiate it from classical mechanics. Quantum mechanics deals with extremely small entities, such as atoms, electrons, photons etc., which are commonly called quantum particles.

An atom as a whole is neutral in charge; which means that there are as many protons (positively charged) as there are electrons (negatively charged) in an atom. Hydrogen is the first element with just one proton and hence one electron; carbon is the sixth element with six protons and six electrons. There are more than 100 elements; each element has equal number of protons and electrons. These electrons are assumed to revolve round the nucleus of the atom. When an electron is dislodged from an atom, it is free to diffuse or drift along the material. When these electrons flow in large numbers through a conducting medium, we get electricity.

Now the technical question that can be posed, is an electron a particle like a miniature ball or a wave like a photon? Quantum mechanics asserts that it can be either – a wave or a particle – depending on the circumstance. In fact, one of the major planks of quantum mechanics is the wave-particle duality. Louis de Broglie in his Ph.D. dissertation in 1924 postulated that if light waves i.e. electromagnetic waves could behave like particles, then particles such as electrons could also exhibit wave properties. Indeed, they do and Louise de Broglie received a Nobel Prize in 1929 for his ground-breaking contribution of wave-particle duality.

Electromagnetic waves propagate through space like waves, as water waves do on the surface of water having crests and troughs. When two waves merge together in harmony, the crests and troughs join together and become larger (the amplitudes of two crests or two troughs add together); this is called the constructive interference. On the other have, if two waves merge in opposition i.e. in anti-phase, the crests and troughs cancel each other and there will be no ripple and that is called the destructive interference.

Double-slit experiment with a light source

If a light source is placed in front of a double-slit barrier and the light is allowed to fall on a screen behind the barrier, the constructive and destructive interferences would show as interference fringes of bright and dark bands, as shown above. So, interference fringe is a definitive proof of wave nature of light – light diffracting through the double-slits. (Of course, light can also have particulate nature, as shown in Einstein’s photoelectric effects.)

Double-slit experiment with an electron source

Now let us get back to the question of electrons. If electrons are fired from a source towards a screen and there is a double-slit barrier between the source and the screen, the screen should show the images of two slits on the screen. That is expected and perfectly normal, as the electrons are behaving like particles going through the slits and then striking the screen. Now if the slits are sufficiently narrowed down and the rest of the arrangement remains same, what is then seen on the screen is a band of bright and dark bands, as if the electrons are behaving like waves producing interference patterns! Now remembering de Broglie’s wave-particle duality, this outcome would not be too surprising or outrageous!

Now let us make an arrangement when just one electron is fired at a time and let that electron have sufficient transit time to go through the slit and reach the screen. The electron can go through either of the slits and one would expect that images of the slits would be produced on the screen, if sufficiently large number of electrons are fired. But amazingly, an interference pattern appears on the screen!

This is bizarre. Remember that just one electron was fired at a time. Even if the electron behaved like a wave, then that electron-wave would just melt away as it reached the screen. It surely could not wait on the screen for the next electron-wave to come through and interfere with it!

Now, could that be that an electron somehow goes through both the slits simultaneously to produce an interference pattern on the screen? Then what on earth is the physical mechanism to have one electron going through two slits at the same time? The other possible picture could be that half of an electron goes through one slit and the other half through the other slit and they produce the interference pattern. But then what is the mechanism of splitting an electron into two halves to make an interference pattern? The whole thing becomes surreal, but the interference pattern is real.

Then the experimenter became more curious and thought that it would be worthwhile to find out exactly which way the electrons are going? Is an electron going through both the slits simultaneously? A detector was placed very discreetly away from the path of the electrons behind one of the slits. As an electron is negatively charged, the flow of the electron would produce current and that current would produce a magnetic field. The detector that had been designed to detect the magnetic field. Thus, a detector placed behind one of the slits would not disturb the electron path and its flow.

The experiment was then conducted with the same setup, but with a detector placed discreetly behind one of the slits. What had been found on the screen? The interference pattern just disappeared completely! Yes, no bright and dark bands; only images of the slits on the screen! It is, as if, the electrons found out that they had been spied on and they decided not to behave like waves any more. Take the detector away, interference pattern return! Science becomes supernatural!

These strange behaviours of electrons were so puzzling that even more than hundred years later (since these experimental evidences) nobody could give a rational explanation. Quantum mechanics came into existence and flourished since then, but even quantum mechanics could not give any sensible explanation of the bizarre electron behaviour. But, nonetheless, quantum mechanics had produced an abstract mathematical formalism to explain this evidence.  

In quantum mechanics, particles or waves are treated wave functions (Schrodinger’s wave equation). When there are two slits, two wave functions go through and interfere and that process is called quantum superposition. That superposition of waves produces interference pattern. Even one wave function – a mathematical formalism – can go through two slits and have superposition and produce an interference pattern.

Niels Bohr, the high priest of quantum mechanics, and his group of fellow quantum physicists produced, what is known as Copenhagen Interpretation of quantum mechanics.  This Interpretation advanced the idea that sheer act of observation of quantum particles disturbed the character of electron-wave flow and that caused the waves to collapse into particles.

Quantum mechanics gives an abstract mathematical formalism of a system. It can predict quite accurately the correct outcome (such as electron fringes), but it does not or cannot give the physical picture of the path of the electron. In fact, the Copenhagen Interpretation insists that asking to know the path of the electron is superfluous and irrelevant. What is relevant is what happens when electrons reach the destination and quantum mechanics has the answer for that. That is the strength of quantum mechanics.    

  • Dr A Rahman is an author and a columnist