1
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format. New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address. THE LAST WORD Biggest and best In simple terms, why does the Large Hadron Collider have to be so physically large, when it is designed to detect particles that are incredibly tiny? n The Large Hadron Collider is a synchrotron, a circular accelerator that uses carefully synchronised electromagnetic fields to accelerate particles to very high speeds. When this involves charged particles on a curved path they release synchrotron radiation, which wastes energy. This is not desirable because most of the particles physicists are looking for, such as the Higgs boson, have large masses and can only be created in high-energy collisions. The large radius of the LHC’s track is big enough to limit the radial acceleration given to the particles, thus minimising the energy that they lose as synchrotron radiation. The superconducting magnets used to control the flow and direction of the particles can accelerate them up to 99.9999991 per cent of the speed of light. Doris Lee Burnaby, British Columbia, Canada n The short answer to your question is “ the magnets”. The size of the LHC is actually a trade-off between three things: the magnets that are available; the energy – or velocity – it is necessary to give the particles; and the feasible dimensions of the structure. The faster the particles are moving, the more likely you are to see something interesting happen in a collision. So it’s important to accelerate the protons as much as possible. The protons need to follow a circular path so they can be continuously accelerated by an electric field, and this is done using magnets positioned around the tunnel. The faster the protons travel, the stronger the magnetic fields need to be to keep them on track. To increase energy there are two possible choices: make the magnets stronger or the ring larger, so that the particles’ path does not need to be bent so much. At some point there is either a technological or financial limit on the strength of the magnet, leaving ring size as the only remaining variable. However, to keep the costs of the project manageable, the LHC was built in an existing tunnel that housed a previous experiment, called the Large Electron-Positron Collider. So the energy to which protons can be accelerated was actually predetermined by limits of technology and funding. When people say the next generation of accelerators needs to be the size of the solar system, they should qualify this by admitting that they are assuming existing magnet technology cannot be improved upon. Michael Luk By email, no address supplied n It is not just the LHC’s circumference that is huge; so are the detectors. It’s here that the collisions take place and physicists look for new particles. A detector is packed with all kinds of instruments to measure mass, energy, temperature and so on, and there are additional trackers and triggers right around the beam. In part these explain the detectors’ size. But there is something else. We know that a theoretical particle can be recognised by the way it decays into particles we already know exist. For instance, in the ATLAS detector, a huge cylindrical device, the Higgs boson will theoretically decay into muons, which are massive relatives of the electron. We can see muons by tracing their paths. But they are very elusive particles and will fly through any substance with enormous speed. So the muon detectors extend from a radius of 4.25 metres from the axis of the proton beam out to 11 metres. If the detectors were closer to the beam or narrower than this, the muons would fly past before they could be identified. So the size of the LHC has much to do with what we can physically measure with the means available to catch the unseen wonders of nature. A friend of mine helped build the hardware for the muon detector in ATLAS, and he and his colleagues kindly took me on a tour of the ring and the detectors when the LHC was almost finished. It was the best trip I have ever been on. Marieke Nelissen Amsterdam, the Netherlands This week’s questions ICE SPY When I put ice into my whisky I see lovely swirling patterns in the liquid. These are obviously to do with temperature and density differences between the melting ice and spirit. But what mechanism allows me to see these differences? What optical effects are at play? Brian Higgs Blackpool, Lancashire, UK ALCO-HOLIDAY If I drink alcohol regularly I build up a tolerance to it. But if I have a decent period of abstinence and then start to drink again, I immediately notice the effect of the alcohol. So where does my tolerance go? Julia Simpson Oxford, UK “To keep the costs manageable, the Large Hadron Collider was built in an existing tunnel” Last words past and present, plus questions, at last-word.com The new book out now: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphin Will we ever speak dolphin? “It is not just the Large Hadron Collider’s circumference that is huge, so are the detectors”
