Chapter Text
“Alright, Rocky. Ready to talk about your engines?”
Rocky stands up and does a brief jazz hands, tilting his carapace toward me. It’s a posture that my students would have described as “locked in.” Well, students are always more engaged when they can relate the material to their own lives and interests, so it makes sense that a ship’s engineer would be extra attentive to this part of the lesson.
“Let’s start with a review: Acceleration is a change in velocity, like speeding up or slowing down. Your engines accelerate your ship at about 2g, or 20 meters per second per second, right? To match the gravity on Erid?”
“Approximately.”
“And that acceleration is constant; it never changes. Which means your velocity changes at a constant, never-changing rate: it increases by 20 meters per second for every second of engine thrust. After one second, your velocity is 20 meters per second. After two seconds, it’s 40 meters per second. After three seconds, it’s 60 meters per second, and so on.”
“Rocky design and build ship. Rocky knows how ship accelerates.” If Rocky had eyes, I’m sure he’d be rolling them right about now.
“I know, I know. But like everything in relativity, the acceleration you experience on the ship is different than the acceleration that someone at Tau Ceti would observe.”
Rocky emits a short hum similar to the raspberry sound, but I think its meaning leans more toward amused exasperation rather than outright irritation. I retrieve a small test tube from one of the supply drawers.
“Let’s start with what you observed. Your engine thrust was normal, correct? The acceleration of your ship felt like a steady 2g of gravity. As far as you could tell, you accelerated just like an object in freefall on Erid would accelerate. Well, ignoring air resistance, anyway.”
I show him the test tube. “This is the Blip-A.” I hold it high above my head and drop it, letting it clatter to the floor and roll away. Not for the first time, I am glad that all of the lab “glassware” is made of... whatever this unbreakable material is.
Rocky taps his fists again in the dismissive gesture. “Rocky no crash ship.”
I roll my eyes; I’ve been doing that a lot today. “You know what I mean. Assuming there’s no floor in the way. You felt like you were accelerating at the same rate forever, always traveling 20 meters per second faster than you were a second ago.
“Now let’s think about what I would see while your ship accelerates if I were on Tau Ceti. Remember how I would see your clock tick slower and slower, the faster you go? From my perspective, each ‘second’ that your ship takes to increase its speed by 20 meters per second will take longer than the ‘second’ before it. If I measure your acceleration using my clock on Tau Ceti, your acceleration would be decreasing.”
I pinch my chin and look around the lab again. “If I had a piece of cardstock or something, I could make a little ramp... Oh! Acetate sheets! Hold on a sec, I know they’re around here somewhere.” I remember seeing acetate sheets when I catalogued the contents of the lab while approaching Tau Ceti, I just need to remember where I saw them.
Rocky lets out an exasperated sort of trill. “Bad human memory waste much much much time.”
I don’t bother to respond. He’s not wrong.
After searching for a few minutes, I find the clear, flexible plastic sheets and bring one over to the lab table, picking up the test tube from the floor on my way. I hold one edge of the acetate sheet and rest the other against the table, flexing the sheet. It looks a little like a quarter-pipe at a skate park. Rocky starts tapping the xenonite barrier to get a better “look” at what I’m doing.
I hold the test tube against the upper edge of the acetate sheet, ready to roll down the ramp. “Ignore the specific path of the test tube. This demonstration is about the velocity of the tube, not its direction.”
I release the test tube, letting it roll down the quarter-pipe, across the table, and onto the floor again.
“Now tell me, what was the test tube’s acceleration while it was rolling across the table?”
“Is zero. Speed not change.”
No snarky comments about easy questions this time? I guess he really is engaged.
“Correct. When I first let go of the test tube, it started accelerating just like when I dropped it to the floor earlier. But as it sped up, its acceleration decreased. Eventually, it reached its maximum speed and coasted.
“That’s approximately what your ship’s acceleration looks like from Tau Ceti. The faster you go, the lower your acceleration gets. As you approach the speed of light, you barely accelerate at all, and you move through space at a steady speed.”
Rocky taps his claws together again, thinking. I pause to give him time to ask his question.
“Alternative: Rocky increase engine thrust. Acceleration increase again. Go fast fast fast.”
I nod. “You’re right, you could do that. But then you’d run into an engineering problem.”
Rocky continues tapping. After a moment, he says, “More thrust needs more fuel.”
“Bingo. And if you want to maintain a constant rate of acceleration from my perspective on Tau Ceti, the rate at which your engine uses fuel would need to increase exponentially. To reach the speed of light, you would need an infinite amount of fuel.”
I walk over to the lab bench against the wall, bending over to find the test tube again. “That’s why the speed of light is a sort of universal speed limit: nothing can travel faster than it. Anything with mass can get very very very close if you give it enough energy, but it can never travel at the speed of light, let alone go faster than it.”
I put the test tube on the table and watch while Rocky clicks away, radiator vents puffing. “Take your time, let me know if you have any questions. I know it’s a lot of information to digest.”
Rocky’s response is immediate. He makes a sound like an angry goose and shuffles his claws against the floor. “How Rocky eat information, question?”
I try to keep a straight face. “It’s an Earth idiom. It means to think deeply about something.”
“More Earth idioms about eating.” He does a dramatic little shiver, like a dog shaking water out of its fur. “Disgust disgust disgust.”
I chuckle. “You know, I’ve been wondering, do Eridians have any idioms about eating?”
Rocky draws his limbs a little closer to his carapace; the posture isn’t one I see often, but I think it’s related to shyness or discomfort. After a moment, he says, “Yes.”
“Oh?”
He goes quiet.
“Come on, you can’t stop there.”
“Need word. Rude word. Use to surprise or offend.”
“Ah. Swear word.”
Rocky sings his word for “swear word,” and I grab the laptop from its perch on a nearby lab stool. “Eridians use idioms about eating. Eating idioms often use swear words. Not use in polite company.”
“I see.”
I suppose that makes sense, for a species with only one orifice. Many swear words and vulgar phrases on Earth are related to bodily waste and reproduction. And for Eridians, eating is very closely related to those other biological functions.
I’m starting to wonder if asking to watch him eat was a little more taboo than I first realized.
After entering the new Eridian word in our database, I turn to look at Rocky. “Well, that’s relativity. Or at least, an introduction to relativity. I haven’t even talked about gravity and the curvature of spacetime, but we can cover that later if you’re curious. Now that I’ve shown you how relativity explains the strangeness of your trip, do you still think I’m stupid?”
“Of course Grace stupid. Grace injure toe three times on journey to Adrian. Collide with open door.”
“Hey!” I glower at him, but I can’t help but smile at the same time. Rocky chortles. “You know what I mean. Do you still think relativity is stupid?”
Rocky is quiet for a moment. “Human science about speed of light is... strange. But Rocky trip to Tau also strange. If trip true… maybe human science also true.”
I’ll count that as a successful lesson! I start gathering the props and putting them away, eating the last two Skittles while returning the pieces of lab equipment to their various storage compartments.
“Grace.”
I turn to look at Rocky. His carapace is lower than usual. “What’s up, bud?”
“If true, time on Erid not same as time on ship.”
“That’s right. Based on what you’ve told me, I would estimate that Erid has experienced seven more years than you have. And they’ll experience another seven extra years on your journey home.”
Rocky lets out a low, mournful whistle, and his voice drops an octave. “Adrian wait fourteen more years than Rocky.”
Suddenly there’s a pit in my stomach. “Uh... Yeah. Yeah, that’s true.”
Rocky is tapping his claws together again. “Is not... need word.”
“What word do you need?”
“Not equal. One person gets good thing, other person gets bad thing.”
I try to swallow, but my throat feels tight. “‘Unfair.’ Our word for that is ‘unfair.’”
The laptop is still open on the lab table. Rocky says his word for "unfair," and I add it to the database.
“Is unfair Adrian wait more time than Rocky. Rocky choose to go on mission. Ask Adrian to understand, to support. Rocky not know what Rocky ask. Not know Adrian wait more time than Rocky.”
His carapace sinks to the floor. “59 years long long long time. 73 years worse. Rocky choose to leave, not Adrian, but Adrian wait more time.”
I hang my head, but I don’t say anything. What is there to say? He has a family and a life to return to. Of course he’s mourning the lost time.
Maybe Steve Hatch was right and relativity is an invitation from God or the universe to explore the cosmos. Heck, the Hail Mary could reach the edge of the observable universe in one human lifetime, if we had enough fuel.
But interstellar travelers will always pay a price in the form of lost time. The farther we travel, the more we become unmoored from our home planets, unable to return to a home that no longer exists.
Compared to Rocky, relativistic travel hasn’t cost me very much. I didn’t exactly have a busy social life back on Earth, even before the “militaries of the world tour” that brought me to Stratt’s Vat. In fact, when we eventually part ways, I think I’ll miss Rocky more than anyone I’ve missed from Earth.
I get to live, but I will never see Rocky again.
I shake my head to clear it.
“Tell you what. When we solve the Astrophage problem, and we’re ready to head home, I’ll calculate your trajectory for you. You’ll know exactly when to reverse thrust, and you’ll know exactly how long the trip will take. No surprises this time.”
It takes a moment, but eventually Rocky lifts his carapace off the floor. “Yes. Thank thank thank. Now Grace Rocky make chain, find predator, save stars.” His voice is still a little shaky, but at least it’s in the usual octave again.
“Yes yes yes. I’ll make more molds while you prepare the xenonite.”