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The story of quantum mechanics Video He [Rutherford] pictured the atom as a tiny solar system. Electrons, tiny particles of negative electricity, orbit around a minute positively-charged object called the nucleus. Rutherford calculated that the nucleus was 10,000 times smaller than the atom itself. That's why only one in 8,000 alpha particles bounced back. They're the ones that hit the tiny nucleus by chance. The rest whizz by without hitting anything. The first astonishing consequence of this idea is that Rutherford's atom is almost entirely empty space. That's why nearly all the alpha particles race through the gold atoms as if there's nothing there. There really is nothing there. Consider the bizarre implications of Rutherford's atom by imagining it on a bigger scale. If the nucleus were the size of a football, then the nearest electron would be in orbit half a mile away. The rest of the atom would be completely empty space. Let me explain it another way. If you were to suck out all the empty space from every atom in my body, then I would shrink down to a size smaller than a grain of salt. Of course, I'd still weigh the same. If you did the same thing to the entire human race, then all six billion of us would fit inside a single apple! The atom was unlike anything we had ever encountered before. And it would only get stranger and stranger! Almost immediately, a problem surfaced, and it was a big one. According to the tried and trusted science of the time, the electrons should lose their energy, run out of speed and spiral into the nucleus in less than the blink of an eye. Rutherford's atom contradicted the known laws of science. The atom didn't care that it defied scientific convention. It's almost entirely empty space and it's gonna stay that way. I show no signs of shrinking down to the size of a grain of salt. And the Earth is, well, the size of the Earth. It's not getting smaller. It's worth remembering the time scale. In six short years from 1905 through to 1911, the atom had announced its existence with the fact that it was unimaginably small. Then it revealed that it was mainly empty space. And now it didn't obey the known laws of physics. Not surprisingly, all the established scientists of the day, including Einstein, were baffled. Scientific ideas they'd put their faith in all their lives had failed completely to explain the atom. The atom now required a new generation of scientists to follow in Rutherford's footsteps. Bold, brilliant and above all, young. It was crucial they had no loyalty or attachment to ideas held by previous generations. One of the first of this new breed was Niels Bohr. He sailed from Denmark in 1911 and made his way to English soil. Having finished his studies in Copenhagen, Bohr decided to move abroad and be at the centre of the new physics. The trail led him to Britain, Manchester University and Ernest Rutherford. Bohr had a brilliant mind, at times hampered by a pathological obsession with detail. In fact, the story goes that Bohr taught himself English by reading Dickens' Pickwick Papers over and over again. Bohr was so captivated by Rutherford's picture of the atom that he made it his mission to solve the puzzles of why the atom didn't collapse and why there was so much empty space. As one of the new breed of theoretical physicists, he was fearless in his thinking and was prepared to abandon common sense and human intuition to find an explanation. So, in a leap of genius, he started to look for clues about the atom's structure not by looking at matter but by examining the mysterious and wonderful nature of light. Now, atoms and light are clearly connected. Most substances glow when they're heated. For centuries people had realised that different substances glow with their own distinctive colours, a bit like a signature. So the green of copper, the yellow of sodium and the red of lithium. These colours associated with different substances are called "spectra". And Bohr's great insight was to realise that spectra are telling us something about the inner structure of the atom, that they could explain all that empty space. Bohr's idea was to take Rutherford's solar system model of the atom and replace it with something that's almost impossible to imagine or visualise. So sensible ideas like empty space and particles moving around orbits fade away. They're replaced with something that is one of the most misunderstood and misused concepts in the whole of science - the quantum jump. Now, it takes most working physicists many years to come to terms with quantum jumps. Bohr himself said that if you think you've understood it, then you haven't thought about it enough. So I'm going to take a deep breath and in under 30 seconds try and explain to you one of the most complicated concepts in the whole of science but one that underpins the entire universe. Bohr described the atom not as a solar system but as a multi-storey building. The ground floor is where the nucleus lives, with the electrons occupying the floors above. Mysterious laws mean the electrons can only live ON the floors, never in-between. Other mysterious laws mean that sometimes they can instantaneously jump from one floor to another. These are what we call quantum jumps. Now, Bohr had absolutely no idea what these laws were. But thinking like this allowed him to make a startling prediction. When an electron jumps from a higher floor to a lower one, it gives off light. More significantly, the colour of the light depends on how big or small the quantum jump the electron makes. So an electron jumping from the third floor to the second floor might give off red light. And an electron jumping from the tenth floor to the second floor, blue light. To test his new theory, Bohr used it to make a prediction. Could it explain the mysterious signature in the spectrum of hydrogen? After months of calculating furiously, he finally came up with the result. And his prediction was surprisingly accurate. For the first time ever, it looked like the spectrum could be explained. And back in 1913, that was big news. But Bohr's new idea rested on a single seriously-controversial supposition. Why should the electrons and the atom behave as though they were in a multi-storey building? And why should they magically perform quantum jumps from one storey to another? There was no precedent for it anywhere else in science. When one physicist claimed that the jumps were nonsense, Bohr replied, "Yes, you're completely right! "But that doesn't prove the jumps don't happen, only that you cannot visualise them." But not being able to visualise things seemed to go against the whole

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purpose of science. Older scientists in particular felt that science was supposed to be about understanding the world, not about making up arbitrary rules that seem to fit the data. Conflict between the two generations of scientists was inevitable. Bohr's weird new atom and his crazy quantum jumps were a shot across the bow of traditional classical science and the old school reacted angrily. Leading the traditionalists was the giant of the physics world Albert Einstein. He hated Bohr's ideas and he was going to fight them. Anything to save the world of order and common sense from this assault by madness. Bohr, though, was undeterred and as the 1920s dawned, the battle lines for one of the greatest conflicts in all science were drawn. Einstein spent much of the early 1920s arguing against Niels Bohr, with mixed success. His celebrity status gave him power so when he said he loathed ideas like quantum jumping that seemed plucked out of thin air, people listened. Then in 1925, a letter landed on his desk that turned out to be manna from physics heaven. Here finally was an idea that described the atomic world with the tried and trusted principles of traditional science. Einstein was ecstatic. He told friends, "Finally, a veil has been lifted on how the universe works." The letter came with the PhD thesis of a young Frenchman. And behind it lay an extraordinary tale. During the First World War, a young French student spent his time at the top of the Eiffel Tower, as a radio operator. His name was Prince Louis de Broglie. He came from French aristocracy but he was devoted to physics. He was so wealthy he built his own laboratory off the Champs-Elysees. After the war, De Broglie became gripped by the mysteries and controversies surrounding the atom. And then his war-time experience as a radio operator gave him an intriguing idea. Perhaps radio waves could explain the atom. Although invisible, they behave very much like water waves. Like ripples spreading out across a pond, radio waves obeyed mathematical equations that were reliable and well understood and had been worked out decades earlier. So for his PhD thesis, De Broglie imagined a kind of radio wave pushing the electron around the atom. He called it a pilot wave. This pilot wave would also hold the electron tightly in its orbit, stopping the atom from collapsing. There were no strange instant quantum jumps, just intuitive common sense familiar waves. The relief felt by the traditionalists was palpable. "The atom is all about waves", they cried, and we understand what waves are. Einstein and the traditionalists felt that victory was within their grasp. They believed they had Bohr and the new atomic science with its crazy quantum jumps on the ropes. But Niels Bohr wasn't the kind of man to roll over and give up. Even though he'd explained the spectrum of hydrogen, with his new revolutionary theory, he had nothing like Einstein's worldwide recognition. But in his native Denmark, his theory was enough to make him a star. Flushed with success, Niels Bohr returned to Copenhagen in 1916, a conquering hero. His new-found celebrity status meant he found it very easy to raise money for research. In fact, it was funding from the Carlsberg brewery that helped build his new research institute. You could say it was beer that helped us understand the secrets of the atom! This institute became a leading centre for research in theoretical physics that survives to this day. I came here in the early 1990s to carry out research on nuclear halos. And even then, this was the place to be to do that sort of research. This is the main lecture room in the Niels Bohr Institute. It doesn't look very impressive as far as lecture halls are concerned, but it's full of great quirky details. I remember lecturing here a few years back and I know that Niels Bohr himself designed some of the machinery that raised and lowered blackboards. There's an incredible series of boards, one underneath the other, of boards filled with his formulae so that he wouldn't ever need to rub out any of his equations. It sort of goes on and on. Bohr's reputation for radical and unconventional ideas made Copenhagen a magnet for young, ambitious physicists. They were keen to make their mark and be a part of Bohr's innovative new science, which came to be known as quantum mechanics. In 1924, in defiance of Einstein and De Broglie's traditional explanation of the atom, the radicals revealed a new theory, based on Bohr's quantum jumps. It was to be their most ambitious and most controversial idea yet. It was first developed by Wolfgang Pauli, one of Bohr's rising stars. Pauli took Bohr's bizarre "quantum jumps" idea and turned it into one of the most important concepts in the whole of science. And I don't say that lightly. Pauli's idea goes by the uninspiring title of the Exclusion Principle. But I think a better title would be "God's best-kept secret" because it explains the vast variety of Creation. The question Pauli's idea tried to answer was this. Every atom is made of the same simple components. So why do they appear to us in so many different guises? In such a rich variety of colours, textures and chemical properties? For instance, gold and mercury. Two very different elements. Gold is solid, mercury is liquid. Gold is inert, mercury is highly toxic. And yet they differ by just one electron. Gold has 79 and mercury has 80. So how does one tiny electron make all that difference? What Pauli did was pluck another quantum rule out of thin air. Remember Bohr's multi-storey atom? The nucleus is the ground floor with the electrons progressively filling the floors above. Pauli said there's another quantum rule which states crudely that each floor can only accommodate a fixed number of electrons. So if we want to add another electron to the atom, it has to check for a vacancy in the top floor. And if that floor is full, another floor or shell is created above it for the electron. In this way, a single electron can radically change the shape of the atom and this, in turn, affects how the atom behaves and how it fits together with other atoms. So Pauli's principle really is the basis upon which the whole of chemistry, and ultimately biology, rests. Pauli's Exclusion Principle was a major breakthrough for Bohr's quantum mechanics. For the first time, it seemed to offer us a real understanding of the incredible variety in the world around us and possibly life itself. Its success blew a large hole in Einstein's defence of the old physics. And like quantum jumping, it was straight out of the weird rule book of atomic physics. Pauli didn't explain why his principle worked. He said it just did.