How is a proton like a bagel?

"How is a raven like a writing-desk?" is not a question physics can answer, but the shape of a proton is. [Credit: John Tenniel]

“How is a raven like a writing-desk?” is not a question physics can answer, but the shape of a proton is. [Credit: John Tenniel]

Protons give every atom its fundamental identity. The number of protons tells you where an atom sits on the Periodic Table, while the number of neutrons and electrons can vary (within limits). However, they are not fundamental: they are made of three quarks, gluons to bind things together (yeah, the name is even derived from “glue”), and a chunky stew of pairs of other particles, all of which combine to create the protons  we know. For that reason, the shape and size of a proton are more complicated properties than we might think at first. My new article for NOVA‘s “Nature of Reality explains:

Yet those questions trouble many particle physicists as well. When it comes to protons, even the simplest questions—How big is it? What is its shape?—turn out to have complicated answers. While we’ve known the basics for decades, the details are more elusive. Take size, for example. Measuring the proton isn’t as simple as getting out a really tiny ruler: Any measuring tool we use is made of other particles, and those interact with the proton we’re trying to measure. The solution: “We do it the same way [Ernest] Rutherford discovered the atom has a very small and positively charged nucleus,” said Alberto Accardi, a physicist at Jefferson Lab and Hampton University in Virginia—that is, by firing other particles at the target particle and measuring how they “scatter.” Similarly, to measure the size of a proton, physicists typically bombard it with electrons or muons, a more massive relative of the electron. The spread of the particles after scattering reveals the size and shape, much as you could use the shadow of a tall building to estimate its height. [Read more…]


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