UCL Lecture: Cosmic Rays, Neutrinos & Micro-Black Holes

Yesterday (16th October) I attended another of the weekly UCL science lectures. This week it was given by Dr Ryan Nichol from the Department of Physics and Astronomy at UCL. It was basically about practical particle physics, discussing experiments such as the LHC and the ANITA experiment (the one with hot air balloons flying over Antarctica). So here are my relatively brief impressions…

The first thing I noticed before the lecture started was the enormous turnout. In fact the lecture was moved from the usual Massey Lecture Theatre to the vastly larger Chemistry auditorium, and even so I ended up sitting on the steps. The audience was largely populated with schools, perhaps explaining the turnout. I also noticed the MIT-style blackboards adorning the front of the lecture theatre.

The talk started with a primer on high-energy physics. This was a pretty standard intro, putting familiar things on a log scale of size, quickly delving into some elementary particle and the ball-exchange explanation of repulsion and attraction in Quantum Field Theory (two people on boats throwing balls to each other), an explanation Feynman would not approve of considering his famous answer to the question ‘what makes two magnets attract’! QFT was elaborated upon slightly in terms of a brief description of the four gauge bosons.

He then moved onto cosmic rays. Apparently it was discovered there were more high-energy particles being detected than there ‘should’ have been. In ~1910 Theodor Wulf attempted to detect a difference in muon detection between the top and bottom of the Eiffel Tower, but his instruments weren’t sensitive enough to record any difference. In 1912 Victor Hess did the same experiment, but using hot air balloons instead. As it turned out, as the balloon’s height increased, the radiation detected decreased owing to a decrease in detection of earth-produced radiation due to an increased amount of atmosphere between the balloon and the surface. But after a certain point, the detection rate actually increased, leading the scientists to conclude this excess radiation actually came from space. He also talked about the Pierre Auger Observatory in Argentina. This is a set-up spanning an enormous area consisting of a grid of particle detectors. If a shower occurs, several of these detectors will be hit at the same time. There are also four telescope stations which can, on a clear moonless night, actually ‘see’ the fluorescence caused by the shower particles hitting air molecules (Cherenkov radiation?) and produce a pretty picture and model 3-D of the event. Dr Nichol was also at one point involved in ‘CREAM TEA’ which stands for ‘Cosmic Ray Extensive Area Mapping for Terrorism Evasion Application’. Supposedly this uses naturally occurring muons to map an area – detectors can build up a good picture of dense objects (like bombs) within an area by measuring scattered muons.

He then moved on to talking about neutrinos, invented in 1930 by Pauli to conserve energy. I knew they didn’t interact with much, but apparently the average neutrino travels through 53 light years of water before interacting! Luckily the sun produces 100 Bn neutrinos per cm2 per second, and the few milligrams of 40K in humans emits about 300 M neutrinos per second. I was pretty surprised by the sheer number of neutrinos being constantly produced – beta decay must be a big thing… He then talked about some neutrino experiments. He mentioned the MINOS experiment in which Fermilab fires a massive stream of neutrinos at Soudan (MN) in an attempt to detect a few. He also talked about the Super-Nemo experiment (interesting name…) which was created to discover whether neutrinos are their own antiparticles through the inspection of radioactive decays. Apparently theory has it that neutrinos can turn matter into antimatter (I’m guessing something to do with beta+ decay), so the discovery that neutrinos are their own antiparticles might help explain why the universe has more matter than antimatter.

The talk moved onto the less well-known field of micro-black holes. He didn’t actually say much about this except that most black holes are several hundred/thousand/million solar masses, but that it’s possible to create tiny ones of only several milligrams. Concerning the media scare about the LHC creating a black hole and eating the earth, he explained that only some exotic models allow collisions in the LHC to produce micro-black holes (which can be subsequently studied by measuring the Hawking radiation produced as they [very rapidly] evaporate), and only the most exotic models allow one such black hole to grow large enough to engulf the Earth. Thinking about it, 14 TeV (now reduced to something like 7 or 8 TeV) collisions are unlikely to do much harm considering the Earth is constantly being slammed into by 300 TeV particles from über-novae and other such awesome events. This actually reminded me of an interesting article I read in Scientific American (my preferred science magazine) which discussed formulating QFT on the hyperbolic geometry of general relativity (semiclassical gravity). Apparently as a star begins to collapse into a black hole, a force gets created which opposes this collapse which grows to infinity when the star tends from above to a certain radius. So according to this article, while singularities are theoretically possible, there is no way of going from a non-singularity to a singularity (a bit like travelling faster than light). So Black Holes are actually Black Stars. All very interesting stuff, though I just don’t know enough about the maths to really appreciate it myself. My only experiences of hyperbolic geometry are from a C4 programme about 10 years ago, and from reading GEB by Hofstadter. Not to mention QFT…

Finally, he talked about ANITA, a project he had recently worked on. I think someone came to Halley Soc recently to talk about ANITA, so this sounded pretty familiar. The idea was to fly around Antarctica in hot air balloons in the hope of detecting a Cherenkov cone of RF radiation produced when a neutrino decays in the ice. Apparently the battery box of ANITA had to be painted half black and half white to prevent overheating / freezing (since fans wouldn’t be very effective at low pressures at high altitudes), so they did an art contest for the best 1/2 black and 1/2 white design! Later they analysed their results and discovered no neutrinos, but did find they had detected 6 ultra-high energy cosmic ray air showers – the radiation was so intense that it actually bounced off the ice and was picked up by their detectors.

The Q&A session afterwards didn’t seem as lively as normal, possibly due to the sheer size of the audience, but overall it was an interesting talk and I learnt about several cool experiments I’d never heard of before. I’m always quite interested in how there’s always lots of good theory in practical experiments (cf. my Imperial work experience – week 1, week 2), so it’s all good.

Afterwards we (the three of us from SPS who went) managed to spend 3 solid hours discussing physics and maths in the regular post-UCL-lecture pub ranging from things from the lecture, CERN and QM to Feynman, Galois, Turing and Boltzmann to Primer (my new favourite film), a suitable conclusion to the evening!

EDIT: Urgh that wasn’t brief at all. I should really stop doing this whole mega-post thing…

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