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COFFEE WITH EINSTEIN AND BOHRWe’ve agreed to meet in the hotel bar, but I’m a bit early. Brussels is bustling busily by outside – it’s about midday, often a hectic moment in Belgian social life. The Hotel Métropole, though, seems unaffected by the frenzied pace of life outside. I have entered the lobby, which looks like a hall of mirrors from the French Renaissance, a place from another time where lead glass, marble, cast iron and gold leaf are all entangled together to create characteristic Brussels Art Nouveau and Art Deco. A red-garbed bellhop hurries over, asking in barely a whisper what Sir requires. The Métropole’s brasserie, a little further into the hotel, is just as impressive as the lobby. Glittering cut-glass chandeliers illuminate a roomful of marble columns with semicircular seats of red leather around them. On the other side of the round tables are comfortable armchairs in white leather. With their well-filled purses on a chain in the back pocket, the black-waistcoated and white-shirted waiters keep a discreet eye on the customers, gliding smoothly over to them when a beckoning hand is raised. People are mostly ordering coffee, or perhaps here and there their first beer. This is where I have agreed to meet Albert Einstein and Niels Bohr, the two men who turned physics on its head a century ago. One of them, Einstein, had exposed a striking conspiracy between space and time with his theory of relativity: together they arrange things so that every observer sees light move at the same speed, no matter how fast the observers themselves are going. That was in his annus mirabilis, 1905. Ten years later, in 1915, that conspiracy of space and time also transpired to be the key to a new theory of gravity that left nothing in the cosmos untouched. Bohr, however, was the man for the very smallest scales, the atoms. In 1913, he suggested a radical new model for the structure of matter, proposing atoms as something like miniature solar systems in which electrons orbited a much heavier nucleus. This was able to explain a great deal about the way certain gases emit or absorb light. But the model came at a price: the electrons were only able to go round the nucleus in very specific orbits. Bohr didn’t know why, but he knew it had to be true and he borrowed an idea that had been introduced to describe it in 1901 by his German colleague Max Planck. The energy of the electrons in the atom, he said, was quantized. Instead of being continuous, energy at the very smallest levels only occurs in packets of a fixed size. Planck himself had felt thoroughly uncomfortable with the concept, but as far as Bohr was concerned, Nature had clearly arranged it that way. Their radical insights and ideas turned both these physicists into major figures at the start of the last century. And not just for their colleagues in the field. Einstein in particular was already a familiar name to the general public by 1927, a man whose compelling theories about the cosmos were regularly reported in the serious newspapers. In the spring of 1927, here in Brussels, Einstein and Bohr both took part in the annual Solvay Conference, for which the formal theme that year was ‘Light and Electrons’. In reality, the big names in physics used the conferences set up by Ernest Solvay to meet up once again in luxurious surroundings to discuss and exchange ideas. They were working, no doubt about it, but in a friendly and relaxed atmosphere. That wasn’t the case for Einstein and Bohr that year, however. Although they had been friends for years, they were attacking each other so vigorously that people around them were starting to find the debates uncomfortable. In the middle of their prolonged and protracted conversations, one of the pair would often get up and stalk off gesticulating, leaving the other behind, grumbling. The dispute could then start up again hours later, no matter what the time of day or what was officially on the schedule. Virtually every breakfast ended up being left untouched. All these clashes between Einstein and Bohr were about the reality of quanta. A great deal had happened since Max Planck launched the idea in 1901, and indeed since Bohr had used it to explain the atom in 1913. Einstein had also been able to use it in 1905, for instance, to explain why light clearly behaved not only as a wave phenomenon but also as a kind of particle. And early in the 1920s, colleagues such as Max Born, Erwin Schrödinger, and above all Werner Heisenberg had set up an extensive theoretical network that seemed to describe the behavior of particles such as electrons and light down to the finest detail. That description was what Einstein didn’t like. The theory did admittedly yield excellent predictions for experiments, for example with electrons. But at the same time, he felt that something essential was missing: the insight. Heisenberg and the others could make excellent predictions of the relationships between variables. But Einstein saw that as little more than mathematical sleight of hand. The physical reality of what was going on deeper down in the system not only remained totally unclear, but also seemed to run counter to all intuition. To put it mildly. Particles were able to be in multiple places at the same time. They were both waves and particles. They could penetrate energy barriers. Niels Bohr accepted the strange quantum phenomena as reality. Einstein kept asking him for deeper insights into the physics. Bohr stubbornly resisted that as a matter of principle, sometimes taking hours to think and then doggedly rejecting all the thought experiments Einstein could come up with. In the end, it seemed to be primarily a question of preference. God does not play dice, concluded Einstein. God’s got nothing to do with it, reckoned Bohr. That was in 1927. Now let’s return to 2015. To Brussels. It’s taken me months of reading, talking and deliberation to work out what I’m going to tell the two of them once they’re sitting at my cafe table on this historic spot. I asked them formally for an interview a couple of weeks ago – a dialog, actually, because I know that it’s impossible to get a word in edgewise once Niels Bohr is talking unless you are as astute and as determined as Einstein. But in fact, I’m not really going to interview the two of them at all. What I want to do is astound them. Astonish them with the aftermath of their own legendary discussions, the arguments that the pair of them had in 1927 at this very spot in Brussels. The aftermath of their fight about what it actually meant, back when quantum theory was still in its cradle. A theory about energy and particles that was not yet fully fledged and that was at the very least an affront to decent scientific intuition, according to Einstein. Whereas Bohr, the younger of the two, kept telling Einstein that scientific intuition was perhaps simply inadequate for understanding the true nature of energy and particles. Accept the strangeness of quanta, he kept on repeating, and a new world will open up before your eyes. Einstein and Bohr were guests in 1927 at the now famous annual Solvay Conference, organized by the Belgian industrialist and chemist Ernest Solvay. He was deeply interested in the latest insights in the sciences, and in 1911 he had invited the cream of international physics to a meeting at the Métropole Hotel in Brussels to discuss issues in modern physics. For the first few years, the Dutch theoretician Hendrik Lorentz from Leiden was the chairman. He was seen back then as the binding force in European physics. The conferences in Brussels always included a group photo. The picture from 1927 shows this gathering of brilliant minds grouped on the terrace steps next to the Hotel Métropole. Einstein, gray-haired already, is sitting pontifically in the middle, with his typically unruly hair that made him a popular symbol of free-thinking genius even while he was still alive. Many young physicists still run their hands unthinkingly through their hair one extra time in the morning... On the far right of the second row, we see a younger looking Niels Bohr, pale and rather introverted. He must have been tired after days of cutting-edge debate with one of the smartest physicists of his time.
