This week, we'll finish up the topic I titled The Matter of Matter. We left off last week with the fact that an atom consisted of electrons, protons (a proton itself composed of two up quarks and a down quark), and neutrons (a neutron being composed of two downs and an up).
As I suggested at the end of the previous message, however, there are many more particles of matter. I will now present these additional types of particles. In doing so, I draw roughly from the structure and organization of the book Supersymmetry, by Gordon Kane. I will also be drawing from an excellent website called The Particle Adventure.
First, the electron has two "relatives." According to Kane:
There are two more particles like the electron. All their properties are identical to those of the electron, except that they are heavier... Although we don't have any idea why that is so, the Standard Model theory still correctly describes the behavior of these particles... They are unstable particles, decaying in less than a millionth of a second... One is called the muon, denoted by the Greek letter mu, and the other is known by the Greek letter tau. Because they behave in very similar ways, the electron, the muon, the tau, and their associated neutrinos [to be discussed next] are grouped together in a class of particles called leptons (p. 26, the bold emphases added are mine, and I'm unable to produce the Greek-letter symbols Kane showed).
The aforementioned neutrinos were referred to as "ghostly particles" by Brian Greene, in his book The Fabric of the Cosmos (p. 346). As discussed by Kane, it was discovered in 1930 that some nuclear decay processes experienced a mysterious loss of energy; he notes further that, "Wolfgang Pauli proposed that the energy was being carried off by an unseen particle..." (p. 25). These stealthy particles were named "neutrinos," and not directly detected until 1958. They are symbolized by the Greek letter nu. The electron, muon, and tau each has its own neutrino.
The lepton class of particles (electron, muon, tau, and their neutrinos) is nicely depicted in this Particle Adventure illustration. This particular linked graphic is part of a slide show, so when the new page comes up, you might also want to click on the forward arrow to see the next few slides, as well. (The Particle Adventure website requests that users of its images credit the Particle Data Group, which I am happy to do.)
Just as the electron has an "extended family," so do quarks. In addition to the two types of quarks mentioned thus far -- up and down -- there are four other kinds. These are the charm, strange, top, and bottom quarks. The full set of quarks is depicted here, in humorous fashion. The six different types of quarks (e.g., up, charm) are known as "flavors," though as best I can tell, they have no connection to sweet, sour, salty, and other tastes.
The top quark was the last to be discovered and documented, during 1994 and 1995, as described in Andrew Watson's recent book The Quantum Quark (p. 248).
According to Particle Adventure's page on quark naming, the name "strange" refers to the strangely long lifetime of a particle. My impression, gleaned from reading The Quantum Quark (p. 141), was that a class of particles was referred to as "strange" because they followed some properties of the strong nuclear force and others of the weak nuclear force (I will discuss forces in a future posting).
The Quantum Quark contains a display (Figure 4.76, p. 266) that attempts to show the relative masses of the six quark types with spheres of different volumes. Although the spheres in Watson's figure all have a three-dimensional, undifferentiated gray appearance, I will attempt to convey their relative sizes in terms of everyday objects, based on my own "retinal inspection" (quarks, again, are sub-atomic particles, so the everyday objects are merely used as an analogy).
Let's say that the smallest quark, the up, is analogous to the size of a pea that might appear on one's plate of vegetables. On this rough scale, the next biggest, the down, might be the size of a marble. The next biggest, the strange, might then be around the size of a ping-pong ball. The next biggest, the charm, might resemble a tennis ball. The next biggest, the bottom quark, might be roughly the size of a bowling ball, and finally, the largest, the top quark, might be analogous to a medicine ball, which people use to work out with in a gym.
All my talk of relatives and families of particles is bolstered by the fact that physicists themselves refer to "generations" of matter particles. As seen here, the first generation consists of the up quark (U), down quark (D), and electron (E), plus the electron's neutrino; remember that the up and down quarks and electrons comprise an atom. I remember the first generation by pronouncing UDE as a word (pronounced "ood"), as in, "Hey dude, the first generation is UDE."
The second generation consists of the charm quark (C), strange quark (S), muon (M), and muon neutrino, or CSM. The third generation consists of the top quark (T), bottom quark (B), tau (T), and tau neutrino, or TBT. I don't have any catchy memorization devices for these latter generations, sorry.
Just when it looks like we might be done, it turns out we're only halfway home; I will try to describe the other half of the particle "census" much more briefly than I have described the first half.
All particles have what are called antiparticles or, in the case of matter, antimatter. Kane notes in his glossary that, "Every particle has an associated antiparticle, another particle with the same mass but all charges opposite."
In his main text (p. 25), Kane explains that, "The particles and their antiparticles are all just particles with different charges. The antiparticles have all been observed."
An electron's antiparticle is called either an "antielectron" or "positron." Some of you may be familiar with the term positron from Positron Emission Tomography or PET scans, used for biomedical clinical and research purposes.
Another physics blog, Orange Quark, discusses an important point about matter and antimatter:
...in the early universe, one expects there to have been equal amounts of both matter and antimatter and then, as the universe cooled, for these particles to find each other, annihilate, and leave our present universe with very little matter around (and an equally small amount of antimatter).
This is clearly at odds with what we observe in the universe, where we have relatively large amounts of matter and essentially no evidence of primordial antimatter.
The name "Orange Quark" is a doubly clever one. First, the blog's operator, Mark Trodden, is a physics professor at Syracuse University; those of you who are sports fans will know that Syracuse's teams are referred to as the Orange. Second, as you physicists will know, quarks have not only "flavor," but "color."
In the Trodden posting I've excerpted, as you'll see if you click on the above link to the full entry, he also has a play on words utilizing the double meaning of the word "matter" (I guess it's not that hard to come up with!). Something I just came up with is that, if he were to become dispirited about his research and switch to studying, not up quarks, but a different kind, he could start a new blog called "Down Trodden."
An additional note: Brian Greene, whom I've discussed frequently in my postings, appeared this past week (March 22, to be exact) on The Late Show with David Letterman.
As I summarized in a message I posted to the Physics Forums discussion board (where my moniker is "PhysicsFan"):
Greene and Letterman seemed to have a nice banter for probably 7-8 minutes. Letterman asked about topics that one would reasonably associate with physics (e.g., Einstein, what science has to say about the origin of the universe). I think it was when Greene was talking about string theory -- about how, at a level smaller than atoms and quarks, things might be comprised of strings -- Letterman asked him, in that mischievous way, "How is my life the better for knowing that?" (paraphrased).
Greene has a good sense of wit and timing, so he rolled with it pretty well.
