Читать книгу Quantum Physics is NOT Weird - Paul J. van Leeuwen - Страница 27

While doing experiments with cathode rays in vacuum glass tubes – the precursors of the once ubiquitous but nowadays outdated TV display tube – and directing these rays through an electric or a magnetic field, John Joseph Thomson (1856-1940) noticed that these rays were deflected by those fields. See figure 4.4.

Figure 4.4: Vacuum tube for cathode ray deflection by an electric field measurement. Plates D and E are electrically opposite charged.

Source: J.J. Thomson – Philosophical Magazine 1897.

Thomson inferred from their deflection in electric fields that cathode rays had to be composed of negatively charged particles. He dubbed these particles electrons. He assumed then that these electrons existed in matter and were evenly distributed within the atoms that made up matter. This made an atom into a positively charged solid little ball with negative electrons scattered through it, like currants in a plum pudding. Concerning his “plum pudding” model Thomson apparently did not realize that, because of their mutual repulsion, all electrons should be evenly distributed on the outside of the atom, each as far away as possible from the others. Also, the magnitude of the electric charge of the electron was still unknown at time of Thomson’s discovery. The electron charge was later determined by Millikan ^[10] by measuring with a microscope the velocities of slowly falling electrically charged oil droplets, a beautiful, sophisticated laboratory experiment.

Experiments ^[11] conducted under supervision of Ernest Rutherford ^[12] (1871-1937) revealed that electrons probably formed the outer ‘hard’ shell of the atom with an almost totally empty void inside. In the centre of that emptiness resided an utterly tiny positive mass – the nucleus. For an electrically neutral atom the positive charge of this nucleus had to be equal to the sum of the negative electron charges. In 1908 Rutherford’s assistants Marsden and Geiger ^[13] shot alpha rays – positively charged and fast moving alpha particles, ejected at high velocity from a radioactive source (radium) – at a gold foil. As Rutherford already suspected, these alpha particles turned out to be helium nuclei, consisting of two protons and two neutrons. Gold can be beaten to an ultrathin foil ^[14] of less than 1 nm thickness (1 nanometer = 0.000000001 m) but Marsden and Geiger used a gold foil of around 210 nm thick ^[15] in their experiment.

Figure 4.5: Shooting alpha particles from a radioactive source through an ultrathin gold foil. Only 1:8,000 particles will be deflected.

A gold atom has a diameter of 0.3 nm, so such a 210 nm thick gold foil will still be around 800 atoms thick when we assume closely packed spheres. From the perspective of these utter tiny alpha particles, the gold foil should therefore be experienced as an impressive solid 800 gold atoms thick wall.

According to Thomson’s "plum pudding” atom model such a solid wall should present a major obstacle for the massive alpha particles. However, the experimental result turned out quite contrary to expectations. Almost all alpha particles shot straight through the foil as if that solid wall of 800 heavy gold atoms thick did not exist at all. But some were deflected. Of 8,000 alpha particles, an average of 1 was deflected and sometimes even completely bounced straight back. The hits were measured by counting the little light flashes made by the alpha particles when they hit a zinc-sulfide screen, that could be moved on a circular rail. Rutherford assumed that those rare deflections were in fact near-collisions of positive alpha particles with incredibly small positive atomic nuclei. He is said to have exclaimed:

“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

Rutherford concluded that the atom had to consist of negatively charged electrons orbiting a very small and positive nucleus. The inside of an atom was virtually empty space. He was able to calculate from the ratio of the deflected alpha particles against the not deflected ones – 1:7999 – that the ratio of the small nucleus to the size of the electron shell should measure approximately between 1:10,000 to 1:20,000. An image now emerged of atoms as miniature solar systems with minuscule negatively charged electrons, orbiting like tiny satellites the positively charged nuclei at high speeds. To give you an impression of the proportions of the Rutherford atom: the ratio of the nucleus to the electron shell can be compared with the ratio of a fruit fly of 2 to 4 mm to the dome of St. Peter's in Rome. Take a moment to visualize that image of a fruit fly hovering in the middle of that dome. So, 99.999999999% of the atom is just empty space, void. The solidity of matter started more and more to look like an illusion. But even still stranger views were soon to come.

The reason why we do not sink through a floor consisting of these ephemeral atoms, which we now know as mainly empty space, has to do with the Pauli exclusion principle ^[16], discovered by Wolfgang Pauli (1900-1958). This principle prohibits electrons from having the same position in space when all their quantum properties have equal values. These properties are connected to the four quantum numbers that describe the quantum state of the electron. One of them is the electron spin. The electron spin is the quantum version of the north-south orientation of a magnet. It was discovered in 1925 by two Dutch PhD students – George Uhlenbeck (1900-1988) and Samuel Goudsmit (1902-1978). They were awarded the Max Planck medal in 1964 for their discovery of the electron spin. The electrons in the outer shells of the atoms of our hand repel the electrons from the outer shells of the atoms of the wall, and vice versa. Their absolute refusal to have the same position in space when their quantum numbers are the same, prevents us putting our hand through a wall, where both hand and wall are in fact almost entirely made up of empty space. The electrons of these atoms repel each other, the nearer to each other the stronger. So, in fact, we don't even touch the wall, even when we hit it hard.

From Rutherford comes the parable of the two desks. One desk is solid and exists in the solid world of our daily experience, the other desk is mostly empty space, ghostly and exists in the world of physical ideas. Which desk would be the real one? What is your opinion?