Re: How do we know about atoms & quarks & stuff when we can't see them?

Dear Casey!

You want to know how scientists arrived at the notion of matter being composed of small objects, like atoms, electrons, quarks etc. There are basically two answers to this question, and they both have to do with the way scientific experiments are performed and interpreted:

1. Indirect evidence. Sometimes it is convenient to explain the outcome of an experiment by hypothesizing something about the objects one has experimented with, without having a serious proof for their existence in the first place. That is often done because other explanations seem too esoteric or complicated - remember that scientists are, in a sense, lazy people: They want their picture of the world to be as simple as possible. When several options exist for the explanation of an experiment, the simplest one is usually favoured, sometimes even if that means the introduction of objects or mechanisms never heard of before.
   One example is chemical reactions. By putting some sulphuric acid into water (the exact quantity is not too important) and sending electric current through it one observes the production of gases at the metallic electrodes: hydrogen and oxygen. It is therefore plausible to conclude that hydrogen and oxygen are somehow `contained' in the water beforehand. If one now mixes the two gases again and adds some energy to it (in the form of a spark or a little flame, for instance), a chemical reaction takes place (quite a noisy one, I might add, if the gases are mixed in the right ratio of quantities) and water is produced again. Many such experiments have led chemists to the conclusion that all matter consists of a set of `pure' substances called `elements', and that those elements can be used to build other substances by means of chemical reactions. Now the natural way to explain this phenomenon is to postulate that every element consists of a certain type of `unsplittable' building block, the atom. Of course, it was still a long way to the proof that atoms really exist and how they `look' like, but finally the conjecture has turned out to be correct.
   Another example would be brownian motion. Placing tiny ink particles in water and observing them with a microscope, one notices that they move in a very peculiar, irregular manner which cannot be caused by simple fluid flow (which would lead to a much more regular motion). The conclusion is that there must be something in the water which we cannot see but which `pushes' the particles irregularly. It's like observing a ship on the sea that is very far away. Seeing the mast moving tells you there is a rough sea with many waves over there, although the water might be calm where you are.
2. Direct examination of substructure. It is true that we cannot build a microscope working with visible light to explore the `atomic' structure of matter. This is because light can be described by `waves' which have an attribute called wavelength to them. Light of different wavelengths has different colours. The problem is that the atomic substructure of matter is so fine-grained that even with the shortest visible wavelengths we cannot see it. That is because light with a certain wavelength can only resolve structures on a scale of at most a wavelength or so, and for blue light this is roughly 400 nanometers. Atoms `live' on scales of roughly one nanometer (billionth of meter), so there's a no-go here. Fortunately, there are ways out of this dilemma: First, one can use even shorter-wavelength light. This is not visible any more, but it can be made visible by using the right equipment (cameras, films etc.). Unfortunately, even this cannot take us to atomic scales (but is very useful in other applications, for example in microchip design). Second, not only light has wavelike properties. All matter can be described as waves, and especially with elementary particles like electrons these properties show up prominently. So what physicists do to explore tiny objects like an atom is to `throw particles at it'! One major application of this method is deep inelastic scattering, where electrons are accelerated to very high energies and then collide with atomic nuclei. The higher the energy of the electrons, the shorter their wavelength and the smaller the features they can resolve in the nucleus. Now it is a tedious mathematical task to extract physical information about the nucleus from such an experiment, but it can be done. In principle, all the expensive (or, regarding average national defense expenses, not too expensive) accelerators like RHIC in Brookhaven or SPS at CERN in Geneva are built to do exactly this: Throwing things at each other to explore their structure. It has worked in the past, and it will work in the future - the drawback is that we need larger and larger machines to get to higher and higher energies to explore the deepest innards of matter.

Hope that helps,
Georg.