I just finished Ender's Game. In the book there is the concept of an Ansible, a device that allows for instantaneous communication across the universe. Traveling to the stars still adheres to the laws of relativity.

So if someone from Earth wants to facetime to a spaceship traveling at near lightspeed speed, will the recipient have to slow down the message in order to understand it? Will the person on Earth get the answers in super slow-motion?

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    Questions like this are why FTL communication breaks relativity. Dec 9 '18 at 13:35
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    I'm at a loss why you've accepted an answer that offers only supposition over an answer that offers direct evidence from the books
    – Valorum
    Dec 9 '18 at 17:27
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    @Valorum realtime holographic maps are what you get when you feed discrete coordinates into a system that can display them as a whole. Think back to the glass displays used in WWII naval context; a big picture put together by a bunch of enlisted men with grease pencils plotting last known location and heading. The actual information transmitted need be no more than a small set of numbers.
    – gowenfawr
    Dec 9 '18 at 18:23
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    @Pelinore - My concern is that if OP is interested in purely how the mechanics would work in real life (noting that as far as we're aware, such a thing isn't actually possible), then they probably should have asked on physics:SE
    – Valorum
    Dec 9 '18 at 19:24
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    By the way, I'm fairly sure that it was Ursula Le Guin who came up with the term ansible first, although it's shown up in many works since then.
    – Adamant
    Dec 10 '18 at 12:22

Within the Ender's Game series, the majority of spaceship travel is conducted below lightspeed (see below). In Book 3 of the series Xenocide we learn that it's possible. albeit hideously expensive and damn-near impossible for civilian ships given the limitation of their on-board computers, to communicate while traveling at top speed.

“Our time here on this starship is just as real as their time out there,” said Jakt.
“Sometimes I wish Ender's friends hadn't figured out a way for our starship to keep up a landside link.”
“It takes up a huge amount of computer time,” said Val. “Until now, only the military could communicate with starships during near-lightspeed flight. If Ender's friends can achieve it, then I owe it to them to use it.”
“You're not doing all this because you owe it to somebody.”

Since the dilation effect when you're at full speed is approx 960:1 (960 minutes pass on-planet for each minute on-ship), a meaningful realtime conversation wouldn't be possible. You can send letters to each other, that's about it.

Note that later in the book series, they discover instantaneous (FTL) travel. Since you move from point-a to point-b instantly, there's no possibility, or indeed need, to communicate while en-route since there isn't an en-route.

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    //It takes up a huge amount of computer time,” said Val. “Until now, only the military could communicate with starships during near-lightspeed flight.// : that's from Xenocide I take it? interesting, I'd always assumed his rationale for the Ansible was quantum entanglement & there'd be no plausible reason (with that) for it to take any more processing power to communicate while moving than while stationary.
    – Pelinore
    Dec 9 '18 at 16:05
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    @Pelinore - "[The Hive Queen's] thoughts were instantaneous; bound, not to synapses, but to philotes that were untouched by the relativistic effects of lightspeed. She passed sixteen hours for every minute of Ender’s time—the differential was too great for him to receive any kind of communication from her." So lightspeed comms are even too hard for the Hive Queen to effectively manage, and she's far more advanced/experienced in these things
    – Valorum
    Dec 9 '18 at 16:17
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    ^ "wikipedia : Philotes (disambiguation) : Philotes, the plural form of a fictional particle in Orson Scott Card's Ender's Game series" : I see, so he threw out all pretense at scientific reality & went completely technobabble on us there then, thanks for that :)
    – Pelinore
    Dec 9 '18 at 16:27
  • So I assume when you say "later in the book series" you mean "something at the end of book four which is never mentioned in the next thirteen books"?
    – ibid
    Dec 9 '18 at 22:33
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    @ibid - I never got past book four. Card turned the whole thing into garbage. To be brutually honest I thought he might be gearing up to launch his own cult religion.
    – Valorum
    Dec 9 '18 at 22:44

Short answer: Yes.

Longer answer: Those on the ship traveling at very very fast speeds would appear to be moving (& speaking) in slow motion on the Earth-side end of the conversation / conversely the people on earth they're talking to will appear to those on the speeding ship to be moving & speaking at high speed.

The exact speed would depend on what fraction of light speed the ship is travelling at, if the ship is travelling at half light speed then people on Earth appear to be moving at double speed to those on the ship & those on the ship appear to be moving at half speed to those on Earth, the closer to light speed it goes the faster those on Earth appear to move & the slower those on the ship seem to be.

So if the ship isn't travelling at very large percent of light speed you probably don't need to make any adjustments, slow your tape player (or more modern device for playing recorded sound) to half speed or speed it up to double speed & it might sound funny but you can still understand the words.

You could get around it by using ordinary technology to speed & slow the feed at the respective ends, then you do get an artificial lag on the Earth-side of the conversation (& none on the ship).

So in summary: Yes (at near light speed) you are going to need to slow down the message as it's received on the ship to be able to understand it, but at slower speeds of merely a significant portion of light speed (say 50%) you probably wouldn't need to.

Side note: FTL communications by the way is a literary device that's as equally impossible as FTL travel, it often relies on properties of quantum entanglement that (to put it bluntly) simply don't exist.

Further side note: Orson may have taken a different tack on things though.

  • I did say near-lightspeed :) The 2 years it took Ender to reach the colony translated into 50 earth years, so relativistic effects were in play.
    – Lodewijk
    Dec 9 '18 at 15:21
  • @Lodewijk : Maybe you did, but that doesn't change the fact that FTL means "Faster Than Light" & it's still in the title :)
    – Pelinore
    Dec 9 '18 at 15:42
  • @Lodewijk //so relativistic effects were in play// : relativistic effects (of in-flight communication with earth) are what my answer is about (unsurprisingly, as that is what you asked about), so I don't understand the point you're making with that bit of your comment?
    – Pelinore
    Dec 9 '18 at 15:51
  • @Lodewijk : Ah! I think I get where you're coming from now, the new summary work for you?
    – Pelinore
    Dec 9 '18 at 16:12
  • You're right. I messed up the title :)
    – Lodewijk
    Dec 9 '18 at 17:14

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