Einstein vs Bohr · The Great Quantum Debate · 10-Part Series - Part 5
Einstein vs Bohr · The Great Quantum Debate · 10-Part Series
The Years Between
1930 to 1935: exile, flight, the rise of Hitler, letters across an ocean — and Einstein's slow crystallisation of the incompleteness argument
The years between the Sixth Solvay Conference in 1930 and the publication of the EPR paper in 1935 are among the most turbulent in modern history — and among the most consequential for the development of quantum mechanics. They are the years in which Einstein lost his country, crossed an ocean, and arrived in Princeton to spend the rest of his life as a refugee. They are also the years in which the nature of his debate with Bohr changed profoundly.
After his defeat at Solvay 1930 — after Bohr had used General Relativity itself to save the energy-time uncertainty principle — Einstein acknowledged that quantum mechanics was internally consistent. He could not find a logical contradiction inside the theory. But he became more convinced than ever that consistency and completeness were not the same thing. You could have a consistent theory that was nevertheless only a partial description of reality, like a statistical description that correctly gives averages while concealing the underlying mechanics.
The question was no longer "Is quantum mechanics self-contradictory?" The question was now: "Does quantum mechanics tell us everything there is to know about physical reality?"
Einstein's Two Principles — The Foundations of His Objection
To understand the EPR argument that Einstein was building toward, one must understand the two deep physical principles that he would never abandon — principles so fundamental to his thinking that they functioned more like axioms than hypotheses:
The Principle of Physical Reality
"If, without in any way disturbing a system, we can predict with certainty the value of a physical quantity, then there exists an element of physical reality corresponding to this quantity."
This is the EPR criterion of reality — formulated precisely in 1935, but present in Einstein's thinking since 1927. It says: if you can predict something with certainty without touching the system, that something is objectively real. It is not created by measurement; it was already there.
The Principle of Locality
Physical reality is local: the real state of system A cannot be instantly changed by what happens to system B when A and B are spatially separated. Events in one region of spacetime cannot instantaneously influence events in a spatially separated region. This follows from special relativity and was, for Einstein, non-negotiable.
It also implies separability: when two systems are separated, they have independent real states. The state of A is not defined by reference to B, and vice versa.
From these two principles — reality and locality — Einstein would construct an argument that quantum mechanics was necessarily incomplete. The argument required two particles that had interacted. It required the quantum mechanical prediction of correlations between measurements on those particles. And it required showing that these correlations, combined with locality, implied the existence of pre-existing definite values that quantum mechanics did not describe.
But in 1930, this argument was not yet ready. Einstein was building it, testing it, refining it in letters to Bohr, to Born, to Ehrenfest. He needed time — and he needed his collaborators at Princeton.
The Catastrophe — Hitler and the Flight from Europe
In January 1933, Adolf Hitler was appointed Chancellor of Germany. Within weeks, the systematic persecution of Jewish scientists — and anyone else who opposed the Nazi regime — began. Einstein was in California at the time, lecturing at Caltech. He never returned to Germany. He resigned from the Prussian Academy of Sciences and renounced his German citizenship.
By October 1933, he had accepted a position at the newly founded Institute for Advanced Study in Princeton, New Jersey — a paradise for theoretical physics, deliberately designed to give Einstein and others the freedom to think without teaching obligations. He arrived in Princeton in October 1933 and would not leave America again.
Hitler appointed Chancellor of Germany. Einstein, in California, writes to his wife that he will not return. He is correct: the Nazis immediately strip Jewish professors of their positions. Einstein's Berlin home is searched; his bank account confiscated.
Einstein formally resigns from the Prussian Academy of Sciences. The Academy's response, drafted under Nazi pressure, is an attack on his character. He calls it "a comedy." He never forgives them.
Einstein spends months in Belgium and England, stateless, refusing to return to a fascist Germany. He lectures at Oxford. He visits Belgium's King Albert and Queen Elisabeth, who are friends. He gives talks on the dangers of fascism.
Einstein arrives in Princeton, New Jersey, to join the Institute for Advanced Study. He is 54. He will live here — at 112 Mercer Street — until his death in 1955. Princeton becomes his world.
At Princeton, Einstein collaborates with Boris Podolsky and Nathan Rosen on the argument that would become EPR. The collaboration is difficult: Einstein thinks primarily in German; Podolsky and Rosen in English. Podolsky writes the final paper — and Einstein, when it is published in May 1935, feels it does not quite capture what he wanted to say.
The Correspondence — Letters That Built the Argument
Through the early 1930s, Einstein and Bohr remained in contact — always warm in tone, always sharply opposed in substance. Einstein's letters are rich documents of his evolving thinking: he probes, proposes, tests ideas, and retreats when they fail. Bohr's responses are dense, sometimes nearly impenetrable, but always sincere.
"Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the 'Old One.' I, at any rate, am convinced that He does not throw dice."
— Einstein to Max Born, December 4, 1926"The Heisenberg-Bohr tranquilizing philosophy — or religion? — is so delicately contrived that, for the time being, it provides a gentle pillow for the true believer from which he cannot very easily be aroused. So let him lie there. But this religion has so damned little effect on me that, in spite of everything, I must exercise my pen in its disparagement."
— Einstein to Schrödinger, 1931"I am still wrestling with the foundational questions of quantum mechanics. My difficulty is not about the correctness of the formalism — that much I concede — but about whether the wavefunction can possibly be a complete description of the physical state of a system. I am developing an argument to show that it cannot be, without assuming any inconsistency in the theory itself."
— Einstein to Born, 1933 (paraphrased from correspondence)Bohr's World — Copenhagen in the 1930s
While Einstein was navigating exile and building toward EPR, Bohr's world was also changing. The Institute for Theoretical Physics in Copenhagen — founded in 1921 and now known worldwide simply as "the Bohr Institute" — had become the global centre of theoretical physics. Young physicists from every country came to work with Bohr: Werner Heisenberg, Wolfgang Pauli, Lev Landau, Paul Dirac, Léon Rosenfeld. The institute had an atmosphere of intense, almost monastic intellectual work combined with constant social interaction — table tennis, football, and long philosophical discussions that sometimes continued until midnight.
Bohr spent the early 1930s extending and defending the Copenhagen Interpretation. His philosophical language became more refined — and more opaque. He developed the concept of complementarity beyond just wave-particle duality into a general philosophical framework for understanding the relationship between any two mutually exclusive but jointly necessary descriptions of a physical system.
Bohr's Extending Vision of Complementarity: By the mid-1930s, Bohr was applying complementarity beyond physics — to biology, psychology, and even ethics. He thought of complementarity as a fundamental feature of human knowledge: whenever you have a situation where a precise description in one conceptual framework necessarily excludes a precise description in another, you have complementarity. The space-time description of an event and the causal energy-momentum description: complementary. The psychological description of a person's inner life and the physiological description of their brain states: complementary. This was controversial — many physicists thought Bohr was over-reaching — but it shows how deeply he was committed to his framework as a general epistemology, not just a patch on quantum mechanics.
The Pivot — From Inconsistency to Incompleteness
The central intellectual event of the 1930–1935 interlude was Einstein's strategic pivot. At Solvay 1927 and 1930, he had been trying to show that quantum mechanics was inconsistent — that its predictions contradicted each other. He had failed. Bohr had shown the theory was consistent, and Einstein knew it.
Now he changed his approach entirely. He would accept that quantum mechanics was consistent. He would grant that its predictions were correct. He would even grant that the uncertainty principle was a correct consequence of the theory. His new question was: consistent with what? Correct as far as it goes — but how far does it go? Is the wavefunction a complete description of physical reality?
This is a subtler, harder question than inconsistency. A theory can be consistent and still be incomplete — think of Newtonian mechanics, which was perfectly consistent but incomplete because it didn't describe electromagnetic phenomena. Einstein's new claim was that quantum mechanics was complete in this sense: it gave correct predictions for statistical averages but systematically failed to describe the real physical state of individual systems.
Why completeness is a harder question than consistency: To show inconsistency, you need to find two predictions of the theory that contradict each other. To show incompleteness, you need to show that there are elements of physical reality that the theory fails to describe. And to do that, you need a criterion for what counts as an "element of physical reality" — which is where Einstein's EPR paper will establish its criterion: if you can predict the value of a physical quantity with certainty, without disturbing the system, then that value is real.
Entanglement — The Key Idea
The weapon Einstein was building toward required a concept that Schrödinger would name — and find deeply troubling — in 1935: quantum entanglement.
When two quantum systems interact and then separate, they can leave the interaction in a quantum entangled state: a state in which the two systems cannot be described independently. Their joint wavefunction is not a product of two separate wavefunctions. Measuring one system instantaneously determines — or correlates with — the outcome of measuring the other, no matter how far apart they are.
Schrödinger, in a paper responding to EPR published later in 1935, would call this "the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought." He also called it "not one but rather the characteristic trait." Verschränkung in German — "entanglement" in English. It was the strangest prediction of quantum mechanics, and Einstein intended to use it as his final weapon.
Entanglement — A Concrete Example (the precursor to EPR): Imagine two electrons prepared in a spin-singlet state: one is spin-up along the z-axis, one is spin-down, but we don't know which is which — and quantum mechanics says neither has a definite spin until measured. When we measure one electron and find it spin-up, the other is instantly spin-down — no matter how far away it is. The correlation is perfect and instantaneous. Einstein will argue: either (a) this involves faster-than-light influence (violating locality), or (b) the electrons always had definite spins that quantum mechanics failed to describe (incompleteness). Take your pick — quantum mechanics is either non-local or incomplete. Neither is acceptable.
The Paper Takes Shape — Podolsky and Rosen
By late 1934, Einstein had the argument in its essential form. He needed collaborators to formalise it in the English-language paper he was planning. At Princeton, he recruited Boris Podolsky, a Russian-American physicist fluent in the mathematics of quantum mechanics, and Nathan Rosen, a young American physicist who had been working on problems of general relativity.
The three worked through the winter of 1934–1935 on the paper. Einstein's core idea was his own; Podolsky did most of the mathematical writing; Rosen contributed technical details. The collaboration was not without tension. When the paper was published in May 1935, Einstein complained to Schrödinger that Podolsky had emphasised mathematical formalism at the expense of the physical argument — that the real point had been obscured.
The title of the paper, when it appeared in the Physical Review on May 15, 1935, was austere and technical: "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" The answer Einstein, Podolsky, and Rosen intended was: No. It could not.
The paper's publication would trigger the most important exchange in the history of quantum foundations — and force Bohr into what he himself called the most difficult and important response of his career. That is the subject of Part VI.
"My concern is not whether the theory is correct, but whether the description of reality given by quantum mechanics is complete. I believe it is not."
— Einstein, in conversation at Princeton, early 1935 (paraphrased from multiple sources)
Comments
Post a Comment