Radius of the proton – a most remarkable baryon


Do we know what the radius of a proton is?

One of the questions selected for both our high school and undergraduate essay contests focuses on medical imaging by way of PET scans (positron emission tomography). Given that PET scans function reliably as a beneficial imaging technique in medicine, a technique made possible by exploiting the positron (the electron’s anti-matter counterpart), our hope is that this question will motivate students who express interest in physics and medicine. So, when choosing this question, students will need to describe how PET scan machines work, such as the processes of pair production and annihilation interactions.

In addition, this question serves to prepare students for more complex concepts in particle physics. In the context of the writing prompt, we introduce research on matter/anti-matter interactions relative to protons and anti-protons. This question, then, creates an opportunity for students to include their thoughts on the significance and implications of ordinary matter in the universe and how this balance favors our existence in light of annihilation outcomes between matter and anti-matter. This question also deals with the Standard Model of particle physics, as well as with ongoing challenges to the Big Bang theory of the universe. Clearly, for an aspiring scientist, gaining a deeper understanding of the proton would be a valuable use of time.

One of the most important properties associated with the proton is that the number of protons present in a chemical element determines the placement of that element on the periodic table. So the number of protons, called the proton number or atomic number (and denoted by the letter Z), uniquely identifies a chemical element. In addition, the atomic number also tells us something about certain physical characteristics of an atom, and it describes an atom’s nuclear charge.

As for proton classification and qualitative schemes, there are a lot of things we know, and yet some things still elude us. For instance,

  • We know that protons fall into the general classification group called baryons and are part of the hadron family.
  • We know that the proton is made up of quarks (two up quarks and one down quark).
  • We also know that the mass of a proton is much larger than the mass of an electron.
  • However, we do not have reliably accurate information about the physical size of a proton.

What is the radius of a proton, anyway? It turns out that there are scientists currently working on this issue, and although we do not yet have a clear and accurate measurement, it is the inherent complexity of measuring the proton’s radius that makes the issue so fascinating.

Could you consider yourself as someone who helps unravel unknown facts about the proton? Starting out now in your own discovery of the intricacies of God’s great creation through scientific study could very well help you in discerning your future career path. Maybe the writing prompt on positron emission tomography was developed just for you!

For more information on the topic of the radius of the proton, we encourage you to read the following two short articles: