Conversations About Science with Theoretical Physicist Matt Strassler
Today a reader asked me “Out of the quantum fields which have mass, do any of them also have weight?” I thought other readers would be interested in my answer, so I’m putting it here. (Some of what is discussed below is covered in greater detail in my upcoming book.)
Before we start, we need to rephrase the question, because fields do not have mass.
For those of you who subscribe to New Scientist, their magazine’s cover story this week is a feature entitled “THE AMAZING THEORY OF (ALMOST) EVERYTHING”. In the feature is an overview of the Standard Model (which describes all known fields and particles, excepting gravity, with amazing accuracy, but leaves a plethora of puzzles unaddressed) and includes a final section (edited by Abby Beall) with short articles by six scientists about their current views regarding the Standard Model, among them myself. [This website’s introductory article on the Standard Model is here; see also here.] . . .
Every book on science focuses attention on a little sliver of a vast, complex universe. In Waves in an Impossible Sea, I had intended to write mainly about the Higgs field, and the associated Higgs particle that was discovered in 2012 to great fanfare. I was planning to explain how the Higgs field does its job in the universe, and why it’s so important for the existence of life.
However, this plan had a problem. The Higgs field would be irrelevant were it not for quantum physics on the one hand and Einstein’s relativity on the other, and to comprehend the latter requires some understanding of Galileo’s earlier concept of relativity. To show why the Higgs field can give mass (more precisely, rest mass) to certain types of particles requires combining all of these notions together. Each of these topics is daunting, worthy of multiple books, and I knew I couldn’t hope to cover them all in 100,000 words!
To my surprise, resolving this problem wasn’t as difficult as I expected, once I picked out a few crucial elements about each of these subjects that I felt everyone ought to know. Lining up those conceptual points carefully, I found I could give a non-technical yet accurate explanation of how elementary particles can get mass from a Higgs field. (A more mathematical explanation has been given previously on this website, in two series of articles here and here.)
Yet what surprised me even more was that the book’s main subject slowly changed as I wrote it. It became focused on the question of how ordinary life emerges from an extraordinary cosmos. Though a substantial section of the book is devoted to the Higgs field, it is situated in a much wider context than I originally imagined.
Once we clear away the hype (see the previous posts 1, 2, 3, 4), and realize that no one is doing anything as potentially dangerous as making real wormholes (ones you could actually fall into) in a lab, or studying how to send dogs across the galaxy, we are left with a question. Why bother … Read more
This has been an exceptional few days, and I’ve had no time to breathe, much less blog. In pre-covid days, visits to the laboratories at CERN or Fermilab were always jam-packed with meetings, both planned and spontaneous, and with professional talks by experts visiting the labs. But many things changed during the pandemic. The vitality … Read more
There’s been a lot of chatter lately about a claim that charm quarks are found in protons. While the evidence for this claim of “intrinsic charm” (a name that goes back decades) is by no means entirely convincing yet, it might in fact be true… sort of. But the whole idea sounds very confusing. A charm quark has a larger mass than a proton: about 1.2 GeV/c2 vs. 0.938 GeV/c2. On the face of it, suggesting there are charm quarks in protons sounds as crazy as suggesting that a football could have a lead brick inside it without you noticing any difference.
What’s really going on? It’s a long story, and subtle even for experts, so it’s not surprising that most articles about it for lay readers haven’t been entirely clear. At some point I’ll write a comprehensive explanation, but that will require a longer post (or series of posts), and I don’t want to launch into that until my conceptual understanding of important details is complete.
But in the meantime, here’s a related question: how can a particle with zero mass (zero rest mass, to be precise) spend part of its time as a combination of objects that have positive mass? For instance, a photon [a particle of light, including both visible and invisible forms of light] has zero rest mass. [Note, however, that it has non-zero gravitational mass]. Meanwhile electrons and positrons [the anti-particles of electrons] both have positive rest mass. So what do people mean when they say “A photon can be an electron-positron pair part of the time”? This statement comes with a fancy “Feynman diagram”, in which the photon is shown as the wavy line, time is running left to right, and the loop represents an electron and a positron created from the photon.
For readers who want to go a bit deeper into details (though I suggest you read last week’s posts for general readers first [post 1, post 2]):
Last week, using just addition and subtraction of fractions, we saw that the ratio of production rates
(where e— stands for “electron” and e+ for “positron”) can be used to verify the electric charges of the quarks of nature. [In this post I’ll usually drop the word “electric” from “electric charge”.] Specifically, the ratio R, at different energies, is both sensitive to and consistent with the Standard Model of particle physics, not only confirming the quarks’ charges but also the fact that they come in three “colors”. (About colors, you can read recent posts here, here and here.)
To keep the previous posts short, I didn’t give evidence that the data agrees only with the Standard Model; I’ll start today by doing that. But I did point out that the data doesn’t quite match the simple prediction. You can see that in the figure below, repeated from last time; it shows the data (black dots) lies close to the predictions (the solid lines) but generally lies a few percent above them. Why is this? The answer: we neglected a small but noticeable effect from the strong nuclear force. Not only does accounting for this effect fix the problem, it allows us to get a rough measure of the strength of the strong nuclear force. From these considerations we can learn several immensely important facts about nature, as we’ll see today and in the next post.
A post for general readers who’ve heard of quarks; if you haven’t, you might find this article useful:
Yesterday I showed you that the usual argument that determines the electric charges of the various types of quarks uses circular reasoning and has a big loophole in it. (The up quark, for example, has charge 2/3, but the usual argument would actually allow it to have any charge!) But today I’m going to show you how this loophole can easily be closed — and we’ll need only addition, subtraction and fractions to close it.
Throughout this post I’ll shorten “electric charge” to just “charge”.
Our approach will be to study the process in which an electron and a positron (the electron’s anti-particle) collide, disappear (“annihilate”), and are converted into one or another type of quark and the corresponding anti-quark; see Figure 1. The rate for this process to occur, and the rate of a similar one in which a muon and anti-muon are produced, are all we will need to know.
In an electron-positron collision, many things may happen. Among the possibilities, the electron and positron may be converted into two new particles. The new particles may have much more mass (specifically, rest mass) than the electron and positron do, if the collision is energetic enough. This is why physicists can use collisions of particles with small mass to discover unknown particles with large mass.
In particular, for any quark of mass M, it is possible for an electron-positron collision to produce that quark and a corresponding anti-quark as long as the electron’s energy Ee is greater than the quark’s mass-energy Mc2. As Ee is gradually increased from low values, more and more types of quark/anti-quark pairs can be produced.
This turns out to be a particularly interesting observation in the range where 1 GeV < Ee < 10 GeV, i.e. when the total collision energy (2 Ee) is between 2 and 20 GeV. If Ee is any lower, the effects of the strong nuclear force make the production of quarks extremely complicated (as we’ll see in another post). But when the collision energy is above 2 GeV, things start to settle down, and become both simple and interesting.