AI Summary
5 min readIn 2015, NASA launched an experiment that Einstein could only dream of making real. They took two men with the same face—the identical twin astronauts Mark and Scott Kelly—and put one on a rocket ship in Kazakhstan while the other drove home to Arizona. Scott spent nearly a year aboard the International Space Station while Mark stayed on Earth as a control. The goal was not to test Einstein's famous twin paradox, which predicts that a twin traveling at relativistic speeds would return younger than the one who stayed home. The ISS moves at only 0.002% of the speed of light, so the time dilation effect would be a measly five to eight milliseconds over 340 days. What NASA actually wanted to know was something far stranger: what does the absence of gravity do to a human body built for its presence?
The body rebels against weightlessness
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What you'll learn
- 1 (00:00) **Episode Setup and the Twin Premise** - Hannah introduces the mystery of two men (twins) where one goes to space and the other stays home, setting up the real-life version of Einstein's twin paradox.
- 2 (11:44) **From Thought Experiment to Real Experiment** - Hannah explains that NASA couldn't send Scott near light speed, so they used the next best thing: a year on the ISS.
- 3 (18:38) **Why NASA Actually Did the Twin Study** - Hannah reveals the study wasn't really about time dilation; it was about the physical and biological effects of being in space.
- 4 (19:22) **Space Ruins Your Eyes** - Hannah describes how microgravity permanently changes astronauts' vision due to fluid shifts.
- 5 (21:07) **Two Liters of Coke Shift to Your Head** - Hannah explains the dramatic fluid redistribution in space: the equivalent of two large bottles of soda moves from your legs to your head.
- 6 (23:34) **Your Body Is Built for Gravity** - Michael and Hannah list the many ways the human body depends on Earth's gravity to function properly.
- 7 (26:27) **The Cave Experiment: Time Without Cues** - Hannah tells the story of a 23-year-old geologist who lived in a cave for 63 days with no time references.
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Guests on this episode
Show Notes
Does space travel make astronauts younger?
Professor Hannah Fry and Michael Stevens explore Einstein’s twin paradox, time dilation and NASA’s famous Twins Study with astronauts Scott and Mark Kelly.
After Scott Kelly spent 340 days aboard the International Space Station, he returned milliseconds younger than his identical twin. But microgravity had also affected his eyesight, bones, blood, gut microbiome, body temperature, genes and telomeres.
So what exactly happens to the human body in space, and which changes remain after astronauts return to Earth?
In this episode, Hannah and Michael discover how deeply the human body depends on gravity. Along the way, they uncover why your head ages faster than your feet, what a 63 day cave experiment reveals about our sense of time, and whether future humans could ever truly adapt to life beyond Earth.
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