Lecture 6 – Seeing atoms at last

From the 1979 lecture programme:

We can say some more about nuclear power stations. Fission provides a huge output of heat that is used to boil water and, make steam to run the electric generators of a power station.

Keeping in mind the proper meaning of radioactive decay with half-life after half-life after half-life… we can comment sensibly on precautions and safety. Neutrons cause fission easily in the rarer kind of uranium (U-235, less than 1% of all uranium from rocks). The more common isotope (U-238) usually grabs a neutron without fission and, in two stages of radioactivity, becomes plutonium, a new element, man-made, which can also make a fission chain reaction.

Breeder reactors make rich use of that and put out more fissionable atoms than they take in. See the great graph which shows the possibilities: only the heaviest atoms can yield useful energy by fission; but at the other extreme the lightest elements could yield energy by fusion — nuclei of hydrogen lithium etc. forced together to join and make helium.

A steady fusion chain reaction for power stations has yet to be achieved, but it is the hope of the future: fusion energy for our great-great-great-grandchildren — in addition to sunshine, which is also produced by nuclear fusion in the Sun's ultra-hot core.

A gang of lasers offers a promising avenue to the necessary high temperature. We cannot show such a gang in action, but we will show a small laser demonstrating its 'bite'.

In an earlier lecture you saw light making double-talk: it is a storm of waves or a shower of bullets — it switches between those two behaviours according to the questions we ask or the measurements we try to make. Now see that solid bullets of matter also give double-talk. 

High-speed electrons, which certainly behave as bullets, can also show wave properties — if we ask them. Shoot a stream of electrons through a thin crystal of carbon and we see a pattern formed on the receiving screen. Compare that with the pattern made by X-rays shot through a crystal or visible light shot through woven cloth. The higher the voltage used in the electron gun, the faster the electrons and the smaller the pattern, the shorter the electrons' wavelength.

Larger things, neutrons, nuclei, even atoms, can show wave patterns like that. With still larger bullets the wavelength is utterly too short to make a noticeable pattern, but for all we know it is still there.

In Lecture V we sketched a nuclear atom model and poured some doubts on it. Now we can make a better model, better because it fits with more experimental knowledge and better because it is more fruitful — it makes more successful predictions. The new model has a tiny massive nucleus with a positive electric charge, just as before — until we try to look inside that nucleus; then we may find waves there also. But the electrons are busy with double-talk. They carry momentum like bullets still — if we ask them — but they no longer have the sharp clear orbits beloved of the storybooks. Instead, a wave pattern tells, of each electron, where it is more likely or less likely to go — like the bright and dark regions in the pattern shown with electrons and carbon crystals.

Are those controlling and predicting waves real? We had better just call them thinking models. We prefer to let mathematicians turn them into wave formulae for prediction. Even a formula can be a good thinking model in the most international of all languages.

There is more to be said about models. They are our way of describing what we have found out about Nature. In the realm of atoms, they are useful pictures for talk, for clever thinking and understanding. But we hope you will not ask "Are they true ?" because in making them we have had to use the words for big things that we can touch or see.

If we say a hydrogen nucleus is a round ball, we only mean symmetrical like a ball. As a warning, we hope to show you a crazy room, built distorted. As you look in through a peephole, you would rather believe it is an ordinary rectangular shape, although you have to swallow some surprising events that you see in it. It is a reminder of the way we attach new unfamiliar knowledge to old familiar things that we already know.

And yet you have our promise from Lecture I: You shall see atoms". You shall see the atoms of the sharp point of a tungsten needle — now that you are prepared to understand the method. It is indirect, yet we hope, now in conclusion, that you will each be able to boast "I have seen atoms".


Natural world




Eric M Rogers



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