Much ink has been spilled over the legendary butting of heads between two of the most eminent scientists of the twentieth century; a rare instance where the smartest man of his time was utterly wrong. Niels Bohr, in a battle that had our most basic conceptions of reality at stake, bested the super-genius by forgoing all pretensions to understanding reality. That’s how the foundation of the paradigm-shifting Quantum Mechanics was solidified, engraving this legendary battle in history and making it relevant even today.

The back and forth initially kicked off in 1927 during the fifth Solvay Conference. Albert Einstein, founder of the theory of relativity, was a recognized quantum mechanics skeptic—best exemplified through his now famous assertion: “God does not play dice with the Universe”¹. At the conference, he attacked quantum mechanics and its popular interpretations by presenting a thought experiment to one of its primary founders: Niels Bohr. To first understand Einstein’s attacks, we must understand their subject: Quantum Mechanics and its philosophical implications as understood by Bohr—also known as the Copenhagen Interpretation.

The Copenhagen Interpretation is one of the earliest and most popular explanations for how physics functions at the smallest level. It lays a few ground rules which force us to jettison many of our presumptions of what reality is or what it ought to be. At its simplest, the Copenhagenites assert the following: that particles like electrons and photons behave as both particles and waves (wave-particle duality); that these particles may exist as a superposition of multiple states at once—the probabilities of which states they may exist in can be calculated by a series of wave functions. However, the observation of the particle leads to the collapse of these wave-functions as the particle assumes a single state. Additionally, a complementarity principle is followed according to which there exist pieces of information about a particle that can never be observed simultaneously (e.g., the more precisely you are aware of its momentum, the less precisely you are aware of its position; and you may observe a particle acting as a wave at one point and as a particle at another, but never both simultaneously).

This is best illustrated by Schrödinger’s cat, which imagines a cat in a closed box with a 50% chance of being poisoned by a radiation emitter in the box. Assuming quantum physics’ rules for reality, the cat would exist as both dead and alive at the same time and would only assume a single state once the box is opened and it is observed—that is the world of implications of quantum superpositions. The key implication that challenges our classical understanding of reality is that the state of something depends on whether it is observed or not. The act of measurement itself is crucial, and it doesn’t just passively reveal pre-existing properties but rather determines the properties of the system. This unintuitive, observer-dependent framework of reality deeply troubled Einstein, and he vehemently rejected it quite famously with the aforementioned statement of God not playing dice. Bohr’s response to it was simple: “Stop telling God what to do”².

Understanding the stakes, we can finally see how Einstein launched his attack against Niels Bohr during the Solvay Conference in 1927. It came in the form of a thought experiment. Einstein posited a modification of the famous double-slit experiment which first confirmed the wave-like nature of particles. Two electron beams are passed through two slits towards a screen and, true to their wave-like nature, the beams diffract and interfere with each other resulting in a striped pattern on the detecting screen. That’s simple enough, but what Einstein suggested was that a single beam first diffracted through one slit and then made to interfere with itself after passing through two separate slits. If a particle indeed exists in a superposition of many states, then once interacting with the detecting screen it must lead to a collapse of the wave-function and the immediate assumption of a single state, at a single point on the screen.

Since the Copenhagen Interpretation claimed that the electron simultaneously exists in all of its probabilities at once, once detected at a single point by the screen, all those probabilities would instantly collapse to zero. This would mean that once a single electron is detected by the screen at one point, it instantly conveys to all its other potential positions that it is not there. This instantaneous communication between multiple points violates Einstein’s theory of relativity, which asserts that nothing can travel faster than the speed of light—nothing can happen instantly. So something had to be wrong, either with Bohr’s superpositions, his observer-based reality, or both.

Bohr responded to Einstein by returning to his complementarity principle, more specifically to Heisenberg’s Uncertainty Principle—you can’t know the position of an electron if you know its momentum and vice versa. Put simply, if the experiment were to be conducted in real life, the measuring apparatus (the detecting screen) would undergo a minuscule movement upon interaction with the electron. This would insert just enough uncertainty in the position and time of the electron’s impact for instantaneous communication to be avoided. Quantum mechanics only provides probabilities and not deterministic outcomes and the process of measurement itself plays a role in defining the outcome. Bohr: 1, Einstein: 0.

Einstein’s second criticism came in 1930, at the next Congress of Solvay, in the form of another thought experiment. This time, Einstein theorized a box fitted with a weighing balance that could measure the smallest changes in masses and a trap door that could open and close quickly enough to allow only one photon to escape. This hypothetical construct could allow for the measurement of the exact time of a photon’s escape via the trap door and its energy as well through the change in mass (as energy and mass are related through Einstein’s famous equation E = mc²). Energy and time should, according to the principle of complementarity, be unable to be known simultaneously. By the next day, however, Bohr had a response to this as well: Einstein had failed to consider his own theory of relativity, which meant that the recoil from the box losing a photon must also affect the time of the event, however slightly. And indeed it would do so just enough to reintroduce uncertainty just as much as predicted by Heisenberg’s principle.

Having failed numerous times already, Einstein still remained unconvinced and made his thoughts abundantly clear in his most significant attack on Quantum Mechanics in 1935. In collaboration with physicists Boris Podolsky and Nathan Rosen, Einstein published a paper titled “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”³ Its answer, of course, was in the negative. It elaborates on the 1927 thought experiment while raising the stakes. To simplify, it presents two entangled particles (meaning they are linked and the properties of one affect the properties of the other), like the ones formed after the diffracted beam passes through the double slits, that separate at a great distance without being observed or disturbed.

If, at this great distance, a scientist were to capture one and observe it, he could come to know a great deal about either its momentum or its position. But in doing so, the scientist would also get to know about the second particle’s position or momentum (as they are entangled) instantaneously without making any measurements on it. According to the current understanding of quantum mechanics, a particle doesn’t have any exact position or momentum until it is measured. According to the theory of relativity, no instantaneous communication between the entangled particles should be possible—nothing should be able to travel faster than light. Einstein called this “spooky action at a distance.”

Bohr’s response came five months later in a paper published in the same journal, with the exact same title⁴. Bohr’s explanation again appeals to complementarity which says the kind of information about the particle we get depends on how we measure it. And even though the act of measuring one particle affects what we can know about the other, there’s no one complete description of the whole system—just pieces of it depending on what we observe. Since our knowledge of each particle is essentially incomplete, we have no way of making use of the correlation. Therefore, though the observer may know something about the state of the second particle, he has no way of conveying that knowledge faster than the speed of light. As Brian Greene put it: “special relativity survives by the skin of its teeth.”

At last, Bohr was considered the winner of these debates, and future experiments in the latter half of the century would only serve to validate his arguments and further prove the weirdness and undeniability of the quantum world.