Invisibility theory

As part of Ra I realised that I needed a character to become invisible. Since this requirement is coming up quite soon I decided to sit down and work out how this cloak of invisibility would work. I ran into some interesting problems. It turns out that becoming invisible is less than half of the problem.

I haven't written any more of Ra yet, so the cloak might not come into the story in the next chapter, or even at all. Even if it does appear, the cloak might not function as described below, and even if that's the case I probably won't spend a huge amount of time laboriously explaining the full mechanism behind it in the story. So, other than the fact that an invisibility cloak might appear in Ra in the future, this article contains no spoilers.

What you should already know

Here are two things that have already been established in the story. The chapter What You Don't Know covers this in detail.

Firstly, there is a special kind of mana called "chi" (the Greek letter Χ) which is given off whenever magic spells are carried out. Chi mana exists only in very small quantities. The universe is almost completely dark in the chi spectrum. Chi mana is also very weakly interacting. It passes through physical matter (including human bodies) with ease. They are most similar to real neutrinos, except that the universe is flooded with neutrinos whereas chi particles are very rare.

Secondly, there are special magic rings called "oracles". An oracle is enchanted so that when a chi particle passes its mouth, it is transmuted into a photon of visible light moving in the same direction. By holding an oracle up and looking through it, you can essentially "see magic". Augmented reality for mages.

A third fact revealed in other chapters (starting in Sufficiently Advanced Technology) is that there exist full-body magic shields. When using one, the wearer is enclosed by a surface set a few centimetres out from his or her skin, which can absorb physical attacks. Naturally, this closed "bubble" contains only a small amount of air, but this is a secondary problem.

Transparency in the visible spectrum

Invisibility cloak #1

Within the rules already described, it's quite easy to conceive of a magic shield ("cloak") whose surface is enchanted to function like an oracle. This would allow you to see magic in whatever direction you looked.

What is also easy to conceive of is an oracle which works in exactly the opposite fashion to a conventional oracle: it converts photons of visible light into invisible chi particles. Looking through such an oracle, you would see only perfect darkness.

Given these two developments, it's easy to devise a cloak of invisibility: Make a shield which operates like an "opposite oracle" on the way in, and like a conventional oracle on the way out.

Watch what happens when a photon of visible light passes through you while wearing this cloak:

  1. The photon of visible light hits the surface of the cloak from the outside. The photon is converted to a chi particle which passes into the cloak interior.
  2. The chi particle hits the surface of your skin from the outside and passes into your body, unchanged.
  3. The chi particle hits the surface of your skin from the inside and passes out into the cloak interior, still unchanged.
  4. The chi particle hits the surface of the cloak from the inside. The chi particle is converted back into a photon of visible light which passes out into reality.

Thus, all the light falling on the cloak is eventually emitted in the same direction from the far side of the cloak, and what a person standing on the far side of you sees is exactly what they would expect to see if both you and the cloak were not there. The cloak and everything inside it is invisible!

Some additional assumptions have to be made here for this to work:

  • That the different visible wavelengths (colours) of photons can be put into a correspondence to different wavelengths of chi particles, and that this transformation can be reversed. If the chi particles are not of different "wavelengths", they must at least be able to differ from each other along some kind of usable spectrum (which, for the sake of argument, we shall refer to as "wavelengths" from here on). Otherwise, at point S when we receive a random chi particle, we have no way of knowing what colour of photon to turn it back into.
  • That the transformations at the cloak and at your skin occur instantaneously, or at least very quickly. Any kind of delay in propagation results in a curious "delay" effect whenever the "invisibility" cloak is moving. The background image projected forwards will be out of date, making it possible to tell where the cloak is, although it would admittedly be much more difficult to see than you just standing there.
  • That chi particles in flight move at the speed of light, or at least very quickly, for the same reason.

I'm going to go ahead and dismiss these issues for now because from a storytelling perspective I can just say "yes, this is all the case" quite easily without it being too implausible given what we already "know". There's one other issue which is much harder to crack.

You're blind while you're wearing the cloak! It's pitch black in there!

Visible light cannot enter the cloak from the outside: it is all converted into chi particles. You're invisible, but you also can't see out of the cloak.

This problem arises in other models of fictional invisibility too: for example, if you have your body chemistry somehow altered so that visible light passes through you, or are somehow "shifted out of phase with the universe".

Human vision works like this: a photon falls on your eyeball, your retina absorbs the photon, and a nerve impulse sends the image to your brain. If you can see, then a person standing behind your retina should at the very least notice a dark patch in mid-air where this absorption is taking place. Conversely, if you are invisible, then photons passing through your eyeball must pass through without being absorbed: thus, you cannot see.

Invisibility cloak #2

The solution to this problem is to modify our cloak so that it lets the light through as well as creating chi particles. Let's see what happens now:

  1. The photon of visible light hits the surface of the cloak from the outside. The photon is left unchanged, but a chi particle is also generated.
  2. The photon and the chi particle hit the surface of your body - your retina, let's say. The photon is absorbed (and seen) but the chi passes through.
  3. The chi particle emerges unchanged from the back of your head.
  4. The chi particle hits the surface of the cloak from the inside and is converted back into a photon.

Great: you can see again. But you're not invisible anymore.

Firstly, when the photon hits your body it may be reflected (or re-emitted) instead of being absorbed. When the re-emitted photon arrives back at the cloak surface, it'll escape without a problem. You're now plainly visible again.

Worse, the cloak is a few centimetres thick. It's quite possible for an incoming photon to pass through the cloak and straight out again without passing through the wearer's body. In this case, see what happens:

  1. The photon of visible light hits the surface of the cloak from the outside. The photon is left unchanged, but a chi particle is also generated.
  2. Both the photon of visible light and the chi particle hit the surface of the cloak from the inside. The chi particle is converted back into a photon while the original photon is unchanged. There are now two photons.

Your cloak of invisibility now not only leaves you completely visible but wraps your whole person in a few-centimetres-thick glowing shroud of amplified light!

Invisibility cloak #3

Let's make a second modification to our cloak. Make it so that visible light exiting the cloak is suppressed. Now, light rebounding off your brightly-coloured clothing doesn't escape, so you can't be seen. And let's look at the edge of the cloak:

  1. Photon enters the cloak. Becomes a photon + chi particle.
  2. Photon and chi particle both exit the cloak. The photon is absorbed. The chi particle is converted to a photon identical to the original.

Great. We're now totally invisible - which is to say that we are transparent in the visible spectrum - but we can also see perfectly!

There's only one small drawback, which is that torches don't work. In fact, philosophically speaking, torches cannot work if you wish to remain invisible. A torchbeam, by definition, gives away your location! Invisibility means that light emitted by you - regardless of source - must not escape the cloak where it can be seen by hunters. A laser sight would have the same problem. Of course, if it's so dark that you need a torch, it's probably also so dark that a cloak of invisibility is of no particular use. So, this would be a good place to stop in most cases.

Transparency in the chi spectrum

Now let's consider a different problem. With cloak #3, you can't be seen if visible light falls on you. But what if there's a chi source nearby?

  1. Chi particle enters the cloak. It passes through unaltered.
  2. Chi particle enters your body unaltered.
  3. Chi particle exits your body unaltered.
  4. Chi particle exits the cloak. It is converted into some sort of visible photon.

Oh, snap! All a hunter needs to do is put a significant chi source behind you (who are normally invisible) and when you walk in front of it you'll light up in brightly visible purple and orange! We need to reconfigure our shield again. But we can't modify the cloak exit transition, because that'll break our transparency in the visible spectrum. We have to do something to the cloak entry transition.

Invisibility cloak #4

Let's change our cloak entry transition so that it destroys incoming chi particles.

  1. Chi particle enters the cloak. It is absorbed.
  2. Darkness.
  3. Darkness.
  4. Nothing exits the cloak.

What we now have is a cloak which is invisible as usual but whose interior is completely dark in the chi spectrum. If you walk in front of a chi source, you'll absorb all of it. The only way the hunter could see you is if the hunter is watching through an oracle of his or her own. Through the oracle, you would show up as a big dark shadow. At least we've made things harder.

Is this the best we can do? Can we be transparent in the entire visible spectrum and the entire chi spectrum simultaneously?

Invisibility cloak #5

Um, yes. But the solution suddenly becomes much less elegant.

The major problem is this:

  • The spectrum of visible and magical radiation falling on your cloak must be exactly the same as the spectrum of visible and magical radiation that leaves it. All of the information stored in this spectrum has to be retained in transit somehow.
  • But when visible light hits your skin, it's absorbed. This destroys some of the information.
  • That means that the information stored in the visible part of the spectrum has to be stored somewhere else in the spectrum while it's in transit through the cloak (and possibly your body), then restored.
  • The logical place to put this information is in a dedicated section of the chi spectrum.
  • But that means that the information previously stored in that dedicated section of the chi spectrum is destroyed instead.
  • Where do we put it? How can we find room?

There are several approaches available here, of differing levels of complexity. Here's one:

  1. Take the entire original spectrum of both visible and magical radiation.
  2. Compress the spectrum horizontally so that the whole thing will fit just in the chi wavelengths.
  3. Store this compressed spectrum in the chi wavelengths as you'd expect, BUT also retain all of the original visible light information in the visible part of the spectrum. Notice how the new spectrum has two copies of the original visible light information.
  4. The new spectrum hits your body, and much of the visible part of the spectrum is absorbed (i.e. you can see it). No matter: we have a copy of this information encoded into the chi part.
  5. When the radiation exits the cloak, reverse this transformation. First, discard all the visible light, whether it's a photon that passed through unaltered, a photon emitted from a torch or simple darkness.
  6. Next, take the chi part of the spectrum and expand it to cover both the chi and visible parts of the spectrum as before.

The above solution assumes - dubiously - that there is infinite granularity in the range of possible chi wavelengths (or, if not, that the two transformations don't result in a noticeable loss of fidelity).

Another solution would involve no "compression" in the spectral information, but just moving all the wavelengths "up by one" so that they are no longer visible. This in turn requires that there are infinitely many available chi wavelengths, such that this wavelength shift can be carried out for incoming information of any wavelength.

Both of these alternatives require us to introduce some further assumptions about the nature of magic. As a writer of fiction, I'm able to wave my hands around as much as I like to justify these assumptions, but I'm sure you'd agree that there's a distinct loss of elegance the more I do this.

These solutions can be broadened to accommodate more forms of incoming radiation, with a commensurate increase in technological difficulty because of the extra compression required. For example, it would probably be desirable to be invisible in infrared as well.

Invisibility cloak #6

These solutions become more troublesome if we add more requirements. We've already established that, while wearing the cloak, you want to be able to see in the visible spectrum. What if you want to be able to see in the chi spectrum as well (i.e. by using a conventional oracle under the cloak, in the form of a monocle or similar)? Suddenly you have a requirement to let the chi spectrum through unaltered as well. It would presumably be possible to use a more advanced transformation to let a narrow band of chi mana through unaltered while storing all of the necessary information in side-bands. But if you want to be able to see the whole chi spectrum, you've run out of places to store this information.

Essentially, the problem becomes one of out-of-band signalling. You have a requirement to be able to observe and therefore absorb the entire spectrum of information falling on your cloak. But you also need to store all of that information unmodified somewhere so that it can be rebroadcast on the other side of your cloak. This means that you have to pick some sort of out-of-band signal to use: not chi particles, not visible light. One option might be radio, but now you're in a situation where any passing radio broadcast causes you to shine like a light, just like the chi source did! Another option is conventional electronic circuits. But in that situation you've not only sacrificed your lightweight magical shield for a physical object which you have to carry, you've also basically built real, fully non-magical thermoptic camouflage, making no use of the convenient chi phenomenon at all!

We can do better. The last solution is to add your selective "oracle" capability to the invisibility shield itself. This means that all the information you desire is crammed into the visible spectrum in some form at cloak entry, then safely discarded at cloak exit.


If you already have the literally magical technology that is necessary to become invisible, you are still going to have a hard time getting practical use out of it, because you have to be able to see.

I now have a reasonable idea of the level of skill and intelligence which is necessary for a fictional character to be able to construct each of these cloaks. For example, I know that none of them are completely impossible, which is something of a relief.

I eventually decided not to put any of the mathematics into this article because mathematically the major problem is just working out exactly what mathematical objects to use to represent the various things. I eventually decided that an incoming spectrum would probably be a function f mapping wavelengths λ to intensities, and that the spectrum would undergo some sort of transformation e.g. P, Q, R, S, T upon passing through the cloak in each direction and upon passing into and out of the human body and on being reflected. After that, the rest was actually quite boring, consisting of observations like "For transparency, we must have S(P(f)) = f for all f" and "If we want to be able to see, we must have P(f) = f" and so on. None of this actually helped me in my work very much.

This is also a classic example of a science fiction invented thing - in my case, magic - which, when taken all the way to its logical conclusion, quickly results in phenomena which are too complicated to be palatably explained to readers or used meaningfully in storytelling. I have the same problem with many other things, usually time travel. The writer has a duty to overthink things.

Once again, I can't guarantee which (if any) of the described magical cloak technologies will appear in future installments of Ra. Still, I hope this has been a vaguely interesting look at how I work.

Discussion (41)

2012-12-27 00:58:36 by DanielLC:

"you are still going to have a hard time getting practical use out of it, because you have to be able to see." You don't have to be able to see. It's just helpful. There's still a lot of interesting stuff you could do with dead reckoning alone. And that's before you start to just use methods they're not looking for. For example, you could set it to only work in the visible spectrum and use night vision goggles, or use sonar instead of vision, etc. You could even just let a tiny portion of the light through, and use some kind of giant magnifying glass to get just enough light to see by. If you're in direct sunlight to begin with, this is a very, very small portion of the light. It would be much easier just to cheat and use a different kind of sensor to find you. A cheaper way to make cloak three: Start with cloak 1, but then wear goggles with oracles acting as cloak three. If you use a special camera rather than human eyes, there's no theoretical limit on how dark it can be, so you could use cloak 2 and have it still work. I wonder if it could be made to fool sonar. Phonons are just as real as photons, so there's no obvious reason it couldn't be done. Cloak four and up have a heat problem. The earlier ones would allow you to cool down by emitting chi particles, but if you're not willing to emit them, you have no way to cool down. You could try to cool down by whatever you're touching. Theoretically, this shouldn't work because the forces necessary to do this are mediated by photons, but even if you get around that, you'd still start heating the ground. This would be visible to infrared sensors. If you're willing to allow flaws like that, you might as well use cloak one. Essentially, you will have a time limit. You could try to last longer by carrying a heat sink, but your time will be limited. Also, air could be a problem. Breathing through the cloak could be noticed, so you'd have to carry around your air. If you're not in it too long, you could just use a big cloak. Since a giant sphere would be easier to make than something human-shaped, there's a good chance you're doing this anyway. If you want to last longer, normal scuba equipment won't help. You could add an air pump so that you can pump the used air into another tank, but you'd have to have some way to make it all silent. Using a re-breather would mean that less air has to be pumped, but I think the filters might make it louder.

2012-12-27 02:10:20 by OtherSam:

My first concern when reading this was how to power the more sophisticated cloaks. #1 can probably run passively off the absorbed light, and maybe also #4 if there are enough incident chi particles. The others would need the mage to actively provide mana (or wear a battery pack, or whatever), which introduces not just another interesting point of failure--fiction is boring when everything works fine!--but also will be generating heat that could be detected by an adversary.

2012-12-27 06:59:37 by Solus:

What's all the fuss about being undetectable in chi, infrared, sonar, radio and all this other crap? I thought you just wanted to be invisible. A device like this would usually be used for stealth, right? If you're doing your stealthing right, then your opponents shouldn't realise that they need to use infrared or sonar or whatever in the first place. Or if it's because they eg. use infrared goggles as standard equipment, just focus on that and the visible part and ignore the rest. It's rather unlikely that they're gonna have every kind of sensor at once.

2012-12-27 10:38:02 by Sysice:

Solus: he is overthinking things, remember? If we're assuming that this is to be used in a story, there's no real reason to go beyond cloak 3 or so- strong enough to be useful, with a weakness that lets it be countered in the story. Everything beyond that's just to see how far he can take this, just what's possible with the assumptions we had at the start.

2012-12-27 12:05:00 by Matt:

I for one found this about as interesting as the fiction side of things. Not quite as engaging, but its primary purpose was a long way from "be an engaging tale".

2012-12-27 13:25:50 by Arik:

Take #1, and add two things to it: 1. There is only one type of oracle. It converts visible light to chi and vice versa, in both directions, on all sides. In other words, the oracle has the property that any particle beam crossing it an even number of times remains exactly the same, while a particle crossing it an odd number of times is converted to the other form, be it photons or chi. This enables the cloak to be, well, cloak-like with folds etc. 2. The user must use another oracle unit, which is again the same, perhaps mounted on goggles, to see outside of the cloak normally. Without it the user will be in complete darkness, unless a chi source exists outside. The elegance from an engineering standpoint is that all of the components of the system are uniform - simplicity is beautiful. You are still in the dark inside but you can see perfectly. The one remaining problem is the detection by chi source. This will have to remain an unavoidable imperfection, as all things are. I recommend that the user only use an oracle on one eye, leaving the other eye in the dark. The user will then easily identify a chi source (as they will look very bright compared to absolute darkness) and avoid passing between such sources and potential observers - otherwise they will see a chi shadow where the body obstructs the visible light inside the cloak.

2012-12-27 14:25:11 by Baughn:

There's another, simpler way to make an invisibility cloak that lets you see, which you may have missed. It uses the same basic principle as half-silvered mirrors. That is, the eye has a much greater range of static adjustment than dynamic range. You can convert 99.9% of the incoming light to chi particles and leave the rest as photons, and (a) you'll still be able to see, without noticing a massive difference, but (b) you'll remain quite invisible to anyone looking at you.

2012-12-27 18:03:10 by Dan:

Cloak #1 works mostly fine, and DanielLC had the same idea I immediately did: map incoming photons to chi, then wear (small) normal oracle-goggles. Yes, this would create a small amount of lost information (a dark spot opposite the eyes). For casual stealth operations this should be more than sufficient, but it really depends on who the user is. If it's Laura, then she's totally shown herself to be impulsive enough that I figure she would stop thinking it through at this point; 99% invisible is good enough to get most people to assume they're seeing things. If it's for the Magic SAS or something, then sure, they probably have a better design. Even if it is, though, there's one remaining problem I'm seeing here: my limited understanding of chi mechanics suggests that all spellcasting throws off chi as a pseudo-entropic byprouct. How are you supposed to stop the shield/cloak spell from radiating chi as a consequence of its own casting and glowing like a beacon to anyone with an Oracle?

2012-12-27 18:51:08 by Andrew:

How are sound waves affected by the cloak? If they can pass through bidirectionally, you would be "visible" to sonar, and/or have to remain incredibly quiet to avoid detection.

2012-12-28 10:25:02 by Eleanor:

If chi is produced whenever there's magic, then in principle cloak #4 is about the best you can do. Even if you retain and reemit all incoming information from the environment, you can't stop an observer with an oracle from noticing you, because he will see the magic that is making your cloak work.

2012-12-28 18:30:13 by Kevin:

Cloak #2 may not work so well as an invisibility cloak, but it sure would be pretty neat to have. Look at me, I'm glowing!

2012-12-28 21:29:15 by Dmytry:

I have a suggestion. Under the cloak, you wear low density oracle goggles that convert 0.1% or maybe 0.01% of the incoming chi radiation into visible light. You can wear night vision goggles under those. The shield converts chi into visible light on ingress, so that you also see chi from inside the shield if you take off your goggles. So, basically, the contrast is far, far too poor to see your goggles. The chi-invisibility can be done with a secondary spell that just bends the chi to pass around the cloak. Then this looks more like engineering and less like overly convenient laws of physics. If you want to pursue alternate laws of physics - perhaps light and chi are unified field. You could then have a lot of different polarization states, and changes in polarization which an oracle may be entirely unable to discern.

2012-12-28 22:37:42 by Silhalnor:

Sounds like this cloak is expending a lot of mana to produce the visible spectrum. As others have said wont the waste chi alone make you fairly brightly lit in the chi spectrum? My first thought while reading this was the same as what Baughn suggested, a one-way magical "mirror". Unfortunately it looks as though you need the cloak to be perfect so this wont work satisfactorily. If not for waste chi you could use an oracle that decodes your compressed data into visible light for the wearer while the cloak decodes it to both the visible and chi bands. I assume it is impossible (or only under special circumstances like superconductors) to have 100% efficient magic with no waste so you have to hide or delay them somehow. I suspect that there is not a magical solution to the problem of hiding your waste chi. Is there any way to interact with chi particles non-magically? What you really need is some kind of passive solution that doesn't expend energy. Something like metamaterials but to my knowledge there is no such thing as "passive magic" in this sense. It's the difference between eye glasses that use no power and emit no waste and a digital camera. Can you build a passive magical device that does not use power to run? Even that wont handle "physical" events like sound and air currents though without constructing a metamaterial type device that manipulates spacetime itself but even if it's possible that would be so exceptionally advanced that you are already a deity. What you are wanting to do is drop out of the universe while still being able to observe it.

2012-12-28 23:07:09 by Silhalnor:

Destroy, or separate yourself from, your body and exist as a spirit or ghost of sorts. Laura's dreams of her mother show her doing essentially that so it would appear that Laura believes it to be possible. In essence create a magical body to shunt your mind into, maybe temporarily. This would surely emit large quantities of chi particles but it could conceal you in every other way. I imagine it could be used for scrying. It would require a magical Montauk sink of some sort. Eyes made of magic. Aliasing to steal people's magic for long term persistence? Horribly horribly complex but it may work absolutely perfectly sans waste chi. But you wanted a perfect invisibility cloak not a way to mimic cessation of existence so this is probably way way overdoing things. What are your design constraints? Do you need to be able to manipulate objects or do you just need to hide? Do you need to hide footprints, air currents, and other physical activity?

2012-12-29 09:10:48 by john:

What you need is a purely magical surface which converts photons to chi-band mana and vice versa, as efficiently as possible, forming a bubble around the thing you want to hide, with any supporting devices inside the bubble. Ideally, waste chi from the invisibility spell itself gets translated to something mostly outside the visible spectrum; radio waves if you can swing it, lots of background noise there. Inside the bubble you can navigate by watching pre-existing chi sources, potentially planting new chi sources specifically as navigational beacons, sound, dead reckoning, or (depending on bubble size) by looking at what you're standing on. Speaking of which, the other flaw in this strategy is that you can spot a divot seemingly cut out of the ground wherever the invisible person is standing, or any other surface they get too close to, if they're not extremely clever about the positioning. Logical fix for that would be to build it into a wheelchair, so distance between the machinery and the ground can be kept constant while moving.

2012-12-29 10:18:33 by JohnF:

What is the propagation speed of chi particles? If they propagate at exactly the speed of light regardless of the medium they travel through, then there would be refraction induced at the conversion surface. I would think that refraction would occur regardless of the mechanism that oracles operate by, because refraction is caused by how the differing propagation speed affects the impacting particle's wavefront. The index of refraction in air at STP is 1.000277, which would give a critical angle of 88.15 degrees. So there would be total internal (actually external because the air outside the cloak is the denser medium) reflection of all incident light which strikes at angles greater than that. I suspect this would be noticeable as a faint silvery outline. You could handwave this away by saying that refraction simply doesn't occur, but that feels unsatisfying to me.

2012-12-29 10:47:07 by Eleanor:

"Divot cut out of the ground" doesn't really apply. Yes the shield is defined as being a few centimeters out from the wearer's skin, but it's also defined as able to block physical objects. That blocking means the shield butts up against the ground rather than penetrating into the surface. No part of the ground is allowed inside, which means you don't have the problem of the floor going invisible beneath you. However that does remind us that you'll still leave conventional footprints in any soft surface, and disturbances in any other soft material you contact. The user who's smart and paranoid enough to make a level-4 or higher cloak must also be smart and paranoid enough never to walk on a grass surface or stand amid blowing leaves. Best they stay in an enclosed environment where everything is bare concrete or linoleum.

2012-12-29 18:09:18 by Silhalnor:

john, my interpretation of how magic works suggests that the waste chi will be emitted from the bubble's surface and within which means that there is not a chance for the waste chi to pass through the bubble. Of course this is speculation based of observations in the story and extrapolation from real world physics (eg. a black body cannot influence the radiation it emits) so I could certainly be wrong (especially if there is hand waving involved, haha) but I don't believe it should work. I would like to say that Arik's solution is the most elegant solution here, at least for photon invisibility. It has a flaw; the user's goggle mounted oracle will cast a shadow but this is easily solved, albeit by reducing the elegance somewhat. What you need to do is have the goggles act like cloak #2 and leave the chi particles undisturbed while producing new photons. When a photon hits it you'd probably want it destroyed to ensure that no visible light escapes your cloak. Ignoring waste chi (and immense energy expenditure and potentially insufficient space for data) I think my addition above is the only one here one that works perfectly in both the chi and optical bands. It is the same as Sam's cloak #5 (the cloak encodes incoming photon and chi particles into compressed data and decodes it back on the other side) but with the addition of an oracle on your person that decodes the chi data out of your compressed data and emits it as visible light, leaving the original chi particles alone of course. Waste chi from both the cloak and the oracle would produce interference though so the chi bands used would have to exceed a minimum brightness or amplitude. This, in effect would both increase energy expenditure even further and take away storage space.

2012-12-29 18:12:17 by ComputerSherpa:

From a storytelling point of view, having the cloak be imperfect turns it from being a boring magical technology into an interesting plot device. A cloak that requires active, skillful management from its user is more engaging than something that makes you automatically undetectable. I like the idea of an operative who has to hide from his pursuers in the electromagnetic spectrum as well as in real space. Give the cloak a knob so its user can choose which frequency light will be "stored" in, as well as a "full black" mode that gives you perfect invisibility in exchange for blindness. This allows you to showcase just how good a stealth operative the cloak's user is, and forces him to resort to luck and dead reckoning when he finally encounters an enemy that can afford full-spectrum jamming.

2012-12-29 19:40:13 by ianso:

Similar to Dmytry, I was going to suggest oracular contact lenses. But doesn't your suit have another problem - it also functions as a normal oracle to anyone looking at it? Chi mana would pass through the visible->chi oracle unchanged and then into visible light on the other side. <br>The alternative is to convert chi mana to visible and back again at the same time? With the downside that you could blind (or fry) someone wearing an invisibility suit using a large spell...

2012-12-31 03:44:19 by Tint:

The compression steps in the later suits seem like they might be unnecessary. I'm assuming that when an ordinary oracle emits photons in place of chi particles that the spell is somehow computing a vector representing the particle's flight (perhaps implicitly, though, since there's no real need to store the information longer than is necessary to generate the photon). So why can't we do something similar here? When a photon hits the suit, generate a vector representing its path and then compute where on the opposite side of the suit the photon would intersect with your force field if you weren't in it, then emit a photon from that point and absorb all photons which attempt to exit the suit. Same goes for chi particles. This does have two obstacles which come immediately to mind: first, you probably don't want to (or can't) emit a photon immediately when one hits your suit (I'm presuming that magic doesn't allow us to get past relativity). Perhaps the time it takes to determine the point and generate the new photon will happen to be proportional to the speed of light, but in any case some work would need to be done to ensure that the particles were being emitted at the correct time as well as the correct place. The second problem is that if the interior of your suit absorbs all particles that attempt to escape, you will quickly overheat from incoming light as well as your own body heat! Power might be an issue as well since any photons absorbed by your skin or clothes wouldn't hit the force field and thus wouldn't be able to contribute any energy to help with generating the extra particles, so the cloak may have a very short lifetime for most mages. Perhaps you could use some of the heat generated by the absorption on the interior of the suit? Even if it wasn't enough to sustain the cloak indefinitely that could help solve the heat problem.

2013-01-01 09:39:05 by Sealeaf:

Sounds like your mechanical approach is leading to more difficulties than it is solving, starting with the invention of chi particles. I prefer a psychological approach to invisiblity. Something on the order of the, " nothing of interest here", field or Douglas Adams' "someone else's problem" field. Tolkien touched on this idea with his elven cloaks that were hard to see "by unfriendly eyes". The advantage to this approach is that the invisiblity applies across all forms of sensing, If you don't see any thing interesting then you don't hear anything interesting either. Also your own senses work fine inside the field. The problem is that you only become invisible to observers who are close to you and inside the effective range of the field. Another problem would be mechanical sensors. The security camera would see you just fine even though the store clerks would not.

2013-01-01 19:31:45 by OvermindDL:

A side effect I see with being able to convert any form of light to chi, especially while modifying the data/waveform/whatever: You could create things that could, for example, take infrared/radio/microwave/whatever, convert it to chi, convert it back to 'visible' light, thus making very effective out-of-band light googles or so for humans. You could even do things such as shifting the waveform around, imagine taking in the visible spectrum but put it back out again after flipping the data, thus making red to blue or whatever.

2013-01-02 21:03:16 by Trord:

I wonder what the boundary conditions would be like. A tangent to a curve hits the curve at one point. Would that mean that photons hitting the very edge of the cloak are converted into chi and not converted back, and vice versa? If so, that would give the cloak an outline that's bright in the presence of ambient chi and dark otherwise.

2013-01-03 14:35:27 by JoetheRat:

What sort of remote viewing magics are there? A lot of the "need to see" issues could be resolved by having your visual input be outside of the cloak, and transmit the signal to a receiver within the cloak. I'm going to assume that you can't simply create an invisible p.o.v. for a crystal ball, and instead rely on magic mirrors (Mirror 1 sends its "view" to display on Mirror 2). Rig up a set of goggles, where Mirror 1 extends to the surface or just past the cloak, and Mirror 2 sits comfortably inside to transmit its view. The closer the two, the better, as it will create less displacement. Having the two physically connected could allow you to avoid the chi/em conversion mechanics as the signal should be safe in its wire (or you could incorporate the appropriate shielding/signal transformation to make it work). The only limit here is how unobtrusive you can make your external camera - either go for a pinpoint style lens, or a ring of hyper-fine wire or (very) low refraction material. Or you could create a second simple shield to hide the Mirror 1 lens-ring, but leave the lens/space free to see. Done right, there shouldn't be more than an odd mote in the air. Depending on how serious the concern, A one-way "sound mirror" - noise passes in, but is absorbed going out - means you will not reflect, and will also be rather quiet. This does create a sound shadow / hole in auditory space, so it's a crude measure for the problem, and a wonderful loophole for the opposition to try and exploit. (Yeah, I think like a pulp comic book writer.)

2013-01-05 06:38:14 by DanielLC:

"When a photon hits the suit, generate a vector representing its path and then compute where on the opposite side of the suit the photon would intersect with your force field if you weren't in it, then emit a photon from that point and absorb all photons which attempt to exit the suit." But how do you transmit the signal to emit the photon to that side of the suit? You could send it around the edge of the suit instead of through the middle, but that doesn't really help solve the problem. "A tangent to a curve hits the curve at one point." Only an infinitesimal portion of lines are tangent. It can't possibly make a difference. "What sort of remote viewing magics are there?" If it's anything like real life, none at all. The closest you could get would be something like light, where you can tell what's in the distance, but only because of something moving along the path between them. There are ways to use this someone effectively if you're willing to stretch the limits of disbelief. Namely: the later versions of the cloaks listed.

2013-01-08 07:00:32 by traverseda:

I'm a little bit confused by the time line. In sufficiently advanced, Laura and Natalie have forcefields that extend above the skin. In ragdoll physicist they are surprised by benji being able to modulate a cone shaped force field. The timeline doesn't quite seem to work, unless you're telling it out of order. If they are completely different tech trees, if binding above a surface is a lot easier then modulating in mid air for some reason, putting a personal forcefield on a big speaker would allow them to replicate benjis research and invoke ra. Or just speed up casting a lot by carrying around an mp3 player loaded with spells.

2013-01-08 09:17:19 by qntm:

A simple piece of sound-reproduction hardware like a music player can't be used to cast spells. There's a reason for this.

2013-01-08 10:57:48 by traverseda:

But a force field apparently can. I was under the probably mistaken impression that this was because magic was the thing actually moving the air. Now a forcefield attached to a speaker, well it's the magic actuating the air still. I can't the fundamental difference between a magical forcefield shaking up and down due to more magic, and a magical force field shaking up and down due to electron or mechanical actuation. Both have the same affect, magic shaking the air. I suppose it makes sense if it's not the magic modulating the air that's important, but the data being encoded magically. In which case you could probably generate the electrical potential to actuate air via a speaker. It also has some interesting implications. It implies that the sound data is encoded in a persons mind using magic, and that benjis disk encoded that data in a similar way as the human mind. In which case I doubt the characters could actually make use of it.

2013-01-10 20:06:20 by Omegatron:

For the timeline question: "until very recently, small-scale non-flat force fields were impossible" From just after Ra/Benj activates the sound thing. I think the only reason it was able to cast spells was because it was Ra doing it.

2013-01-10 20:10:23 by Omegatron:

""It turns out that two things in the universe can use magic," Benj explains. "One is sentient humans. The other is magic itself." And the base ring says, "Ibra oniki ra. QUINIO QUINIO QUINIO alef a ra." Laura backs away a step and almost trips on the uneven ground. "But... nobody cast that spell. Without a human mind behind it, it's just pressure waves in air. There've been dozens of experiments. Thousands. Machines can't spell. Machines can't do magic. It has to be a human being.""

2013-01-17 04:39:44 by traverseda:

If magic itself can cast spells, is it important that something magical is actuating the air, or is it important that the data is encoded magically? Some third option, both of them perhaps?

2013-11-30 12:32:04 by Vladimir:

The distinction between visible light and chi particles seems to be irrelevant to the problem of invisibility cloaks. Let's say the "spectrum" is the set of all possible signals: light, sound, infrared, chi, etc. Parts of the spectrum will be called "ranges". In particular, there's the "perceived range" which is all signals a human can easily perceive (light, audio, etc). Let's assume there's also the "non-interacting range" which is all signals a human can neither absorb nor emit, e.g. some subset of chi. These just pass through. Obviously the perceived range does not overlap with the non-interacting range. Let's make an additional assumption that the spectrum is "compressible", i.e. you can build a device that maps the whole spectrum into some smaller range, and another device that maps it back. Now the problem of invisibility cloaks has been properly framed, the solution is easy. On entering the cloak, the whole spectrum is mapped into the non-interacting range, and another copy of it is mapped into the perceived range. On exiting the cloak, the non-interacting range is mapped back to the whole spectrum, and everything else is ignored. So to the outside observer it looks like everything just passes through and nothing is emitted, while the person inside can perceive all parts of the spectrum at once with no special goggles needed (because everything is mapped into the perceived range). You can fine-tune the mapping of the spectrum into the perceived range to highlight different parts of it, etc.

2014-02-02 09:54:58 by Sparky:

A relatively simple cloak could simply flip all photons into chi when it crosses the boundary, and all chi into photons(in either direction. the person would be invisible to non-magic detection, but could see mana sources. If you wish to see visible light with this cloak, you can still use a standard oracle within the cloak, preferably one that's very inefficient, so it doesn't create a noticeable shadow(only ~5% of the light goes missing, similar to a partially silvered mirror).

2014-08-03 06:41:55 by MilesOfSand:

Why bother using chi particles at all? All this discussion makes me think it would be a lot easier to use wires and electricity to transmit the encoded versions of the chi & light waves. And it would only need 2 spells: 1: Let me know what materials I need to make an [description of the suit] 2: Assemble these materials into an [description of the suit]

2014-09-04 08:49:08 by Stan:

"Why bother using chi particles at all? All this discussion makes me think it would be a lot easier to use wires and electricity to transmit the encoded versions of the chi & light waves." That would cause catastrophic failover fanfiction, since you'd need to invoke the "Heisenberg compensator" technology from the Star Trek transporter. You'd need to know both the position and momentum of the photons (and the chi particles, of course, though separately -- and here I only assume that chi particles more-or-less follow quantum electrodynamic principles but are at best weakly interacting with ordinary matter) in a frame in which they interact in order to reconstruct them on the other side. Otherwise you don't have invisibility, just cuttlefish-level camouflage (which, I'll admit, is nothing to sneeze at). One can take some literary license to hand-wave the oracle technology into being, and it's even fair to posit conservation of momentum through an oracle (in both directions), but saying that you can ignore all of the inconveniences of physics BECAUSE MAGIC sort of goes against the spirit of the work so far.

2014-10-02 10:45:25 by sunshine :

lol eyes do not work that way my friend ... its the complete opposite ... light is emitted from your liquid crystal brain through your eyes projecting the world that a one conscious grid views of as one with individual will .,,,, giving your material body the sense of reality .... conscious spirit is not invisible is at a different spectrum of light .. our math is wrong ... bright light at its brightest will still continue a higher infinite path to the spectrums of what we call dimensions ... in other words light spectrums are infinite to the point of what we think is invisible light and darkness will continue its shade spectrum to the point of dimensional invisibility also ... they will not tell you that our math is set to be a roadblock ..... use your common sense ,,,, resonance, light , sound, vibration , etc continue infinitely,,, so therefore defy the rules of reason and create your own form of math theory and see what u get ;) ps a sphere must be turned inside out to understand weightlessness within the gravitational force ... everything thought to exist but not seen is the total opposite of our reality at a dimensional frequency hidden within the shades of the light spectrums ... governed by TIME and your box below will not let me put anything but i" (i) is not valid ITS WHAT I WANT IT TO BE for the reasons i want it for ,,,

2015-07-03 12:29:48 by darth_biomech:

I am probably late for this, but it is possible to make enchanted contact lenses, that take up chi particle from the front side, and then emit identical particle plus a photon from the other side. There, you can use #1 model of the cloak, no chi loss and no blindness.

2015-09-12 03:35:44 by readerMan:

or you could take a cloak that converts all light into chi and back again while wearing googles that function as the reverse.

2018-12-28 19:12:07 by tahrey:

I'm now wondering if some of this might be the inspiration for the Cloak Of Grey Tomorrow in Prequel Adventure, which is *almost* an invisibility cloak that blinds you whilst you use it... but the particular magic imbued in the cloak *displaces* the blinding effect... until the point at which you take it off. Wearing the cloak, you can see, for a certain maximum period. When you take it off, you're blind for the same amount of time, but can now be seen... Animation on the character of Katia testing it out suggests that it may do something weird and physical like displacing your pupils (and so provide a portal for light to pass into your eyes), and when you take the cloak off they're displaced into the cloak itself...

2019-01-31 19:09:52 by AGM:

There's a far simpler solution to becoming invisible that uses no chi while active: give the cloak a full-spectrum negative refractive index. Partial-spectrum negative RI has been achieved IRL for each frequency range on the EM spectrum up to the blue/green section. The longer the wavelength, the easier it is to create something with a negative RI for it. The catch there is that one for multiple sections - such as the whole Visible spectrum (RGB) and the IR band - is even harder than making a negative RI material for the Gamma radiation band. At least Marvel had a more practical approach that solves the vision problem at the expense of stealth. The Invisible Woman from the Fantastic Four can see while invisible to the RGB band by becoming visible to Cosmic Rays (though Dr. J. Kakalios points out in The Physics Of Superheroes that the illumination source is wrong - it should be UV radiation).

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