The Einstein-Bohr Debate

One of the most fascinating debates in scientific history was the ongoing disagreement between Albert Einstein and Niels Bohr about reality and quantum theory, particularly between “realists” and those who accepted the Copenhagen Interpretation (CI) proposed by Bohr. See for a very brief summary of the CI and other metaphysical models of quantum mechanics. Einstein was an unwavering realist. He believed reality was fundamentally knowable if we had enough information. “God does not play dice with the universe,” one of Einstein’s famous quotes, sums up his refusal to believe that probability was the fundamental basis for physical reality.

Einstein’s refusal to believe in the fundamental nature of probability led him to conclude that quantum theory was incomplete and that the wave function was not the final word on a particle’s behavior. It was, instead, the result of insufficient understanding of the quantum world. Once we knew more about the nature of the quantum world, we would be able to describe a particle in more than probabilistic terms.

Bohr argued that the wave function was the most complete description possible, and that trying to dig deeper into ‘hidden realities’ was meaningless within quantum mechanics. One of the specific issues around which this debate raged was the question of what happens to a pair of particles created together– for example, a pair of photons created simultaneously. Quantum theory requires that these photons have complementary characteristics, but it also requires that those characteristics cannot be known until they are measured.

The difficulty for Einstein (and other realists) comes from how those photons are connected. According to the CI, the photon pair characteristics exist only as a wave function (any characteristic can be anywhere within the range of all possible characteristics). But, and this is hugely important, both photons must have corresponding characteristics, whatever they turn out to be. Using a simplified example, let’s say that the photons have a characteristic of ‘up/down.’ When a photon pair is created, both photons exist as a wave function that can be either ‘up’ or ‘down,’ but whatever it turns out to be will be the same for both photons. However, we can’t know if they are ‘up’ or ‘down’ until we measure one of them. At that point the wave function collapses, and the photon is revealed to be “up”. Quantum theory says that we now know that the other photon is also “up”. But if both photons existed as equal probabilities of being ‘up’ or ‘down’ until one of them is measured, how does the second photon know that it is ‘up,’ as required by quantum theory. The first photon could have just as easily been measured ‘down,’ in which the second photon would also be ‘down.’ Of course, photons don’t ‘know’ in any conscious sense, but this is the puzzle the mathematics presents. To put it another way, it’s as if you flipped two coins on opposite sides of the universe, and every time one lands heads, the other does too—instantly.

This connection between the photons is called “entanglement” in quantum theory, and was referred to by Einstein as “spooky action at a distance”. Because nothing can travel faster than the speed of light, it is impossible for the photons (moving away from each other at the speed of light) to communicate with each other about the state of “up/down”. Einstein maintained that there were “hidden variables” within each photon that determined the state of the photon pair. The fact that we don’t know about those variables doesn’t mean that they aren’t there; and how else could the photons know about their mutual state? Einstein remained firmly in the realist camp, joined by many high-caliber colleagues.

The physics is clear—we observe entanglement in experiments. What remains unsettled is how to interpret what the mathematics and experiments really mean about the nature of reality. Entanglement is at the heart of cutting-edge technologies, including quantum computing and cryptography. It also shows how ideas that once seemed remote and abstract lie at the very core of how we understand the world–and our place in it.