Of Particular Significance

Category: History of Science

[This is a follow-up to Monday’s post, going more into depth.]

Among the known elementary particles are three cousins: the electron, the muon and the tau. The three are identical in all known experiments — they have all the same electromagnetic and weak nuclear interactions, and no strong nuclear interactions — except for effects that arise from the fact that they have different rest masses:

  • electron rest mass: 0.000511 GeV/c2
  • muon rest mass: 0.105658 GeV/c2
  • tau rest mass: 1.777 GeV/c2

[These differences arise from their different interactions with the Higgs field; to learn more about this, see Chapter 22 of my book.]

[Note added for clarity: these particles do exhibit slightly different magnetic moments, dramatically different lifetimes, and a number of other differences — but all of those variations can be traced back to the difference in their masses.]

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POSTED BY Matt Strassler

ON May 7, 2024

The Sun has been acting up; a certain sunspot has been producing powerful flares. In the past three days, several have reached or almost reached X-class, and one today was an X4.5 flare. (The letter is a measure of energy released by the flare; an X1 flare is ten times more powerful than an M1 class flare, and an X4.5 flare is almost three times more powerful than an X1 flare.)

From the https://www.swpc.noaa.gov/ website

With so much solar activity, it’s possible (though certainly not guaranteed) that one or more coronal mass ejections might strike Earth over the next 48 hours and might generate northern and southern lights (“auroras”). If you’re in a good location and the weather is favorable, you might want to check every now and then to see if the atmosphere is shining at you.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON May 6, 2024

Within the Standard Model of particle physics, one finds three almost identical types of particles: the electron, the muon and the tau. Their interactions with the electromagnetic force, the weak nuclear force and the strong nuclear force are exactly the same. In particular, all three have electric charge -1 (which in first-year physics classes we would usually write as “-e”).

They’re not entirely identical, however. For instance, they have different masses.

  • Electron: 0.000511 GeV/c2
  • Muon: 0.105658 GeV/c2
  • Tau: 1.777 GeV/c2

Compare these to a hydrogen atom, which has mass 0.938783 GeV/c2 . (For the definition of “GeV”, which is an amount of energy, click here.) [Specifically, these are their “rest masses”. Rest mass is the type of mass that is intrinsic to objects and does not change with speed; see Chapter 5 of Waves in an Impossible Sea.]

Why does nature have these three similar particles, collectively called the “charged leptons“? We don’t know. But they’re not alone.

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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON May 6, 2024

Are you sitting or lying down? Perhaps you’re moving around at a walking pace? I probably am. And yet, unless you live in the northeastern US or in southern South America, you and I are moving relative to each other at hundreds of miles per hour.

In Chapter 2 of the book “‘Waves in an Impossible Sea”, I remarked on the this fact. (See below for the relevant passage.) At first glance it might seem puzzling. After all, the distance between your town and my town is constant; it never changes. And yet our relative motion is comparable to or faster than a jet aircraft. How can both of these things be true?

And then there’s another question: if we’re all moving so fast relative to one another, why don’t we feel the motion?

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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON May 2, 2024

A couple of days ago, I posted an article describing how the size of a quantum object, such as a proton or electron, can be measured. This isn’t obvious. For example, scientists say that an electron spreads out and is wave-like, and yet that it has no size. This apparent contradiction needs resolution. While I addressed this puzzle in the book‘s chapter 17, I didn’t do so in detail, and so I wrote this article to fill in the gaps.

Now, in response to a reader’s question, I’ve added a section to the end of the article, entitled “Estimating the Object’s Size From Its Excited States”. There I explain in more detail how one goes from simple measurements, which confirm that a proton’s size isn’t zero, to an actual estimate of a proton’s size. The discussion is a little more technical than the rest of the article; you will probably need first-year physics to follow it. But I hope that some readers will find it useful!

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON May 1, 2024

Quantum physics is certainly confusing.

  • On the one hand, electrons are wave-like and can be quite spread out; in fact, as I’ve emphasized in my book and in a recent blog post, a stationary electron is a spread-out standing wave. (I’ve even argued that these “elementary particles” should really be called “wavicles” [a term from the 1920s].)
  • On the other hand, scientists say that electrons have no size — or at least, if they have a size, it’s too small to be measured using current technology. They are often described as “point particles.”

How can both these things be true?

Well, to clarify this, let’s look at objects that do have an intrinsic size, such as protons and neutrons. How are their sizes actually determined? While this question is addressed in the book’s Chapter 17 (see Figure 40 and surrounding text, and footnote 2), I didn’t go into much detail there.

To supplement what’s in the book, I have written a webpage that outlines two classic methods that are used to measure the intrinsic size of a proton, or of any ultra-microscopic object.

When these same methods are used on electrons, one finds no evidence of any finite size, and so one concludes that their intrinsic size (if any) is too small to measure.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON April 29, 2024

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