‘How long does it take to get to the sun?’ Jenna asked, catching me off guard with the question. The workday had ended, but we were stuck in the office because a book I was supposed to approve for printing had run into a snag, and Jenna always went home later than almost everyone because she had to wait for her fiancé to pick her up.
I gave the only answer I had off the top of my head. ‘Around eight minutes at the speed of light, but I kind of doubt that’s what you wanted to know.’
Jenna edited most of the English language titles (books about the English language, which were, in this case, in English) and I fielded most of the STEM stuff (also in English), especially the engineering and maths parts of it. We made a good team.
She leaned back into her chair and squinted at her monitor. ‘Here’s a story about people in a spaceship. They’re taking one million minutes to get to the sun. I know it’s fiction, but I want to make sure it’s plausible, at least.’ She pointed at the screen. ‘Doesn’t that seem too long to you?’
I glanced at the story but couldn’t make out any details. It looked like it had a lot of dialogue. ‘The travel time? Seems not long enough, honestly.’ I personally would be more worried about the ‘going to the sun’ part and measuring time in reasonable units, but I let it go. ‘How long is one million minutes?’ I asked her. ‘Never mind,’ I said at her puzzled expression, and started to punch numbers into my calculator. ‘Huh, that’s about two years. I take it back; it’s plausible, but not with anything we have right now, I think.’
‘Two years!’ she exclaimed, looking even more perplexed. ‘But it’ll be much faster in a spacecraft, right?’
I couldn’t quite process that. ‘Come again? Two years is two years.’
‘Wouldn’t time be faster for the people on the spaceship than it is for us? For the people on Earth, I mean.’
It took some time for that to click. ‘Oh. No, no, it doesn’t work like that. You have to be travelling very, very fast for there to be a difference in how time elapses. Besides, when it does work like that, it’s usually the other way round—time on Earth passes faster than on the spaceship.’
‘So time doesn’t go faster in space?’
‘No, not in space itself. It’s the relative motion that makes the difference.’ There was no way I could explain the inertial frame of reference and observers and clocks ticking at different speeds without further confusing the both of us, but I gave it a go anyway. ‘It’s relative, you see. A clock on the spaceship ticks slower relative to one on earth because the ship’s velocity is greater.’
She shook her head with a laugh. ‘I don’t think I understood that.’
‘It’s like one of those classic science fiction tropes,’ I tried, despite it being a long shot. Jenna didn’t care for science fiction—she didn’t even know the basic plot to the original Star Wars trilogy. ‘You know, the guy goes off into space and comes back to find everyone else is much older than he is.’
‘Yes,’ she said excitedly. ‘That’s what I meant!’ That was a surprise; perhaps I had misunderstood her dislike for the genre.
‘That’s because the spaceship is going very fast. If the trip takes two years, it still takes two years; time doesn’t go any quicker for the people on it. For someone observing from Earth, though, they’re taking longer than that. The closer you are to the speed of light, the greater that time dilation between you and someone on Earth. It’s great for stories, but confusing in physics textbooks.’
She laughed again. ‘I’ll take your word for it. That’s what happened in Interstellar, right? He goes away and doesn’t age, but his daughter stays and grows older?’
I racked my brain for the plot of that movie. I remembered that I liked the beginning, but thought the resolution was nonsense. Great, more confusion ahead. ‘Essentially, except that one was because of gravity.’
I made a face. ‘I don’t quite remember how the story goes, but there was a black hole, right? The closer you get to a black hole, the stronger the gravity field is, and the stronger the gravitational pull, the slower your clock ticks.’ I paused and watched as Jenna absorbed that. ‘The dad got too close to a black hole, so time slowed down considerably for him, so his daughter grew up alone.’
She was delighted by that. ‘And that’s why the guy they left on the spaceship got older!’
I didn’t quite remember that detail, but it sounded right. ‘Yeah, because he was further from the black hole than the rest of the crew was.’
She shook her head. ‘I couldn’t make sense of that part when I watched it.’ She tapped the Oxford Dictionary for Scientific Writers and Editors that had a prominent spot on my desk. ‘I don’t really like science fiction and I was never good at science.’
I probably understood more of relativistic time dilation and black holes from science fiction than I did from my textbooks. ‘Why did you watch it, then?’ I asked.
She shrugged. ‘My fiancé is like Christopher Nolan’s greatest fan. It’s all too clever for me.’
‘No, it’s not,’ I protested, my voice rising sharply. Jenna was as smart as anything. I wondered what she would say if she knew we were discussing the basics of Einstein’s theory of relativity. As far as I could recall, that wasn’t even part of the standard school syllabus, not even at pre-U or STPM level—it only crept into undergraduate courses if you were studying physics (unavoidable) or aerospace engineering (usually to contrast it against classical mechanics, along with a note that it was out of the scope of that particular discussion).
‘He was just capitalising on common tropes,’ I insisted. ‘I understood that part because I read too much science fiction, not because he was being spectacularly clever.’ Jenna didn’t seem convinced. ‘Like I said, that trope is used everywhere: The boy goes on a space adventure, and comes back to find his girl dead. Or a grandma. Or there’s a girl who looks a lot like the girl he left behind, but is actually his great-granddaughter.’
Jenna looked surprised. ‘Really?’ Whoops. Maybe I had spoiled half of the time-dilation sci-fi stories for her. Oh well.
‘Don’t ask me about that ending, though,’ I warned her. ‘It made no sense, and I have no idea how to explain it.’
‘Love transcends everything,’ she said dramatically, turning back to her screen. I hoped her short story had a more satisfying conclusion.
I rolled my eyes. ‘Very scientific. I’d like to see a peer-reviewed paper on that.’