Abracadabra

"They could now clearly see the ship for what it was simply because they knew it was there. It was quite apparent, however, that nobody else could. This wasn't because it was actually invisible or anything hyper-impossible like that. The technology involved in making anything invisible is so infinitely complex that nine hundred and ninety-nine thousand million, nine hundred and ninety-nine million, nine hundred and ninety-nine thousand, nine hundred and ninety-nine times out of a billion it is much simpler and more effective just to take the thing away and do without it.... The Somebody Else's Problem field is much simpler and more effective, and what's more can be run for over a hundred years on a single torch battery. This is because it relies on people's natural disposition not to see anything they don't want to, weren't expecting, or can't explain." Douglas Adams, Life, The Universe, and Everything.

The Premise

Invisibility and illusions have been integral to AD&D from the start, possibly because of one of the most famous magic items in literature, ever. Invisibility was a first level spell, and illusionists were the first mage specialty class (favored by the gnomes, and the illusionist-thief combination seemed too easy). Illusions had their own progression, going from basically 2D images without sound at the cantrip level all the way to holodeck-style illusions at high level. At high enough level, you could kill with an illusion (the Phantasmal Killer, which could be looked at as a cool spell, or a blatant way to buff up the illusionist).

But, easy access to invisibility and illusion has its price. This article examines that price, the effects, and the issues surrounding these spells.

The Facts

But, first some physics and biology. First, you need either something emanating light (the Sun, or a torch or lantern), or something reflecting light (the Moon, ambient light). This light source will be emitting light in multiple wavelengths -- this could be a narrow spectrum (blacklight or tinted light), or a wide spectrum (sunlight or typical light bulbs). A graphic QA technician can tell you about light booths (which I'm shocked that wikipedia hasn't covered this topic, so I lead you instead to http://www.solux.net/edu7.htm#What%20is%20a%20light%20booth?, and who is the wiseguy that decided to include a "?" in the URL anyway?) that are used to compare two samples under different light conditions. If I look at a product sample and the Pantone color (a color guide), the sample is suppose to match, the two colors may look the same under indoor light. However, indoor light has a color temperature of 2800K - 2900K, while direct sunlight has a color temperature of 5000K, and since the two light sources are different, different frequencies are being absorbed and reflected, and the color is perceived to us as different.

So, anyway, the light hits objects, and some frequencies of light bounces off of individual objects and scatters. Some of those light waves/particles hit the eye. You eye muscles change the lens and various liquids in the eye to change the wavelength and angle of the light so it reaches the retina in focus. The iris also expands and contracts to allow less or more light in -- less light means less light reflecting off of objects, which means a grainier picture. Cones and rods in your retina contain photosensitive chemicals (rhodopsin for rods, and color pigments for cones). When light hits these chemicals, they trigger electrical impulses which are transmitted to the brain, which interpret these impulses as color and objects.

And the brain does a lot of interpretation. In the area where rods and cones do not exist (the blind spot contains blood vessels), you would think the picture would just be blank. But, the brain is smarter than that. Instead, it extrapolates what the picture would be, and just paints more of that. If you're looking at a series of blue dots and there's an empty space due to your blind spot, your brain will fill in a blue dot. Some images are processed by reflexes before they are consciously revealed to you. Here's some fun with blind spots. It's also this interpretation that allows us to construct pixels into a identifiable image.

Sound and smell are not nearly as exciting, in my opinion. Again, there's a physical piece (sound waves, or particles) that get interpreted by a sense, and then further interpreted by the brain. Touch is more interesting, because there are multiple nerve endings within the skin. Different nerves produce different sensations, and some of those sensations, like vision, produces involuntary reactions. Along with the sensation of pain, the brain will lower blood pressure and numb the area, and the person may lose consciousness.

The How

Well, that was exciting. What does this all mean?

There's several ways to perform the effects of illusions and invisibility. TV works by producing light and colors, and magic could mimic this. However, it wouldn't look that realistic, because it's not matching the light sources available -- if everyone extinguished the torches and lanterns, you would see it as a ghostly image giving off its own light. It is a TV screen. Something more realistic is to take the existing light waves and manually reflect a portion of them to simulate an object that isn't there. Because you are doing this "at the source", so to speak, everyone will see the same object, and physical recordings will interpret the same thing. A second way is more mind control. Trick the brain to see what's really there, or what's not there. Each person may see something different, based on what they expect to see. A good example is in the movie The Shadow, where a building was hidden by people just looking over it. I had a superhero character whose ability was to erase people's memory of her (her superhero name was Deja Vu). Yes, she wasn't invisible, but she could walk through an area and since no one remembered her, she could be all but invisible. A third way is to manipulate the electrical impulses in the eye, as if light were hitting the rods and cones in a particular sequence. This isn't mind control per se, but it is giving a unique view to each person, but is not dependent on the mind interpreting and extrapolating information.

This brings the "save to disbelieve" idea, or the idea that a person can dismiss an illusion just by looking hard at it. The practical game-related reason is to allow higher level characters an escape route against a first level illusionist. However, this concept is directly related to how the illusion is created. If it is created by manipulating light waves or by triggering rods and cones, you can't disbelieve that, any more than you can disbelieve your real vision. AD&D 4th edition changed this -- you still see the illusion, but know it looks off (they don't mention how it looks off though -- is it a "orcs aren't that shade of blue" or "the monster looks...pixelated").

Another issue: monsters. They are going to perceive the world in a different way, see different wavelengths as we do. If an illusion is going to be created by manipulating light, then an illusion will need to manipulate light waves we can't even see in order for some monsters to be fooled. However, if illusion is mental or works on rods and cones, then there's less of a need of this.

And this is just vision. Hearing and smell works the same way. Touch again is a bit different. There's no energy you can transmit that can be interpreted by the nerves within the skin. You can trigger the nerves directly, or the brain. As the subject puts more pressure on an object, you trigger more of the nerves, leading to increased pressure. Touch even more nerve endings, and you can trigger some of the involuntary actions.

Finally, a thought experiment. A person takes an Polaroid of an invisible person. What does he see?

The Why

The obvious use is information gathering. We do it already in the modern era with two-way mirrors and small cameras. There's not much difference between an impossibly small camera and an invisible spy. If illusion is mental, you can create a system where the bad guys can't see the good guys, but each good guy can see each other.

Entertainment is a second option. TV and the holodeck is nothing but a fancy set of illusions. Television is such a staple in modern day life; if your world has disposable income and a bit of free time, some enterprising young magician may set up magic items that record stories, possibly in a serial fashion. Imagine a module where your mission is to get the series finale to the next big city...a series that everyone's been dying to find out the ending for.

There's also illusion as user interface and communication. First, the obvious -- secret messages can be sent through illusionary means. However, there's a lot more. The computer screen could be seen as a 2d illusion containing a basic user interface. Expand this idea, and you can get a Minority Report style interface through illusions. Expand this even further, and whole training scenarios can be set up using a fancy illusion. Expand this further, and you can set up a Matrix-style world, where everything is an illusion.

The Results

"Uh oh, guys, I think we have a problem."
"What? What is it?"
"The bane of all invisible characters ever -- a closed door"Order of the Stick

One word: paranoia and mistrust. I'm reminded of the GURPS Transhuman Space scenario where a small percentage of the population has a neural implant, implanted without their knowledge and consent. How much fun can you have with that? I quote: "In one instance, they made it appear that every woman that he saw had full head of red hair; in another, they made him hear a simultaneous Chinese translation of everything that was said to him over the course of a day. More recently, they've started to wake him up in the middle of the night with visions of Zarubayev, dressed only in a long nightshirt, sitting on Kharkov's bed, lecturing him about politics. Kharkov believes this to be a recurring nightmare and has been drinking more heavily in order to have uninterrupted sleep." If you can't trust your own experiences, everything is in doubt, a la Gaslight.

Another issue is the court system. If a person's experiences cannot be proven, then any defense lawyer worth his or her money can use illusion as a possible issue. The courts may require a defense lawyer to show connection to magic users, or they may just rely on DNA and physical evidence. Or, there may be magical residue, and one of the first thing police may do is check for illusionary effects.

Finally, security itself will shift, from physical viewing to barred doors. If you're invisible, you still can't get past a wall, or you still have issues with the other four senses. Special "invisible detectors" of fine dust, smoke, or other substance will find themselves on the market. Sonar and radar becomes more prevalent.

The Next Column

This column was about tricks of the mind, how to bewitch the person next to you. Next column is about the tricks of the body -- shapeshifting, polymorph other, and creating objects out of thin air.