Third in an occasional series on the nature of reality from the perspective of quantum physics.

In my second post of this series, we discussed the apparent paradox that quantum physics presents to us, in that the world we perceive seems to be so profoundly different from the world of atoms and subatomic particles. The question we posed was this: At what point, if any, does the fuzzy quantum world of disconnected quantum objects becomes the world of balls, cars, and plants? There is no obvious demarcation point.

Beyond the inevitability of quantum fuzziness, the hard question goes even deeper: Should we set aside expectations of describing the probabilistic nature of quantum theory, as Niels Bohr, Werner Heisenberg, Max Born, and their followers would have it, and accept the inherent unpredictability of Nature at its very core? Or should we side with Einstein, Schrödinger, Planck, de Broglie, and their followers, and search for an underlying order beneath the quantum dance? These two groups represent a true clash of worldviews, epitomized in the famous Einstein-Bohr discussions that ran for decades until Einstein’s death in 1955.

And they haven’t stopped. Quite the contrary, they have intensified during recent decades, as more and more rigorous experiments have consistently ruled out the possibility that local effects may explain strange quantum phenomena—such as what Einstein referred to as “spooky action at a distance,” when two quantum objects (such as photons), even if separated by large distances, maintain a kind of coherent behavior as if one “knew” what the other is doing, faster than the speed of light. This property of “entangled” quantum objects is sometimes called quantum nonlocality.

As MIT physicist Seth Lloyd wrote in his book Programming the Universe, it is as if twin brothers at two bars—a great distance apart and with no means of communicating—always ordered the opposite of one another: if one says “beer,” the other instantaneously says “whisky”; if one says “whisky,” the other says “beer.”

Here is how Maximilian Schlosshauer, Johannes Kofler, and Anton Zeilinger summarized the situation, after polling attendees at a 2011 conference on “Quantum Physics and the Nature of Reality” held in Austria:

Quantum theory is based on a clear mathematical apparatus, has enormous significance for the natural sciences, enjoys phenomenal predictive success, and plays a critical role in modern technological developments. Yet, nearly 90 years after the theory’s development, there is still no consensus in the scientific community regarding the interpretation of the theory’s foundational building blocks. Our poll is an urgent reminder of this peculiar situation.

We can see why Einstein considered quantum nonlocality “spooky:” Something “influencing” something else far away without exchanging information in any conventional way is spooky indeed. It defies “reasonable.” It adds a dimension to reality completely foreign to our everyday perception of time and space. In fact, it does away with time and space, since it acts instantaneously (or at least superluminally) and at any distance (at least as has been measured thus far, up to astronomical distances).

A fundamentally rational world

If you believe, as Einstein did, in a fundamentally rational world—that is, understandable through a chain of cause and effect relations—nonlocal phenomena simply don’t belong. They seem to be mocking the whole edifice of science, built upon the solid foundation of what we see around us. In December 1926, Einstein wrote to Born:

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 natural phenomena could not be the final word. There was an objective reality out there, independent of the observer. The inherent nonlocality typical of quantum theory upset him deeply. He conceded its value as an efficient way to describe the results of experiments dealing with small-scale physics. Still, there had to be a deeper formulation of physics, which could do away with such “incomplete” theory. Quantum mechanics should be incorporated into this more complete theory, but could not serve as a basis for it. Einstein believed that accepting nonlocality and the probabilistic nature of quantum physics as an end in itself was accepting defeat in our quest for knowledge.

I wonder what Einstein would say of the current experiments essentially establishing the reality of quantum nonlocality. Bohr, most probably, would consider it a victory of his worldview. Nature is inherently probabilistic, and our attempts to build some sort of deterministic account of the quantum world are doomed to fail. Behind the Bohr-Einstein debate were their opposing beliefs in what physics was about, and what the physicists’ goals were when building theories of Nature. Theirs was a “religious war,” fed by the two very different (and noncomplementary!) ways by which their scientific creativity was inspired. Einstein’s more idealistic worldview clashed with Bohr’s more pragmatic view. There is no resolution in sight, although Einstein’s dream of a local theory for quantum physics is in jeopardy.

Austrian physicist Anton Zeilinger, whose experiments have pushed the boundaries of quantum nonlocality like few others, summarized the situation well in his book Dance of the Photons:

We have tried for centuries to look deeper and deeper into finding causes and explanations, and suddenly, when we go to the very depths, to the behavior of individual particles of individual quanta, we find that this search for a cause comes to an end. There is no cause. In my eyes, this fundamental indeterminateness of the universe has not really been integrated into our worldview yet.

The implications are vast, in particular to the grand project of using science to probe deep into the very core of physical reality. If current ideas stand, we will have no choice but to embrace indeterminism for the long haul. And accept, with it, the unknowability of the very essence of the world.

Einstein-Bohr credit: Emilio Segre Visual Archives/AIP/SPL