As I understand it, thanks to the General Relativity it is absolutely impossible to have both relativity and FTL without the latter opening you a way to get into your past thanks to the relativistic frame shifts, thus destroying the causality, which in turn destroys the entirety of physics and science, undoing everything we've achieved in an instant.

But can the time-travel aspect be obfuscated enough to shift it into "hypothetically possible according to the abstract math, practically impossible according to the complexity of the reality" area, making its reality-unraveling capabilities be unreachable at least for the normal characters in the setting?

The FTL system in question works via the existence of an expensive device that creates a wormhole of a greatly reduced (But never zero) length between two points in space, presumably by taking a shortcut through either another universe or through some high-dimensional witchery (It's a replicable black box device that nobody exactly understands). The spaceship then needs to travel through the wormhole under its own thrust, carrying the device with it along the journey. The wormhole supports normal spacetime only in the vicinity of the device, so it can't be used for FTL communications or attacks (You can't strap the device to a radio wave or laser beam).

Current limitations imposed by the setting is that the system cannot establish a wormhole to and from an area with a significant gravity gradient (preventing jumping to and from near planets and other heavy objects, forcing you to go away from them before initiating the jump), and it cannot establish a wormhole between two points that exceed a certain relative velocity from one another, which hopefully should prevent the "jump to the object moving at 0.999c, jump from it to your own past" situations. Both attempts result in the wormhole being unstable and collapsing instantaneously.

Would these restrictions be enough to prevent the time travel from being practically achievable?

  • $\begingroup$ Possible duplicate of worldbuilding.stackexchange.com/questions/172007/… or worldbuilding.stackexchange.com/questions/46873/… The second question most closely resembles yours, but my answer to the first question I think will hit the nail on the head for you. $\endgroup$ – Nosajimiki Nov 13 '20 at 20:18
  • $\begingroup$ Not quite right. It's special relativity where causality problems arise with FTL. General relativity, on the contrary, does allow for some hypothetical ways of FTL travel. Wormholes, for one, as is part of the subject of your question. $\endgroup$ – a4android Nov 13 '20 at 21:40
  • $\begingroup$ Well, not exactly, those ask about FTL without time-travel, while I ask about FTL that doesn't allow the characters to use its time-travel functionality. $\endgroup$ – Darth Biomech Nov 13 '20 at 23:04

One of the less discussed laws of nature is that if something is possible, it's already happened. If it were possible for your FTL "black box" to put you into your own past, it could never be invented.

That means the black box must have some internal restriction that keeps it from opening a wormhole that could be used to send any mass or information around a "timelike loop" pathway.

One such restriction would be that, in fact, the externally observed transit time through the wormhole is no faster than that of light through inertial space between the end points -- even though the duration to pass through is much shorter. It only takes four days, six hours, nine minutes, eight point seven seconds (rounded to the tenth) ship time to go from the outskirts of Sol system to the outskirts of Proxima Centauri system -- but it still takes a few weeks less than nine years, Earth time, to make the round trip.

This limits the usefulness of the drive a bit -- you still can't ever really come "home" from a long voyage, everyone you knew will be dead by the time you return -- but you don't need ships that are 99.999% fusion fuel and reaction mass to make even the shortest trip or a generation ship to get humans there alive; if you've got a few years to spare, you could go one way with a Falcon Heavy and a Centaur upper stage. It opens the galaxy -- even the universe -- to colonization -- though only to a point; other galaxies, eventually, will be burned down to only the red dwarves by the time you get there, and the Milky Way will be worse off than that by the time you come back.

On the bright side, the universe won't implode ab initio the first time someone powers up one of the boxes.


There are still are time travel exploits available.

Communication and travel must be at the same speed otherwise there are "future knowledge" exploits one can do from travelling faster than news of an event.

The first one that comes to mind:

  • Travel to Planet A. Attend a sporting event live.
  • Watch the match and thoroughly enjoy it. Note the winner.
  • Get in your space craft and travel to planet B, located one light day away.
  • Attend a casino offering betting on live sports feed.
  • Place a bet on the match occurring tomorrow - you know who will win cause you were there.
  • Come back tomorrow to collect your winnings.

Now a simple fix if FTL is common is to not offer betting on foreign events - there are stock market or other economic moves available with this kind of information. I could witness a company have a massive disaster on one planet, FTL to another planet and short-sell the stock.

A simple way to understand this is to imagine a pulse coming from a fixed point in space, and the time at any point in space being the number of pulses that point has observed. If you can ever travel to a point with a lower count, you've travelled back in time, if you ever travel to a point with a higher count, you've travelled forward in time.

Allow FTL communication and these exploits will not be possible - the pulse count will never decrease no matter where you go in the universe - and you'll arrive at the casino after the match has already taken place.

  • $\begingroup$ FTL radio/laser may not be possible, but there’s nothing saying FTL ships can’t carry messages with them. FedEx has more bandwidth today than the entire Internet, and the latency ain’t that bad in astronomical terms. $\endgroup$ – StephenS Nov 13 '20 at 18:26
  • $\begingroup$ @StephenS in which case the time traveller needs to make sure they FTL faster than the FedEx regular internet-delivery. This could result in "high frequency traders" rapidly travelling in FTL back and forth between two markets faster than the official data exchange to take advantage of price arbitrage. $\endgroup$ – Ash Nov 13 '20 at 18:30
  • $\begingroup$ The data dump would be traveling on the same ship as the trader, and transmitted as soon as the ship emerged from the wormhole. His only advantage would be having the transit time to digest the data and prepare his orders. $\endgroup$ – StephenS Nov 13 '20 at 18:41
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    $\begingroup$ This feels more like a "ride your horse faster than the postal carriage goes" exploit rather than time-travel exploit. You aren't exactly arriving in somebody's past - rather just operating on information others don't know yet. $\endgroup$ – Darth Biomech Nov 13 '20 at 19:54
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    $\begingroup$ Nothing in this is a breach of causality. It’s exploiting travel time, not time travel. $\endgroup$ – Joe Bloggs Nov 13 '20 at 20:38

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