Guestblog : Alice down the rabbit hole
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.
