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My understanding of special relativity is that a clock that flies one lap around the earth then returns to its starting position measures less time than a clock that "stayed put" relative to the earth.

Can we imagine a world where a couple might pay to experience "traveling into the future" by flying a lap in a spaceship then returning to earth? When they return, more time has passed on earth than for the travelers. What are some of the issues with this concept? What technology is required to make it work?

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    $\begingroup$ A thought on your title: It is true, in a sense, that one travels to the future via near FTL travel. However, one does not prolong life. If a person would live to, e.g., 93 years age if staying always on earth, then they would do so even if they "jump" to the future. Others, who stayed, will see it as if the travelers prolonged their lives, but they didn't according to the clock that traveled with them. The travelers will still only experience 93 years worth of events, but they will experience a time shift around them and, thus, appear to live longer. $\endgroup$ – Mrkvička May 30 '17 at 7:28
  • $\begingroup$ @Mrkvička true, but medicine and technology will advance on the planet while they are gone. So it's possible that the changes to civilization that took place while they were away would result in an elongation of life EVEN in their own frame if reference. $\endgroup$ – Isaac Kotlicky May 30 '17 at 11:31
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    $\begingroup$ @IsaacKotlicky It is not necessarily true that progress is inevitable; many things could cause regress instead. A simple solar mass ejection (on average Earth is hit by one every 100 years) would fry all electric appliances, including all generators and vaporizing all electric power lines. It would not kill us directly, but we so depend on electrics for food, transportation (cars have batteries and electric elements), medicine, heating & A/C, communications, manufacturing, water and sewage pumps, gasoline pumps, banking - We'd fall into a new dark ages before we could repair it all. $\endgroup$ – Amadeus-Reinstate-Monica May 30 '17 at 14:39
  • $\begingroup$ Worth reading Orson Scott Card's Enderverse series. This concept is present throughout, and is fairly critical in some of the later books. $\endgroup$ – Joe Bloggs May 30 '17 at 15:40
  • $\begingroup$ @Amadeus true, but you'd live longer than if you were in earth during such events... :) $\endgroup$ – Isaac Kotlicky May 30 '17 at 15:55
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Your basic understanding of special relativity is correct. By travelling very fast, in fact, by travelling at velocities extremely close to the speed of light time passes slowly compared to "stay put" people and places.

Yes people with access to near-lightspeed spacecraft could travel away from the Earth and return to what is effectively its future.

Now, while the theory is simple, the practical application is mind-bogglingly difficult. For example, energetically a spacecraft of this sort would consume more power than the human species has used during its entire history to date. This means near-lightspeed spaceships won't be possible for a very long time. This also might be if ever. It is quite possible the human species will never build near-lightspeed spaceships. Travel to the stars may take place at lower, much more practical velocities.

The energetics question for near-lightspeed vehicles is only one of many problems that will need to be solved before this form of space travel is a practical possibility. There are a multitude of other problems all of which make it an effective near impossibility.

There are questions like like the economic cost. Who makes the choice of those sent into space in fast spaceships?

Plus it isn't often realized how close to lightspeed a spaceship will need to go to achieve large time differences during its flight, For example, a spaceship travelling at 99.5% of the speed of light will only have a time dilation of ten. This means if your couple to return to Earth two hundred years in their future, their travel time in the spaceship will be twenty years. This assumes the spaceship's average velocity will be 99.5% of lightspeed. Spending twenty years together in a spaceship might trying for even the most devoted of couples.

Since you are creating a fictional world of your own there is nothing to stop you, as its creator, to assume all the technical problems have been solved. Now you can have a future full of near-lightspeed spaceships and couples can effectively travel into own futures.

This a common assumption of many science fiction stories where special relativity and spaceships travelling at close to lightspeed enable their characters to go into the future. Most of that fiction isn't interested in exploring the details of this kind of travel, it usually assumes it can happen and leaves it at that. So can you. The basic principles are sound. While the science is good, the engineering is unbelievably difficult. Know when to leave well enough alone. Just let it happen and focus on the story about what your couple finds in the future.

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    $\begingroup$ «isn't often realized how close to lightspeed a spaceship will need to go to achieve large time differences during its fligh» I made an interactive web page for that, some years ago. I get a result of 22, not 10, in your example case. $\endgroup$ – JDługosz May 30 '17 at 8:32
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    $\begingroup$ The more I've learned about physics, the more I cannot take a science fiction work seriously if they talk about instant teleportation and faster-than-light travel. Even an Einstein-Rosen bridge seems more likely than teleportation or light travel for me. I don't know if it is such good advice to disregard the practical impossibilities of FTL travel. If it is not an important aspect of the writing, I wouldn't even include it. $\endgroup$ – Neil May 30 '17 at 8:52
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    $\begingroup$ @JDługosz A gamma of 22! In that case I have a bone to pick with my physics lecturer who said 0.999 c had a gamma of ten. At least, this means less time cooped up inside a spaceship to get two centuries into the future. $\endgroup$ – a4android May 30 '17 at 11:08
  • $\begingroup$ @Neil. I know exactly how you feel. My advice was to ignore the practical impossibility of relativistic, near-lightspeed travel.If the OP wanted to get his characters into the future this is a plausible way of doing so scientifically. It may be incredibly impractical, but I don't think the OP was interested in the technical issues. An astronomer said he read science fiction because it didn't get caught in the limits of real science. He was reading SF for relaxation and fun. $\endgroup$ – a4android May 30 '17 at 11:25
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    $\begingroup$ @Neil Your comment touched on an important point. How much realism should there be in SF? Sometimes only sufficient realism, like the engineering, is needed. Other times, the story can be bogged down in getting all the technical details right. My rule of thumb is: If getting the science & engineering right makes for a better story, do it! Otherwise the story's background should be right without deluging the reader with detailed mechanics. There is SF where the science doesn't matter, then the story had better be enjoyable. $\endgroup$ – a4android May 30 '17 at 12:13
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Your couple is probably going to be in for the culture shock of their lives when they return. They probably have no marketable skills, although it very well might be that they end up in a basic income type society and that's not too big a deal.

It's also possible that they end up on the far side of the singularity, what they find then is inherently beyond our ability to predict.

Some fundamental breakthroughs are going to be needed before this is even possible, thus what technology or when is beyond our ability to predict. While we can at least envision the technology needed to push something to relativistic velocity, to get meaningful time travel requires pushing it into a range that we can only currently envision in atom smashers. Even antimatter is far below the energies needed.

Lets take a perfect antimatter-powered stardrive and take a hop to the stars. Specifically, Alpha Centauri, out and back with no time at the destination. We aren't going all that fast, Lorentz factor of 1. That will give 8.6 years Earth time and 4.3 in ship time--and at launch our craft is 15/16ths fuel.

Lorentz factor 2. 8.6 years Earth time, 2.15 years ship time, 255/256ths fuel.

Lorentz factor 3. 1.07 years ship time, 4095/4096ths fuel.

See how prohibitive it gets without pushing you that far into the future?

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  • $\begingroup$ That'd be one expensive trip then. It seems far more likely that they'd create a sort of time capsule into the future where the items contained within would be less corroded. At least a ship weighing little would be easier to accelerate at those speeds. $\endgroup$ – Neil May 30 '17 at 8:55

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