Chris Lintott’s Universe

In the fourth and final part of the guestblog from Alice Sheppard, we conclude our trip to CERN with a pictorial look round the ATLAS detector. If you’d rather start at the beginning, then part one is here, part two here and part three here. I’d like to thank Alice for assuaging somewhat my annoyance that I’ve never made it to CERN, apologise again for taking so long to put these up, and to invite anyone else who might fancy guestblogging to email me.

CERN – reception floor! “Cosmic Song”, 1986, Serge Moro. Apparently a detector recording cosmic rays, with a programme modulating light information and timing cycle, with 9 lighting circuits arranged under metal floor panels.

As you can probably tell, the walkway was halfway up the wall, which prevented any of us from taking a panoramic photo. Hence, it is impossible to reproduce what ATLAS - the detector - really looked like, which was a bit annoying. The “clock-face” thing was, the guide said, 40m high, though it looked an awful lot bigger.

I’m afraid the technical details of how the detector worked would probably have gone straight over my head even if the roar of machinery hadn’t been so loud; all I can tell you is that there is a lot of aluminium involved, and that they are especially concentrating on capturing muons and to do so are making lots of layers “like an onion”. In the shed-like area was a scary-looking instrument I would have ascribed to the torture trade: a sort of rectangular aluminium rod full of bits of wire, which apparently helps with detection . . . There were hundreds of feet below and above us in that room, and people hurrying back and forth everywhere and doing the most amazing gymnastics on and between machines!

We had finished by lunch time, though there were several hours of fun left in the Microcosm museum. There was something especially uplifting about the message of openness and international concerted effort. There was much emphasis on how CERN was founded by several nations who had recently been fighting each other in World War II, and how Palestinian and Israeli scientists now happily work together. It was also evident that students can come here and find the doors are open to them.

The only annoying thing about the visit was how very little I felt I’d learnt and seen in comparison to how much there actually was there. All the notes I didn’t take, all the times my mind had wandered while the guide might just have been saying something especially interesting! So I’ll end with one of my favourite quotes, by someone called Richard Jeffries:

“In the heart of most of us there is always a desire for something beyond experience. Hardly any of us but have thought, some day I will go on a long voyage, but the years go by and still we have not sailed.”

For more information, see the CERN website.

This is part 3 of Alice Sheppard’s trip to CERN, our first guest blog. Part 1 is here, and Part 2 here.

The main tunnel is 100m underground, and 27km in circumference, with detectors at various intervals like beads on a bracelet. The circular tunnel lies under both France and Switzerland, so, as someone commented, sub-atomic particles must have very fast passport controls. The building work underway at the moment has to be done with precision of micrometres, and has to be flexible in accordance with the effect of the Moon’s gravity on the mountains. The temperature stays fairly constant down there; however, they shut down from November-March when it is coldest – which seemed interesting and contradictory to me, since the infrastructure needs to be kept cold. That just goes to show that natural conditions are never good enough for sophisticated experiments.

The experiment we are currently waiting for is the same principle as an earlier one, when electrons and positrons were accelerated around the tunnel and made to collide close to the speed of light, thus, as we heard, “recreating the conditions of the Big Bang”. A positron is an antielectron. When a particle meets its antiparticle, they annihilate each other, releasing energy. CERN’s proton/antiproton smasher releases so much energy that new particles are created – and discovering what these are is what the detectors are for. The experiments have to be timed to unbelievable precision, given that the newly created particles break up in, for example, a millionth of a second. Even neutrons have a half-life of only about 15 minutes if left on their own – that is, not in a neutron star, where they are held together by gravity, or in an atom, where the strong nuclear force stabilises them.

Anyway, the Large Hadron Collider, the new experiment, works on the same principle, but with protons and antiprotons not electrons and positrons. This is much more ambitious, as protons are composed of three quarks each, and are a thousand times heavier than electrons. One thing struck me and has stayed in my mind: the speed at which they will be accelerated around the circuit – 11,000 times per second. As the tunnel’s circumference is 27,000m, this does indeed come very close indeed to the speed of light – 297,000,000 m s-1 as opposed to 299,000,000.

And it will be double this speed at which they collide . . . presumably their time will have to slow down quite a lot relative to our own to make sure that from their perspective, the other is not travelling faster than the speed of light !

This was also the first time I heard about relativity causing an increase in mass rather than weight. I had always heard about increases in weight when travelling at relativistic speeds, and my very basic physics background had given me the idea that this was not the same as an increase in mass – so hearing about these particles gaining mass made me realise how much I still have to learn. The lecturer also said the temperature would reach several billion Kelvin, much hotter than the Sun, and indeed would be the hottest temperature in the Universe since just after the Big Bang. I wondered how they could contain such heat. Someone I asked shrugged off this question on the grounds that they would be putting in more energy than they would be getting out, but I thought that the same could be said of a radiator, or indeed any energy transfer . . . I don’t know how much matter they’ll be sending around; it’s not as if it’s a massive power station doing this to millions of tonnes of protons and anti-protons, of course. Thus musing, we then headed towards two little buses for the tour to Atlas, one of the detectors around the tunnel.

Continuing our first set of guest blogs at Chris Lintott’s Universe, this is the second
part of Alice’s trip around CERN. The first part is here.

The talk began with a description of the international co-operation involved, the 20 member states, and the Big Question they are investigating: “How Nature Really Works”. The lecturer described their branch of science as “Astro-Particle Physics”, or the origin of matter. The list of topics he put up at the beginning sounded so interesting that I just have to put up a list: particle physics, neutrino oscillation, lead nuclei collisions, anti-hydrogen radiation and spectroscopy, radioactive isotopes and neutron beams, accelerators and detectors.

The lecturer went on to focus on one of these: the accelerators and decectors. He claimed that the antihydrogen produced is the first that exists in the Universe since the “grand shoot-out” between matter and antimatter that occurred in the first minutes after the Big Bang, and that it is not yet confirmed whether it will show the same spectroscopic lines as hydrogen. This made us all sit up and frown because we’d had a lecture claiming that antimatter is exactly the same as matter, just with the charges reversed – that a galaxy we see through our telescopes could be antimatter, that the person sitting next to us could be antimatter; the only way we could tell is if we touched these things. Unfortunately all the antihydrogen so far created has far too much kinetic energy for it to be useful, and research is currently underway to see how to cool it. (We all wanted to know how it can be contained before it meets matter and they annihilate each other! I am still wondering. I also wonder if aliens have ever created antimatter or recreated the Big Bang, too, in a galaxy far, far away . . .)

We then focussed on one major experiment to come in 2008, the Large Hadron Collider. Particle accelerators such as the LHC were compared to electron microscopes: both can “see” smaller things than visible light. The particles they accelerate gain kinetic energy up until a point at which the kinetic energy can be interpreted as an increase in mass (E = mc2). Electrons accelerated by CERN, for example, have reached 20,000 times the mass of an ordinary electron. They then hit He(l) nuclei. This generates new particles – so new matter is created out of kinetic energy.

Energy and momentum have to be conserved. If very big particles are created, they may just “flip away” little ones, so it is necessary to make the heavy ones collide with each other. One method of detection is Cherenkov radiation – distorted atoms left behind a trail, like a visual sonic boom. In the LEP detector, the only thing that is not absorbed is muons. They can see them falling down like rain. The tour guide, later, talked a lot about muons . . .

In a previous CERN experiment, when an electron and a positron, they created a new pair of quarks. The lecturer spoke about quarks changing ‘flavour’ from up to down, for example in the creation of C14 in the Earth’s upper atmosphere, and the weak nuclear force and how it allows changes of particle (quark) identity, and the sizes of neutrinos and generations of matter which literature can describe better than I can. This was given as an explanation for matter rather than antimatter surviving in the Universe. (I’ve heard quite a few of those, most of which were stated to be the established truth! The most intriguing to date claimed that Dirac’s equation suggests that, for antimatter, time runs backwards*, and that antimatter decays faster than matter, hence the one in a billion matter particles left over after all the annihilations . . .)

* Someone told me two weeks ago that that’s tachyons, not antimatter.

Coming up over the weekend : The experiment itself

Alice Sheppard will be well known to any of you who’ve been to the Galaxy Zoo forum where she does a sterling job as moderator. Not content with putting her to work there, when I heard she was going to particle physics lab CERN in Geneva along with her cohort of trainee teachers, I insisted that she write up her experiences for this site. I’m afraid this is long delayed - and that’s my fault, for which I apologise - but here’s the first installment of her trip diary. The rest will follow daily.

It was one end result of a stimulating and memorable six months for twenty-six future Chemistry teachers – an Enhancement Course at Sussex University for those whose degrees were a long time ago, or did not contain quite enough chemistry, to immediately qualify as a teacher of that subject. “I’ll bet you’ve all been told that matter is just protons, neutrons and electrons,” said Tim, our course leader. Well, science is never that simple, is it? Many of us now know about neutrinos, streaming out in their billions from the Sun and other stars. What few of us knew was quite how many fundamental particles can exist (if only for a few millionths of a second at a time), and just how much has been discovered 100m underground around the mountains of France and Switzerland.

Anyone can go to CERN. It’s a few minutes on the no. 9 bus from Geneva, and it’s easy to find your way around. You can visit the reception, shop and Microcosm (their museum); or groups can book a lecture and tour. We were able to do so, and in English – they are prepared for a range of nationalities. They even have a restaurant, but mind out for the small pieces of octopus in the salads.

On Monday 30th July, amid a bit of a shoe crisis (somebody’s shoes had broken and her feet were very painful; a few more were later momentarily floored to discover that open shoes are not allowed – until we discovered CERN has shoes visitors can borrow), we made our way into Room 33, the reception area. We had been preceded by a flurry of excited questions along the lines of, “You mean it’s true, that Dan Brown book?” and jokes such as “Have a smashing time.” At nine o’clock we had a superb lecture and video – though I recommend a little reading up on particle physics first! The few posts that follow are a mixture of the lecture notes I took and my additions, speculations and random ramblings.