i did an online course on particle physics, but i barely mention it in my personal statement (its not finished yet) i mainly played on a research paper that i wrote and had published. But if you wanna play heavily on that go for it, milk it for everything its worth. put it this way it cant hurt you so i say go for it. Even talk about a book you've read, or the works of someone you admire in the field and how they've impacted your views and why. but remember to also spend a little para on extra curriculars not related to your subject. You want to show yeah you know your stuff but you're also well rounded. I talked about about a non-profit for lgbtq ppl i set up and run.