Einstein's big mistake: "God doesn't play dice."

GenevaThe dining room of the Hotel Métropole in Brussels gave off the usual aroma of morning coffee. The most brilliant minds of the early 20th century gathered around the tables, deep in intense and profound conversations, commenting on and refuting the presentations given the day before during the final session of the Fifth Solvay Conference. This series of meetings between eminent scientists was initiated by the Belgian chemist Ernest Solvay and, since 1911, was dedicated to delving into the most complex problems of physics and chemistry.

Among the attendees was a physicist who, for more than a decade, had earned the label of revolutionary genius. Albert Einstein had already conquered eternity after bringing about a paradigm shift in the conception of the Universe by laying the foundations of the theories of special and general relativity. For the German physicist, that 1927 conference represented a golden opportunity to demonstrate his opposition to the new generation of young physicists who had been seduced by a new theory that described the subatomic world with an unprecedented level of precision: quantum physics.

Despite being one of its founding fathers, for Einstein the quantum mechanics It was an incomplete theory. He didn't believe it was the ultimate description of nature.

One of the main postulates of quantum mechanics establishes that before observing it, a particle's position is not defined; rather, it is found in more than one place according to a series of probabilities. "Orthodox quantum mechanics says there is no deeper explanation for why when we make a measurement, in one case one result comes out and in another case another," comments Enric Pérez, professor of physics at the University of Barcelona and expert in the history of physics. Much of the scientific community embraced this probabilistic description of nature, but for Einstein, this view was an aberration. With his famous quote, "God does not play dice," he made his position clear.

This interpretation bothered the German physicist so much that he went from one end of the Hotel Métropole to the other trying to convince the rest of the audience that the new theory was incomplete and that nature was hiding something deeper. "Ach, was, das stimmt schon, das stimmt schon" ("What do you say? Oh, yes, that's true, that's true...").

Thought experiments

Throughout the days of the conference, Einstein devised several thought experiments that attempted to challenge certain aspects of quantum theory. These challenges were primarily directed at Danish physicist Niels Bohr, another of the main creators of the new theory that envisioned a new status quo where uncertainty dominated the subatomic world.

"Einstein was more of a realist, while Bohr became the defender of an orthodox interpretation of quantum mechanics, in which he embraced uncertainty, lack of determinism, and the influence of the observer," explains Pérez, who adds that "the price to pay is that you often can't build up accurate images of what's happening, it can't be healthy, and you can't get a handle on what's happening; where you'll get to, but not what happens in between."

At one of the sessions, Einstein presented a thought experiment that challenged Heisenberg's uncertainty principle, which prevented two variables, such as the position and velocity of a particle, from being known simultaneously and with complete certainty. The German physicist's well-crafted and well-argued presentation had a direct impact on Bohr, who couldn't believe the possibility that Einstein had finally discovered an inconsistency in his theory.

"Einstein sought out different ways to find internal contradictions in Bohr's and Heisenberg's arguments," Pérez comments. That night, Bohr wouldn't go to sleep until he found a counterargument to that thought experiment. And the next day, he returned the argument even more forcefully, refuting the German physicist using the theory of relativity that Einstein himself had developed years earlier. After that exchange, Einstein had to withdraw, accepting that the uncertainty principle was an inherent characteristic of nature. "Bohr and Heisenberg came out well in those debates," Pérez assesses.

Einstein does not give up

Einstein was wounded but not defeated. He had lost that battle, but the war would be long and intense. The German physicist then focused on another of the most controversial aspects of quantum physics: entanglement between particles, in which two particles separated by a great distance can affect each other. Einstein believed that such behavior not only defied logic, but also violated the principle he himself had constructed twenty years earlier, according to which nothing can travel faster than the speed of light. Hence, with a certain degree of sarcasm, he called this property "spooky action at a distance."

Following this line of thought, in collaboration with two physicists from Princeton University in the United States, Einstein developed what is known as the EPR paradox (from the initials of Einstein, Podolsky, and Rosen, the authors of the work). This type of information physicists were overlooking would make sense of it all. They called this hidden information "hidden variables."

Years after his death, John Bell, a physicist at CERN, the European Laboratory for Particle Physics, in Geneva, developed mathematical relationships that should allow the hidden variables theory to be finally verified or ruled out. "Bell found theoretical relationships that made it possible to distinguish in the laboratory whether the state of objects is determined before measurement or not," explains Pérez. However, no one knew whether these mathematical relationships could be implemented experimentally. "You don't need to be an expert on quantum mechanics to see an interesting result in Bell's article, because it allowed us to experimentally distinguish between two theories," comments this UB physics professor.

It took several decades until scientists were able to demonstrate, using Bell's results and complex experimental mechanisms, that hidden variables definitively did not exist. The implications of this discovery were recognized with the 2022 Nobel Prize and represent the starting point for the development of quantum technologies. "It now seems fairly accepted that nonlocality exists in quantum mechanics," Pérez emphasizes.

Quantum mechanics is so capricious, and all we have to do is embrace it. As physicist Stephen Hawking summed up, alluding to Einstein's own quote: "God throws dice where no one can see them."

A solid but incomprehensible theory

The interpretation of quantum mechanics is, one hundred years later, a topic of intense debate among physicists and philosophers. "Quantum mechanics is a very sophisticated theory, very well done and very well plotted, in which many have tried to find holes and inconsistencies," explains Pérez, who notes that in the educational field, the interpretive aspect of quantum mechanics is overlooked. "There are quantum mechanics teachers who don't interpret it."

For Pérez, this debate should make us rethink what it means to conduct an experiment or observe nature, from which we can learn from other branches of science beyond physics. "These are problems that have been previously raised in other humanities."

The series of conceptual disputes between Einstein and Bohr gave rise to one of the most intense debates of the 20th century on the foundations of nature. These conversations and subsequent experimental and theoretical developments transcended the boundaries of physics to delve into the deepest aspects of the philosophy of nature.