This book is fascinating and jam-packed with history, science, sociology, psychology, and more. I feel like Newitz must have decided to throw every cool idea into one book, and the result is, as you might expect, nerdily impressive. There's even a section in the back that summarizes the real science, history, etc. that is touched on through the various events in the book. And the premise, that ancient machines enabling humans to travel in time were discovered centuries ago and spawned a new branch of scientific research plus a lot of "timeline editing," is brilliant. I found the ideas in the book to be fascinating and thought-provoking.
However...while I admire that depth of knowledge and passion, it also makes the narrative a bit disjointed at times.
But let's start with the positive:
First off, this book is written from multiple perspectives, but mainly from the first-person POV of both Tess and Beth. Beth is a teenager living a difficult life and learning the impacts of that difficulty on her life, while Tess is a time-travel scientist and an under-the-table activist who wants to edit the timeline. Beth's chapters are written in a strong, compelling voice that rings with truth, and that truth is often painful. These chapters flow beautifully and are well-paced and consistently-toned.
The book is also centered almost entirely around women and nonbinary characters, which is a lovely relief from the typical mostly-male casts that are often found in SFF. The themes and interactions are also woman-centered (as appropriate to the time periods they are in) without being either stereotypical or preachy.
And of course, the science in the story is plausible, at least in part because there is definitely real science that much of the premise is based on, but there are also aspects of the Machines that remain a mystery and cannot be explained by the characters from any of the timelines represented. This, to me, seems far more realistic than books which attempt to explain every single detail of the science, since there is still much about our real world we don't fully understand.
On the downside:
The Tess chapters range a bit in pacing and style: Some are well-paced and emotionally impactful, but some get bogged down in too much science and exposition. There were some scenes that felt rushed, too; my main complaint with the story overall is that Newitz sometimes just needed to slow down and let the character live the scene instead of (as it felt, at least) trying to get through to the plot point. It would have been more immersive with more "moments" for the character, though some chapters express their moments beautifully.
Still, this was an interesting read with a satisfying resolution. I look forward to reading more of their work.