Page 105 of The Paradox Club

“Gravity and velocity can both dilate time.”

“You’re not helping. It’s like when I said, ‘Explain magnetic fields’ and you told me that they’re fields that are magnetic.”

“Okay. Let’s take velocity time dilation. Imagine you’re sitting here on Earth.”

“I don’t have to imagine. I am sitting on Earth.”

“But imagine that you have a twin sister, exactly the same age as you.”

“I’d like a sister, but you and Marie never take the hint.”

“This is an imaginary sister, and she’s an astronaut.”

“A ten-year-old astronaut?”

“Yes, the world’s youngest astronaut. And she’s going on a journey through space. She takes off today, when you’re both ten years old.”

“Ten years and sixteen days.”

“Right. Ten years and sixteen days. And she travels through space at half the speed of light. From her perspective, she’s gone for ten years. How old would she be when she came back?”

“Twenty years and sixteen days.”

“Correct. And how old would you be?”

“The same.”

“Wrong. Because the faster you move through space, the slower you move through time. Your sister…”

“Flora.”

“I beg your pardon?”

“My sister’s name is Flora.”

“Okay, by her own clock, Flora has been gone for ten years. But from your perspective, she has been gone for over eleven and a half years. She’s twenty years old, but you’re twenty-one and a half.”

“That doesn’t make any sense.”

“Which is why I didn’t want to explain it to you. If she traveled at the speed of light, from your perspective she’d stop aging altogether.”

“Can an astronaut travel at the speed of light?”

“No.”

“Can astronauts travel at half the speed of light?”

“Not so far.”

“Then what difference does it make?”

“Okay, imagine your sister, instead of traveling through space at half the speed of light for ten years—”

“Which she couldn’t do.”

“Right. Imagine instead she spent six months on the International Space Station, zooming around at about five miles per second. When she came home, you would still be older than her.”

“How much?”