What We Cannot Know: Explorations at the Edge of Knowledge. Marcus Sautoy du
Чтение книги онлайн.
Читать онлайн книгу What We Cannot Know: Explorations at the Edge of Knowledge - Marcus Sautoy du страница 24
Thomson managed to achieve a similar effect with other gases: helium, nitrogen, oxygen. The masses were all whole-number multiples of the positive particle produced by the hydrogen atom. Atomic harmony yet again. As yet, there was no reason to believe that there weren’t just many sorts of positive particles, just as there were many sorts of atoms. Thomson had suggested a model of the atom known as the plum pudding. The positively charged part of the atom, which was more massive than the negative electron, formed the pudding making up the bulk of the atom, while the electrons were the tiny fruit inside.
Then the age of the bombardment of the atom began which would eventually lead to the ultimate atom smasher: the Large Hadron Collider at CERN. The New Zealand-born British physicist Ernest Rutherford is generally credited with the discovery of the proton, the particle that was the building block for all these positive particles that Thomson had investigated.
Rutherford became fascinated by the new subject of radioactivity. Uranium atoms seemed to be spitting out particles that could be picked up by photographic plates. There appeared to be two types of radiation, and these became known as alpha particles and beta particles. The alpha particles were more easily detected. Rutherford found that using a magnetic field he could deflect these alpha rays in the same way that Thomson had deflected the negative particles. Calculations showed that they had the same mass as the stripped helium atoms. Their hunch that the alpha rays being emitted by the uranium were actually bits of helium atoms was confirmed when the alpha rays were combined with a shower of electrons, which resulted in a stable gas being formed. Chemical analysis soon confirmed that the gas was indeed helium.
TISSUE PAPER BALLISTICS
It was when Rutherford’s student Hans Geiger placed a thin sheet of gold foil between a stream of alpha particles and the plate detecting the particles that the evidence once again contradicted the theoretical model of the atom. In the model of the atom which has positive charge distributed evenly like a pudding, positive alpha particles passing through the metal would be repelled by the positive charge in the atom. Given that the charge is distributed over the full extent of the atom, you wouldn’t expect much deflection.
Alpha particles being deflected by the nuclei of atoms of gold.
Geiger found that, on the contrary, some of the alpha particles were deflected wildly, to the extent that some bounced back off the gold foil in the direction they’d been fired from. Rutherford was staggered: ‘It was as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.’
Again it was mathematical calculations that gave rise to a new model. By counting how many alpha particles were deflected, and by how much, they discovered that the data was consistent with the charge and mass being concentrated in a tiny centre of the atom, which became known as the nucleus. It still wasn’t clear whether this nucleus was indivisible or not.
When Rutherford bombarded lighter atoms with alpha particles evidence emerged that the nucleus wasn’t a single entity but made up of constituent particles. Tracing the paths of the alpha particles in a cloud chamber, he detected paths that were four times longer than they should be. It was as if another particle four times as light was being kicked out of the nucleus by the impact of the alpha particles. Different gases produced the same result. Indeed, Rutherford found that pure nitrogen was being converted into oxygen by the impact. Knock out one of these particles and the element changed.
Here was evidence for a building block from which all nuclei of atoms were built. It behaved just like the hydrogen atom with its electron stripped off. Rutherford had discovered the proton. The nuclei of atoms were built by taking multiples of this proton. The only trouble was that the charge on the atom didn’t make sense. Helium had a nucleus that was four times as heavy as the hydrogen atom, yet the charge was only twice as big. Perhaps there were electrons in the nuclei attached to protons, cancelling out the charge. But the physics being developed to explain the behaviour of these particles precluded electrons and protons in such close proximity, so that couldn’t be the answer.
This led Rutherford in the 1920s to guess that there might be a third constituent, which he called a neutron, with the same ball-park mass as the proton but no charge. Producing evidence for this particle proved very tricky. He used to discuss with his colleague James Chadwick crazy ways by which they might reveal the neutron. Experiments conducted in the 1930s in Germany and France eventually picked up particles being emitted when various nuclei were bombarded with alpha particles, and, unlike the proton, these particles did not seem to possess any charge. But the experimenters mistakenly believed it was some sort of electromagnetic radiation, like the high-frequency gamma rays that had been discovered by French physicist Paul Villard at the beginning of the century.
Chadwick, though, was convinced that these particles must be the neutrons he’d discussed with Rutherford. Further experiments revealed that they had mass just slightly bigger than the proton, and without charge this new particle was the missing ingredient that made sense of the numbers. With Chadwick’s discovery it seemed as if the building blocks of matter had been revealed.
It was a very attractive model. Fire, earth, air and water, the four elements of Aristotle, had been reduced to three particles: the electron, proton and neutron. With these three building blocks scientists believed they could build all matter. Oxygen: 8 protons, 8 neutrons and 8 electrons. Sodium: 11 protons, 12 neutrons and 11 electrons. It was as if the music of the spheres was singing out and the foundations of matter were these notes: protons, electrons and neutrons. All matter seemed to be made up of whole-number combinations of these three particles. Why would you expect these particles to be made up of smaller entities? If they were, you might expect to see fractional pieces between the elements in the periodic table.
Except the dividing didn’t stop there. It turned out that there were experimentally and mathematically very robust reasons to think that protons and neutrons were not indivisible. But the building blocks of the proton and neutron have a strange property: they don’t like to be seen in isolation. They only come in groups, making up something like a proton or a neutron. Safety in numbers. So if they have never been seen on their own, why do scientists think there are even smaller bits into which we can divide protons and neutrons?
Everything we call real is made of things that cannot be regarded as real.
Niels Bohr
At the end of the 1920s it seemed as if the basic building blocks of matter had been tracked down. The atoms of the periodic table could all be built by taking combinations of electrons, protons and neutrons. The electron has withstood any attempts to divide it further. But revelations over the next decades would lead scientists to believe that there was another layer of reality hiding below the other two building blocks.
The principal reason for the realization that protons and neutrons might not be as indivisible as the electron came not from more sophisticated technology but from the mathematics of symmetry. It is striking that time and again mathematics appears to be the best microscope we have to look inside my casino dice. A mathematical model began to emerge to explain the proton and the neutron, and it was built on a mathematical concept that could be divided. If the mathematics came apart into smaller pieces, the feeling was that the same should apply to the proton and the neutron.
The mathematical model responsible for this belief in the divisibility of the proton and neutron arose