Twin Paradox
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Basic Information

The Twin Paradox refers to a thought experiment (later confirmed by physical experiments involving airplanes, and by GPS satellites) that illustrates that someone who takes a journey at speeds approaching that of light ages much more slowly than someone who stays home and avoids that trip.

Let's say we have two Twins, Adam and Brad. Adam becomes an astronaut, and hops into a spacecraft for a voyage to the nearest star other than our own sun. It's 4.45 light years away, and his spaceship travels at 86.6% of the speed of light.

  • From Brad's point of view back on earth, the voyage takes his brother 10.28 years. So, if Adam left in 2050, his return would be expected in 2060.
  • From Adam's point of view, however, the trip is much shorter. Time dilation occurs, and he's left thinking that the trip took only 5.14 years. It's still 2060 when he gets home, but in the meantime he's only experienced 5 years of aging and memories. He now looks noticeably younger than his twin brother.

Explaining the Paradox

This whole situation can be rather confusing and counter-intuitive at first glance. Relativity seems to suggest that each would think the other was undergoing the time dilation - so why does one age faster than the other? Shouldn't they each think the other is aging more slowly, and thus it would even out in the wash? The answer is "no, the astronaut twin really does age more slowly and experience less time".

The Twin Paradox isn't really a paradox - it's natural and logical, it just looks paradoxical until the math and physics behind it are explained. The reason that relativity doesn't apply is because the two brothers do not share an inertial frame of reference. Yes, either can be seen as traveling relative to the other, but only one of the two actually shifts his frame of reference and experiences acceleration.

Adam, the astronaut, accelerates to relativistic speeds and feels immense g-forces. He does so again when he reverses course at the midpoint of his trip. He decelerates again at the end of his trip, and once again feels g-forces. During all those periods of acceleration and course-correction, his frame of reference was shifting significantly.

Meanwhile, Brad, his twin back home, never experiences such large-scale accelerations, and never leaves his original inertial reference frames. As a result, his timeline does not contract, but his astronaut brother's timeline does.

By the way, the g-forces are not the cause of the twin paradox, they're just tangentially useful to the conversation. The cause of the disparities in aging is actually the change of reference frames. I just mention the g-forces because they're essentially symptomatic of the change of reference frames. Trying to understand the frames is tricky - it's sort of an abstract concept, and you might not grasp why one Twin is changing frames and the other isn't. However, g-forces are something familiar you feel every time you step on the gas in your car. It's a force you can feel that clues you in that you're changing frames of reference. If a random car driving past my house accelerates, I (in my living room) don't feel any g-forces from it, because the car changed frames of reference, not me. By the same token, the guy in space ages slowly because he changed frames of reference, and the guy back on earth did not.


The same will happen to anyone traveling at relativistic speeds between the stars. The trip across the galaxy takes a certain amount of time for the astronaut / passenger, and that amount of time is always less than the amount of time that is experienced by the inhabitants of the planets they travel to and from.

If you have children back home when you leave on a spaceship, you're probably going to miss their childhood.

Given long enough distances and high enough speeds, astronauts may return home to find entire civilizations have risen and collapsed in their absence.

The first mission sent to a distant planet may actually be the second to arrive. This could happen if significant advances in stardrive technology had occurred after the first vessel left home. It may not be practical to cancel or reroute the first mission, even if the later missions can outpace it.

If Adam were traveling at 99.995% the speed of light, and traveled for what he perceived to be a year, he'd arrive back at the earth 100 years later (local time) than when he left. Compare that to the early scenario where traveling 86.6% the speed of light resulted in just 2 years passing on earth for every one spent in transit. It's clear that there's a steep curve to the dynamic linking speed of travel and the variation in years experienced. The faster you go, the less time you experience.

See Also:



  • Joe Haldeman's The Forever War deals intimately with the effects of relativistic travel and the disconnect between the men sent out to fight and the dystopia that is emerging back home.

Here's two videos that might help explain this if my layman's text above wasn't intelligible enough: YouTube video 1 and YouTube video 2.

Game and Story Use

  • The "Implications" section above has a few ideas.
  • Fun characterization for a pair of identical twins - one is ten years older and wiser than the other (though the other is no slouch, either, since they qualified for the astronaut program).
  • The Twin Paradox has some implications regarding space merchants, interstellar militaries, and astronaut programs. Whereas the hardship of splitting up a family for months or even a few years is something modern militaries don't shy away from, when relativistic travel is involved, you'll almost certainly never see your family again. Either space militaries will have to recruit bachelors/bachelorettes exclusively, or they'll have to accommodate families onboard spaceflights.
  • Many versions of FTL will be able to sidestep this issue by making space travel instantaneous for both the traveler and the planetary inhabitants.
    • However, some FTL concepts may still require long travel times, especially in-system sublight maneuvering flights, and thus may hit upon such issues. The GM has a lot of freedom in this, since you're talking about alternative science and technology that defies physics anyway.
    • In Relativity, travel at FTL speeds is basically the same thing as Time Travel. If some miracle phlebotinum allowed you to accelerate to speeds faster than light, you'd actually be able to arrive before you left. The faster you accelerate to, the less time you experience, and at FTL you'd (theoretically) experience negative time.
  • Don't forget the Planet Of The Apes Ending - the Twin Paradox is what justifies it!
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