Written by OC Wanderer, author of Destiny Among the Stars, a LitRPG sci-fi series set in the real stellar neighborhood. Published August 21, 2026.

In the grain-of-sand post, we looked at shielding a ship against dust at 20% of light speed. Suppose that protection works: the crew still faces about 22 years in transit, with time dilation trimming only about 2 percent. I want to push that possibility further. How fast would we have to travel before relativity opened up stars we could otherwise never reach within a lifetime?

Long-voyage science fiction is a big part of why this question grabs me. Alastair Reynolds's House of Suns and Pushing Ice are touchstones for me because they make deep time part of the scale of travel. Their crews cross enormous distances and come back to a universe that has aged far more than they have. In an interview, Reynolds says he often keeps his space opera within Einstein's speed limit because it gives the stories their sense of scale.1 That makes me wonder how far Einstein's rules could carry a living crew, and what sort of homecoming those rules would allow.

Time dilation really does leave that possibility open for the people aboard. Imagine spending 40 years traveling to a star a thousand light-years away, or reaching Alpha Centauri in six weeks of ship time. The arithmetic allows both, if a ship could reach the required speeds. What grabs me is the difference between reaching another star and still belonging to the world you left behind.

The clocks let a crew cross a thousand light-years in 40 years

At speeds close enough to light, a crew's clock records only a small fraction of the time that passes on Earth. My post on aging at 10% of light speed worked through the formula and the airliner and GPS clocks that confirm it. Optical atomic clocks have since measured the effect at everyday speeds, below 10 meters per second.2 At speeds much closer to light, that small clock difference grows large enough to change which stars a crew could live to see.

Suppose we set our sights on a star 1,000 light-years away and give the crew 40 years to reach it. For the moment, let the ship start and stop instantly: an idealization that isolates the clock arithmetic, not something a real crew could endure. The speed we need is about 99.92 percent of light speed. The travelers age 40 years; roughly 1,001 years pass on Earth.3

The part I love is that the crew also measures a shorter distance to the destination. Einstein's length contraction means that, in the ship's frame, the 1,000 light-years between the stars has contracted to about 40 light-years. Crossing that distance at nearly light speed takes about 40 years. Earth's astronomers describe a thousand-year journey, while the crew can cross the distance in a working lifetime. Both descriptions agree on departure and arrival; they give the travelers very different calendars to live by.

That opens up an enticing question: how far could we go if we had 40 years aboard? A star 10 light-years away needs about a quarter of light speed. Stretch the destination to 1,000 light-years and we need the 99.92 percent above. A crew's lifetime can accommodate either journey on paper, but the farther star demands a far more punishing speed.

For Alpha Centauri, the same arithmetic gives my book's timeline. At 99.965 percent of light speed, a crew would cover the 4.37 light-years in about six weeks of ship time. That is the Triumph's travel time in Destiny Among the Stars. I find it thrilling that real relativity allows the same six weeks aboard a sublight ship, provided it could reach that speed.4

Reaching 99.92 percent of light speed costs 516 megatons per kilogram

At 99.92 percent of light speed, each kilogram of ship carries the kinetic energy of about 516 megatons of TNT, ten times the largest nuclear bomb ever tested.5 Arrival requires removing all of it again, and every kilogram of fuel and shielding raises the energy needed. This is where the trip starts to look daunting even before we choose an engine: André Füzfa's models of radiation-powered ships identify the tremendous energy cost as the central problem.6

Now imagine scaling that kilogram up to a ship with room for people. Füzfa's 100-tonne rocket on a trip of several years needs about 15 times the world's energy production for a full year. Our 100-tonne ship at 99.92 percent of light speed would carry about 24 times the energy of that modeled rocket as motion alone. That is the scale of the project before we add the energy cost of carrying fuel and shielding, or work out how to shed all that motion at the destination. Giving travelers somewhere to live makes a formidable difference from the gram-scale sail in note 6.

For the Triumph, the energy question becomes how much its fictional drive can draw from the capacitors. In Book 1, Danny explains that the FTL drive first raises a Reality Anchor Field and then skips the ship between folds of spacetime, and Ryan warns that a field too big draws exponentially more power from the capacitors, leaving less for the jump. Even in fiction, I like having a crew whose route depends on what the ship can afford to do.

Every atom along the route hits like a cosmic ray

I picture all that apparently empty space ahead of the bow, and then remember what even its sparse gas means at this speed. At 99.92 percent of light speed, every hydrogen atom in the ship's path strikes with the energy of a cosmic-ray proton. In relativistic-flight models, the gas becomes a beam of fast particles hitting the bow, growing more punishing as the ship accelerates. Radiation inside the shielding is already a concern for probes like Breakthrough Starshot's at about 0.2c; a shield sized for that speed would need to be redesigned for 99.92 percent. The dust from the grain-of-sand post hits more than a thousand times harder than it did at 0.2c.7

How fast could we go before the route itself became the barrier? Yurtsever and Wilkinson put the threshold for antiproton production in collisions with interstellar protons at a Lorentz factor of 9.1. Our ship's factor is about 25, already in what they call the extreme range. A journey that fits a human lifetime would ask us to send those humans through a stream of particles energetic enough to create antimatter in collisions.8

The dust follows my crew into fiction, too. Before a short FTL hop from Proxima to Alpha Centauri A in Book 3, Ryan warns that every dust grain shocks the containment bubble. The Reality Anchor keeps the ship tethered to the known laws of physics, but pushing harder drives its load past design spec. A faster route still leaves him watching what the ship can take.

Two crew members at the controls of the Triumph as stars streak past the window

Earth ages 1,001 years while the crew ages 40

After 40 years aboard on the 1,000-light-year trip, the crew steps out into a world where Earth's clock has run on by 1,001 years. A crew that turned around and flew home at the same speed would return to an Earth about two thousand years older, so a voyage like this is a one-way emigration.9

The Alpha Centauri case shows it at human scale. The crew's six weeks each way total about twelve weeks aboard, while Earth counts about 8.7 years for the same round trip, before any time spent at the star.10 A crew of twenty-year-olds would come home to friends nearing thirty, and every conversation with home would lag by more than four years each way.

For Destiny, I wanted a crew who could survey another star for three months and come home to the same people. That is the homecoming I want to explore: the discoveries matter to people who are still there to hear about them.

So is a lifetime enough?

Time dilation leaves me with a remarkable possibility: in the idealized instant-start, instant-stop case, a crew at 99.92 percent of light speed could reach a star a thousand light-years away in 40 years aboard. The propulsion and shielding remain formidable obstacles. But even if we solved them, Earth's roughly 1,001 years would separate the travelers from everyone who watched them leave.

I want the distant stars and the homecoming. Could there be a way to get both? That question brings me to the limit we have been approaching: Why is the speed of light the speed limit, exactly?

If you want to see a crew that can travel and still go home, start Destiny Among the Stars.

Endnotes

  1. Alastair Reynolds, official novels list; Michael Flett, “Alastair Reynolds: Revelations from Beyond the Aquila Rift,” interview, September 28, 2014. ↩
  2. Chin-Wen Chou et al., “Optical Clocks and Relativity,” Science 329 (2010), 1630–1633, DOI. ↩
  3. Albert Einstein, “On the Electrodynamics of Moving Bodies” (1905), English translation. For constant-speed cruise, crew time is distance divided by speed and the Lorentz factor. Setting 1,000 light-years and 40 crew-years gives a Lorentz factor of 25.019992, speed of 0.999200959c, and Earth time of 1,000.79968 years, with instantaneous acceleration and braking; a real ship would spend part of the trip accelerating and braking. Length contraction, the shortening of a moving body along its direction of travel by the same factor, is in section 4 of the same paper; the crew-frame distance is 1,000 ÷ 25.02 = 39.97 light-years. For a 40-crew-year trip, the required speed is D ÷ √(D² + 40²) in units of c: 10 light-years gives 0.2425c and 41.2 Earth years, 100 light-years gives 0.9285c and 107.7 Earth years. ↩
  4. Author calculation with the relation in note 3, for 4.37 light-years: six weeks (42 days) of crew time requires a Lorentz factor of 38.0, or 0.99965c; Earth time is 4.3715 years one way. ↩
  5. Author calculation: (γ − 1)c² with γ = 25.019992 gives 2.158810 × 10¹⁸ J/kg; at the NIST TNT energy equivalent of 4.184 × 10¹⁵ J per megaton, 516 megatons. The largest nuclear test, the Soviet Tsar Bomba of 1961, yielded about 50 megatons. ↩
  6. André Füzfa, “Interstellar Travels Aboard Radiation-Powered Rockets,” Physical Review D 99 (2019), 104081, abstract, which gives the gram-scale sail flyby of Proxima Centauri as about one day of a 1 GW power plant; the 100-ton, several-year mission (about 9 × 10²¹ J, about 15 times world energy production in 2017) is in the paper's conclusions (arXiv full text). Author calculation for the ship: 2.158810 × 10¹⁸ J/kg (about 2.16 quintillion joules per kilogram) × 100,000 kg = 2.16 × 10²³ J, about 24 times Füzfa's 9 × 10²¹ J. ↩
  7. Oleg G. Semyonov, “Radiation Conditions in Relativistic Interstellar Flight,” Acta Futura 12 (2020), abstract; Philip Lubin, Alexander N. Cohen and Jonathan Erlikhman, “Radiation Effects from ISM and Cosmic Ray Particle Impacts on Relativistic Spacecraft,” The Astrophysical Journal 932 (2022), 134, abstract, which addresses spacecraft like Starshot’s; the 0.2c speed for those probes is the figure used in my grain-of-sand post. Author calculation: dust impact energy per unit mass, 2.1588 × 10¹⁸ ÷ 1.8533 × 10¹⁵ J/kg ≈ 1,165 times the 0.2c value. Proton energy (γ − 1) × 938.272 MeV is 19.3 MeV at 0.2c (γ = 1.0206) and 22.5 GeV at 0.99920c (γ = 25.02). ↩
  8. Ulvi Yurtsever and Steven Wilkinson, “Limits and Signatures of Relativistic Spaceflight,” Acta Astronautica 142 (2018), 37–44, arXiv. Yurtsever and Wilkinson give a Lorentz factor of about 1.24 × 10⁸ for pair production in an ordinary hull by microwave-background photons, 9.1 for antiproton production in proton collisions, and 1.3 for pion production, which they call the safe limit in the interstellar medium (arXiv full text). ↩
  9. Author calculation: 1,000.8 years to arrive, plus 1,000 years for a radio reply to cross 1,000 light-years. A round trip at the same speed takes about 2,002 Earth years. A message about the arrival would need another 1,000 years to reach Earth, so Earth would receive news of the landing about 2,000 years after departure. ↩
  10. Author calculation with the relation in note 3, at 0.99965c over 4.37 light-years each way: Earth time 8.74 years, crew time 84.5 days. ↩