A Room Full of Geniuses: The Fifth Solvay Conference

A picture of all the attendees at the Fifth Solvay Conference in Brussels.

Imagine a room filled with the brightest people our planet has ever known. Many people have not heard of what the Solvay Conferences are, yet alone what was special about the 5th one. Still, it was the most important scientific conference of the 20th century. From around the world, all pioneering physicists were invited to this meeting organized by one Ernest Solvay. The 5th of these conferences, held in 1927 in Brussels, Belgium, marked a turning point in scientific history, solidifying quantum mechanics while also bringing up fundamental debates about the nature of reality. 

Prior to the 1900s, Newtonian physics was considered as a complete understanding of our universe. Everything we experimentally observed seemed to line up almost perfectly with Newton’s equations and predictions, at least until the year 1900, when Max Planck released his revolutionary paper arguing a radical new explanation for a concept known as blackbody radiation. He claimed that energy was emitted in discrete packets, known as quanta, and that all energy was quantized. Then, soon after in 1905, Einstein similarly explained that the photoelectric effect proves that light can be both a particle and a wave, and that each particle of light is a packet of energy known as a photon. Many more discoveries that Newtonian physics could not explain came around this time, and flipped modern physics onto its head. By 1927, the field quantum physics had been developed, and mathematical explanations were starting to arise. The issue was, no one could agree on what those equations actually meant. These confusions set the stage for the Fifth Solvay Conference. 

The concept of a blackbody led to many confusions in the early days of quantum physics.

The conference is often considered the most intellectually concentrated meeting in scientific history. Of the only twenty nine attendees, seventeen had won or would soon win a Nobel Prize.  Attendees include Einstein, Bohr, Schrodinger, Heisenberg, and Marie Curie, to name a few. The focus was on quantum mechanics, but rather than debating the equations, the debate was mainly on interpretation. Did these equations prove that probability is a fundamental characteristic of our universe? Can we really never know where anything is? The main argument was held between Einstein and Bohr. Einstein argued on the side of determinism, saying that “God does not play dice.” He believed that probability had no place in physics. Bohr on the other hand argued that probability is a fundamental root of nature, and that uncertainty is built into the world we live in. In the debate, Einstein cooked up many, many proposed scenarios and thought experiments that would help to expose flaws in Bohr’s ideologies. Often, it took Bohr multiple days trying to figure out how to properly respond to him. One of Einstein’s favorite objections involved a thought experiment that would disprove Heisenberg’s uncertainty principle. He claimed that by carefully measuring certain properties, one could pinpoint a particle with position and momentum far more accurately than quantum mechanics allowed. Bohr eventually, after hours of work, showed that the scenario was inherently wrong, and the world still obeyed the uncertainty principle. Through exchanges like these, the debate grew more and more memorable.

The outcome of this meeting is nothing awesome, where everyone completely agreed with one side. Bohr’s probabilistic views gained acceptance, with many physicists adopting the Copenhagen Interpretation. The Copenhagen Interpretation simply stated that quantum systems can be described in terms of probability up until the point they are actually measured, at which point it becomes deterministic. Einstein and many others still disagreed with him, and remained unconvinced. In 1935, Einstein released the EPR paper, which argued that quantum mechanics was still incomplete. In essence, he continued to debate Bohr’s view years after the conference. Still, the influence of this meeting extended far beyond the immediate effects. 

Physicist John Bell built off the debates of the conference to formulate Bell’s Theorem.

In 1964, physicist John Bell developed Bell’s Theorem, a claim that transformed the Einstein-Bohr debate into an experimentally testable question from a philosophical disagreement. Einstein believed that in quantum mechanics, there were “hidden variables” at work that we did not know about, leading to results we see in experiments. Bell showed that if that was true, then mathematically, entangled particles would have a limit to how correlated they could be. Experiments in the 70s and 80s however, showed that entangled particles were far more correlated than that mathematical upper limit. Further results increasingly showed more support for quantum mechanics in a way that Einstein had once considered impossible. They provided strong evidence that the probabilistic nature of quantum mechanics is a fundamental feature of the universe. 

In the modern day, the influence of this conference extends even further. Technologies such as quantum computers rely heavily on principles like superposition (two places at once) and entanglement (particle interconnection), which were concepts heavily discussed and debated during the Solvay conferences. What began as just a disagreement over some equations eventually set the foundation for a whole new field of science and technology. Nearly a century later, physicists continue to use the mathematics discussed in Brussels while still exploring deeper questions first raised in that building. As a result, the Fifth conference is remembered not only as a gathering of brilliant minds, but also as an event that shaped the future of physics.