If

They say that Virginia is for lovers. It’s a song, according to Cassie, and I have to admit as we glide across the humid green hills beneath a suncrusted tree line that it is rather beautiful, the way that only small towns are. We drive past corner stores, brown and papery, past the cardboard cutout shadow-men in the streets whose illuminated backs turn into darkened fronts as our view revolves around them. It reminds me of the Doppler shift, I say, and Cassie points out that the Doppler shift is for sound waves, but to me the meaningful part is in the passing: sound, or in this case a shadow, stretches differently depending on whether you are approaching or fleeing the source.

We roll by horse fields with their glossy chestnut creatures, and Cassie tells me about how she always wanted a horse, but her parents insisted that it was too hard to maintain. “Do you know that’s why white elephants are an unwanted gift?” she says. “Historically they were so precious and rare that the cost to keep them usually ruined the receiver.”

Cassie feels the same to me. I appreciate this when we arrive at my cottage, a place of perpetual twilight where the mosquitoes hover above the water and the stars peer through the clouds. It’s always the same: The cottage stands in stone, the creek runs through rock, and the trees tower above us when we slip back into our favorite summer night, stargazing from the hood of my car. Here we talk about space, this vast romantic emptiness. Nobody likes the idea of emptiness, Cassie declares, but I argue that therein lies the romance: the unknown, waiting to be filled, is what gives people hope.

We wanted to be astronauts when we were kids, both of us, but now we’ve realized that the true discoveries are made in science and math, which are the most abstract places of all. The abstraction, though, gives them immortality. It frees numbers from our flesh and earth which keep changing, and it saves us because any notion of immortality provides an anchor, something to hold on to.

Except that this time, Cassie isn’t the same. Cassie isn’t the same at all, and there she is still sitting in my car, her seatbelt still buckled, suddenly looking very different. There is her hand pulling at her hair, and her mouth repeating her sadness about Mike, and her head filled with the awful idea that if it’s cowardly, it isn’t love. There she is in her new boarding school in Connecticut, stretched out on the turf field with him the night before he graduates, and she wants to give him everything, but she’s afraid. Do you love me? he asks her, and the awful thing is that she’s never loved him as much as she does now, but she won’t love him the way he wants her to because he is about to leave and he will not wait. So no, she doesn’t love him. She is afraid, and she saves herself.

“You can’t live for other people,” I tell her.

“But isn’t that what love is?” she replies.

She’s asking me again, as she’s asked me for months, except that this time, it’s different. This time, she is wretched, and this time, she cannot bear herself, and this time, she says, this time she wants to lose her virginity, and this time it won’t be to the boy she loves but it’ll be to me, her best friend, the boy she trusts, and this time it’ll be at a place to which she’ll never return, this time is the last time. Why here? is all I want to know but I never get the chance, and instead I get only the sense that Cassie will destroy things at the height of her loving them most, which might be her way of saving them.

The first thing to understand about four dimensions is that space (three dimensions with which we are...