The Founding Physicists

Open any textbook on quantum mechanics written in the United States or the United Kingdom. Flip to the history section. You will find the same list — Planck, Einstein, Bohr, Heisenberg, Schrödinger, Dirac, Born, Pauli. You will almost never find Bose. You will not find Saha. You may find Raman relegated to a single sentence about scattering. You will not find Bhabha at all.

This is a strange gap, because the physics is not in dispute. Bose statistics — the statistical rule that photons, gluons, and the Higgs boson all obey — is named after Satyendra Nath Bose. Particles that obey it are called bosons for the same reason. The Saha equation — the equation that lets you read the temperature and composition of a star from its spectrum — is the first place the new quantum theory was applied to something at industrial scale, and it sits on the first page of every textbook in stellar astrophysics. Raman scattering is an experimental signature of the quantisation of molecular vibration so sharp that every undergraduate spectrometer lab in the world reproduces it. Bhabha scattering is one of the standard test processes of quantum electrodynamics, the theory that became the template for the Standard Model.

If quantum mechanics was built in the 1920s, then a full quarter of the founders were Indian, working in Dacca and Calcutta and Allahabad, often in poorly equipped labs, often without the resources their European contemporaries took for granted. Their work was not peripheral — it was foundational. This article is a quick tour through the nine people who built the subject, with equal weight given to each. You will notice, by the end, that the names are not decoration — the physics they did is still in use.

The narrow window: 1920 to 1928

Quantum mechanics as a usable theory was created in a shockingly narrow window. By 1900 Planck had introduced the quantum to fix the ultraviolet catastrophe (you met this in the old crises of physics). By 1925, nothing rigorous yet existed — only the old quantum theory of Bohr and Sommerfeld, full of ad-hoc rules about quantised orbits. Then, in the space of four years, three independent formulations of a full quantum mechanics appeared, were proved equivalent, and were stitched together into one theory that has needed no fundamental revision since.

That is the period this chapter covers. It opens in 1920 in Allahabad with Meghnad Saha, closes in 1935 in Cambridge with Homi Bhabha, and in between moves through Dacca (Bose), Göttingen and Copenhagen (Heisenberg, Bohr, Born), Zürich (Schrödinger), and Cambridge (Dirac). Nine people, all alive at once, reading each other's papers and arguing about them across the international mail.

Meghnad Saha — ionisation in the stars (1920)

Meghnad Saha was born in 1893 in Shaoratoli, a village in what is now Bangladesh, to a shopkeeper's family. He was the son of a shopkeeper because his upper-caste schoolmaster had refused to teach Dalit students, and Saha's family — of the Nomo-Shudra caste — would have had no formal schooling at all if not for a single reformist teacher, Ananta Kumar Das, who took him in. By 1920, at 27, Saha was a lecturer at Allahabad University and had produced what remains the most important early application of quantum theory to astrophysics.

The problem: why do different stars show different spectral lines? The Sun has strong hydrogen and calcium lines; the very hottest stars (now called O-type) show barely any hydrogen and instead show helium and heavily ionised metals; the coolest stars show molecular bands. Before Saha, astronomers classified stars by their spectra but had no theory of what the classification meant.

Saha's insight, published in Philosophical Magazine in October 1920 [1], was to apply the brand-new Bohr quantum theory of the atom combined with statistical mechanics to the ionisation equilibrium. At a given temperature, some fraction of atoms are ionised — the hotter the gas, the more ionisation. The precise fraction is set by a balance between two competing rates, and Saha wrote down the equation.

Reading the equation. The exponential e^{-\chi/k_B T} is the Boltzmann factor — it says that the ratio of ionised to neutral atoms rises sharply once k_B T approaches the ionisation energy. The \Lambda^3 factor is purely quantum — it counts the number of accessible quantum states for the freed electron. Without the 1/\Lambda^3 (which is \propto T^{3/2}), you have classical statistical mechanics, and it gives the wrong answer. With it, you recover the spectra observed from actual stars.

Saha's equation is the reason every modern stellar-atmosphere model works. It was also, historically, the first serious industrial-scale application of the new quantum theory — years before matrix or wave mechanics existed. Saha was nominated for the Nobel Prize several times; he did not win, a common pattern for Indian scientists of the era.

Satyendra Nath Bose — the statistics of identical photons (1924)

Satyendra Nath Bose, born 1894 in Calcutta, was teaching at the University of Dacca (then part of British India, now in Bangladesh) in 1924. He was lecturing on the Planck formula for blackbody radiation and realised that the standard derivation made an error: it assumed photons could be distinguished from each other, the way two billiard balls can be. Bose suspected the assumption was wrong. He rewrote the counting treating photons as genuinely indistinguishable — two photons in the same state do not double-count — and out fell the Planck formula without needing any of the handwaving of the original derivation.

This is a sharper point than it looks. Before Bose, the Planck formula had been derived by treating electromagnetic radiation as a collection of classical oscillators and then quantising the energy per oscillator. That is how Planck himself had done it in 1900, with obvious discomfort. Bose's 1924 derivation was the first to work from the photon directly — from the counting of quantum states of identical particles — and get the right answer.

He sent the paper, Planck's Law and the Hypothesis of Light Quanta, to the Philosophical Magazine. It was rejected. Bose then posted it to Einstein in Berlin with a covering letter in English, asking him to translate it into German and find a journal. Einstein did both — he translated the paper, submitted it to Zeitschrift für Physik with a recommending note, and then wrote three papers of his own extending Bose's counting to atoms of an ideal gas. The result is what you now call Bose–Einstein statistics, the rule obeyed by all integer-spin particles. They are called bosons because of Bose.

The counting argument

The setup is a cavity with electromagnetic modes labelled by frequency \nu. Each mode can hold 0, 1, 2, \ldots photons. The question is: at temperature T, what is the average number of photons per mode?

Classical statistical mechanics treats the photons as distinguishable and uses the equipartition theorem. It gets the wrong answer (the ultraviolet catastrophe — see the old crises). Bose's fix: enumerate the states of the mode, not the photons. A mode with n photons is one state, regardless of which n photons are in it. Then use Boltzmann weights on those mode-states.

The deep point: Bose's statistics is one of the two statistics that all particles obey (the other being Fermi–Dirac for half-integer spin). When you write a multi-qubit quantum state, the assumption that qubits are interchangeable systems — that you can permute them without physically doing anything — is the same kind of assumption Bose was formalising. Identical particles are a foundational input to quantum computing, and Bose is the reason the framework exists.

C. V. Raman — inelastic scattering and a Nobel (1928)

Chandrasekhara Venkata Raman was born in 1888 in Tiruchirappalli, Tamil Nadu. By 1928 he was the Palit Professor of Physics at Calcutta University and had already published widely on acoustics and optics. His Nobel-winning discovery is one of the cleanest in quantum physics, because you can see it with cheap equipment.

The observation

On 28 February 1928 (now celebrated in India as National Science Day), Raman and his student K. S. Krishnan observed that when monochromatic light — a single sharp frequency — passes through a liquid, the scattered light contains, in addition to the original frequency, faint spectral lines at shifted frequencies. The shifts are a fingerprint of the molecule doing the scattering, and they are quantised.

The quantum explanation, immediate once you believe in photons and molecular vibrational levels:

• A photon of frequency \nu_0 arrives at a molecule.
• The molecule is in some vibrational state with energy E_i.
• The photon interacts, and the molecule is left in a different vibrational state with energy E_f.
• Energy conservation: the scattered photon has frequency \nu_s satisfying h\nu_s = h\nu_0 - (E_f - E_i).

If the molecule gains vibrational energy, the photon loses it — Stokes lines, at \nu_0 - \nu_{\text{vib}}. If the molecule loses vibrational energy, the photon gains it — anti-Stokes lines, at \nu_0 + \nu_{\text{vib}}. The shift is a direct read-out of molecular quantum energy levels.

Raman had predicted the effect theoretically from the old quantum theory in 1923, and had been trying to observe it since 1921. The experimental confirmation took a mercury lamp, a prism, a well-purified sample of benzene, and patience. The spectrum showed the Stokes and anti-Stokes lines exactly where Raman predicted.

He received the 1930 Nobel Prize in Physics — the first Nobel in any science awarded to an Asian scientist. The prize citation names the effect explicitly: "for his work on the scattering of light and for the discovery of the effect named after him."

What Raman scattering means today

Raman spectroscopy is now the standard technique for identifying molecular structure in chemistry, biology, and materials science. Every pharmaceutical company in India — Sun Pharma, Dr. Reddy's, Cipla, Biocon — has a Raman spectrometer on the factory floor for identity verification of raw materials. ISRO uses Raman on Mars- and Moon-bound instruments to identify minerals. Every time you see a headline about "molecular fingerprinting" or "rapid sample identification," the underlying technology was discovered in a Calcutta lab in 1928 using a mercury lamp.

Heisenberg, Schrödinger, Dirac — the three formulations (1925–1926)

Now the European spine, briefly. The Indian contributions above came first chronologically; the European formulations came after.

Werner Heisenberg, 23 years old in 1925, was on Helgoland — a small North Sea island where he had gone to recover from hay fever. In two weeks he worked out that the quantum observables like position and momentum were not numbers but non-commuting arrays of numbers. Max Born, reading Heisenberg's manuscript back in Göttingen, realised that these arrays were matrices — and that Heisenberg had rediscovered matrix algebra without knowing the word. Born, Heisenberg, and Jordan then formalised matrix mechanics: observables are Hermitian matrices, dynamics is matrix-valued, position and momentum satisfy [x, p] = i\hbar.

Erwin Schrödinger, 38 years old in 1926, was in Zürich. Motivated by de Broglie's 1924 idea that matter has a wavelength, he asked: what wave equation does an electron's wave function satisfy? In a six-month burst of work in 1926 he wrote down

— the Schrödinger equation. This is a partial differential equation for a complex-valued function \psi(\mathbf{r}, t), and Schrödinger showed that its solutions for the hydrogen atom reproduced the observed spectral lines exactly.

Matrix mechanics and wave mechanics looked like completely different theories. Within three months of Schrödinger's first paper, Schrödinger himself and independently Carl Eckart proved they were mathematically equivalent: Heisenberg's matrices are the representations of Schrödinger's operators in a specific basis, and vice versa.

Paul Dirac, 24 years old in 1926, was at Cambridge, reading both theories in parallel. He realised that both were special cases of a more abstract structure: quantum mechanics is about operators on an abstract Hilbert space of states. In November 1925 he wrote a paper making this precise, inventing along the way most of the notation you now see in every quantum-computing textbook — in particular the bra-ket notation \langle \psi | \phi \rangle, which you use every time you compute an inner product on a quantum computer.

Dirac went further. In 1928 he wrote down the Dirac equation, a Schrödinger-like equation that is consistent with special relativity and automatically gives the electron its spin-1/2 structure. The Dirac equation predicted the existence of antimatter — the positron, observed by Carl Anderson in 1932 — four years before it was seen in a lab. It is the cleanest example in physics of a correct equation predicting something nobody had asked for.

Niels Bohr and Wolfgang Pauli complete the European team. Bohr (Copenhagen, Denmark) framed the interpretation — the Copenhagen interpretation with its notions of complementarity and observer-dependent outcomes — and ran the Institute for Theoretical Physics that was the epicentre of the field. Pauli (ETH Zürich, then Princeton) produced the exclusion principle (two fermions cannot share a quantum state) and the neutrino hypothesis. These two framed the philosophical and structural side of the new theory.

Homi Bhabha — electron–positron scattering (1935)

Homi Jehangir Bhabha was born in 1909 in Bombay, to a wealthy Parsi family. He read mathematics and then theoretical physics at Cambridge, arriving just as Dirac's theory of the electron was generating the first predictions about antimatter. In 1935, collaborating loosely with Walter Heitler, he published the calculation of electron–positron scattering — the quantum-electrodynamic process where an electron and its antiparticle bounce off each other. The cross-section for this process is today called the Bhabha cross-section, and it is used daily at particle-physics experiments (including the Large Hadron Collider) to calibrate luminosity.

Bhabha also worked out, again with Heitler, the theory of cosmic ray showers — the cascade of electron-positron and photon creation that happens when a high-energy cosmic particle hits the atmosphere. This is still the standard model of cosmic-ray phenomenology.

Bhabha's scientific importance is sometimes overshadowed by his institutional importance. He returned to India in 1939, founded the Tata Institute of Fundamental Research in 1945, then the Atomic Energy Commission of India, then what is now the Bhabha Atomic Research Centre. India's entire nuclear and particle-physics establishment is his creation. He died in a plane crash near Mont Blanc in 1966 at age 56.

Common confusions

• "Quantum mechanics was invented in Europe." A quarter of the foundational work was done in India, in Bengali- and English-speaking institutions, often with poor equipment and no travel budget. Saha (1920) came before matrix mechanics (1925). Bose (1924) preceded wave mechanics (1926). Raman (1928) was published alongside Dirac's equation. The "Copenhagen-centric" narrative is a historical artefact of who wrote the textbooks, not of who did the physics.

• "Bose won the Nobel for Bose–Einstein statistics." He did not. The Nobel Committee considered the work several times but the prize was not awarded — an omission widely regarded as one of the most striking in Nobel history. Einstein, who extended the statistics to atoms, had already received his Nobel in 1921 for the photoelectric effect. Particles that obey Bose statistics are bosons; the statistics are Bose–Einstein; but the Nobel Prize was not.

• "Dirac synthesised everything already worked out." Dirac's equation was new, not a synthesis. The Schrödinger equation is non-relativistic — it does not respect the rules of special relativity. Dirac in 1928 wrote down the first relativistic quantum equation for a single electron, and in doing so discovered both spin and antimatter. Matrix and wave mechanics were the warm-up; the Dirac equation is its own discovery.

• "Raman's effect is obscure." The Raman effect is the active signal behind every Raman spectrometer on Earth, and every pharma factory, ISRO mission, and forensic laboratory in India uses one. National Science Day (28 February) commemorates its discovery. It is one of the most widely-deployed applications of quantum physics in industry.

• "These names are just history — what do they have to do with quantum computing?" Every one of them. Bose statistics is the input to photonic quantum computing (§17 of this track), where the qubits are literal bosons. The Dirac equation's Clifford algebra is the structure of error-correcting codes (§13, Steane code). The Saha equation is the input to simulating stellar plasmas on a quantum computer. The Raman effect is used to read out trapped-ion qubits (§18). The Bhabha cross-section is a standard benchmark process for simulating quantum electrodynamics on a quantum computer (§15). The 1920s physics is not decoration — it is the upstream source of everything in the rest of this track.

Going deeper

Bohr versus Einstein

Between 1927 and 1935 Niels Bohr and Albert Einstein conducted what is sometimes called the most important philosophical debate in 20th-century physics. Einstein believed that quantum mechanics was incomplete — that "God does not play dice" and that a deeper deterministic theory would eventually replace it. Bohr defended the Copenhagen interpretation: measurement is fundamental, outcomes are genuinely probabilistic, and there is no "deeper" layer.

The debate went through several rounds. Einstein would propose a thought experiment that he thought broke quantum mechanics; Bohr would spend a sleepless night working out what he had missed, and refute it the next day. The most important exchange is the EPR paper (Einstein–Podolsky–Rosen, 1935), which introduced the phenomenon you now know as entanglement and argued it showed quantum mechanics was incomplete. Bohr's reply was famously difficult to parse. The issue was not resolved until John Bell's 1964 theorem and Alain Aspect's 1982 experiments — by which time both Einstein and Bohr were dead. You will meet this history again in Part 7 of the track, on Bell's theorem.

Chandrasekhar — quantum statistics and stellar death

Subrahmanyan Chandrasekhar, nephew of C. V. Raman, did his PhD at Cambridge under R. H. Fowler. In 1930, on the ship from Madras to Cambridge, he calculated what happens to a white-dwarf star when you apply Fermi–Dirac statistics (the other quantum statistics, for fermions like electrons) to its degenerate electron gas. The answer: above a certain mass, about 1.4 solar masses, no amount of quantum pressure can stop the star from collapsing. This is the Chandrasekhar limit, the mass boundary between white dwarfs and neutron stars / black holes.

Eddington famously attacked the result at a Royal Astronomical Society meeting — he did not want to believe black holes could form. Chandrasekhar was right. He received the Nobel Prize in 1983, over fifty years after the calculation.

The National Quantum Mission and historical continuity

India's National Quantum Mission (2023) has a ₹6000 crore budget and explicitly cites the foundational contributions of Bose, Raman, and Bhabha as the historical continuity it is extending. The Mission's four pillars — quantum communication, sensing, materials, and computing — each map to a 1920s-foundational area:

• Communication (QKD, post-quantum crypto) ← Bose statistics (photons as qubits)
• Sensing (magnetometry, atomic clocks) ← Raman effect (spectroscopy) and Saha equation (plasma diagnostics)
• Materials (superconductors, topological matter) ← Dirac equation (relativistic electrons in graphene), Bose–Einstein condensates
• Computing (gate-model hardware, algorithms) ← all of it, via quantum gates acting on qubits that are identical particles in the Bose/Fermi sense

When ISRO discusses satellite-QKD experiments, or when TIFR Mumbai sets up a Raman-readout trapped-ion platform, the thread back to the 1920s is not sentimental — it is technical. The devices are direct descendants of the physics these nine people built.

Where this leads next

• The Old Crises of Physics — chapter 208, the problems that forced the quantum revolution to happen in the first place.
• The Quantum Information Turn — chapter 210, the reframing that made quantum mechanics a computational resource.
• Feynman's Argument — chapter 2, where the 1920s founders' framework gets turned into the original case for quantum computing.
• Bell's Theorem and CHSH — Part 7, the resolution of the Bohr–Einstein debate.
• Four Postulates of Quantum Mechanics — Part 3, the 1920s formalism rewritten for computer scientists.

References

1. Meghnad N. Saha, Ionization in the solar chromosphere, Philosophical Magazine (1920) — Wikipedia summary.
2. Satyendra N. Bose, Planck's law and the hypothesis of light quanta (1924) — Wikipedia; original translated by Einstein, Zeitschrift für Physik.
3. C. V. Raman and K. S. Krishnan, A new type of secondary radiation, Nature (1928) — Wikipedia, Raman scattering.
4. P. A. M. Dirac, The Quantum Theory of the Electron (1928) — Wikipedia, Dirac equation.
5. Homi J. Bhabha, The scattering of positrons by electrons with exchange on Dirac's theory of the positron (1936) — Wikipedia, Bhabha scattering.
6. John Preskill, Lecture Notes on Quantum Computation, historical context — theory.caltech.edu/~preskill/ph229.