The Structure of Matter series continues: last week’s article on the basics of atomic nuclei is now supplemented with an article discussing the “residual” strong nuclear force which binds protons and neutrons inside of nuclei. It further explains why nuclei are so small compared to atoms. Or rather, it explains it in part, because I have to also explain why protons and neutrons themselves are so small — which I will do soon enough.
As always, readers are encouraged to comment on things that don’t seem clear or correct. And any nuclear physics experts who want to weigh in on my presentation — suggesting how it might be improved or extended, or identifying misconceptions on my part — are encouraged to speak up (publicly or privately as you prefer).
Meanwhile, we’re entering the March conference season, when many new results from the Large Hadron Collider [LHC] (based on analysis of last year’s data) and from other important experiments will start appearing. Since the LHC’s proton-proton collisions went til December in 2012 (in 2011 they stopped in October) the time for LHC data analysis has been rather short. I therefore think it likely that any really surprising results from the LHC will be delayed for extra scrutiny — and may not appear until late spring or summer, when there are other conferences. But I could be wrong! And one thing we’re all waiting for is the measurement by CMS (one of the two general purpose LHC experiments) of the rate for the recently discovered Higgs particle to decay to two photons. However, we won’t see that result until CMS is absolutely confident in it.
Posts have been notably absent, due mainly to travel with very limited internet; apologies for the related lack of replies to comments, which I hope to correct later this week.
Meanwhile I’ve been working on a couple of articles related to the nuclei of atoms, part of my Structure of Matter series, which serves to introduce non-experts to the basics of particle physics. The first of these articles is done. In it I describe why it was so easy (relatively speaking) to figure out that nuclei are made from certain numbers of protons and neutrons, and how it was understood that nuclei are very small compared to atoms. Comments welcome as always!
A related article, which should appear later this week, will clarify why nuclei are so tiny relative to atoms, and describe the force of nature that keeps them intact.
Fig. 1: A hydrogen atom consists of a tiny proton “orbited” by an electron.
There’s been a lot of reporting recently on a puzzle in particle physics that I haven’t previously written about. There have been two attempts, a preliminary one in 2010 and a more detailed one reported just this month, to measure the size of a proton by studying the properties of an exotic atom, called “muonic hydrogen”. Similar to hydrogen, which consists of a proton orbited by an electron (Figure 1), this atom consists of a proton and a short-lived heavy cousin of the electron, called the muon (Figure 2). A muon, as far as we have ever been able to tell, is just like an electron in all respects except that it is heavier; more precisely, the electromagnetic force and the strong and weak nuclear force treat electrons and muons in exactly the same way. Only the first two of these forces should play a role in atoms (and neither gravity nor any force due to the Higgs field should matter either). So because we have confirmed our understanding of ordinary hydrogen with very high precision, we believe we also understand muonic hydrogen very well also. But something’s amiss. Continue reading
I’ve been quite busy with some physics research this week, but I have nevertheless managed to finish a new article on electrons, part of my Structure of Matter series, which aims (among other things) to introduce a non-expert to particle physics, step-by-step. The completion of this article feels like a significant step for this website. After all, the electron was the first subatomic particle and the first of the apparently-elementary particles to be discovered, about 115 years ago, and its discovery really gave birth to the field of particle physics we know today. Moreover, it was the failure to describe the behavior of electrons within and outside of atoms that forced physicists to go beyond Newtonian views of physics processes, and introduce the theory of quantum mechanics. Electrons, tiny as they are, are enormous in human life; they play a key role in all chemical reactions, including those that sustain our bodies. Beyond that, they lie at the heart of much modern technology — electronics! And there’s more. So no particle physics website can be complete without an electron webpage.
Looking ahead, a question I sometimes get asked is whether I’m sure electrons (or any other elementary particles that physicists talk about) really exist. After all, it is true I’ve never seen a picture of one taken with any sort of microscope! Well, in answer to this question, I want to write an article on why we particle physicists are so confident that electrons (and atomic nuclei) exist… explaining the types of experiments and the types of logical reasoning that lead to this conclusion. I suspect a lot of readers will find such an article interesting; after all, why should one take expert knowledge for granted just because it appears in a textbook or on a website? Readers should demand to know where the knowledge came from — and a writer should be prepared to answer.
I’ve been adding to my series of layperson’s articles on The Structure of Matter, which eventually will serve as an introduction to particle physics for those coming to this site for the first time. You might recall that in early December I supplemented my older article on molecules with an article on atoms. I got some terrific reader feedback, in the form of incisive constructive criticism, which allowed me to greatly improve the latter article. Well, readers, you’ve got another chance to help me out if you would like to — or you can just enjoy the read. I have three new articles (two of them short) which were put up over the last few weeks. These are:
Incidentally, the next stage in this series will be to describe electrons, and then I will turn to atomic nuclei, to the neutrons and protons that they contain, and eventually to the quarks and gluons that make up the neutrons and protons.
It took me over six months, following my article on molecules, to write the sequel, on atoms. These are just two in a series, intended to introduce the structure of matter to novice readers who want to learn what particle physics is about. Atoms aren’t the main focus; future articles will focus on electrons, on protons and neutrons, on quarks, and on the forces that hold these objects together. But the essay on atoms might be the hardest of the set to write (at least I hope so). The long delay reflects the challenges involved, and as my readers’ wise and helpful criticisms of Friday’s first version confirmed, I didn’t meet them on my first try.
So after some thought, I’ve made another attempt. Critique still welcome from anyone who wants to make suggestions.
Aside from the fact that I fell into a couple of pedagogical traps that anyone who’d taught chemistry would have known about, I also struggled to describe atoms briefly, clearly and accurately because their features are determined by quantum mechanics — that weird but fundamental behavior of our world that we don’t encounter in daily life but is essential to the structure of matter. What’s profoundly confusing to the non-expert (and somewhat confusing even for experts) is that electrons are, on the one hand, best described in many circumstances as point-like particles (much smaller than atoms, and smaller even than atomic nuclei) yet around atoms they are in some way spread out in a very non-particle-like fashion. Well, indeed, thinking of elementary objects like electrons as “particles” will get you into trouble; for one thing, they are really “quanta” of quantum fields, and in most circumstances they behave much more like waves. And yet it is essential to explain that one can try to measure their size — essentially by forcing them, through an appropriate experiment, to reveal whether they, like baseballs, rocks and dumplings, have internal structure.
Ok, I can’t even figure out how to write this paragraph clearly. There needs to be a way to explain this issue, one that is both moderately intuitive and based on accurate and clear physical reasoning…