Related to Invisibility should cause blindness: how does hard sf cope?, I'm wondering whether the cloaking device implementation in Star Trek includes an explanation as to why cloaked ships can still observe their surroundings.

  • Rufus: "Modern technology, William"
    – komodosp
    Jan 17, 2020 at 14:36

6 Answers 6


The cloaking device in Star Trek is imperfect. If you look closely either with eyes or sensors you can observe a disruption where the ship is. This is because it isn't successfully bending all of the light around the ship. Some of it is going into the ships sensors, being absorbed or being reflected at random. And this creates a detectable disruption, but it also means that the ship can still see.

It's only with total invisibility that all of the light has to go through and you'd have issues with seeing.

This might apply to the invisibility question as well. The only light that has to go through you to be invisible is the light coming from behind (wrt the observer). As long as you don't reflect the light coming toward your front (wrt the observer), then you won't be visible. Whoever is looking at you will see only the light coming from behind you. Of course, this is an issue if there are people on all sides of you.

However, if you only allow certain points to absorb the light instead of bending or transmitting it, then for all practical intents and purposes, you'd still be invisible. There'd only be a very, very slight disruption right around the sensors. Or eyes.

The bit about not reflecting on coming light only works in space actually. Unless you count 'casting a shadow' as being invisible. Which sometimes happens and sometimes doesn't. Depending on the story.

  • 1
    I do seem to remember scenes where, beyond cloaking, the ship goes completely still and powers down most systems, presumably as a way to reduce emitted interference? Feb 1, 2011 at 15:41
  • Probably, yeah. Thought I don't remember those scenes, it would make a certain amount of sense. Feb 1, 2011 at 15:44

I'd like to propose another answer.

Looking at the cloaking technology itself, there are two ways that it could work:

  • First Case: Light hitting the ship is absorbed and emitted on the other side of the ship without alteration from all directions.

    In this case, your technology is actually capturing the light, so it IS "seeing" everything as normal... it's just not reflecting it back for everyone else to see them. Problem Solved.

  • Second Case: Light hitting the ship is bent around the ship and then straightened again on the other side without any other alteration by the ship.

    In this case, you're going to need a small inconsequential amount of light to pass through the cloak barrier and be absorbed (not reflected) by the ship as sensor data.

So the bottom line, as long as you have technology that absorbs light without reflecting it back... you can "see". But the more light you absorb without passing it to the other side, the more distortion there's going to be... and that is a way for the other ships to detect you. (Of course, any energy being emitted by the ship is detectable... so you have to turn the ship into a closed sealed system).

  • I'm sure that they're using other scanners, not just light based ones - the cloak would have to avoid all scanners. Feb 2, 2011 at 15:30
  • I agree. I'm using the term "light" to mean electromagnetic energy... but your point is valid. The cloaking technology would have have to treat all known energy equally and it would really become an arms race between cloaking technology and sensor technology.
    – Adam
    Feb 2, 2011 at 15:42
  • This also means that it would have to account for matter (e=mc^2). I can see an active sensor that shoots molecules out in all directions looking for a disruption. To get around this, a cloak would have to also be interphasic: memory-alpha.org/wiki/Interphase_cloaking_device
    – Adam
    Feb 2, 2011 at 15:47
  • Another issue is anti-matter based sensors: memory-alpha.org/wiki/Quantum_beacon . Basically, anything hit by a positron would create a burst of energy...
    – Adam
    Feb 2, 2011 at 15:51
  • This issue is handled far and away better in the Lensman series. Doc Smith first decided how detection of ships was going to work, and then worked on ways that a ship could become stealthier. This required trade-offs (more stealth == less performance in other regards) because Doc Smith was an engineer in his own right.
    – EvilSnack
    Apr 6, 2017 at 13:32

Out-of-universe, it's easy to explain: The writers wrote down to their expected audience, who they didn't expect to want to watch episodes about invisible enemies guessing where their foes were. Alternatively, the writers could have been unable to come up with appropriate stories for cloaked ships being blind.

In-universe, it's probably explained in the same vein as the sensors (which can 'see' events light-hours away in real time). My guess is 'subspace'.


Sensors positioned outside the cloak would "see" for the ship and wouldn't be detectable in and of themselves as long as they were small passive sensors.

  • 4
    I don't see how passive sensors become undetectable - a chunk of metal is detectable by sonar, light and other scanners just fine. Assuming this works, you'd still have the problem of attaching the sensors to the rest of the ship through the cloak. Feb 1, 2011 at 15:30
  • 1
    but it would only have to be a very small piece of metal in what is, after all, a very big bit of space :)
    – HorusKol
    Feb 1, 2011 at 22:34
  • 1
    Exactly what Horuskol said. All you really have to worry about is extending the sensors past the cloak limit. If you don't have to many of them in any sort of pattern, the small pieces of metal would be functionally invisible in the vastness of space.
    – Adam
    Feb 2, 2011 at 14:54
  • 1
    You're talking about hiding from sensors which can, in the time span of a few minutes, find a specific person, on the surface of a very large planet. Typically the person is identified by life-signs. With todays' technology we can quickly identify one individuals' face, on video, in a crowd of thousands - in the vast emptiness of space, small bits of metal would have to be really really really small to be hard to detect Feb 3, 2011 at 16:39
  • 1
    Anything that requires power, generates heat. This is a fundamental rule of physics: Entropy always wins in the end. In the vastness of space, there's no where to dump waste heat (Air is a horrible conductor of head, and empty space is even worse) so a simple Infra-red visual camera would see any sensor (even passive) with no problem. Add in radar, lidar, and the magitech sensors used to see things light-hours away, and those 'nearly invisible' tiny passive sensors would stand out like a neon sign.
    – Jeff
    Feb 3, 2011 at 18:43

This is sort of like the invisible man problem. If the invisible man is totally, 100% invisible, he'd have to be blind. If he can see, then at least his retinas must attenuate light passing through them, and his lenses have to refract properly. Artificial sensors would have similar requirements.

But the thing is, in the case of an invisible man, you've reduced the visible area from a square meter or two, to a square centimeter or two, and in the case of a ship, from (probably) several hundred square meters to (possibly) less than a single square meter. The cloaking doesn't have to be perfect to be effective.

Of course, if you have a ship that's going to go even just a few percent of C, it should probably be capable of detecting quite small particles, and should therefore be able to detect even the tiny reflection/distortion from the sensors of a cloaked ship.

Of course there's also the possibility of 'active cloaking', that if you know where the enemy you're trying to hide from is, and what their sensor systems are, then you can actively beam just the right signal at the other guy to fool his sensors into not seeing you.

  • No, this only matters if the invisible man's retinas are reflecting light back. If his retinas are absorbing the light energy, then he can see... Most of the light will still pass through in between the retinal cells... he'll still be invisible AND see...
    – Adam
    Feb 2, 2011 at 15:27
  • @Adam - in order to see, you have to absorb something. If you absorb something, you render yourself detectable.
    – JustJeff
    Jun 20, 2011 at 23:31

There is one difference between this question and the invisible man question. The invisible man issue is purely limited to the visible spectrum of electromagnetic radiation.

For a space ship, it is conceivable that the cloaked ship is hidden in one part of the spectrum (e.g. visible light) but detects other ships using a different part, perhaps X-rays.

  • 2
    This is a bad answer because it assumes that the engineers of the cloaking technology are dumb. Obviously, if cloaks don't hide from all the viable spectrum of light, then I'm going to build a sensor that detects the other spectrums (as if I wouldn't already). To go a step further, I'd develop an active "sonar" sensor that shoots all those other spectrum out of my ship in all directions and watches for a blip.
    – Adam
    Feb 2, 2011 at 15:31

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