## When, where and to who did Einstein Say It Too?

The quote “God does not play dice” was spoken by Albert Einstein to Niels Bohr at the famous 1927 Solvay Conference. The Solvay Conferences were a series of international conferences in physics that were held in Brussels, Belgium. The first conference, which took place in 1911, was organized by Ernest Solvay and attracted some of the most famous physicists of the time, including Albert Einstein, Marie Curie, and Max Planck. The conferences were held approximately every five years and were designed to bring together leading physicists to discuss the latest developments in the field. Some of the most important discoveries in physics, including the theory of relativity and the birth of quantum mechanics, were presented and discussed at the Solvay Conferences.

The argument concerned whether or not a fundamental feature of the universe was uncertainty (that a particle, such as a photon, moves probabilistically) or whether there’s some hidden variable that can explain why we can only calculate a problem probabilistic.

## What Was Einstein Doing At The Time?

In 1927, Einstein was working on the development of a unified field theory, which was a theoretical framework that aimed to reconcile the laws of electromagnetism with the laws of gravity. At the time, it was believed that these two fundamental forces were incompatible with each other, and Einstein spent much of his career trying to find a way to unify them. In 1927, Einstein published a paper in which he proposed a new theory that he believed would lead to the unification of electromagnetism and gravity. Despite his efforts, however, Einstein was ultimately unsuccessful in his quest for a unified field theory, and the problem remains one of the most important and challenging unsolved problems in physics to this day.

## What Was Niels Bohr Up To In 1927?

Niels Bohr was, like Einstein, also a revolutionary physicist who made significant contributions to the understanding of atomic structure and quantum mechanics. In 1927, he was working at the Institute for Theoretical Physics at the University of Copenhagen, where he had been since 1916. He was also a professor at the University at this time. In that same year, 1927, Bohr published a series of papers that became known as the “Bohr model” of the atom, which explained the structure of atoms in terms of the behavior of their constituent electrons. This model was an important step in the development of our understanding of atomic structure and provided a foundation for the development of quantum mechanics.

## What Were They Arguing About? The Simple Explanation

A more detailed explanation will follow, but given the complexity of this subject, we’ll first give a simple one. Essentially a fundamental feature of the theory of quantum mechanics is the element of uncertainty. I know it doesn’t seem this way, but certain particles (such as electrons or photons) can only be tracked statistically. In other words, we don’t know where they are at any particular moment in time.

This is what Einstein objected to, the notion that it just **is** the case that the absolute location of these particles can’t be known and we can only know where they are probabilistic. Einstein would have none of it. He was convinced that we just lacked the experimental ability to get at what’s really going on at the quantum level. This is why he favored a ‘hidden variables’ interpretation of quantum mechanics (ie- some yet undiscovered aspect of the world is what is causing this counter intuitive result).

Unfortunately for Einstein, Bell’s Inequality Theorem (or simply Bell’s Theorem) would effectively rule out any interpretation that relied on hidden variables (well, technically local hidden variables). Therefore just is the case that, as Bohr quipped back to Einstein, “Stop Telling God What To Do.”

## Quantum Uncertainty

To understand the details of the disagreement, one needs a brief introduction to quantum mechanics. Don’t worry, just three easy pieces. The first one only needs to know that light travels in small, discreet packets of light called photons. I know it doesn’t seem that way, and it was extremely controversial when first posed, but that’s how light moves.

The second piece is most commonly explained through the double-slit experiment, but let’s try a different example that may be easier for Americans to understand. Imagine a gun that fires photons. When the gun is set to automatic, like a machine gun, and you shoot it at a wall the resulting pattern on the wall (if photons made the same impact on the wall as regular bullets do) looks exactly like what would happen if a machine shot at the wall. Nothing counterintuitive so far. However, it’s what’s next that has baffled thinkers for over a century.

If one sets the gun to fire as a single shot and fires at the wall, the pattern on the wall changes. It looks like, exactly like, a giant ocean wave has hit the wall. Perplexed at how a single photon could make such a pattern, you decide to stand over it or put a camera (or any attempt to follow this particle) and say “I’m gonna get you, you photon! I’m going to see how you’re doing that.” Then you resume firing the gun again, as a single shot, and the photon doesn’t do it anymore. It doesn’t act like a wave. It goes back to acting as if a particle had hit the wall. The question underpinning interpretations of quantum mechanics is “why did it stop doing the thing (acting as a wave)? And why did it act as a wave in the first place?”

These are merely problems of interpretation. The theory itself, as physicists never tire of mentioning, is unbelievably successful. Quantum mechanics is perhaps the most successful theory in all of science, at least descriptively. It accurately predicts, mathematically, what is going on. However, the question of what it means is still very much up in the air.