Of Particular Significance

How Do You Measure a Quantum Object’s Size?

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/29/2024

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.

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