How Much Would a Crew Age Differently Traveling to Alpha Centauri at 10% Light Speed?

Written by OC Wanderer, author of Destiny Among the Stars, a LitRPG sci-fi series set in the real stellar neighborhood. Published August 17, 2026.

We explored how a ship built around the dust might cross the Oort Cloud, and that at one percent of light speed the crossing alone would take generations. Speed that ship up to ten percent of light speed, and I start wondering about the crew's clocks.

“I wanted my story to be FTL, otherwise we'd never get there!” That was my reason for giving Destiny Among the Stars a faster-than-light drive, and it left me curious about the trip I skipped. Relativity says a moving clock runs slow. If a crew flew to Alpha Centauri at one tenth the speed of light, how much less would they age than the people they left on Earth, and would any of them feel it?

Forty-three years aboard a starship, and the crew gains only about eighty days on the people back home. I find that wonderfully strange: we can cross the gap to another sun and come away with a time difference shorter than a summer. Relativity is doing exactly what it promises. The question is how much of a life that promise buys us.

Forty-three years for eighty days

Suppose we set a course for Alpha Centauri at one tenth the speed of light. The system is about 4.37 light-years away, and a light-year is the distance light crosses in a year. Our ship needs ten years for each of those light-years, so Earth records 43.7 years between departure and arrival. Imagine setting off at twenty-five and knowing that arrival will come in your sixties.¹

The ship's clock does give the travelers a little time back. At 0.1c it records about 0.995 years for every year in Earth's frame: 43.48 years aboard against 43.7 at home. Those almost identical numbers hide the whole prize, about eighty days.²

A crew member who launched at 25 would step out at about 68 and a half, some 80 days younger than a twin who stayed home. In Destiny Among the Stars, the same trip takes weeks. Early in Book 1, Luca runs the numbers for the crew: four weeks down, two weeks out from Alpha Centauri, and a little over three months for the survey before they head back to Sol. That is the kind of voyage I wanted for them: enough time to discover another system and still come home to familiar faces.

We already carry relativity around with us

What delights me is that we can test this without a starship. Fly precise clocks around Earth and compare them with clocks left on the ground, and their readings come back out of step. Hafele and Keating did that with airliners in 1972; the differences were tens to hundreds of billionths of a second, close to relativity's predictions.³

We can even put a person on the traveling side of the comparison. Scott Kelly's year on the International Space Station brings the effect down to a human life. Over his 340 days circling Earth at about 17,000 miles an hour, he aged about nine milliseconds less from his speed alone, far less than a blink.⁴

GPS depends on the same physics every day. Speed and height in Earth's weaker gravity pull in opposite directions on a satellite's clock, and the net result is a clock running about 38 microseconds a day fast, a microsecond being a millionth of a second. The system is built to correct for that. Left uncorrected, the positions it reports would drift by about 10 kilometers a day.⁵

But hang on: from the crew's seat, Earth is the one flying away. Why shouldn't its clocks be the slow ones? The journeys are different. The traveler changes frames while speeding up and slowing down; Earth stays in the home frame we used for the calculation. Comparing the elapsed time along those different journeys gives the crew the shorter interval.⁶

I like the thought that all this strange clock behavior would accompany perfectly ordinary shipboard days. In Book 1, before the Triumph reaches the Oort Cloud passage where it will ignite the FTL drive, the days are maintenance and calibrations, with movie nights that Joey, one of the crew, insists are critical for morale. A voyage at 0.1c would run on days like those. Breakfast, repairs and Joey's next film would all take their usual time aboard; the eighty-day difference belongs to the comparison with Earth.

What if we push much harder?

What happens if we turn up the speed until the crew's calendar really starts to separate from Earth's? Getting people to even 0.1c is beyond anything we can build today. Breakthrough Starshot, a proposal to reach another star, would use a powerful beam from Earth to push gram-scale lightsails to about 0.2c.¹⁰ Half the speed of light saves the crew about 1.2 years of aging.⁷ At 0.9c the gap becomes much more striking: about 2.12 years aboard while about 4.86 years pass at home, a saving of about 2.74 years.⁸ I can imagine packing for two years away much more easily than for forty-three. Yet everyone at home still has nearly five years to wait. At any speed below light, the aging saved is less than the time that passes on Earth, and the outward voyage takes longer than light's roughly 4.37-year crossing.

In Book 1, the crew sends a probe home at ninety percent of light speed. It would take roughly five years to reach Sol, and Luca, the captain, expects the Triumph, their ship, to beat it home by years.

Now imagine putting the return ticket on that original 0.1c flight. A round trip at 0.1c takes about 87 years on Earth's calendar, before counting any time spent at the star, and about 87 years minus 160 days for the crew.⁹ A crew that launched at 25 would come home past 110, and most of the people who saw them leave would be gone. That is why the trip home matters so much to me as a storyteller. The drive in my story lets the crew survey another star for three months and return to the people they left.

Memories of home across a long voyage.

So how much younger?

About eighty days younger, after living roughly forty-three years aboard. That answer leaves me wondering what a crew would make of its small victory over Earth's clocks. They would have reached another sun, grown old together and gained a few months on home. For the kind of adventure I wanted to write, I needed a way to bring that sun within reach of a much shorter voyage.

Going faster also puts the route at risk. At twenty percent of light speed, a single milligram grain hits with the energy of about 443 kilograms of TNT. What would hitting a grain of sand do to a ship traveling at 20% light speed?

Endnotes

1. NASA Jet Propulsion Laboratory, “The nearest star system” (2013), gives 4.365 light-years; this essay rounds it to 4.37. Travel time is the stated distance divided by 0.1 light-year per year. ↩
2. Markus Pössel, “From light clocks to time dilation,” Einstein Online, Max Planck Institute. The factor is √(1 − v²/c²), which is √0.99, about 0.994987, at v = 0.1c. Author calculation: 43.7 × √(1 − 0.1²) = 43.4809510016 years; (43.7 − 43.4809510016) × 365.25 = 80.00764668 days (0.219 year). ↩
3. J. C. Hafele and R. E. Keating, “Around-the-World Atomic Clocks: Observed Relativistic Time Gains,” Science 177 (1972), 168–170, DOI. Eastbound clocks lost about 59 nanoseconds and westbound clocks gained about 273. ↩
4. Jeffrey Bennett, “Scott Kelly’s Time Dilation,” 0.00946 seconds per year at the station’s speed of about 17,000 mph, or 0.0088 seconds over Kelly’s 340 days. This counts the speed effect alone; the station’s height in weaker gravity pushes the other way, as with GPS satellites in note 5. ↩
5. Richard W. Pogge, “Real-World Relativity: The GPS Navigation System,” Ohio State University Department of Astronomy. The speed effect is about 7 microseconds a day behind and the height effect about 45 ahead. ↩
6. Markus Pössel, “The case of the travelling twins,” Einstein Online, Max Planck Institute. ↩
7. Author calculation with the relation in note 2: 4.37 ÷ 0.5 = 8.74 Earth years; multiply by √(1 − 0.5²) = 0.866025 for 7.569062 crew years; difference 1.170938 years, or 2.34 years out and back. The Lorentz factor γ, the number by which the crew’s clock runs slow, is 1.1547 at 0.5c and 1.005 at 0.1c. ↩
8. Author calculation using the same special-relativistic relation explained by Pössel: 4.37 ÷ 0.9 = 4.855556 Earth years; multiply by √(1 − 0.9²) for 2.116488 crew years; difference 2.739068 years. ↩
9. Author calculation with the relation in note 2, for a steady coast out and back with no time at the star: 2 × 43.7 = 87.4 Earth years; 87.4 × √(1 − 0.1²) = 86.9619 crew years; difference 0.4381 year, or 160.0 days. ↩
10. Breakthrough Initiatives, “Breakthrough Starshot Project to Develop 100 Million Mile per Hour Mission to the Stars,” announcement, April 12, 2016. ↩