Page 79 of Project Hail Mary

Rocky taps the tape some more.

I’ve always assumed he had eyes somewhere and I didn’t recognize them. But what if he doesn’t have eyes at all?

The airlock of theBlip-Awas dark, and Rocky didn’t have any problem with it. So I figured he saw in frequencies of light I can’t see. But the tape measure has white tape with black markings on it.Anyvision inanyspectrum should be able to discern black on white. Black is the absence of light and white is all frequencies equally reflected.

Wait—this doesn’t make sense. He knows what I’m doing. He mimics my gestures. If he doesn’t have vision, how can he read my clock? How can he readhis ownclock?

Hmm…his clock has thick numbers. Like an eighth of an inch. And, thinking back, he actually did have some trouble with my clock. He needed me to tape it to the divider wall. When it floated an inch away he got upset. Just being close to the divider wasn’t enough. The clock had to betouchingit.

“Sound?” I say. “Do you ‘see’ with sound?”

It would make sense. Humans use electromagnetic waves to understand our three-dimensional environment. So why couldn’t a different species use sound waves? Same principle—and we even have it on Earth. Bats and dolphins use echolocation to “see” with sound. Maybe Eridians have that ability, but on steroids. Unlike bats and dolphins, Eridians havepassivesonar. They use ambient sound waves to resolve their environment instead of making a specific noise to track prey.

Just a theory. But it fits the data.

That’s why his clock numbers are thick. Because his sonar can’t perceive things that are too thin. My clock was a challenge to him. He can’t “see” the ink, but the hands are solid objects. So he knew about them. But the whole thing is encased in plastic….

I slapped my forehead. “That’s why you needed the clock pressed against the wall. You needed the sound waves bouncing around in it to get to you more easily. And the tape measure I just handed you is useless. You can’t see the ink at all!”

He plays with the tape measure some more.

I hold up a finger. He’s more focused on the tape measure toy, but he absently returns the gesture with one of his spare hands.

I fly back into the ship, through the control room, and into the lab. I grab a screwdriver and head farther down to the dormitory. I detach a storage panel from the floor. Simple aluminum sheet stock. Maybe one-sixteenth of an inch thick, with the edges rounded so we don’t cut ourselves. Strong, durable, and light. Perfect for space travel. I fly back to the tunnel.

Rocky has wrapped one end of the tape around one of his tunnel’s grab-handles and tied it in a somewhat crude knot. He hangs on to the dispenser with one hand and uses the other four to climb backward along the bars.

“Hey,” I say. I hold up my hand. “Hey!”

He stops playing with the tape measure for a moment. “????”

I hold up two fingers.

Rocky holds up two fingers.

“Yeah. Okay. We’re in mimic mode again.” I hold up one finger, then switch to two, then back to one, and then finally three.

Rocky repeats the sequence, just as I hoped he would.

Now I put the aluminum panel between my hand and Rocky. Behind the panel, I hold up two fingers, then one, then three, then five.

Rocky holds up two fingers, then one, then all three. He brings in a second hand to hold up two more fingers for a total of five.

“Wow!” I say.

One-sixteenth-inch aluminum will stop pretty much all light. Some absurdly high frequencies can get through, but those frequencies would also pass right through me. So he wouldn’t see my hands. But sound travels through metals just fine.

That’s proof. He’s not using light to perceive what’s going on. It has to be sound. To Rocky, that metal plate is like a glass window. Maybe it muddles the image a little, but not much. Heck, he probably knows what theHail Mary’s control room looks like. Why not? The hull is just more aluminum.

How did he see me out in space? No air in space. So no sound.

Wait. No. That’s a dumb question. He’s not a caveman wandering around in space. He’s an advanced interstellar traveler. He has technology. He probably has cameras and radar and stuff that translate data into something he can understand. No different from my Petrovascope. I can’t see IR light, but it can and then it shows it to me on a monitor with light frequencies I can see.

TheBlip-Acontrol room probably has awesome-looking Braille-like readouts. Well, I’m sure it’s way more advanced than that.

“Wow…” I stare at him. “Humans spent thousands of years looking up at the stars and wondering what was out there. You guys never saw stars at all but you still worked space travel. What an amazing people you Eridians must be. Scientific geniuses.”

The knot in the tape comes loose, recoils wildly, and smacks Rocky’s hand. He shakes the affected hand in pain for a moment, then continues messing with the tape measure.