Time Isn’t As Fixed As You Think
Imagine this: You’re born just six seconds after your twin. Decades later, after a trip to space, you return to Earth slightly younger than them.
Sounds like science fiction, right? Well, for astronaut Scott Kelly, this bizarre twist on aging is real.
While Scott spent 340 days aboard the International Space Station (ISS), his twin brother, Mark Kelly, remained on Earth.
When Scott returned, he had aged five milliseconds less than Mark. The once six-second gap between them had stretched ever so slightly.
This isn’t a sci-fi plot; it’s Einstein’s theory of relativity in action.
More specifically, it’s a phenomenon called time dilation—which means that time moves differently depending on how fast you’re going and how close you are to a strong gravitational field.
So, what exactly happened to Scott Kelly? And could this effect ever be large enough for us to notice in everyday life?
The Secret to Aging Slower
Einstein’s special theory of relativity, published in 1905, laid the foundation for understanding how time isn’t absolute.
Instead, it’s relative—it changes depending on how fast you’re moving.
One of the key takeaways from Einstein’s work is the idea that the faster you move, the slower time passes for you compared to someone at rest.
This effect becomes significant at speeds approaching the speed of light, but even at lower speeds—like orbiting Earth at 28,200 km/h (17,500 mph)—it can still be measured.
Scott Kelly’s extended stay aboard the ISS meant that he was moving much faster relative to his brother on Earth.
As a result, he experienced time just a tiny bit slower than Mark did. That five-millisecond difference might not seem like much, but it’s real and measurable.
A Mind-Bending Thought Experiment
The Kelly twins’ experience brings to life a classic physics problem: the Twin Paradox.
Imagine two twins—one stays on Earth while the other travels to space at near-light speeds and then returns.
According to relativity, the space-faring twin would have aged less than their sibling due to time dilation.
But here’s the twist: from the astronaut’s perspective, their sibling on Earth is the one moving away. Shouldn’t that mean that Earth’s twin ages slower instead?
This paradox is resolved when we factor in acceleration and deceleration.
The twin on the spaceship experiences acceleration when leaving Earth, changing direction, and returning.
This acceleration breaks the symmetry and confirms that the astronaut ages more slowly.
Scott’s experience wasn’t extreme enough to be noticeable without precise instruments, but it confirms that time dilation isn’t just theoretical—it happens in real life.
Proof Beyond the Kelly Twins
The Kelly twins’ case isn’t the only time scientists have tested time dilation. In the 1970s, physicists conducted the Hafele-Keating experiment, where atomic clocks were flown on airplanes around the world.
When compared to stationary clocks on Earth, the airborne clocks showed a small but measurable lag—just as Einstein predicted.
More recent experiments with GPS satellites have reinforced this concept.
GPS systems require corrections for time dilation because they orbit Earth at high speeds and experience weaker gravity than we do on the ground.
Without these adjustments, GPS locations would be off by several kilometers per day.
The Bigger Concern
Scott Kelly’s time dilation was minuscule, but his experience in space was still profound.
NASA used the Kelly twins for a landmark study to examine how long-duration space travel affects the human body.
The results were eye-opening:
- Telomere Length: Scott’s telomeres—DNA caps linked to aging—were longer in space but shortened quickly after returning.
- Gene Expression Changes: Over 7% of Scott’s genes behaved differently after his trip.
- Vision and Bone Density Issues: Space travelers often experience vision loss and weakened bones due to prolonged weightlessness.
In short, while time dilation was a fun scientific curiosity, the real concern for astronauts is how space alters the body over extended periods.
Can We Exploit Time Dilation?
If time dilation affects astronauts at speeds of 28,200 km/h, what would happen at near-light speeds?
Theoretically, if you could travel at 99.999% the speed of light, you could experience one year while centuries pass on Earth.
This concept fuels sci-fi ideas about interstellar travel—where astronauts could potentially explore distant galaxies without aging much themselves.
But for now, we’re far from reaching such speeds.
The fastest human-made spacecraft, NASA’s Parker Solar Probe, moves at 692,000 km/h (430,000 mph)—still only 0.064% the speed of light.
Wrapping It Up
Scott and Mark Kelly’s age gap is now six seconds and five milliseconds instead of just six seconds.
This tiny difference might not change their lives, but it proves that time dilation is real.
More importantly, their experiences give us a glimpse into the challenges of space travel.
Beyond the mysteries of time, we still need to solve the biological and psychological impacts of living beyond Earth.
As Mark Kelly joked when his brother returned:
“If 10 years from now, I look like I’m 60 and he looks like he’s 80, you’ll know what happened.”
One thing’s for sure—when it comes to aging, speed is everything.
