How much can we know of the world? Some believe we can go all the way and find the answers to the most penetrating questions, at least those concerned with the natural world. Others think there is only so much we can know, that there are some very concrete limits to how much information we can gather about reality. These limits are not just a consequence of our brains or the tools we use to extract knowledge. They are Nature’s trademarks.
So, which one of the two views is the right one?
Perhaps nowhere in the history of science this split is better expressed than in the famous Einstein-Bohr debates. The two giants of twentieth century physics, with a deep intellectual respect for each other, locked horns on several occasions trying to make sense of the puzzling new science they helped developed, quantum mechanics.
In 1905, Einstein wrote what he considered his most revolutionary paper, where he proposed that, contrary to the accepted view, light could be seen as being comprised of little bullets, later called photons. The prevailing view then, after centuries of disagreement, was that light is a wave. More precisely, an oscillation of the electromagnetic field. This dual nature of light, corpuscular and wavy, was like nothing else anyone had seen. When, in 1924, Louis de Broglie suggested that this dual nature was not restricted to light but was a property of electrons, protons, and all particles of matter, things became even more mysterious.
The new quantum mechanics imposed two fundamental restrictions on knowledge: 1. we can only know the probability of finding a particle in a given place; 2. the observer interacts with what is being observed. As a consequence, the determinism of classical physics is an approximation to a reality where the notion of complete knowledge seems to be an impossibility.
Einstein couldn’t accept this. In a letter to Max Born, who had recently proposed the probabilistic interpretation to quantum mechanics, he wrote:
Quantum mechanics demands serious attention. But an inner voice tells me that this is not the true Jacob. The theory accomplishes a lot, but it does not bring us closer to the secrets of the Old One. In any case, I am convinced that He does not play dice.
To Einstein, the probabilistic description of the natural world couldn’t be the final word. There had to be an objective reality out there, independent of the observer. Quantum mechanics, useful as it was, had to be an incomplete theory. He believed in a deeper layer of physical reality where the normalcy of classical physics—determinism and the separation of observer and observed—would prevail.
Niels Bohr, on the other hand, saw quantum mechanics as an expression of the world of the very small. To him, there was no reason why the rules that apply to the world around us, that is, the rules of classical physics, should also apply in such a different realm. What physicists were finding was the way things were. At some point, Bohr apparently said to Einstein: “Stop telling God what to do!”
As I wrote in my book The Dancing Universe, behind the Einstein-Bohr debate we find opposing beliefs of what physics is about and, more than that, on the nature of ultimate reality. Theirs was a “religious war,” fed by two very different ways to think about Nature and our relationship with it.
Einstein couldn’t accept what to him was akin to intellectual defeat, an acknowledgment that there is only so much we can know about the world and, at a deeper level, that Nature doesn’t follow determinism all the way down to its core. To Bohr, the success of quantum mechanics spoke for itself. The theory described the data extremely well, and that was enough. Furthermore, Bohr saw the relationship between observer and observed as an expression of our connection with the world. When awarded with the Order of the Elephant from the Danish crown in 1947, he chose as his coat of arms the Taoist symbol of the Yin and Yang, with the Latin inscription Contraria sunt Complementa, “opposites complement each other.”
At this juncture, things remain uncertain. Experiments to uncover an einsteinian deeper structure of reality have so far failed. On the other hand, quantum mechanics does display certain properties that are quite bizarre, whereby two separate systems, if initially prepared in a certain way, may affect each other’s behavior instantaneously even if separated by huge distances, a seeming violation of causality. (It’s really not.) Einstein called this effect “spooky action at a distance,” although careful analysis shows that no information is being exchanged between the two systems. And yet, a persistent nonlocal behavior remains, that is, a connection that defeats the limits of space and time. If Einstein and Bohr were still alive, they’d be delighted to find out that, in spite of much progress, the debate lives on.