Are we to believe that the scanners and sensors used on Star Trek are:

  1. Instantaneous
  2. Faster than instantaneous
    • Travel into the future
    • Travel into the past
  3. FTL but slightly less than instantaneous

How does Star Trek Canon address the issue of Violating Causality?

  • I don't think canon ever directly addresses the issue of why FTL doesn't routinely lead to causality violations, but it's worth noting that connection between FTL and causality violations comes from the fact that different inertial frames have different definitions of simultaneity, along with the fact that the laws of physics are though to work the same way in all inertial frames, as I explained here. So if FTL "subspace" phenomena didn't work the same way in all frames, if they had a "preferred" frame, you could avoid causality problems.
    – Hypnosifl
    Jan 27, 2015 at 19:30
  • The scanners never operate "faster than instantaneous".
    – Valorum
    Jan 27, 2015 at 19:40
  • For more on the point above about how a preferred frame (with a preferred definition of simultaneity) would allow FTL without causality problems, see the discussion here, especially sections 9.5.4 (which refers back to a discussion of FTL bullets in Chapter 8) and Chapter 10.
    – Hypnosifl
    Jan 27, 2015 at 19:44

1 Answer 1


The sensors on the Enterprise operate in two modes;


"The major external sensors employed at sublight include stellar graviton detectors, stellar pair coordinate imagers, pulsar/quasar counters, far infrared scanners, and Federa¬tion Timebase Beacon (FTB) receivers. These devices also communicate with the structural integrity field and inertial damping field processors, inertial sensors, and main comput¬ers to obtain an adjusted awareness of the ship's location."


The majority of instruments in the long-range array are active scan subspace devices, which permit information gathering at speeds greatly exceeding that of light. Maximum effective range of this array is approximately five light years in high-resolution mode. Operation in medium-to-low resolution mode yields a usable range of approximately 17 light years (depending on instrument type). At this range, a sensor scan pulse transmitted at Warp 9.9997 would take approximately forty-five minutes to reach its destination and another forty- five minutes to return to the Enterprise. Standard scan protocols permit comprehensive study of approximately one adjacent sector per day at this rate. Within the confines of a solar system, the long-range sensor array is capable of providing nearly instantaneous information.

Since the scanners can never operate at a faster speed than simultaneity (e.g. if close enough, they see what's happening right now) there's no obvious issue with causality.

  • There would be a problem with causality if subspace respected the theory of relativity, because in relativity simultaneity is relative, and this means that any signal which is moving faster than light but forward in time in one inertial frame (the signal reaches its destination after it is sent in the time coordinate of that frame) is moving backwards in time or "faster than simultaneity" in another (the signal is received at an earlier time than it's sent). But as I said in a comment, you can avoid this by imagining a preferred frame.
    – Hypnosifl
    Jan 27, 2015 at 19:44
  • @Hypnosifl - Yes. At most it would be issue of perception rather than violating causality since subspace communication simply increases the top speed of light (or whatever). The only way to then end up with a causality problem is if something travels "faster than subspace".
    – Valorum
    Jan 27, 2015 at 19:53
  • In terms of real-world physics, if you imagined just plugging a higher maximum speed into the equations of relativity it would alter all the equations where the constant c appears (the speed of light) like the time dilation equation, so it would conflict with existing observations of things like the way particles take longer to decay when moving at higher speeds. I think you have to imagine that slower-than-light phenomena still behave symmetrically in all the inertial frames of relativity (whose relation to one another involves the constant c), but subspace phenomena do not, though they could
    – Hypnosifl
    Jan 27, 2015 at 19:59
  • (continue) behave symmetrically relative to some different set of coordinate systems with a different constant relating them. Physicists have occasionally speculated about FTL particles having a preferred frame while non-FTL phenomena still behave symmetrically in all inertial frames, as in this paper.
    – Hypnosifl
    Jan 27, 2015 at 20:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.