Lost and Wanted by Nell Freudenberger

Love this author. I struggled with the "physics" of the book (the protagonist is a physicist) and I think I missed some of the meaning of that profession in the context of the book. But no matter. Loved it. Reads well, interesting characters.  A great find.
Here's what Kirkus says:

A physicist at MIT receives a text from her dead best friend.
"In the first few months after Charlie died, I began hearing from her much more frequently," Helen Clapp explains at the outset of Freudenberger's (The Newlyweds, 2012, etc.) third novel. Charlie Boyce and Helen met freshman year at Harvard. Though they were "an upper-middle-class black girl from Brookline"—Charlie—"and a work-study white science nerd from Pasadena"—Helen—their friendship took flight, powered by in-jokes, catchphrases, shared ambitions, and theories about life. After graduation, Charlie moved to LA and became a screenwriter, married a surfer, had a little girl. Helen stayed in Boston and became famous as one of the authors of the Clapp-Jonnal model "for quark gluon plasma as a dual black hole in five-dimensional space-time." She wrote two bestselling science books and gained an endowed chair at MIT; her 7-year-old son, Jack, whose father was an anonymous sperm donor, became the "love of [her] life." As the novel begins, Charlie has just died of lupus. Though they hadn't spoken for over a year, Helen is now receiving texts from Charlie's cellphone, which her husband hasn't been able to find since she died. Strangely, they seem like they could only have been written by...Charlie? Meanwhile, said husband and daughter come to stay with Charlie's parents in Boston; also back in town is Neel Jonnal, Helen's college boyfriend and collaborator, now with a fiancee. Complications ensue, though not the predictable soap-opera ones you'd imagine. Freudenberger is good at explaining physics, but her real genius is in the depiction of relationships. Each one in the novel, whether between adults, adults and children, or among children, is unique, finely calibrated, and real. The title is a line from a poem by W.H. Auden which doesn't fully hit until the end of the book, when it takes on heart-rending poignancy.

Brimming with wit and intelligence and devoted to things that matter: life, love, death, and the mysteries of the cosmos