The Large Hadron Collider
A Victorian joke proposed that a suitable subject for a 20 minute talk to a group of church members was, "The Past, Present, and Future of God, Man, and the Universe." Professor Paul Halpern's talk to PhACT on October 17 was almost as ambitious.
Dr. Halpern covered the 100 years of physics research that led to the Large Hadron Collider, showed what might be discovered and explained what exploring the tiniest of particles tells us about the beginning of the universe. The Large Hadron Collider (LHC) is the particle accelerator now installed (but not yet working) in a 17-mile circular tunnel under the French countryside between Geneva and the Jura mountains. It is the latest and greatest of the particle accelerators designed and built and operated since 1959 by the European Center for Nuclear Research (CERN). Installed in an existing tunnel and using existing machines to do part of the acceleration work, the LHC will achieve more cheaply many of the aims of the Superconducting Supercollider whose construction in Texas was abandoned in 1993. Dr. Halpern started with a summary of the present status of the LHC.
During integration tests in 2008 a faulty superconducting magnet heated up, causing an explosive vaporization of its liquid helium coolant. The damage has been repaired and tests are due to restart soon. However, in the near term the accelerator will run at half power. This still allows much useful physics to be done.
The LHC is the latest tool in a century of investigations into the ultimate constituents of matter. In 1911 Ernest Rutherford reported that when he bombarded gold foil with alpha particles from a radioactive source, some particles bounced straight back. This indicated that atoms comprised positively charged nuclei with distant, negative electrons, that is, tiny dense lumps surrounded mostly by empty space. To explore further required the bombarding particles to have a controllable energy and to be, for example, electrons or protons, rather than the naturally available but fixed-energy alpha particles. By 1932, particle accelerators using high voltages were in the forefront of research into the structure of the nucleus.
Early accelerators were linear but it soon became clear that if particles were forced into a circular path by a magnetic field they could be raised to a higher energy by a series of low-voltage kicks rather than one high-voltage one. At low energies the particle's velocity increases with its energy and the construction of a circular accelerator must take this into account. At higher energies, particles travel at close to the speed of light: as Einstein predicted, any further input of energy increases their mass, not their velocity. This simplifies the accelerator design. Until the 1970's the particles circulating in the accelerating ring were diverted into fixed targets.
Collisions with the nuclei of the target produced a shower of new particles which travelled in the same direction as the input beam and were analyzed by a series of detectors. Although nuclei consist only of protons and neutrons (and the virtual particles that stick them together), each collision produced a zoo of new particles whose properties had to be determined. Any particle whose creation is allowed by the conservation laws may appear; the more energy one puts into the accelerator the more massive the new particles can be.
There are practical limits to how much energy an accelerator can provide but there's a way to get more bang for the buck. When a moving particle hits a stationary one, most of the input energy goes into knocking-on the target nucleus rather than into creating new particles. If two particles having equal energy hit head-on, all the input energy (twice that of each particle alone) goes into making new and interesting stuff. That's why accelerators built in the past 25 years have counter-rotating beams that are allowed to collide at certain points around the ring.
Halpern summarized the four forces of nature and the families of new particles that have been discovered. As particle energies increase, three of the forces merge. Gravity, the weakest force, remains the odd one out. Physicist Peter Higgs suggested that the mass of sub-nuclear particles is due to a pervasive field which became known as the Higgs Field. This field implies the existence of a fundamental particle, the Higgs boson which, in theory, is just too massive to be created by the current generation of accelerators; hence the itch to build more powerful ones. Halpern mentioned that the Higgs was apparently dubbed "The God Particle" by the publisher of Leon Lederman's 1993 book of that name. Lederman made a case, sadly unavailing, for continued funding of the Supercollider, the main rationale for which was that America should be first to discover the Higgs boson. CERN's LHC, although less energetic by about a factor of three, should also be able to do this job. Even if the Higgs is not found at CERN, this fact adds a point to the data on which theorists base their speculations about new families of particles. Of course the LHC will do a lot of other useful physics too.
Even though the particles being studied are very tiny, high-energy experiments have given us the data on which our models of the structure of the universe are based. The nuclear matter in collisions achieves densities and temperatures typical of the Big Bang itself. As Halpern pointed out, 96% of the mass of the universe consists of so-called dark energy and dark matter which are yet to be detected. We still have a lot to learn.
Dr. Halpern has visited the tunnel in which the LHC was assembled. He was immensely impressed by the huge detector assemblies that have been built into underground caverns at four points around the 17 mile ring. The LHC is by far the biggest, most complex science experiment ever built and uses the largest assembly of raw computer power ever linked together to process the billions of measurements arising from each collision.
A questioner raised the issue that we skeptics have heard most about, the groups who are suing CERN to prevent the LHC from being started up on the grounds that it will create miniature black holes that will swallow the Earth. Halpern pointed out that though the energy density at the moment of a collision rivals that at an early state of the Big Bang, the total mass and energy involved in one collision is microscopic. The only black holes known to exist are more massive than the Sun. If tiny black holes could be made they would vanish in a tiny fraction of a second.
Another question was about security at CERN. As an intergovernmental research organization, everything done there is published freely. There are even public tours. There is security only to protect people and to avoid expensive equipment being damaged. And there was the inevitable question: What practical good is it all? Halpern mentioned the recent Nobel Prizes for devices such as digital cameras that grew from discoveries made at accelerator labs. He gave, as the ultimate answer, that we can't tell what might be found.