At the end of April 1926, the then only twenty-five-year-old Werner Heisenberg gave a lecture at the prestigious Physics Symposium at the University of Berlin. The title of his speech had attracted all the eminent German physicists of the time, among others the already very famous Albert Einstein. In front of this demanding audience, the young physicist presented a new theory on the behavior of atomic particles that he had recently developed in cooperation with other young colleagues. Even though everyone was interested to hear what theory these young scientists had come up with, the lecture turned out to be quite unusual.

At the time physicists were preoccupied with the problem of explaining why certain microscopic particles sometimes behaved like normal particles, and at other times as waves. This strange double nature of atomic particles posed a question to which nobody could provide a satisfactory answer. What was then unusual about Heisenberg’s lecture was that the young physicist seemed to try very hard not even to mention what might be happening inside the atoms themselves. He limited himself strictly to demonstrating a mathematical theory which might be used to predict the results of experiments on such atoms, but without giving any explanation of what was actually happening to them. That was because the physicist himself did not know the answer to the burning question itself, but he had found a way to predict mathematically the atomic behavior which had raised the question in the first place.

After the lecture, Einstein himself came up to the young physicist and invited him to accompany him on his way home. Of course, Heisenberg was ecstatic about the offer. He was certain that Einstein would be very pleased with his approach to the problem of quantum physics, as this field of science had been named, because he had adopted a similar approach when working on his theory of relativity. Just as Heisenberg intentionally avoided delving into what actually happened inside the atoms and restricted himself to the characteristics of atomic activity that could be measured and calculated, Einstein, some decades earlier, when dealing with the question of time and space had systematically limited himself to individual readings and direct measurements, leaving to one side the deeper questions of what time and space actually were.

### “It is the theory that determines what can be observed”

The innovative approach with which Einstein restricted himself to what could actually be measured, temporarily leaving other questions aside, enabled him to start a veritable revolution in physics and to redefine completely the discussion about the nature of time, space, energy and matter. Heisenberg hoped that, by applying this method to the world of quantum particles, he could make a similar breakthrough to that of his idol some two decades earlier.

Surprisingly, Heisenberg’s idea of avoiding theoretical models and limiting his work to observable phenomena did not please Einstein at all. It is interesting that the older man’s doubts did not stem from the concern that a purely mathematical theory was insufficient to explain what happened in nature. His reservations arose from his view that one should still try to apply available scientific theories when working from directly accessible physical data. This would include, for example, explaining the movement registered by a measuring device when an atomic particle collided into the instrument. Einstein claimed that theoretical frameworks were always essential to explaining empirical data, for the compelling reason that the data made no sense without them. For Einstein, that is, there was no such thing as the direct observation of the world independent of a theory to interpret it.

As Heisenberg later wrote down in his memoirs, one of things Einstein said was: “But on principle, it is quite wrong to try founding a theory on observable magnitudes alone. In reality the very opposite happens. It is the theory which decides what we can observe. /…/ Only theory, that is, knowledge of natural laws, enables us to deduce the underlying phenomena from our sense impressions.”^{1}

Heisenberg defended his newly discovered theory of quantum mechanics, but he and Einstein quickly reached the conclusion that they still knew far too little about what went on in the world of atoms to reach any definitive conclusions. It was also apparent that, because of the great age difference – Einstein was almost twice his younger colleague’s age – they were unable to have a completely relaxed discussion. In the following years it was Niels Bohr, an older Danish physicist closer to Einstein’s age, who took over the debate on the important issues raised by quantum physics and also became one of the key scientists exploring the inner world of atoms. The discussion between Einstein and Bohr is regarded as one of the greatest intellectual debates of the twentieth century. It lasted almost thirty years, from the fifth Solvay Conference until Einstein’s death in 1955.

### Einstein would then become a little upset …

In September 1927, at the Como Congress in Italy, Bohr presented his principal idea for explaining the world of atoms to his colleagues. Einstein was not present at this meeting, but he supposedly did not miss much, because Bohr’s lecture, according to those who were there, was so condensed that nobody could understand it very well. Today, however, this very lecture is considered a milestone in the history of physics, because it established the central idea upon which the interpretation of quantum mechanics is focused. It still has a place in many physics textbooks today.

According to Bohr, physics can only base its theories on what we can say about the world and not on what the world itself really is. While Heisenberg ignored the problem of how to imagine the world of atoms, Bohr took a step forward by showing that one can get a relatively good idea of what is going on by combining the two opposing models explaining the nature of atoms. Bohr’s principal idea was that the world of atoms cannot be described using one consistent model only, but by applying several together.

Naturally, Einstein strongly disapproved of Bohr’s idea, so he decided to put all his efforts into revealing a paradox between his theory of relativity and the claims made by the new quantum physics. This is how Heisenberg remembered the events that took place at the fifth Solvay Conference in Brussels in 1927: “We all stayed at the same hotel, and the keenest arguments took place, not in the conference hall but during the hotel meals. /…/ The discussion usually started at breakfast, with Einstein serving us up with yet another imaginary experiment by which he thought he had definitively refuted the uncertainty principle. We would at once examine his fresh offering, and on the way to the conference hall, to which I generally accompanied Bohr and Einstein, we would clarify some of the points and discuss their relevance. Then, in the course of the day, we would have further discussions on the matter, and, as a rule, by suppertime we would have reached the point where Niels Bohr could prove to Einstein that even his latest experiment failed to shake the uncertainty principle. Einstein would look a bit worried, but by next morning he was ready with a new imaginary experiment more complicated than the last, and this time, so he avowed, bound to invalidate the uncertainty principle. This attempt would fare no better by evening, and after the same game had been continued for a few days, Einstein’s friend Paul Ehrenfest, a physicist from Leyden in Holland, said: “Einstein, I am ashamed of you; you are arguing against the new quantum theory just as your opponents argue about relativity theory.” But even this friendly admonition went unheard.”^{2}

### Can we learn more about atoms?

In 1930, Einstein arrived at the sixth Solvay Conference with an elaborate and carefully designed theoretical test which he had prepared in advance and was convinced could prove that something was wrong with quantum mechanics. However, after a detailed analysis, Bohr saw that this time Einstein had failed to take the effect of his own general theory of relativity into consideration, an omission which caused him to think he had found an error in quantum physics. If Einstein had taken the general theory of relativity into account, there would be no inconsistency.

After another failure Einstein no longer tried to prove that quantum mechanics itself was incorrect, but went at the problem from a different angle. He tried to prove that quantum theory was incomplete. In other words, he wanted to show that, in theory, it would be possible to say more about atoms themselves than quantum physics would allow. He wished to show that atomic particles had characteristics that cannot be explained by quantum physics, but could possibly fit into a different theory and could be measured as well.

He needed no less than seven years to find a way of proving theoretically that quantum physics was an incomplete theory which failed to account for all the characteristics of atomic particles. In 1935, he and his colleagues Boris Podolsky and Nathan Rosen at Princeton published a paper in which they described a theoretical experiment to prove that nature must know more about itself than what the equations of quantum physics can reveal.

When Bohr found out about Einstein’s new paper, he immediately dropped everything he was doing and dedicated himself to finding an error in Einstein’s theoretical experiment. Three months later, an article with Bohr’s answer to Einstein was already published. Nevertheless, Einstein did not agree with Bohr’s defense of quantum physics and their debate came to a standstill. Both were certain that it was more or less a matter of a “philosophical” debate. A few decades later, though, another physicist, while reading Einstein’s paper, realized how it would be possible to verify whether quantum physics was a complete theory or not. But that is another story.