How do you discover an element in the sun and what does it have to do with quantum physics?

It is an interesting fact that the element helium, that beloved of elements that makes your voice go all squeaky when you inhale it, was discovered in the sun. Now, that's pretty impressive. What are some other things that have been discovered? Thousands of years ago, humans discovered that gone off fruit juice makes you feel merry, which we now know is because yeast ferments the sugar into alcohol and that makes us drunk. Now there are a number of reasons why that particular discovery could not have been made in the sun, the most important being that the fruit, the yeast and the discoverer would have been vaporised long before they got within even 100 miles of the sun.

So how do you discover an element in the sun, and what does it have to do with quantum physics? People often say that quantum physics was the greatest discovery of the 20th century but what exactly does the expression mean? It's a straight forward question but tricky to answer.

Physicists commonly say that a big part of quantum physics is that small things don't behave like big things. They mean that sub-atomic particles, the things that atoms are made of, don't behave like objects that we can see, like snooker balls. Snooker balls follow Newtonian physics, the laws discovered by Isaac Newton. For example, if you set a snooker ball moving, it will move at a constant velocity until a force slows it down or changes its direction. In practice, if you roll the ball along a snooker table, it will immediately be slowed down by the friction between the ball and the surface of the table, as well as the air resistance. If it hits another ball, both will move away from each other with an equal and opposite force. So usually, however much force is supplied to the two balls during the collision will be shared equally between the two of them, causing them both to move off in different directions and, usually, at similar speeds. These are key examples of Newtonian physics.

But sub-atomic particles do not necessarily obey quantum physics. The electron is an especially good example of this. When we draw diagrams of atoms, like the one below, they often look as if the electrons orbit the nucleus the way that planets orbit the sun. Orbiting planets do demonstrate Newtonian physics - it is the force of gravity between the sun and the planets that keeps each one in its individual orbit. No matter how the diagrams look, however, electrons do not orbit the nucleus of the atom.

Figure 1: Diagrams showing the atomic structures of lithium, boron and fluorine. The electrons appear to orbit the nucleus but do not. These electron shells can also be called energy levels.

A great analogy for the way electrons behave around nuclei is the way moths behave around light bulbs at night. When moths detect a light bulb, they fly towards it and end up flying around and around it until the bulb is switched off. (The actual reason for this is that they are adapted to use stars for navigation, so they basically confuse the light bulb for a star.) Now, magnets are attracted to each other, or more accurately, the north pole of a magnet is attracted to the south pole and vice versa. If you put two magnets near each other on a table top, and nothing is in between them, they will move towards each other until they are touching. Isn't this what it means to be attracted to each other?

Figure 2: A moth flying in an erratic path near the lightbulb

On the other hand, the moths are attracted to the lightbulb but they do not fly towards it until they are touching it and then stay put on its surface. Instead, they periodically crash into the lightbulb before immediately flying away again but continuing to buzz around nearby. This is very similar to how electrons move near nuclei. The negative electron is attracted towards the positive nucleus but it does not move towards it until they are both touching and then stay put. Instead, it follows an erratic path in the vicinity of the nucleus, just like the moths near the light bulb.

As a final confuser, it is interesting to consider that actually, even the two magnets are not technically touching. If you could see the atoms in each of the magnets, you would see that they are not touching. At the atomic level, there are both attractive and repulsive forces between particles, so they simultaneously pull each other towards themselves while also pushing each other away. Therefore, you end up with a little gap between them. At the atomic level, there would be a little gap between the two magnets so actually they are not touching. You may also have heard the idea that the human body would be the same size as just a grain of sand if all of the space between the subatomic particles were removed. The main source of this space is the gap between the electrons and the atomic nuclei. This unexpected behaviour of the electron forms a huge part of quantum physics.

To consider the way electrons move in more detail, it will be helpful to consider what the word quantum means. A quantum is a discrete quantity of energy. What that means is that it is a certain amount and it is only that amount. You can imagine it like the price of some product that you might want to buy in a shop. In some parts of the world, you can haggle over the price, bargain until you get it a little bit lower. Well that kind of caper wouldn't fly if it were a quantum price. If the price were £10 and you said, "Will you do it for £8?" the answer would be: "No!" So discrete means that you cannot divide the quantum into smaller parts; it represents a certain quantity and it has to be just that quantity.

Going back to the electron, what do those circles in atomic diagrams mean, the ones that make them look like orbiting planets? They represent electron shells or, to use a really high level term, energy levels. These energy levels provide some information about where the electron is but they are more like zones than  map coordinates. Electrons dwell in something called an orbital and these are the things like zones. What an orbital represents is a volume of space in which you are likely to find an electron. It's like meeting a friend in town. If you say, "I don't know exactly where I'll be but I'll be somewhere in town so if you also come to town, at some point we'll run into each other," that's kind of like the orbital. It doesn't say exactly where the electron is, but gives you a rough idea of where it will be.

The further the zone, or orbital, is from the nucleus, the more energy the electron has, so we say the electron is at a higher energy level. But this is where the quantum part comes in: each energy level has a discrete value. The electron cannot be half way between two energy levels, it has to be at one or the other. Electrons can move between energy levels but they have to absorb exactly the right amount of energy, and we call this amount a quantum of energy. But then if the electron drops down an energy level, it gives out a quantum of energy of the exact same size. And now, here's the really weird bit, when the dropping electron emits the quantum of energy, it is released in the form of a beam of light.

Figure 3: As the electron drops from the second energy level to the first energy level, a beam of electromagnetic radiation is emitted.

Now light is very complex stuff. Depending on what stage you're at with learning science, you may know that white light can be separated into the colours of the rainbow. You may also know that there are lots of kinds of light that we cannot see, such as microwaves, which we can use to quickly heat up food; then there is ultra violet light, that we can use to tan ourselves; and infra red, which we can use to help our remote controls communicate with our televisions. In fact, the proper name for all of these different kinds of light is electromagnetic radiation.

So, if you have some electromagnetic radiation, how do you know which type it will be? If something emits electromagnetic radiation, what dictates the type that you get, whether it be ultra violet, infra red, microwaves or one of the others? It is linked to the wavelength of the light or how long each wave is.

Figure 4: The wavelength of a wave. The value y represents the amplitude. (Image adapted from the following Wikimedia Commons file: Wave-i18n.png Author not known.)

Radio waves have very long wavelengths whereas microwaves, as the name suggests, have really teeny wavelengths. Here's another interesting fact, it turns out that these different kinds of electromagnetic radiation also have different energies. This makes sense if you think about it because if you leave a jacket potato next to a radio, the radio waves won't make it much hotter, but if you put the potato in a microwave, then it will get piping hot pretty fast. (Although this would not be a “fair test” as the concentration of radiation would be different in each case.)

Now we're ready to think about quanta (plural of quantum). When an electron drops from a higher energy level to a lower energy level, it emits a beam of light that corresponds exactly to the difference in energy between the two energy levels. The value of that quantum of energy dictates the wavelength, and therefore the type, of the beam of light. For example, if it is a big quantum, it might emit a beam of ultra violet light, whereas a smaller quantum could produce a beam of lower energy infra red light.

"Hang on a minute," readers might be thinking, "I thought this quantum could have one value and one value only, so how come different wavelengths of light can be produced?!" A quantum can indeed have only one value but you can have different quanta. A key reason why quanta can be different is that elements are different. If an electron drops from the second energy level of a carbon atom to the first energy level, it is always the same amount of energy that is produced. However, if the electron drops from the second energy level of an oxygen atom to the first energy level, it will give off a different amount of energy than the electron in the carbon did. So, the quantum of energy is always the same for the same element, but if you use a different element, the quantum will be a different size.

Now we are ready to consider how you discover an element in the sun. Basically, helium was discovered by analysing the light emitted from the sun. There is a lot of helium in the sun, for reasons which also relate to quantum physics. When the light was analysed, scientists found a wavelength of light that they had not detected from any element that had already been discovered.  The wavelength corresponded to a quantum of energy that no known element had previously emitted so the only possible conclusion was that there must exist in the sun an unknown element, which was thus named helium, after Helios the sun god in ancient Greek mythology. Helium was subsequently detected on earth a few years later, confirming the prediction. And that is how you discover an element in the sun.

©Tom Husband 2014 All writing and all images, excepting the image of the wave, attributed to Wikimedia Commons