Crossposted at Politicook.net
With all of the current attention towards and controversy surrounding the Large Hadron Collider (LHC) at CERN, it occurred to me that most people really do not understand for what they are used, or how they work. This installment of Pique the Geek will cover this topic.
What if I were to tell you that almost everyone in the developed world uses, or used to use a particle accelerator on a daily basis? It is true, and it is the old-fashioned cathode ray tube (CRT) display, used universally on computers and televisions until the advent of the new flat panels that work on one or more different principles.
All a particle accelerator consists of is a set of circuits and magnets (sometimes) designed to make a charged particle increase in velocity with control of direction. The CRT takes electrons boiled off of a hot filament because of a current running through it and, with a series of electric and magnetic fields, directs the electrons to a target, in this case a luminous screen. When an electron impacts a screen element, it lights up for an instant, converting the energy of the accelerated electron into light.
Here is a picture of a CRT display. The electron gun in on the left (electron beams are shown in false color), the acceleration and focusing elements in the middle, and the target at the right.
One reason that CRT displays are dangerous to work on is that the electrons have to be accelerated to a high velocity by a high voltage power source, and the capacitors that form part of that source can retain a lethal amount of charge after being turned off for weeks. So never work on a CRT display unless you really know what you are doing. Sometimes that is not even enough. I friend of mine from high school was a professional television repairer and, in that split second of nonattention, got a lethal charge from a television on which he was working. Sometimes there are no second chances.
There is one catch about accelerating particles: only charged particles can be accelerated since we have not figured out gravity that well yet. Whilst particle accelerators are often called atom smashers, it is not possible to accelerate neutral atoms. However, ions (atoms deficient or in excess of one or more electrons) can be accelerated, as can electrons, positrons, protons, and antiprotons, along with other exotic charged particles. These charged particles can be directed onto targets of neutral atoms, thus "smashing" them.
The first modern particle accelerator was the cylclotron, developed by Ernest Lawrence at Berkeley in 1929. It consists of two metal cans, called dees because of their shape is like the capital letter, a high frequency oscillator, and a magnet with the north and south poles above and below the dees. Like a CRT display, the entire particle path is under high vacuum to prevent energy wasting collisions with air molecules. Here is the drawing for the one that he patented in 1932:
His first one was small enough to hold in your hand, and the dees were actually silvered onto the surface of a boiling flask that he had the glassmaker flatten. Here is how it works:
A charge of ions is introduced into the evacuated cavity, and the oscillator is energized. The oscillator causes each half of the pair of dees to become first positive, then negative. The ions (let us assume that they are positive, having lost an electron) are attracted by the negative dee, then tht dee becomes positive because of the oscillator, and the ions are repelled towards the (now) negative dee. The cycle reverses, and the ion is then repelled from where it is and attracted to the other dee. This process continues and the ions gain energy.
There is a lot more to it, as you can assume. The oscillator frequency has to be matched to the mass of the ions to keep up the "push-pull" forces or unstable revolutions occur, and the whole process crashes to a halt. Looking back at the drawing, you can see that the ions start in the middle, and with every cycle go faster and trace a larger orbit, until finally they come to the edge, where there is an exit port. A target normally is just outside the exit port. Because the ions get faster with each cycle, the larger orbit compensates for the greater speed, so the oscillator operates at a constant frequency for a particular ion.
That is unless you take into consideration relativity. That changes everything, and the most compelling case for the increase in mass with velocity was demonstrated in the cyclotron. When particles are accelerated to a significant fraction to the speed of light, they gain mass (kinetic energy being transformed to mass in accordance with E = mc squared), and the process decays. By adjusting the frequency of the oscillator to match relativistic predictions, very large cyclotrons were designed and successfully operated.
Cyclotrons are no longer research instruments, but are still useful in flinging ions into targets to create unstable nuclei. They look complicated, but the principles are really simple: put an ion into a chamber with an oscillating electric field perpendicular with a magnetic field, and you get this orbit, with increasing energy until relativity takes over, then you have to compensate.
Later, the idea of the linear accelerator was devised. It is more like the CRT display, in that the ions are injected into a straight tube, and a series of capacitors are charged and discharged along the length of the tube, accelerating the particles along the way. No magnet is involved, so a straight line is made (although in big ones the earth's magnetic field has to be taken into consideration). The disadvantage with linear accelerators is that you only get one straight shot through, not the additive effects of millions of passes through a cyclotron. The length of the accelerator tube determines the energies imparted, and the longest one with which I am familiar is two miles long.
You can think of a linear accelerator as a long gun barrel with more gunpowder here and there to accelerate the bullet over and over. That is a crude analogy, but not entirely inaccurate. There are some theoretical advances that may make the new generation ones even better than the LHC, but that is still in the planning stage.
To overcome the limits of the cyclotron, the synchrotron was devised. In a cyclotron the magnetic field has to be essentially homogeneous over the entire dee array, and making large, stable magnets is not easy. In a synchrotron, the the orbits of the ions are confined to one radius, so the magnetic field has to be controlled only over a relatively small space in comparison. There are several advantages to this approach. By using superconducting electromagnets, exquisite magnetic field control is possible at extremely high field strengths, and since the volume elements in the acceleration radius is constant, it is simpler to design and build oscillators to compensate for relativistic mass increase.
There are some problems, in that particles in a narrow orbit interact with each other and the magnetic field and emit synchrotron radiation, which degrades the efficiency of energy transfer into them. However, that can be overcome and a well designed synchrotron of a given size is always more efficient in pumping energy into ions than a cyclotron. To tap the accelerated particles, an electromagnet or other focusing element is activated at the proper time to divert them towards the target. The initially undesired synchrotron radiation effect can be maximized, and this is now exploited for producing high energy X-rays for several applications, so it just goes to show that lemon aide is indeed sometimes a byproduct of lemons.
So these are the basics. What are the uses? Good question.
Few are still for research into fundamental physics, if the CRT display is included, and there are other uses are made. One is to use synchrotron radiation for three dimensional imaging in a non invasive manner for inanimate objects, like insects embedded in ancient amber. Another is to use the ion beam for direct irradiation of cancer cells in ill patients.
But the research is the most important so far, at least to me. In the past, new elements and new isotopes of known elements were produced by particle accelerators bombarding targets with ions. Fun things can be done with high energies.
Now to the LHC, the Large Hadron Collider. This essentially a synchrotron with an attitude. Its purpose at present is to collide two beams of protons (hydrogen ions) that move in opposite directions at energies never before attained. Actually, it is essentially two synchrotrons, with beams of protons moving in opposite directions. Each unit will accelerate protons to as near the speed of light as our technology currently will allow, then direct the two beams towards each other. The diameter of the chambers is 17 miles, a far cry from Lawrence's hand held initial cyclotron. Future plans call for accelerating heavier ions as well.
This begs the thought: just what is a hadron, and what is large? A hadron (please be careful with your typing for this word, or you will get something completely out of place with a simple transcription error) is simply a bound system of quarks. The most familiar hadrons are the proton and the neutron. The proton is a system of two up quarks (each with an electric charge of +2/3), and one down quark, with an electric charge of -1/3. A neutron is a system of two down quark and one up quark. Thu, the proton carries a net charge of +1, whilst the neutron has no net charge. There are others, but these are most familiar. The large part is from the fact that the diameter of the chamber is 17 miles or so, making is the largest scientific instrument currently existing. For you purists, this is the same chamber used at CERN several years ago for the electron / positron collider, now decommissioned.
The resulting collisions will be at energies never before created by humankind. Will it be a recreation of the Big Bang? No. Even the most optimistic predictions indicate that it might model conditions sometime after event zero. However, it will produce phenomena never before replicated in a measurable location and time on earth.
Will it produce black holes? What is a black hole? Popular science says that it is a collection of matter so dense that its gravitational well is great enough not even to allow light to escape, thus pulling everything around it into oblivion, and allowing nothing to escape. But that is not correct.
Black holes are indeed superdense collections of matter that tend to consume any matter and energy that crosses over the event horizon, the point of no return. What is not commonly understood is that they decay by the so called Hawking radiation, and so are not as stable as we have been led to believe. Now to some semiquantitative math: the largest black hole possible to created in the LHC would be tiny, only a few protons squeezed into a singularity. Of course, that singularity would immediately begin to intake mass and energy, so goes the conventional, popular concept. But would that actually happen?
But because it is so low in mass, the Hawking decay would very quickly destabilize it, and it would disintegrate within picoseconds into fundamental particles, because it does not have enough gravity to be a "proper" black hole. Black holes work because of gravity, and the less gravity, the less stability. Such tiny particles, even though very dense, would evaporate in much less than the wink of an eye. There is no reason to have any concern except for the Luddites wanting to halt these experiments, and I worry more about Luddites than tiny black holes.
Never underestimate Hawking. He is a visionary, and has an extreme amount of insight. Likewise, never be uncertain about Heisenberg. (Tips to those who catch this incredibly subtle play on words).
Finally, there is the question of the Higgs boson, unfortunately sometimes called the "God particle". I really dislike that term, because it is misinterpreted by almost everyone except the community of particle physicists. It was a good joke for them, but the popular literature distorted it.
According to modern physics models, all of space is permeated by what is called the Higgs field, and this field is the cause for mass in particles that possess mass, like us, protons, quarks, and leptons. It does not interact with photons and other massless particles. Modern theory indicates that for an interaction to occur, there must be an intermediary particle. Examples include photons for electromagnetic interactions (shown to be true many times), gluons for nuclear attractive interactions (pretty much accepted), and gravitons for gravitational interactions (well accepted in theory, but as yet not observed).
The upshot is that if the Higgs field be (please note the correct usage of the subjunctive mood of the verb) correct, there must be a similar intermediary particle, the Higgs boson. The reason that it was dubbed the "God particle" is that the Higgs field has to be omnipresent, everywhere, or it would not fit with theory, so it in that sense was like God, everywhere. It was a joke, and way too many people took is seriously. That is all that it means.
Finally, I am very sad to learn of the suicide of the young lady in India last Wednesday who drank pesticide because of fear of the earth being engulfed by a black hole created by the LHC. This is tragic in a plethora of ways, the most important to me being her fear and suffering. But it does point out that there is still a juxtaposition of superstition and science, and religion and reason. This troubles me very much, and I have nothing but my sincere condolences to her family. This was an unnecessary death.
I could philosophize of that for some time, but let it just be said that we scientists are not doing as good of a job as we could to make our case. One point to be made that only one ring of the LHC was activated, so there was not another beam of protons with which to collide. They were just testing the unit for functionality, but that was not communicated to her, or misrepresented to her. We have to a better job to counter unfounded ideas.
This sounds almost trite, but as always, I am dancing naked here, and will not look anything during the comment period unless it is essential for the discussion, and if that is the case I will put a disclaimer on my response to indicate so. I did it for the first time last week, and it seemed to add to the discussion. I changed my sig line to reflect it.
Please pelt me with questions, comments, corrections, and tips and recs.
Warmest regards,
Doc