[eesmith]

... what? It's been part of SF for decades.

The first three that come to mind are: Heinlein's "Time for the Stars", Haldeman's "The Forever War", and Anderson's "Tau Zero".

From those titles, a DDG search finds http://sf-encyclopedia.com/entry/relativity listing more, also containing the line "Very many sf stories use relativistic time dilation for one-way Time Travel into the future."

[akvadrako]

There are very few sci-fi movies or books where relativity is important and they treat it seriously. Tau Zero is one, I'm not sure about Time for the Stars, but Forever War does have FTL.

I created a list of all these works I could find a few years ago. Here is one link:

https://tvtropes.org/pmwiki/discussion.php?id=l6q90mvssgscsj...

I think this is a complete list of the films and tv series:

  - The Star Lost
  - Pandorum
  - Passengers (2016)

[rsynnott]

Virtually all of Larry Niven's novels that don't have FTL touch on this. Also shows up in most of Alastair Reynolds' novels, and in the Three Body Problem series.

[akvadrako]

I know of Niven's "The Legacy of Heorot", but it only takes place on one planet.

Most of Alastair Reynold's novels have FTL - I only know of one that counts, which is Pushing Ice.

The 3-body problem has FTL also.

[rsynnott]

> I know of Niven's "The Legacy of Heorot", but it only takes place on one planet.

The chronologically earlier novels in his Known Space universe (the Ringworld one) are mostly around near-c Bussard ramjets.

> Most of Alastair Reynold's novels have FTL

Hmm, wait, which ones? Only one I can think of is House of Suns, in a very restricted manner.

> The 3-body problem has FTL also.

It has FTL communications, but not travel.

[akvadrako]

FTL communications is not less problematic than travel.

[eesmith]

Niven's "A World Out of Time" is sublight, with time-dilation.

[grkvlt]

alastair reynold's revelation space series certainly doesn't have ftl, apart from maybe [jumper clowns spoiler deleted]; the conjoiner drives use [science] to achieve 1g acceleration to get to near light speed, and time dilation is part of many of the plots.

[rsynnott]

> the conjoiner drives use [science] to achieve 1g acceleration to get to near light speed

For [science] read [magic], more or less, but yeah. In the very first stories they were Bussard ramjets, but this got retconned out (and a Bussard ramjet actually shows up in a later book as a failed experiment). In most of the books they're more or less applied magic, though.

This, incidentally, seems to be a common theme, as later discoveries tended to fall down on the side of Bussard ramjets not working (due to insufficient density etc). The last of Niven's Known Space books have some special pleading for how the pilot has to carefully direct the ramjet to get sufficient combustion volume, a detail that was never present in the old ones.

[eesmith]

Time for the Stars has FTL at the end. (Physical FTL, that is. Telepathic FTL exists at the beginning.) But it's otherwise it's deliberately structured around the twin paradox - the main character is even a twin.

Indeed, while I remembered the importance of time dialation in Forever War , I didn't remember the FTL travel between collapsars.

[NateEag]

Spider Robinson's "Variable Star" is not hard sci-fi, but relativistic speeds and time dilation are relevant to the plot.

[edna314]

I'm sure it has been, but I'm mainly talking about popular sci-fi movies. I'm sure you have at least one in mind where they have these 'sleep' devices.

[eesmith]

I'm not much into visual media, so I can only come up with a few examples of 'sleep' devices:

Star Trek's "Space Seed" - 200 years in sublight stasis, no mention of speed or distance. Could be 0.5c for all we know. 100 ly at Enterprise's warp 6 cruising speed would take about 4 months, which seems not unreasonable.

The movie "Alien" - stasis, but seemingly with FTL given the times and distances involved. (https://avp.fandom.com/wiki/LV-223 says it was a 2 year voyage to the moon LV-223 in Prometheus.)

The TV series "Red Dwarf" - sublight, though it did break the light barrier once. Science in general is only relevant for comedic effect. https://www.ganymede.tv/2004/04/the-science-of-red-dwarf/ .

None of these seem to be characterized as "it takes you 150 years of travel at light speed to get to a place that is 150 light years away".

Which one am I missing?

[edna314]

What came to my mind immediately was 'Passengers'. But, reading up the plot on Wikipedia, it is possible that it never actually was mentioned how far away the destination was or how fast they were going. So, maybe it wasn't a misconception on the sci-fi side, but on my side. For sure, any light speed travel shouldn't take any time and infinite amount of time should pass for the rest of the universe.

[eesmith]

From Wikipedia, 60 light years and "a journey lasting 120 years".

It doesn't say if that's 120 ship years or Earth years. I'll assume ship years as that makes the most sense in context.

Assuming constant acceleration to the 1/2-way point, flip, deceleration, and using http://www.projectrho.com/public_html/rocket/slowerlight3.ph... :

  T = (c/a) * ArcCosh[a*d/(c^2) + 1] (given acceleration and distance)

  60 years = (c / a) * arccos(a * 30 ly / (c^2) + 1)
  1893456000.0 = (3E8/a) * arccos(a * 3.15576 + 1)

Using Wolfram Alpha to solve - https://www.wolframalpha.com/input/?i=1893456000.0+%3D+%283E...

  a = 0.14748 m/s^2 or about 1.5% g

Time elapsed on Earth is only a bit longer than ship time:

  t = (c/a) * Sinh[a*T/c] = 1.15 * ship time ("proper time")

Final velocity at flip is:

  v = c * Tanh[a*T/c] = 0.73 * speed of light

[simonh]

Having sleep pods and such isn't silly if the travel time really is many years.

[philipswood]

Sleep pods are also not silly if you assume your speeds aren't going to be a significant fraction of the speed of light.

[edna314]

I mean yes, but there is a rather specific window where they make sense: at ~1-80% of c. Lower speeds it doesn't make sense to engage in interstellar travel at all, because even travelling 100s of years won't get you anywhere. At higher speeds it doesn't make sense because of time dilation.

[rsynnott]

But that's a... pretty important window. There are not-completely-bonkers hypothetical spacecraft designs that could get to, say, 0.2c. There's nothing particularly plausible that could get to 0.99c.