Primary Sources Guide

An original paper lets you see a result before later terminology made it familiar. Read it beside a modern explanation: the paper tells you what was proposed and argued at the time, while the modern treatment helps you calculate and recognize later corrections.

Choose one question before opening a paper

Start with a narrow question such as “What did Millikan plot?” or “What did Born say the scattering amplitude predicts?” It is easier to follow the author’s reasoning when you know which claim you are looking for. The primary papers index provides publication details and links; the historical reading lists pair sources with broader accounts.

For an experiment, identify the apparatus, controlled variables, measured quantity, and conversion from a detector reading to a physical conclusion. For a theory paper, identify the assumptions, mathematical step, and prediction. In either case, separate the argument in the paper from an interpretation supplied by a later author.

Translate notation carefully

Source language	Useful modern translation	Distinction to retain
Planck’s energy elements	The energy scale $h\nu$ in thermal radiation	The counting argument is not a theory of modern photon states.
Bohr’s stationary orbits	Discrete energies and transition frequencies	An orbit is not a stationary wavefunction.
Transition quantities in matrix mechanics	Matrix elements of operators	A transition array is not an ordinary function along a classical trajectory.
A scattering-wave amplitude	A probability amplitude	Check whether the probabilistic interpretation is present in the source or supplied later.
Directional quantization	Discrete angular-momentum projections	The 1922 atomic-beam experiment predates the electron-spin hypothesis.

For example, the projective Born rule for a normalized pure state is now written

$$P(a)=\langle\psi|P_a|\psi\rangle,$$

where $P_a$ projects onto the eigenspace associated with outcome $a$. Born’s 1926 scattering paper introduced a probabilistic interpretation in a particular problem; it did not state this entire modern projector formulation. See the history of the Born rule and the modern rule for the two treatments.

Three manageable starting points

1. Photoelectric measurements: read Millikan’s experiment, then inspect his stopping-potential data and conclusions. Distinguish support for Einstein’s frequency law from acceptance of every part of the light-quantum picture.
2. Atomic structure: read the Bohr model, then follow the assumptions of the first 1913 paper. Compare its energy calculation with the modern hydrogen solution.
3. Quantum correlations: read the EPR argument before Bell’s historical contribution. Identify the locality and statistical assumptions before interpreting an inequality violation.

Translations are useful, but record the translator and edition when the wording matters. Publication, submission, experimental, and Nobel-award dates answer different questions; a retrospective Nobel lecture is a primary account of its author’s recollections, not a substitute for the original experiment paper.

Exercises

1. A source gives an amplitude $A$, while a textbook writes an observed probability proportional to $|A|^2$. What must you check before attributing the probability interpretation to that source?

Solution

Check whether the source itself connects the squared modulus to the measured frequencies, including its normalization and experimental meaning. The appearance of an amplitude does not by itself tell you how the author interpreted it. A later textbook may supply a valid modern interpretation that the original paper did not yet contain.

2. Why is agreement with Bohr’s hydrogen frequencies insufficient to justify citing his paper as a derivation of modern hydrogen wavefunctions?

Solution

The 1913 model uses stationary orbits and transition rules, not the Schrödinger equation and its boundary conditions. It supports a historical claim about those frequencies and their derivation. The modern wavefunctions require a different calculation, even where the leading energy levels agree.

References

• J. S. Bell, “On the Einstein Podolsky Rosen paradox,” Physics Physique Fizika 1, 195–200, 1964, doi:10.1103/PhysicsPhysiqueFizika.1.195.
• N. Bohr, “On the Constitution of Atoms and Molecules. Part I,” Philosophical Magazine 26, 1–25, 1913, doi:10.1080/14786441308634955.
• M. Born, “Zur Quantenmechanik der Stoßvorgänge,” Zeitschrift für Physik 37, 863–867, 1926, doi:10.1007/BF01397477.
• M. Jammer, The Conceptual Development of Quantum Mechanics, 2nd ed., American Institute of Physics, 1989.
• R. A. Millikan, “A Direct Photoelectric Determination of Planck’s h,” Physical Review 7, 355–388, 1916, doi:10.1103/PhysRev.7.355.