Last night, I headed down to the beautiful Bell House in the Gowanus section of Brooklyn for the latest Secret Science Club lecture, featuring Dr Ray Jayawardhana of the University of Toronto's astrophysics department. While Dr Jayawardhana specializes in the search for exoplanets, last night's lecture was on the topic of neutrinos, a topic he wrote about in his bookNeutrino Hunters. Before I start the recap, I have to get my little joke out of the way- neutrinos are a lot like Switzerland, small and neutral.
Dr Jayawardhana began his lecture, with a quip about the Higgs boson hogging the limelight (insert joke about Hoggs boson). He joked that the Higgs had done well for itself, even being dubbed (by non-physicists) the "God particle" in a good PR move. He then asked us to give neutrinos a chance. Dr Jayawardhana likened the "measly but magical" neutrinos to a "cosmic chameleon", indicating that they come in three flavors.
This introduction was followed by a history of the discovery of neutrinos, beginning with Henri Becquerel's discovery of radioactivity emitting from uranium. Becquerel's discovery of "radioactivity" was followed by by the work of Marie and Pierre Curie, who realized that radioactivity was not limited to uranium, but was a more common phenomenon. The first named types of radioactive "decay" were alpha decay, beta decay, and gamma radiation. Alpha decay involves the emission of two protons and two neutrons, a particle identical to the nucleus of a helium atom. Gamma radiation involves the emission of electromagnetic rays. In Beta decay, a neutron loses an electron and becomes a proton, losing energy in the process. In physics, what goes in must go out, and beta decay seemed to violate the Law of Conservation of Energy. Faced with this conundrum Neils Bohr considered giving up the Law of Conservation of Energy. Austrian physicist Wolfgang Pauli proposed (nine days after his divorce) a solution to the beta decay problem in a letter titled "Dear Radioactive Ladies and Gentlemen". His proposal was that there existed an undiscovered neutral particle (at the time, only protons and electrons were known)- jumping from two to three subatomic particles was a radical step. Pauli recognized the enormity of his proposal, writing, "I have done a terrible thing, I have proposed a particle that cannot be detected." He even bet a case of champagne that the particle would not be detected. The neutron was subsequently discovered by James Chadwick in 1932.
Italian physicist Enrico Fermi proposed the term "neutrino", a diminutive of neutron, to explain beta decay- a neutral particle with almost no mass (the neutron has mass) was needed to square beta decay with the Law of Conservation of Energy. While Fermi had a sound theoretical basis for the neutrino hypothesis, he had no observation of neutrinos. It wasn't until the fifties (World War 2 sidetracked the work of a lot of physicists) that neutrinos were detected by Frederick Reines and Clyde L. Cowan, Jr. Reines and Cowan realized that a nuclear explosion would release neutrinos, originally they wanted to detonate an atomic bomb on a tower to release neutrinos, then they decided that a bomb in a hole would be a better neutrino source. They eventually concluded that a nuclear reactor would serve the same purpose.
Raymond Davis, Jr proposed using underground detectors to find neutrinos emitted by the sun- detectors on the surface of the planet were stymied by too much "noise" from other sources, and the ground would act as a filter to reduce this background interference. Neutrinos rarely interact with other matter. With collaborator John Bahcall, Davis conducted the Homestake experiment. It was theorized that neutrinos interact with chlorine to produce argon, so a quantity of chlorine (basically, cleaning fluid) could be placed underground and the amount of argon could be measured and the number of neutrinos interacting with the chlorine could be extrapolated. For thirty years, neutrinos were "counted" one by one, and it was discovered that only a third of the expected neutrinos were detected.
Another major neutrino detector was built in Kamioka, Japan in order to solve the solar neutrino deficit problem, which is due to neutrino oscillation. Another major neutrino detector is located in a mine in Sudbury, Ontario, it is billed as the world's deepest physics lab.
Neutrinos come in three flavors (electron, muon and tau) which can change as a neutrino moves from the sun to the earth. Initially, only one flavor of neutrino was detected and the morphing of neutrinos, a hard to explain quantum level occurrence, was shocking to observers. Neutrino oscillation was first proposed by Italian physicist Bruno Pontecorvo. Neutrino oscillation depends on the medium through which the neutrinos pass.
Neutrinos are elusive, they are able to leave the "scene of the action" without interacting much with other matter. Dr Jayawardhana joked, "don't just stand there, let the neutrinos through". Trillions of neutrinos pass through our bodies harmlessly over the course of a lifetime. In 1987, with the discovery of Supernova 1987A, neutrinos from outside the solar system were detected. Because neutrinos rarely interact with other matter, they arrived three to four hours before the visible light from the supernova, they left the core of the dying star with no resistance. About two dozen neutrinos were detected from this source.
Dr Jayawardhana then went on to introduce the audience to the Ice Cube Neutrino Observatory, a one square kilometer lab in the Antarctic with eighty-six strings of detectors, with a total number of 5,160 optical sensors. The Ice Cube opened two years ago, and about two weeks ago the first results came in- twenty-eight neutrinos, the most energetic ones ever detected, were found. These neutrinos likely came from outside the solar system, emitted by supermassive black holes or massive stars that produce gamma ray bursts. This array of neutrino detectors has opened a new window on the universe.
A new experiment, dubbed KATRIN, to determine the mass of neutrinos is on tap in Germany. The transportation of the huge spectrometer central to this experiment is an epic in and of itself- this piece of equipment, constructed about 400 kilometers from its destination, was too big for local roads and canals, and had to undertake an oceanic journey of almost 9,000 kilometers.
Physicists are now trying to determine if neutrinos and anti-neutrinos interact with matter in similar ways- did neutrinos play a role in the ascendance of matter over anti-matter?
The talk concluded with the practical use of neutrinos- perhaps neutrino detection can play a role in discovering covert nuclear testing. Because neutrino oscillation is connected to the medium through which neutrinos pass, neutrinos could possibly be used to locate mineral or oil deposits. Some dreamers have even proposed using neutrinos in an attempt to communicate with extraterrestrial life (though I would be remiss if I didn't point out the pitfalls of such a scheme).
In the Q&A session, some bastard in the audience asked about the location of the Ice Cube lab- why Antarctica? Did this have something to due with the configuration of the earth's magnetic field? Dr Jayawardhana indicated that ice can sometimes capture neutrinos, and that the depth of the ice and the resultant pressure removes air bubbles which can affect detection. Bubble free ice is needed for best results.
Once again, the Secret Science Club dished up a fantastic lecture, and Dr Jayawardhana is a very engaging, charismatic fellow. I was able to talk with him briefly after the lecture and he is a nice, nice guy. Hopefully, he will return to Brooklyn to talk about the search for extrasolar planets, which is his main field of inquiry.