October 6, 2016 7:51 AM EDT

Some books ought to come with a warning–not for the reader but for those nearby, who are bound to be interrupted with passages read aloud. Mind-blowing ideas demand to be shared.

Such a warning ought to come with both James Gleick’s Time Travel: A History and Richard A. Muller’s Now: The Physics of Time. The central question of Muller’s book is how to define “now,” and crucial to the answer is figuring out why (or whether) time moves in only one direction. Muller, an acclaimed physicist at the University of California, Berkeley, posits a theory that seems at once plausible and–surprisingly, for a book with equations–one worth not spoiling. His style is friendly to those without a science background, though the feeling of understanding quantum physics may be fleeting. Gleick, meanwhile, takes a cultural view of time travel, going back to H.G. Wells. If you want to know what now is like–as opposed to what now is–you must examine its visions of the future and the past.

Both books quote St. Augustine, who said he was sure what time was until he was asked explain it. And yet we keep trying to pin time down, for good reason. To Muller and Gleick, the question of time leads to something even bigger: free will. What is the link between our pasts, ourselves and our futures? In different ways, each makes a fascinating argument that the most important time is the present.

This appears in the October 17, 2016 issue of TIME.

Write to Lily Rothman at lily.rothman@time.com.

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