Einstein vs Bohr · The Great Quantum Debate · 10-Part Series - Part 6
Einstein vs Bohr · The Great Quantum Debate · 10-Part Series
The EPR Paper
May 15, 1935: the most famous paper in quantum foundations — a three-page argument that quantum mechanics must be incomplete, and the "spooky action at a distance" that would haunt physics forever
On May 15, 1935, the journal Physical Review published a three-and-a-half-page paper with the quietly provocative title: "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" Written by Albert Einstein, Boris Podolsky, and Nathan Rosen — immediately nicknamed EPR — it became one of the most cited, discussed, and argued-over papers in the entire history of physics.
The paper landed like a grenade. Bohr, when he heard about it, immediately dropped all other work and spent six weeks composing his response — according to Rosenfeld, he was unable to think about anything else. Schrödinger wrote to Einstein that the paper had brought him "the most animated pleasure." Pauli wrote to Heisenberg that Einstein had "once again publicly voiced his objections to quantum mechanics in the style of a chess champion." The New York Times ran a front-page story: "Einstein Attacks Quantum Theory."
But what, precisely, did EPR argue? The paper is surprisingly technical, and its central point is often misunderstood. Let us trace the argument step by step.
The EPR Criterion of Physical Reality
The paper opens by establishing a criterion — the EPR criterion of reality:
"If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to that physical quantity."
— Einstein, Podolsky, Rosen, Physical Review, 1935
This criterion is deliberately conservative — it does not claim to give a complete definition of reality, only a sufficient condition: if you can predict with certainty, without touching the system, then the thing you're predicting must already be real. It is the kind of criterion that almost everyone would agree with in a classical context.
Combined with a second assumption — the completeness criterion for physical theories ("every element of physical reality must have a counterpart in the physical theory") — EPR sets up its argument.
The EPR Thought Experiment — Two Particles, Entangled
The EPR argument uses two quantum particles that have interacted in the past and then separated. The specific setup Podolsky formalised involves position and momentum correlations (though the spin version, developed by David Bohm in 1951, is more commonly discussed today and is physically clearer). Let us trace the original position-momentum version as EPR presented it.
The Argument — Step by Step
The EPR argument proceeds with the precision of a logical proof. Let us follow each step:
Two particles are prepared in an entangled state. If their momenta are measured, they are always equal and opposite: p₁ + p₂ = 0. If their positions are measured, the difference is always fixed: x₁ − x₂ = L. This is an exact quantum mechanical prediction: the correlations are perfect.
Observer A measures particle 1 and finds momentum p₁. Since p₁ + p₂ = 0, observer B can predict with certainty that particle 2 has momentum p₂ = −p₁ — without in any way disturbing particle 2. By the EPR criterion of reality: p₂ is an element of physical reality.
Observer A could have chosen differently — measuring particle 1's position x₁ instead. Since x₁ − x₂ = L, observer B can predict with certainty that particle 2 is at position x₂ = x₁ − L — without in any way disturbing particle 2. By the EPR criterion of reality: x₂ is also an element of physical reality.
Crucially, the particles are far apart. Observer A's choice of what to measure cannot, by locality, have any instantaneous effect on particle 2. Particle 2 is untouched either way. So particle 2 must have had definite values of both position x₂ AND momentum p₂ all along — before and regardless of what A chose to measure.
The uncertainty principle ΔxΔp ≥ ℏ/2 says no quantum state can have both a definite position and a definite momentum. Yet EPR has shown that particle 2 has definite values for both. Therefore quantum mechanics — which cannot describe both values simultaneously — is incomplete. The wavefunction is not the whole story.
The EPR Conclusion: "While we have thus shown that the wave function does not provide a complete description of the physical reality, we left open the question of whether or not such a description exists. We believe, however, that such a theory is possible." — EPR paper, 1935
The Real Einstein Argument — Locality is the Key
It is important to note that Einstein, in his later writings, felt that Podolsky's formulation of the argument had not captured its essential point. For Einstein, the crucial step was not the criterion of reality, or the uncertainty principle — it was locality.
Einstein's version of the argument is cleaner. He presents Bohr with a dilemma: the quantum formalism predicts that if you measure particle 1, particle 2 is instantaneously affected — its wavefunction collapses to a definite value. But if the particles are far apart, this cannot be a physical influence (locality rules that out). Therefore, the wavefunction collapse on particle 2 cannot represent a real physical change — it can only represent a change in our knowledge. But if particle 2's state was already determined before the measurement, then quantum mechanics was not describing it completely. Completeness lost; locality preserved.
Option A: Non-locality
Quantum mechanics is complete. The wavefunction is everything. Measuring particle 1 physically affects particle 2, instantaneously, across any distance. This is "spooky action at a distance" — spukhafte Fernwirkung in Einstein's phrase. Einstein found this deeply unacceptable: it violated the spirit, if not the letter, of special relativity.
Option B: Incompleteness
Quantum mechanics is incomplete. The particles always had definite properties — hidden variables — that quantum mechanics did not describe. The correlation is not due to action at a distance; it is because the particles were pre-correlated, like gloves in separate boxes. Measuring one tells you the other's property because they were paired all along.
Einstein chose Option B. He could not accept non-locality. He believed in Option B so strongly that he spent the rest of his life hoping someone would find the "hidden variables" theory that would complete quantum mechanics. Bohr chose — if it can be called a choice — to deny the premise: to argue that the concept of "particle 2's real properties" before measurement was meaningless within the quantum framework.
Schrödinger's Reaction — Entanglement Named
Erwin Schrödinger read the EPR paper in May 1935 and immediately wrote to Einstein: the paper had crystallised something he had been uneasy about for years. In a series of papers published later that year, Schrödinger responded to EPR with two contributions that would prove enduring.
First, he named the phenomenon at the heart of EPR: Verschränkung — entanglement. He defined it precisely and argued that it was not a quirk or a special case, but the characteristic feature that made quantum mechanics fundamentally unlike classical physics. He wrote: "If two separated bodies, each by itself known maximally, enter a situation in which they influence each other, and separate again, then there occurs regularly that which I have just called entanglement."
Second, he proposed a thought experiment designed to dramatise the absurdity (as he saw it) of extending quantum superposition to the macroscopic realm: a cat in a box, whose life or death depends on the quantum decay of a radioactive atom. This is the origin of Schrödinger's cat — which we will examine in depth in Part VIII.
Schrödinger's 1935 papers on entanglement were crucial in framing the EPR problem. He showed that entangled states are generic — they arise whenever two systems interact. He showed that they violate what he called "the separability principle" — the classical assumption that two separated systems have independent real states. And he expressed the view that quantum mechanics, in predicting entanglement without explaining it, was exposing its own incompleteness — a view closer to Einstein's than to Bohr's.
The New York Times — The Public Sensation
The New York Times published a front-page story about the EPR paper on May 4, 1935 — actually before the paper appeared in print. The headline read: "Einstein Attacks Quantum Theory: Scientist and Two Colleagues Find It Is Not 'Complete' Even Though 'Correct.'" The story described the argument in non-technical terms and presented it as a challenge to the quantum theory that had dominated physics for a decade.
The scientific community's reaction was mixed. Most working physicists — trained in the Copenhagen framework, comfortable with "shut up and calculate" — were either dismissive or indifferent. Pauli wrote to Heisenberg: "Once again, Einstein has publicly gone against quantum mechanics. One should not pay too much attention to this." Heisenberg, privately, agreed with Pauli's dismissal.
But Bohr could not dismiss it. He stopped all other work and spent six weeks writing his response. He described the EPR paper as "a very fundamental attack" and acknowledged that it had forced him to sharpen his thinking about the meaning of quantum mechanics. His response — published in October 1935, also in Physical Review — is the subject of Part VII.
"Spooky action at a distance — spukhafte Fernwirkung — is something I cannot believe in. If it is real, then physics has taken a wrong turn somewhere."
— Einstein, letter to Max Born, 1947Why EPR Was So Important
EPR was important not just as an argument against quantum mechanics. It was important because it forced physicists to be precise about what they meant by "completeness," "reality," and "locality." For the first time, these philosophical concepts were given enough mathematical precision that the argument could, in principle, be tested.
It would take thirty years — until John Bell's 1964 theorem — before anyone found a way to actually test whether "hidden variables + locality" could reproduce quantum predictions. Bell's theorem transformed the EPR thought experiment from a philosophical debate into an experimental question. And the experiments — from Clauser in 1972 to Aspect in 1982 to Zeilinger in 1998 — gave an answer that neither Einstein nor Bohr had anticipated.
But first, we must follow Bohr's response to EPR — the most difficult paper he ever wrote, and the one that reveals most clearly both the power and the limitations of his philosophical framework. Part VII.
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