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## Particle Accelerators and huge tanks

Gigantic, fascinating experiments are being performed every day in many different places on our planet, by thousands of scientists. We discuss some of the most impressive aspects of cutting edge particle accelerators and other large man-made machines..

### Giant microscopes

There is a golden rule in small scale physics that says that the smaller the object is that you want to study, the bigger the machine is that you need to study it. We can easily see a kitchen table, for instance, with as small an apparatus as our naked eye. When we want to study cells of our body though, we need an extension, called a microscope. To see even smaller objects, like the atoms the cell is made off, we need even bigger machines, for instance Scanning Tunneling Microscopes, that can feel the presence of an extra atom on a surface through an electric current it induces through a needle that hovers over the surface.

We have been talking about even much smaller objects though. A nucleus of an atom is ten thousand times smaller than the atom itself. (One could think of the electron clouds as occupying the space between atomic nuclei in the air, the kitchen table, and everywhere where there is matter.) Roughly, the apparatus that we need to study these nuclei and the particles within them is about ten thousand times bigger than the machine we needed to see an atom. The microscope that lays bare the secrets of the atomic nuclei, the particle accelerator, has a length in the order of kilometers. Thus, we can think of particle accelerators as giant microscopes. Note that the largest structure we can build on earth would be about ten thousand times bigger. Out there, in the universe, we have of course a lot more room.

Why is it that we need bigger machines to observe smaller structures ? Well, almost always when we say we observe something, it is through indirect means. We don't see the kitchen table itself, but we see the light that is reflected on its surface. (The kitchen table itself does not fly towards our eyes.) The sun shines photons on the surface of the table, which reflects or absorbs it in its own characteristic fashion, and from the little information that comes our way we can deduce that we are dealing with a kitchen table.

Thus, we can send a beam at an object and observe what kind of radiation or particles come out. We can send small particles at an atom and observe that the atom only significantly scatters the small particles when these particles hit the nucleus -- otherwise they fly past almost unaffected. From that we can deduce that an atom is made of a huge cloud of electrons and a tiny nucleus.

One property we need when we send a beam at an object to observe its structure, is that the characteristic probing wavelength of the beam is smaller than the object. Indeed, suppose we send a long wavelength electromagnetic beam at a tiny target. Than it is fairly obvious that the beam will miss the object entirely. (In fact, radiowaves for instance have no effect on the atoms in our body.) For a tiny object, we need an electromagnetic beam with an even tinier wavelength to be able to observe the substructure of the object. Otherwise, the outgoing scattered waves will carry no information about the substructure of the object under study.

Next we note that the shorter the wavelength of the electromagnetic radiation, the higher the frequency (since the product of the frequency and the wavelength is equal to the speed, which is the constant speed of light), and the higher the frequency, the more energy is stored in the radiation. (Imagine having to produce a high frequency wave by moving an electron up and down very fast -- it will cost you a lot of energy.) Thus, the higher the energy of the incoming beam of radiation, the smaller the structures we can probe.

The mechanism in accelerators is slightly different than the one just described, but the basic law that we derived remains valid. The total energy in the incoming beams will be an excellent measure for the distance scales that can be probed by the accelerator. In an accelerator, the beams might be made of electrons and positrons, protons and antiprotons, or ionized atoms. The faster they smash into each other, the finer the physics we probe with that experiment.

We typically smash electrically charged particle beams into each other, for the simple reason that it is the easiest thing to do. Accelerating charged particles can be done by electromagnetic fields (e.g. generated by superconducting magnets), since they interact electromagnetically. Neutral particles are more difficult to accelerate. (The reader should compare this discussion with the discussion of how particles of the standard model interact with each other, and as an exercise, can try to invent accelerators based on the weak force, and find several reasons why they would be problematic.)

Note also that from the above abstract description of experiments, we conclude that we should try to catch every bit of outgoing matter and radiation that is produced when we aim a beam at a target, or two beams at each other. When the outgoing matter is rapidly decaying, or very light and neutral, or extremely fast, that is a highly non-trivial task. If we catch none of the outgoing matter, it is rather evident that we will have gained no information from our contrived experiment.

### Cern and Fermilab

We can be much more concrete about how these experiments are performed. There are in fact only a few places in the world with a microscope in the largest class, since they are extremely difficult to build, and cost a lot of money. There's the Tevatron in Fermilab (Chicago-Ilinois) and the LEP-LHC in Cern, Geneva, Switzerland. (There are many slightly smaller ones, which are admirable works of art too, of course.) I strongly urge the reader to check out the web sites of these huge experiments to uncover more of the detailed fascinating technique that goes into these machines. These accelerator usually have a very accessible web site, with lots of interesting information for the public and the press, including a gallery of impressive photographs of the accelerator. You'll see the tunnel, the way it is implanted in the local landscape, and usually you will find a map of the locations next to the tunnel where big labs are located. These are the places where particle bundles smash into each other and where the debris is gathered with large detectors. The large detectors and their associated labs usually have catchy names. The detectors are like onions, with different layers of the detector being suited for gathering different kinds of particles, or particular types of information on the particles that result from the collision (=scattering) of particle bundles, like their energy or charge. Usually you will find little information on the fascinating stories that tell how the impressive accuracy of the machine was technically achieved. (For example, the accelerator in Cern gave results with small deviations from expectations, and the deviations seemed to depend on the period of the month in which the results were obtained. After a long search, it turned out that the tidal forces of the moon had a tiny effect on the earth's crust that distorted the shape of the accelerator slightly, leading to the unexpected observations -- you see that once one is involved in extremely accurate measuring, one has to take into account all kinds of environmental effects. The more accurate the experiment, the more difficult it becomes to truly isolate it from outside circumstances.)

### The universe

There are many other giant experiments that operate at the moment, including a ground experiment that tries to detect gravitational waves, ground experiments that involve huge tanks that are carefully monitored to observe whether protons are truly stable, or whether they fall apart after a very very long time, or for measuring how many neutrinos produced by nuclear reactions in the sun reach us, or what types of neutrinos we find hundreds of kilometers away from nuclear reactors (for different reasons), or huge wire networks buried in the ice in Antarctica for detecting neutrino's, as well as a planned impressively accurate space-gravitational wave detector, and moreover there has been a lot of recent excitement around the detection of the cosmic microwave background (CMB). All of these impressive international collaborations are worth checking out on the web. (Type the appropriate keywords in Google and surf.) One basic principle underlying a subset of these experiments is that the universe can act as a giant accelerator, and that we will hardly be able to reproduce such violent events as the big bang on earth. The message here is that if we really want to learn about extremely high energy processes, it will be worthwhile to turn our eyes at the sky, and to observe intently the evolution of the universe, the gigantic experiment over which we have very little control.