Non-causality violating sci-fi scenario….. you come up with a Title, OK?

Consider as a thought experiment two scenarios involving time-travel.

In the first it is possible to communicate with the future but not to travel faster than the speed of light.

In the second, communication is only possible with the past as starships travel back in time if they travel faster than light.

The first scenario: Stars A and B are twenty light years apart and each contain an advanced, space-faring society.

They can communicate by tachyons, particles which travel faster than light and backwards in time (depending on perspective) and by radio or laser signals which travel at the speed of light.

A has a small but deadly fleet of warships that can accelerate to 0.05c

(note: it can be shown that a starship can be accelerated to 0.42c by the use of a powerful orbital laser, which then decelerates to maneuvering velocities using a multiple-propulsion system, utilizing fusion and small amounts of anti-matter. Theoretically, a starship could accelerate almost to the speed of light using Zero Point (or Vacuum) Energy with enough propellant).

A can communicate instantaneously with B, as far as either of them is concerned, by sending a radio signal to B. B then sends a reply by sending a reply back via tachyons traveling back in time at a rate of one year per year. A would receive this reply 20 years before B sent it but the only noticeable delay in a conversation would be in the time it takes B to think what to say!

B’s solar system is invaded-they send a distress signal to A via tachyon. These tachyons ‘travel’ back at a rate of twenty years per year. They reach A four thousand years in B’s past: giving A enough time to send a rescue fleet in time to repel the invaders-assuming A has had both a war fleet and the capacity to receive and interpret signals transmitted via tachyon for this long.

In this particular scenario, A’s fleet cannot return to their own time. However, a wormhole connection could be made (assuming they have the capacity to create exotic matter?), linking B with A four thousand years previously.

If A also sends a signal (of any kind) to B, it would obviously be received centuries before the arrival of the invasion fleet. This wouldn’t necessarily violate causality, but it would give B time to prepare its defenses.

If we then consider a scenario whereby ships can travel back in time, even if tachyons were used, instantaneous communication is impossible. Also, there is the very real possibility of causality violation, as a returning starship is just as likely to return to its past as to its future; removing the possibility of wormholes for this instance, a starship’s crew can never return to their own time after a mission.

A less confusing scenario would just involve tachyons traveling back in time at a rate of one year per year, while starships utilize ZPE drives, enabling them to almost reach the speed of light. Communication can be instantaneous and delays in trade are only as long as acceleration time plus deceleration time. Obviously, contact could be made with a civilization before it’s even begun to industrialize but by the time a ship has been sent to prevent said industrialization, they’ve already sent the signal!