# Time dilation question for short scifi story

Hi all! This is a neat forum; wish I'd found it sooner...

I'm writing a short scifi story, and am having trouble understanding time dilation, which happens to be an integral part of the story. Anyway, here's my question:

Its common scifi staple where starfarers travel at close to the speed of light from point A to point B, and back. Upon their return to point A, the travellers aged little compared to those who remained on A. Since motion is relative (so you could say that the travellers remained in place, while point A accelerated away), then why couldn't the folks who remained on A be the ones who experienced a shorter passage of time?

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ZapperZ
Staff Emeritus
Hi all! This is a neat forum; wish I'd found it sooner...

I'm writing a short scifi story, and am having trouble understanding time dilation, which happens to be an integral part of the story. Anyway, here's my question:

Its common scifi staple where starfarers travel at close to the speed of light from point A to point B, and back. Upon their return to point A, the travellers aged little compared to those who remained on A. Since motion is relative (so you could say that the travellers remained in place, while point A accelerated away), then why couldn't the folks who remained on A be the ones who experienced a shorter passage of time?

The motion of A isn't relative. A will have to accelerate away from B, slows down, turns around, and then accelerates again towards B, and then slows down again to meet B it B's reference frame. B continues to be in an inertial frame thoughout this ordeal by A. So the situation here isn't symmetric.

Zz.

A and B are fixed points, relative to each other. So the motion in question is A relative to the spaceship (let's call this S), not to B. So either S travels from A to B, then back to A; or A and B simultaneously move, relative to S, then retrace their motion. Either way, at the end of the exercise, A and S are back on the same spot. So why should S experience less passage of time, instead of A?

I can't escape the feeling that I'm missing a key element in the above scenario. Sorry if I sound so ignorant in this matter, because I am. :(

The motion of A isn't relative. A will have to accelerate away from B, slows down, turns around, and then accelerates again towards B, and then slows down again to meet B it B's reference frame. B continues to be in an inertial frame thoughout this ordeal by A. So the situation here isn't symmetric.

Zz.
I don't disagree with what you wrote. But I wonder if it would satisfy a curious mind who might then point out: But each one sees the other one accelerate. For this reason, I would emphasize that acceleration is an effect that can be felt and only one of the two feels the effect. That is what is not symmetric.

Hello jimmy. I didn't realize it was important to establish who actually "feels" the acceleration. Anyway, couldn't the scenario be setup so effects of acceleration is neutral? We could start with S already at near-lightspeed as it whizzes by A, then later have A and S then execute a similar (but opposite) acceleration to meet a second time.

Chi Meson
Homework Helper
The time dilation effects of SR are meaningless until the two subject return to the same "space-time." If one of the subjects never changes their "space-time" then there is a fundimental difference between the motion of the two.

As Richard Feynman explained it, give the two twins a brimming cup of hot coffee. One twin sits in an easy chair and waits, the other gets into his rocket ship. Upon blastoff, which one gets coffee all over his lap? That's the guy who is changin his space-time. Then he has to change again to return.

And if S whizzes by A at near lightspeed, that is not "acceleration," that is just "very fast constant speed." And they can never compare clocks while going at this relative speed (each will see the other guy's clock running slow). The process of clock synchronization would be very difficult, requiring some pre-planned process. Either way, one of them would have to accelerate to come into the other's space-time, and in so doing, the one who accelerates will get the benefit of travelling through less time.

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ZapperZ
Staff Emeritus
A and B are fixed points, relative to each other. So the motion in question is A relative to the spaceship (let's call this S), not to B. So either S travels from A to B, then back to A; or A and B simultaneously move, relative to S, then retrace their motion. Either way, at the end of the exercise, A and S are back on the same spot. So why should S experience less passage of time, instead of A?

I can't escape the feeling that I'm missing a key element in the above scenario. Sorry if I sound so ignorant in this matter, because I am. :(
I read your question a bit to quickly, but it doesn't change anything. So let's call those two people 1 and 2, where 2 stayed behind.

Acceleration isn't relative. An inertial reference frame is where you can't tell if you're moving or not. But if you are accelerating, you can perform experiments to detect this, and so can others to detect that you are accelerating. So the one that had to go out and come back HAD to undergo at few periods of acceleration and deceleration. This is what breaks the symmetry between 1 and 2.

Zz.

Hello jimmy. I didn't realize it was important to establish who actually "feels" the acceleration. Anyway, couldn't the scenario be setup so effects of acceleration is neutral? We could start with S already at near-lightspeed as it whizzes by A, then later have A and S then execute a similar (but opposite) acceleration to meet a second time.
Consider twins who leave A, one headed for B, the other headed for C

B <-------------------------------- A --------------------------------> C

Upon arrival, both turn back (which means both accelerate) and meet again at A. Then the situation will be symmetric, and both twins will be the same age when they reunite. If anything is done which is not symmetric, then you cannot guarantee that the twins will be the same age when they meet again.

Keep this in mind. In order for two people to meet at the same place twice, either they stay together, or if they part, then at least one of them must accelerate. See if you can think of a counterexample. It is related to the fact that two straight lines intersect in no more than one point.

Chi Meson
Homework Helper
I just wanted to point out that Orson Scott Card is one of the few SF writers that correctly use time dialation in their novels

Chi, your (Feynman's) example makes practical layman sense. Thanks! Fortunately, Card is one of my favorite authors. I also have his book "How to Write Science Fiction and Fantasy", part of which he talks about different modes of interstellar travel.

Consider twins who leave A, one headed for B, the other headed for C

B <-------------------------------- A --------------------------------> C

Upon arrival, both turn back (which means both accelerate) and meet again at A.
Ah, here's my second question. Do you read minds, jimmy?

During their near-lightspeed travel apart, they each see each other's clocks moving slower, right? If during their return trip, then then travel at normal sublight speed, how do their perceptions ever reconcile? As far as I can see, they never perceive each other's clocks moving faster.

During their near-lightspeed travel apart, they each see each other's clocks moving slower, right? If during their return trip, then then travel at normal sublight speed, how do their perceptions ever reconcile? As far as I can see, they never perceive each other's clocks moving faster.
If they could turn on a dime, then each one would see the other's clock jump forward by just the right amount at the moment that they change direction. In the more reasonable scenario that they turn around more gradually, the clock jump is more gradual too.

"See" in this context is such an ugly word. They will have to wait until the recording clocks send their reports in before they know what they would have seen if only they could have.

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In the more reasonable scenario that they turn around more gradually, the clock jump is more gradual too.
Let's say they both gradually decelerate to a dead stop at points B and C, then cruise back to A at normal speed. So you're saying that even if the return trip is "gradual", there will still be time dilation effects that eventually reconcile their clocks, but this time spread out over a much longer time and distance?

Let's say they both gradually decelerate to a dead stop at points B and C, then cruise back to A at normal speed. So you're saying that even if the return trip is "gradual", there will still be time dilation effects that eventually reconcile their clocks, but this time spread out over a much longer time and distance?
Exactly. The slower they go, the smaller the effect, but the longer it lasts. I recommend that you read an introductory text like Taylor and Wheeler's book on Spacetime physics if you are up to a little math. It doesn't go beyond high school level. Perhaps other people here may suggest some other books. A little effort will pay off. If not, there may be an embarrassing ratio between science and fiction in your science fiction.

Thanks for your help, jimmy. Its a bit clearer now, but just to be safe, I'll still be hitting some physics books. The Taylor&Wheeler book looks interesting. I wish there were cheaper alternatives, though. Besides picking your good gentlemen's brains, that is.

The website that got me interested in it (and physics as a whole actually) is: http://www.einstein-online.info/en/elementary/index.html , it's very simplified, but it's a great place to start. It gives a general map of the ideas of relativity so that once you read more specific texts you know where you are.

I joined this website for the exact same question, here's the thread: https://www.physicsforums.com/showthread.php?t=150894 . it's also filled with links and great explanations that definitely helped me (I remember someone explaining the twin paradox without even having to take acceleration into account that really clarified the whole thing for me).

Einstein's book on relativity is also available online for free http://www.gutenberg.org/etext/5001 (though ironically, I think his book is the worst source for understanding his theory. He might have been a genius at physics, but certainly not at explaining things clearly— sorry for the blasphemy).

The wikipedia page on special relativity also is filled with links to sites that explain relativity in many different ways, some websites have really cool animations (I can't remember where I saw "relativistic art," 3D renderings and animations of various relativity-related situations)

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Chi Meson
Homework Helper
Chi, your (Feynman's) example makes practical layman sense. Thanks! Fortunately, Card is one of my favorite authors. I also have his book "How to Write Science Fiction and Fantasy", part of which he talks about different modes of interstellar travel.

Ah, here's my second question. Do you read minds, jimmy?

During their near-lightspeed travel apart, they each see each other's clocks moving slower, right? If during their return trip, then then travel at normal sublight speed, how do their perceptions ever reconcile? As far as I can see, they never perceive each other's clocks moving faster.
If you can find a copy of Paul Hewitt's "Conceptual Physics" text, you will get perhaps the best "elementary" treatment of Special Relativity, and you won't need much math to follow it. He has a simple algebraic explanation of how the two systems "reconcile" motions of each other's clocks. He made an animation of this explanation. I can't find it on Youtube but you can purchase the DVD of his relativity lectures which includes the animation, but it's an absurd \$145.
http://www.arborsci.com/detail.aspx?ID=780