February 2012 Sky from the Keeble Observatory We promised last month to say more about the Standard Model, which represents our current best understanding of how nature behaves at extremely small length scales, what used to be called the subatomic realm. First, let me bring you up to date on our recurring motif of “the sky is falling!” The Russian Mars probe (Phobos-Grunt, literally Phobos-Ground) which failed to leave Earth orbit reentered and broke up on January 15th. The latest reason given for its failure was damage to its electronic control systems from intense cosmic rays. Which is better than the dark suggestion that American radars brought it down!
The Standard Model is an attempt to organize and explain the structure and behaviors of the myriad subatomic particles. In the 19th century we learned that electrons were a component of the ordinary matter around us. In the early 20th century we first learned that essentially all of the mass of an atom is concentrated in its tiny, positively charged nucleus. (An atom’s diameter is about 10-10 meters; the nucleus is 10,000 times smaller, i.e. 10-14 m.) Then, in the 1920s, Chadwick discovered the neutron, which makes up the nucleus along with the positively charged proton. At one point, we thought that these were the only particles, and were nearly arrogant enough to claim that we had solved the mysteries of the atom. But, neutrons are not intrinsically stable particles. Left to themselves outside a nucleus, they will decay into a proton and an electron. When this happens inside some nuclei, we see the atom change to a different element and say it has undergone beta decay. One puzzle to be solved was that the neutron cannot be “made of” an electron and a proton, because their bound state is a hydrogen atom, not a neutron. Another puzzle is that the energy of the decay products was not always the same, which suggests a third (at least) constituent in the process.
Over time we found more and more “fundamental” particles. Instead of simplifying our picture to one where atoms were made of electrons, protons and neutrons – i.e. only three basic building blocks – high energy physics found itself studying a veritable zoo of particles.
In quantum mechanics, forces are carried by “virtual exchange particles” which exist only long enough to transmit the force, and then vanish before the universe has to find the energy to make real particles. In the 1940s, Yukawa suggested that the strong interaction which binds the nucleus would be carried by a particle of a certain mass. He called it a meson. However, the first particle discovered with the right mass did not have the correct other properties to be Yukawa’s meson. It was dubbed the muon, and when I. I. Rabi learned of it, his response was “Who ordered THAT?” As the particle zoo proliferated, it was obvious that there needed to be some underlying scheme to understand them all.
In 1964, working independently Murray Gell-Mann and George Zweig developed what became the basis for the Standard Model. In their scheme, the known particles were built up from a small set of building blocks, which Gell-Mann called quarks. In its original form, there were three quarks. They have the rather whimsical names of up, down, and strange, but the key is that bay arranging these in twos and threes, Gell-Mann was able to not only explain the known menagerie, but was able to predict as yet undiscovered particles and their properties. When a particle was discovered in 1974 that didn’t fit the model, another quark was added, given the name charm. When the mathematics of working with four quarks didn’t pan out, two more were added – top and bottom. Using the mathematics of group theory it is possible to show that there can only be six quarks or the theory doesn’t work, so the Standard Model is built up from six quarks, their antiparticles, and three leptons and their antiparticles. The good news for you and me is that all normal matter is made from only two quarks (up and down) and one lepton (the electron). For example, a proton is two ups and a down, the neutron is two downs and an up. Atoms are made of electrons, protons, and neutrons.
The table below identifies the components of this model. We’ll have more on this next month.
Lunar phases for February: Full Moon on the 7th, at 4:54 pm; Last Quarter on the 14th, at 12:04 pm; New Moon on the 21st at 5:35 pm; and First Quarter on the 29th (Happy Leap Year!) at 8:21 pm.
Pre-dawn planet watchers will have both Saturn and Mars to enjoy this month. Look for the ringed planet about 40 degrees above the southern horizon early in the month, settling to about 30 degrees above the southwest by the end of February. Mars is lower and further west, 37 degrees above southwest early, sinking to the west by month’s end. Jupiter and Venus will both be spectacular this month after sunset. Jupiter starts the month about 70 degrees altitude and to the southwest at the beginning of February, with even brighter Venus about 45 degrees lower, and noticeably brighter. As the month progresses, Jupiter will settle to about 40 degrees and more westerly, with Venus climbing a little. Mercury is just above the horizon – you’ll probably not be able to see it through haze and ground clutter.
Our midmonth view, about two hours after sunset, finds the constellation Auriga closest to zenith, with its brightest star Capella about 10 degrees to the east-northeast. Turning to the south, it will be hard to miss Orion’s familiar asterism. Sirius is that bright star to the left of Orion. Castor and Pollux, in Gemini, are high to the east. Below them we see Leo (the Lion), marked by its brightest star, Regulus. At this time, we can find the Big Dipper in Ursa Major to the northeast, appearing to “stand on its tail” – conversely, to the north we see Ursa Minor “hanging from its tail.” The familiar tilted W of Cassiopeia is to the northwest. Turning again to face southwest, you’ll see the bright star Aldebaran marking the constellation Taurus (the Bull). A bit below and more to the west lies the familiar asterism of the Pleiades.
Copyright 2012George Spagna