THE EINSTEIN ESSAYS
The Bohr-Einstein Debate
A narration through the “Debate of the Century”
Jørgen Veisdal
Sep 25, 2019 · 18 min read
The year is 1905. Newly graduated with a Ph.D. in physics, Albert Einstein publishes the paper Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichttspunkt (“On a Heuristic Viewpoint Concerning the Production and Transformation of Light”). In it, he proposes a revision to one of the fundamental laws of physics to account for the behavior of light as both a particle and a wave, work for which he would later be awarded the Nobel Prize (1921). Eight years later in 1913, in the paper On the Constitution of Atoms and Molecules, Part II Systems Containing Only a Single Nucleus, Danish physicist Niels Bohr adapts Ernest Rutherford’s 1911 model of the atom to Max Planck’s quantum theory to introduce a new model of the atom — the Bohr model, both earning himself his own Nobel (1922), as well as setting the stage for a coming quantum revolution in physics.
Fast forward 12 years. Building on the work of both Einstein and Bohr, Werner Heisenberg introduces matrix equations which remove the foundational elements of space and time from the then increasingly popular quantum mechanical model of physics. Building on this work, Max Born in the following year proposes that mechanics are most effectively understood not as causal links but as actions resulting from probability, not distinct causation. With Heisenberg’s solution of the Schrödinger equation for a scattering problem the following year, the now well-known Heisenberg uncertainty principle is introduced, leading Born and Heisenberg to declare that quantum mechanics was now “complete and irrevocable”.
In less than 25 years, starting with Planck’s 1900 discovery of the black body radiation law, to Einstein’s discovery of the photon, to Bohr’s redefinition of the model of the atom, to Heisenberg and Born’s refinement of quantum mechanics, physics in the first quarter of the twentieth century went from fully deterministic to seemingly indeterminate.
The Fifth Solvay International Conference (1927)
The Bohr-Einstein debate is generally considered to have begun during the Fifth Solvay International Conference on Photons and Electrons. The conference was held in October 1927 in Brussels, Belgium. Continuing on since the successful inaugural conference of 1911, the Solvay gatherings are devoted to outstanding preeminent open problems in physics, and occur approximately every three years. From 1913 to 1961, every gathering revolved around open problems in quantum theory. Chaired by Hendrik Lorentz in 1927, the official topic of the conference was “photons and electrons”. In practice, the 1927 conference revolved around the growing dispute between two then nascent schools of physics: those fascinated and enthralled by the new developments in quantum theory, and those still clinging to the superseded deterministic paradigm. The former was lead by Niels Bohr and the latter by Albert Einstein.
The Copenhagen interpretation
The open problem during the 1927 Solvay conference was how physicists should interpret the recent results of physicists Werner Heisenberg and Max Born, the now so-called “interpretation question” of quantum mechanics. Born and Heisenberg, fervent in their view, promoted the following (simplified) view:
“Physical systems do not have definite properties prior to being measured. Quantum mechanics can only predict the probability distribution of given measurements’ possible results.”
This because, as the view goes, the act of measurement affects the system being measured. This causes the set of probabilities to reduce to only one of the possible values immediately after the measurement — the so-called wave function collapse. In other words, prior to the measurement of (for instance) the position of an electron, its location is best described by a probability distribution (a wave function). In the act of measuring the position of the electron, the device measuring or observing the electron influences the probability distribution. After the measurement, due to the influence of the observer, the position of the electron is now best defined by a single value (e.g. a Cartesian coordinate).
Definition
“Despite an extensive literature which refers to, discusses, and criticizes the Copenhagen interpretation of quantum mechanics, nowhere does there seem to be any concise statement which defines the full Copenhagen interpretation.”
Despite statements such as the one given above by John G. Cramer in 1986 and many more both before and after it, for the purposes of this article we can colloquially state the Copenhagen interpretation as
The Copenhagen Interpretation of Quantum Mechanics
Physical systems generally do not have definite properties prior to being measured, and quantum mechanics can only predict the probability distribution of a given measurement's possible results. The act of measurement affects the system, causing the set of probabilities to reduce to only one of the possible values immediately after the measurement.
More specifically, we can define it as synonymous with a sum of the concepts of indeterminism, Bohr’s correspondence principle, Born’s statistical interpretation of the wave function and Bohr’s complementarity interpretation of certain atomic phenomena. The term itself stems from Heisenberg who worked as an assistant under Bohr at his institute in Copenhagen while he formulated his uncertainty principle, and can been traced to Heisenberg’s 1930 textbook The Physical Principles of the Quantum Theory in which he states that
"On the whole, the book contains nothing that is not to be found in previous publications, particularly in the investigations of Bohr. The purpose of the book seems to me to be fulfilled if it contributes somewhat to the diffusion of that Copenhagen spirit of quantum theory if I may so express myself, which has directed the entire development of modern atomic physics."
- Excerpt, “The Physical Principles of the Quantum Theory” by Werner Heisenberg (1930)
History
In the years from 1925 up until the conference in 1927, the quantum revolution that had been taking place had been propelled mainly by three revolutionary ideas:
In 1925, Werner Heisenberg introduced matrix equations that removed the Newtonian elements of space and time from quantum mechanics;
In 1926, Max Born proposed that quantum mechanics were best understood by probabilities;
In 1927, Heisenberg had formulated his uncertainty principle defining the mathematical model to describe the fundamental limit of the precision with which certain pairs of physical properties of a particle (known as complementary variables) can be known.
Heisenberg’s first breakthrough idea was first proposed in his paper Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen (“Quantum Theoretical Re-interpretation of Kinematic and Mechanical Relations”) which appeared in Zeitschrift für Physik in September 1925. Reportedly, Heisenberg in correspondence with Wolfgang Pauli had been working on the paper while recovering from hay fever. The purpose of the paper was to attempt to describe the energy levels of a one-dimensional anharmonic oscillator via observable parameters such as transition probabilities for quantum jumps (Segrè, 1980). The paper laid the groundwork for what is now known as matrix mechanics, which Heisenberg later developed in collaboration with Born and Pascual Jordan.
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https://www.cantorsparadise.com/the-bohr-einstein-debate-baa0929a78b5
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