One hundred years ago, in July of 1924, a brilliant Indian physicist changed the way that scientists count. Satyendra Nath Bose (1894 – 1974) mailed a letter to Albert Einstein enclosed with a manuscript containing a new derivation of Planck’s law of blackbody radiation. Bose had used a radical approach that went beyond the classical statistics of Maxwell and Boltzmann by counting the different ways that photons can fill a volume of space. His key insight was the indistinguishability of photons as quantum particles.
Today, the indistinguishability of quantum particles is the foundational element of quantum statistics that governs how fundamental particles combine to make up all the matter of the universe. At the time, neither Bose nor Einstein realized just how radical his approach was, until Einstein, using Bose’s idea, derived the behavior of material particles under conditions similar black-body radiation, predicting a new state of condensed matter [1]. It would take scientists 70 years to finally demonstrate “Bose-Einstein” condensation in a laboratory in Boulder, Colorado in 1995.
Early Days of the Photon
As outlined in a previous blog (see Who Invented the Quantum? Einstein versus Planck), Max Planck was a reluctant revolutionary. He was led, almost against his will, in 1900 to postulate a quantized interaction between electromagnetic radiation and the atoms in the walls of a black-body enclosure. He could not break free from the hold of classical physics, assuming classical properties for the radiation and assigning the quantum only to the “interaction” with matter. It was Einstein, five years later in 1905, who took the bold step of assigning quantum properties to the radiation field itself, inventing the idea of the “photon” (named years later by the American chemist Gilbert Lewis) as the first quantum particle.
Despite the vast potential opened by Einstein’s theory of the photon, quantum physics languished for nearly 20 years from 1905 to 1924 as semiclassical approaches dominated the thinking of Niels Bohr in Copenhagen, and Max Born in Göttingen, and Arnold Sommerfeld in Munich, as they grappled with wave-particle duality.
The existence of the photon, first doubted by almost everyone, was confirmed in 1915 by Robert Millikan’s careful measurement of the photoelectric effect. But even then, skepticism remained until Arthur Compton demonstrated experimentally in 1923 that the scattering of photons by electrons could only be explained if photons carried discrete energy and momentum in precisely the way that Einstein’s theory required.
Despite the success of Einstein’s photon by 1923, derivations of the Planck law still used a purely wave-based approach to count the number of electromagnetic standing waves that a cavity could support. Bose would change that by deriving the Planck law using purely quantum methods.
The Quantum Derivation by Bose
Satyendra Nath Bose was born in 1894 in Calcutta, the old British capital city of India, now Kolkata. He excelled at his studies, especially in mathematics, and received a lecturer post at the University of Calcutta from 1916 to 1921, when he moved into a professorship position at the new University of Dhaka.
One day, as he was preparing a class lecture on the derivation of Planck’s law,
he became dissatisfied with the usual way it was presented in textbooks, based on standing waves in the cavity, and he flipped the problem.
Rather than deriving the number of standing wave modes in real space, he considered counting the number of ways a photon would fill up phase space.
Phase space is the natural dynamical space of Hamiltonian systems [2], such as collections of quantum particles like photons, in which the axes of the space are defined by the positions and momenta of the particles. The differential volume of phase space dVPS occupied by a single photon is given by
Using Einstein’s formula for the relationship between momentum and frequency
where h is Planck’s constant, yields
No quantum particle can have its position and momentum defined arbitrarily precisely because of Heisenberg’s uncertainty principle, requiring phase space volumes to be resolvable only to within a minimum reducible volume element given by h3.
Therefore, the number of states in phase space occupied by the single photon are obtained by dividing dVPS by h3 to yield
which is half of the prefactor in the Planck law. Several comments are now necessary.
First, when Bose did this derivation, there was no Heisenberg Uncertainty relationship—that would come years later in 1927. Bose was guided, instead, by the work of Bohr and Sommerfeld and Ehrenfest who emphasized the role played by the action principle in quantum systems. Phase space dimensions are counted in units of action, and the quantized unit of action is given by Planck’s constant h, hence quantized volumes of action in phase space are given by h3. By taking this step, Bose was anticipating Heisenberg by nearly three years.
Second, Bose knew that his phase space volume was half of the prefactor in Planck’s law. But since he was counting states, he reasoned that this meant that each photon had two internal degrees of freedom. A possibility he considered to account for this was that the photon might have a spin that could be aligned, or anti-aligned, with the momentum of the photon [3, 4]. How he thought of spin is hard to fathom, because the spin of the electron, proposed by Uhlenbeck and Goudsmit, was still two years away.
But Bose was not finished. The derivation, so far, is just how much phase space volume is accessible to a single photon. The next step is to count the different ways that many photons can fill up phase space. For this he used (bringing in the factor of 2 for spin)
where pn is the probability that a volume of phase space contains n photons, plus he used the usual conditions on energy and number
The probability for all the different permutations for how photons can occupy phase space is then given by
A third comment is now necessary: By assuming this probability, Bose was discounting situations where the photons could be distinguished from one another. This indistinguishability of quantum particles is absolutely fundamental to our understanding today of quantum statistics, but Bose was using it implicitly for the first time here.
The final distribution of photons at a given temperature T is found by maximizing the entropy of the system
subject to the conditions of photon energy and number. Bose found the occupancy probabilities to be
with a coefficient B to be found next by using this in the expression for the geometric series
yielding
Also, from the total number of photons
And, from the total energy
Bose obtained, finally
which is Planck’s law.
This derivation uses nothing by the counting of quanta in phase space. There are no standing waves. It is a purely quantum calculation—the first of its kind.
Enter Einstein
As usual with revolutionary approaches, Bose’s initial manuscript submitted to the British Philosophical Magazine was rejected. But he was convinced that he had attained something significant, so he wrote his letter to Einstein containing his manuscript, asking that if Einstein found merit in the derivation, then perhaps he could have it translated into German and submitted to the Zeitschrift für Physik. (That Bose would approach Einstein with this request seems bold, but they had communicated some years before when Bose had translated Einstein’s theory of General Relativity into English.)
Indeed, Einstein recognized immediately what Bose had accomplished, and he translated the manuscript himself into German and submitted it to the Zeitschrift on July 2, 1924 [5].
During his translation, Einstein did not feel that Bose’s conjecture about photon spin was defensible, so he changed the wording to attribute the factor of 2 in the derivation to the two polarizations of light (a semiclassical concept), so Einstein actually backtracked a little from what Bose originally intended as a fully quantum derivation. The existence of photon spin was confirmed by C. V. Raman in 1931 [6].
In late 1924, Einstein applied Bose’s concepts to an ideal gas of material atoms and predicted that at low temperatures the gas would condense into a new state of matter known today as a Bose-Einstein condensate [1]. Matter differs from photons because the conservation of atom number introduces a finite chemical potential to the problem of matter condensation that is not present in the Planck law.
Paul Dirac, in 1945, enshrined the name of Bose by coining the phrase “Boson” to refer to a particle of integer spin, just as he coined “Fermion” after Enrico Fermi to refer to a particle of half-integer spin. All quantum statistics were encased by these two types of quantum particle until 1982, when Frank Wilczek coined the term “Anyon” to describe the quantum statistics of particles confined to two dimensions whose behaviors vary between those of a boson and of a fermion.
References
[1] A. Einstein. “Quantentheorie des einatomigen idealen Gases”. Sitzungsberichte der Preussischen Akademie der Wissenschaften. 1: 3. (1925)
[2] D. D. Nolte, “The tangled tale of phase space,” Physics Today 63, 33-38 (2010).
[3] Partha Ghose, “The Story of Bose, Photon Spin and Indistinguishability” arXiv:2308.01909 [physics.hist-ph]
[4] Barry R. Masters, “Satyendra Nath Bose and Bose-Einstein Statistics“, Optics and Photonics News, April, pp. 41-47 (2013)
[5] S. N. Bose, “Plancks Gesetz und Lichtquantenhypothese”, Zeitschrift für Physik , 26 (1): 178–181 (1924)
[6] C. V. Raman and S. Bhagavantam, Ind. J. Phys. vol. 6, p. 353, (1931).
[7] Anderson, M. H.; Ensher, J. R.; Matthews, M. R.; Wieman, C. E.; Cornell, E. A. (14 July 1995). “Observation of Bose-Einstein Condensation in a Dilute Atomic Vapor”. Science. 269 (5221): 198–201.
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