It’s fundamental and natural to ask this question about an object: “how big is it?” For many things—most everyday objects, people, planets, stars—size is easy to measure. However, other things are more challenging, including the size of a proton: one of the three particles that make up every ordinary bit of matter. The major challenge is its tiny size, which precludes using light of any kind to measure it. To make matters worse, the size of a proton may depend strongly on what method you use to measure it, as I explained for Double X Science.

The simplest way to measure the size of a proton involves shooting electrons at it, and measuring the paths the electrons take as they feel the influence of the various forces. Because of those forces, in fact, the proton can’t be said to have a single size! Instead, physicists use three different size measurements, which are all pretty close to each other, but not exactly the same. The one most important to us for this post is the charge radius. Electron bombardment measurements found that to be about 0.88 femtometers.

However, electron bombardment only gets us so far; if we want better accuracy, we need another method. [Read more…]