The search for the building constituents of our world has come a long way. Democritus, sometime around 500 BCE1 was the first to theorize that there is a fundamental indivisible entity which he called átomos - the 'uncuttable'. Today we know that what was later termed 'atom' isn't uncuttable at all, but actually mostly empty, the rest being a small core of protons and neutrons, orbited by electrons. And even the protons and neutrons aren't fundamental particles.
The invention of magnifying glasses, and the light microscope was the first step. Roughly spoken, a microscope uses photons that are focused with lenses. These photons are either reflected on, or transverse a sample.
The photons are then caught on a screen, or a film, and give you a picture. The resolution that one can achieve with light is limited by its wavelength. It is impossible to resolve structures finer than that. Using light of higher frequency (gamma rays) increases the resolution. The average microscope allows us to see cells, or the structure of crystals (for some stunning images see here) .
More efficient than using photons is to use focused beams of massive particles. Due to their quantum properties, massive particles also have a wavelength which considerably smaller than that of massless particles like photons. In addition, charged particles like electrons, can be nicely directed by electromagnetic fields. Indeed, magnetic fields can be used for beams of charged particles like lenses. The electromagnetic fields can also be used to accelerate the charged particles. The advantage of this is that faster particles have a higher energy, or equivalently, a smaller wavelength. Therefore, the faster one accelerates a particle, the better the resolution.
Modern electron microscopes can roughly resolve distances as small as an Ångström - that is about the size of an atom.
However, if you hit the sample with particles of higher and higher energies, you'll eventually alter what you want to observe. If the energy gets sufficiently high, electrons will not only elastically scatter from the sample, but the beam will react with the sample to form new particles. Needless to say, the faster the particle, the more complicated it then becomes to reconstruct an image.
A particle accelerator is nothing but a giant microscope.
Particle beams are accelerated to highest energies, and then either crash into a sample (fixed target) or head on into another beam (collider). The particles that come out of the collision are detected. And here the physicist enters the stage and reconstructs particle's trajectories to understand what has happened. The outcome of such collisions depends on the structure of the elementary matter, and from detecting the particle traces one can confirm, or falsify, models about the stuff that we are made of.
It is quite a detective work. Extracting information about the structure of matter from hundreds of scattered particles whose initial motion is only know to a certain precision is like examining the outcome of a car crash, and trying to find out where the driver had dinner the night before. But over the last decades, physicists have become quite good at this. They've even grown a subclass of the species called: high energy physicists.
The outcome of their detective work is a list of identified objects at the crime scene, published annually in the particle data book, which recently celebrated its 50th anniversary. This essential reading for the high energy physicists also lists the usual suspects for physics beyond the standard model, and it has a very useful table with the technical data of accelerators of the past, present and future.
The February issue of the Discover Magazine has a very well written and researched article 'The Big Bang Machine' by Tim Folger about one of the most interesting currently running experiments, the Relativistic Heavy Ion Collider (RHIC). The article explains the properties of the hot plasma of quarks and gluons that is investigated there, why these findings are so exciting, and what this has to do with string theory and the AdS/CFT correspondence2 (see also our previous posts about the Quark Gluon Plasma and what string theory has to say about it).
If you're to lazy to read, on Wednesday we lucky guys here at PI had a very nice colloqium by Brian Cole from Columbia who will tell you what we can learn from the experiments. You find video and audio at this website.
The world's largest microscope is currently under construction at CERN and is called LHC - the Large Hadron Collider. It looks like this
The LHC is scheduled to start in September. Its main task is the collision of two proton beams with an energy of roughly 10 TeV, that corresponds to a resolution of 1/1000 femtometer. It will allow us to look closer into the structure of matter than ever before. With this, we hope to finally find the Higgs-particle that is our current explanation how particles get mass. But we also have the possibility to find evidence for supersymmetric partners of the standard model particles, and who knows - maybe quarks turn out to be not elementary particles after all?
Besides the proton-proton collisions, the LHC will also run collisions of heavy ions similar to the ones at RHIC, but with higher energy. Though the single particle's collisons have less energy that in the proton-proton collisions at LHC, using larger clusters of colliding particles with the heavy nuclei one can create blobs of matter with extremely high density and temperature. In such a way, LHC is able to re-create conditions that have not existed since the beginning of the universe. The above mentioned Discover article quotes Bill Zajc from the PHENIX experiment at RHIC:
- 'One question that screams out to be answered is whether we'll see the same sort of perfect fluid that we see at RHIC'.
For more info about the LHC's heavy ion program, see e.g. the websites of the ALICE experiment.
If you're not yet totally fascinated by the LHCs prospects you're probably German, so you can have a look at this nice video about the LHC (thanks to Andi). Among other things it shows how the detective's work looks like - and what's essential for it :
However, the protons that are collided at the LHC are themselves made out of three (valence) quarks that are bound together with gluons, also called the 'partons' of the system. So, the detective needs to know something about the distribution of the proton's constituents that is called as the 'parton distribution function'. This complicates matters and increases uncertainties. In addition, this also means that the total energy of the accelerated beams doesn't fully go into the elementary parton collisions, but the energy is actually distributed over these partons. And the energy of the single collisions of these constituents is consequently less.
The easiest way to get rid of this annoyance is to use elementary particles and examine their collisions. The planned International Linear Collider e.g. would collide electrons with anti-electrons. Clifford at asymptotia explains brilliantly why this matters, and JoAnne at CV tells you how to design the next big thing.
However. Having told you why this is fascinating and exciting stuff, I'd also like to bring up an issue that is usually not discussed in design reports, and which I was recently reminded of through this article 'Wer soll das bezahlen' - Who's supposed to pay for that? (again in German, unfortunately)
- "The only things physicists always have are problems. At least when they try to understand the world. They don't get the most obvious stuff: Why do things have weight? Are there really only three dimensions?"
(if you can, I encourage you to read the comments) whichs bring up the question whether it's justified to spend such an amount of money, while there are still people starving elsewhere in the world.
It is of course a tough question, one that I ask myself repeatedly, being aware of my privileged position in Somewhere, North America. Wouldn't all that money be better used otherwise (like, you could give it to me ;-)). One can ask that about every possible investment a country makes, and to begin with I am perfectly sure there are better places to doubt the wisdom of these decisions. However, taking money and - in a mood of generosity - just giving it to those in need, whether in your own or other countries, sounds like a good idea, but isn't going to help on the long run. The reason is simply that we still can't eat money. Investments are only sensible if they permanently affect the infrastructure. It's not as easy as just scraping some billions here and giving them to the homeless. Whether or not we like the current government, the very purpose of politics is to optimize the use of tax money.
This might sound obvious, but I think it's necessary to point out every now and then, so this is now. Yes, experiments in high energy physics are a luxury of our societies, and we are very lucky that we can afford them today. The world is not a system in equilibrium. It has never been. I doubt it will ever be, but we might be able to get closer than we are now. Working towards equilibrium however isn't done by scraping money here and giving it to somebody else over there. It requires, well, a thoroughly investigated plan as to whether the investment is sensible, and not just a feeling of guilt.
No, building large particle colliders isn't necessary for the survival of our species, but it is the way to answer questions that men have asked since thousands of years. There will always be parts of this world ahead of others. But to close with a quotation by Isaac Asimov:
'There is a single light of science, and to brighten it anywhere is to brighten it everywhere.'
Footnote 1: Before the Common Era, Christ! - There goes another 'E'.
Footnote 2: I can really recommend the article, it also tidies up with the myth of the man-made black hole that swallows Long Island.
TAGS: PHYSICS, LHC, SCIENCE, PARTICLE PHYSICS