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Basically I'm thinking of a strategy game, something like starcraft, and you have a unit that emits a field that causes the area around it to appear in grayscale. Is there any kind of potential physics explanation for how you could change the color of light reflected by an object without physically changing it?

Maybe it could involve slowing photons, something like how blueshift or redshift works?

Also, this would be a temporary change, the field could move or shut off and the area would return to it's normal color spectrum.

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    $\begingroup$ A time machine that brings the immediate vicinity back into a time where color was not invented yet? \s $\endgroup$
    – bukwyrm
    Oct 28 '21 at 10:47
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    $\begingroup$ It's called a bomb. Just fill it with large quantities of dust. $\endgroup$ Oct 28 '21 at 13:03
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    $\begingroup$ What is the tactical utility of a field that makes everything in it seem grey? Or is it just a side effect of its activity? > If so, you could sidestep the issue by having the greyscale effect be an "enhanced reality" artifact overlaid by the troops hi-tech battlefield goggles to highlight for them the affected area rather then an actual physical property of the emitted field. $\endgroup$
    – Lafayette
    Oct 28 '21 at 15:56
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    $\begingroup$ @bukwyrm You are missing the requisite Calvin and Hobbs reference. $\endgroup$
    – Glen Yates
    Oct 28 '21 at 19:32
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    $\begingroup$ I once saw a stage production of The Wizard of Oz. Just like the film, the the beginning and end which are set in Kansas were made to appear in B&W, while Oz is all brilliant full color. How they did it I'm not 100% sure - I suspect it was mostly a matter of the sets and costumes being grey-scaled, combined with very flat lighting, though Dorothy did not appear to change her dress between the 2 locations, so the light probably had a bit of a yellowish (sepia tone) tint to make it appear more monochromatic (since the dress is blue). $\endgroup$ Oct 28 '21 at 20:25

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What is black and white?

From the perspective of the physics of color, there is always color save in one condition: when there is no emission of light. Everywhere else along the infinite spectra there is color.

White's even worse. There is no point along the spectra that is "white." White is either a condition of reflected light when all light is reflected or a condition of emitted light when multiple spectra is emitted in perfect balance.

Grayscale (a pure "shade" of color between white and black) is the messiest of all - and I know this because I've mixed "gray" paint. Oh, it looks gray in the can, but put a red chair in front of the wall you painted with it and you'll see the red in the gray paint. Put blue carpet in the room and you'll see the blue in the paint.

And what you're looking for is a way to explain the lack of color, but the presence of luminescence.

You don't want physics. You want biology. You want a field that affects the perception of color by the brain. From the perspective of rationalizing the effect, you can do this in a few ways (and it's IMO a lot simpler than explaining how you changed physics...).

1. Change the way the cones in your eyes work

There are two types of photoreceptors: rods and cones.

Rods are responsible for peripheral vision, and are located outside of the central part of the retina. There are some 120 million of them, and they are responsible for night vision, because they are highly sensitive to low-intensity light. They are completely blind to high-intensity light, so they are not important for daytime vision or for visual acuity. Because they are not able to distinguish colours, they produce achromatic vision.

Cones, which vary in number from 6 to 7 million, are responsible for the visual acuity of the human eye (the ability of the eye to resolve and to pick up the minor details on an object) and for distinguishing colours. They are concentrated in the small central part of the retina known as the fovea centralis, measuring 0.3 millimetres across and devoid of rods. (Source)

Your field disables the cones in the human eye, leaving only the rods (responsible for peripheral vision and unable to detect color) as the only means to see anything. A natural consequence (if you care about that) is that your people would lose their fovea vision, meaning the world would look just a bit blurry.

2. Your field disables two of the three types of cones, making your vision monochromatic.

You have three types of cones: red, green, and blue. Your brain interprets signals from the three types in a way that's similar to how an LED display works (kinda, I'm simplifying things a lot). In other words, if the three cones see their preferred colors at the same intensity, the brain interprets that as grey. Unfortunately, it's basically impossible to guarantee with your field that all spectra would be reduced to just three. But if you removed, for example, the blue and the green cones, what you'd see is everything in red. Not shades of red, but in luminosity of red. You'd get exactly what you want... it's just not gray.

3. Your field affects the way the brain processes color signals.

This is the solution that humans understand the least, but is most likely to explain exactly what you want. The brain takes those shades of red, blue, and green perceived by the cones and creates a picture that has oranges and browns and yellows and purples and puce and all kinds of amazing colors. But if that interpretive process stopped interpreting color... what you'd see is a world of gray.

So, don't change physics. Bad Things happen when you screw around with physics. All of reality could implode! But biology! You can goof around with biology all day long and what's the worst thing that can happen?

Well... zombies... but let's ignore that.

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    $\begingroup$ if there is just one single wavelength around (like under sodium lamps) the scenery appears monochrome (not b/w, but at least monochrome) so maybe the device sucks up all wavelengths but one, or, more feasible, it drowns everything in ultra-intense monochromatic light? $\endgroup$
    – bukwyrm
    Oct 28 '21 at 10:50
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    $\begingroup$ @bukwyrm's comment is indeed the answer. $\endgroup$ Oct 28 '21 at 14:02
  • $\begingroup$ @Join JBH on Codidact So this is the "Emerald City spectacles" effect. The limitation here is that observer must be present within the field. For such observer, even outside objects (like setting sun, for example) would also appear monochromatic, while for an outside observer objects within the field would appear normally colored. $\endgroup$
    – Alexander
    Oct 28 '21 at 16:16
  • $\begingroup$ 4. Instead of disabling color processing in the brain, disable it in the eyeball. Still no idea how to target them, but at least they're better understood. $\endgroup$ Oct 28 '21 at 16:51
  • $\begingroup$ @bukwyrm I intentionally avoided a physics-based answer because such an answer would be, IMO, unbelievable. I'll leave the physics-based answers (if there is one that doesn't cause the universe to implode) to others. $\endgroup$ Oct 28 '21 at 16:58
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I'm afraid this might be difficult. The perception of color is given by the wavelength, but grey means one of two things. The first is that all three opsins in the eye are equally stimulated. So, an object that emits (say) ten quanta of light in the blue range has to also emit ten quanta in the red and ten in the green range:

Source: Wikipedia

This is what happens in photopic vision, i.e. in broad daylight, where humans see using the cones in their retinal fovea. Your "field" would have to "clone" photons in different frequencies maintaining their direction and possibly phase and polarization (or you'd get very funny, and coloured, reflections), and depending on what other photons are traveling the same way (i.e. a green + blue pair would "beget" a red one, and a red one - if alone - would have to beget a blue + green pair).

This seems just, well, magic.

But, when light intensity falls below the mesopic threshold, we see only using the more sensitive rods. This is the scotopic vision, that needs less light, but has no color discrimination; which is what we perceive as seeing in grayscale.

The simplest solution would therefore be to absorb all light in the affected area above a given intensity threshold, so that what is left only allows seeing in grayscale.

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  • $\begingroup$ I like that idea. I'm thinking of an explanation that says the field reduces the energy of everything inside of it, and a side effect of that is the grayscale effect, something like that. Thanks! $\endgroup$ Oct 27 '21 at 23:04
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    $\begingroup$ @JavaRevengeance just keep in mind that how you explain it might also shut down all the cells of the people in the circle, killing them instantly. At that point, might as well just use a grenade ;) $\endgroup$
    – Drake P
    Oct 28 '21 at 17:07
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The aliens have a really, really good stealth system.
(with one fatal flaw)

Their stealth field completely absorbs all the light that tries to enter/leave a sphere around the emitter. Any light that is so absorbed is recorded, analyzed, and re-broadcast with a bit of photoshopping. In effect, the aliens "Edit out" any information that they do not want to pass out through the visual interface of the stealth field.

This sounds like the be-all, end-all of ultimate spy gadgets, right?

Unfortunately, the aliens can only see in monochromatic light. To them the rebroadcast light is a perfect mimicry of reality, and completely indistinguishable from what they want the target to see. Every texture is exactly right, every intensity is perfectly calibrated.

And the output is in monochrome, because the aliens simply cannot conceive of any species capable of seeing several different conflicting images of the same scene, in different wavelengths, at the same time!? Why, the mind just boggles at the thought of explaining why such a ludicrous sense organ would make any evolutionary sense, when an ultrahigh resolution, ultrasensitive single-receptor visual system is so superior.

TL;DR;
The aliens are color-blind, and don't realize their stealth field turns everything into shades of grey.

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    $\begingroup$ Interestingly, these aliens are also highly susceptible to an incredibly simple and low-cost stealth system - just paint yourself a color the aliens can't see to become invisible to them! $\endgroup$ Oct 28 '21 at 19:27
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    $\begingroup$ @NuclearHoagie That's not how color blindness works. Not even remotely $\endgroup$
    – PcMan
    Oct 29 '21 at 6:17
  • $\begingroup$ Someone who "can only see in monochromatic light" doesn't sound like regular monochromatic color blindness - a typical monochromat has cone cell dysfunction, but can still perceive a wide range of wavelengths by their rod cells. What you've described as "only seeing in monochromatic light" is not how color blindness works - monochromats' rods respond to many wavelengths, but lack the ability to distinguish them. Someone who can truly only see monochromatic light (i.e. a single wavelength) would have a visible spectrum shrunk down to a very narrow range, rendering most things invisible. $\endgroup$ Oct 29 '21 at 13:51
  • $\begingroup$ @NuclearHoagie No indeed. is SOUNDS LIKE someone that has a wide-spectrum-sensitivity eye with only a single receptor type. A photon of blue or green or red or infrared or UV would all excite the receptor equally, and be completely indistinguishable to the viewer. $\endgroup$
    – PcMan
    Oct 29 '21 at 14:14
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Smoke.

H4 Thermal Camera Line | Smoke Test | Part 2

smoke test

The thermal camera can see the people through the smoke.

https://viewspace.org/interactives/unveiling_invisible_universe/forms_of_light/seeing_through_smoke

Here, a firefighter stands in a room filled with smoke, which obscures what can be seen in visible light. However, when we switch to an infrared light view, it is possible to see through the smoke. Human eyes can’t see infrared light without the assistance of tools, like a specialized camera, but we can feel it as heat. And since wavelengths of infrared light are longer than those of visible light, they pass through the smoke, which is why they provide a clear view of the setting.

The device is a smoke bomb. Those would work better in space than on Earth because the smoke would hang around a long time. The smoke blocks visible light. If you want to see through it you need infrared light. There is no color information with the infrared image and so it is grayscale. There is thermal information which is interpreted as the varying grays.

If the smoke moves away you can see past it. Or you can disperse it with another explosion that does not make smoke. Energy weapons will not be useful against the smoke.

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    $\begingroup$ Well, you have NIR, MIR and FIR so you get "colors". Just completely different ones. $\endgroup$ Oct 28 '21 at 8:42
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    $\begingroup$ To quibble with a minor detail in your answer, AFAICT smoke bombs wouldn't "work better in space than on Earth" at all. Sure, you can have a device that emits "smoke" (i.e. a mix of dust and gas) in space, but in vacuum the gas will just disperse in all directions at approximately the thermal velocity of the gas molecules (i.e. really fast) and drag the dust along with it. (In fact, we've done that experiment already, since a solid rocket motor is basically a giant high-power smoke bomb. AFAIK the plumes from those are basically invisible in vacuum once they cool down enough to stop glowing.) $\endgroup$ Oct 28 '21 at 11:42
  • $\begingroup$ @IlmariKaronen - I think you are right about rocket plumes but I also suspect that these motors put out almost all gas and hardly any solids - solids mean unburned stuff which means an inefficient rocket motor. A smoke bomb should put out lots of small solid particulates. $\endgroup$
    – Willk
    Oct 28 '21 at 17:23
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    $\begingroup$ @Willk: Many solid rocket motors use APCP fuel, which contains aluminum and has a lot of of aluminum oxide dust in its exhaust. That's for example why the NASA Space Shuttle solid rocket boosters produced thick plumes of glowing smoke, while the main Shuttle engines (burning hydrogen and oxygen into water) had nearly invisible plumes. Yes, having solid particles in the exhaust is kinda inefficient, but the energy density more than makes up for it. (And yes, many orbital solid rockets, like the Star 48, also use APCP.) $\endgroup$ Oct 28 '21 at 19:41
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Any truly monochromatic light source will cause objects to only appear in that colour: more than just a red light making things seem reddish, a monochromatic light will only show values in that one colour hue.

This of course won't be greyscale - for example the sodium discharge lamps, which were common for street lighting, are nearly monochromatic orange so everything appears in intensities of orange. Being monochromatic also means astronomers can filter them out easily compared to the newer white LED ones.

The human eye stops seeing colour at very high and low brightness, so if you want the appearance of greyscale rather than monochrome you might try a low level of monochromatic blue, or a very bright monochromatic yellow.

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If you shine red light on a green object, it appears black - Or at the very least the green colour disappears.

How about a device which scans the colour of the objects around it, calculates the 'opposite' colour on the spectrum, and then projects the new colours onto each object, cancelling out the object's true colour and making it appear grey?

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    $\begingroup$ This wouldn't work the way you think it would. It's not the presence of red light that makes the green thing look black, it's the absence of red light. $\endgroup$
    – Hearth
    Oct 28 '21 at 14:35
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    $\begingroup$ @Hearth I take it you mean the absence of green light? $\endgroup$
    – Drake P
    Oct 28 '21 at 17:10
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    $\begingroup$ You'd need to eliminate other light sources for this to work. Shining wavelengths that get absorbed by an object won't really do anything if the reflected wavelengths are still present (white light already contains the "opposite color" you're shining, but since the reflected color is there too, that's the color you see). Basically, this will only work in darkness... and since you're shining light that only gets absorbed, it'll look very much like the darkness you started with, for zero net effect. $\endgroup$ Oct 28 '21 at 17:57
  • $\begingroup$ @DrakeP Yes, it was early in the morning. $\endgroup$
    – Hearth
    Oct 28 '21 at 18:37
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Darkness.

Human eyes have two types of photosensitive receptors: cones, which come in three types and can differentiate colors, and rods, which cannot. Cone cells, however, do not work well in low light conditions, so if you're in a dark room, even after your eyes adjust to the low light, you won't be able to differentiate colors effectively. Try it yourself sometime!

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Clarification: what do you mean by "field that causes the area around it to appear in grayscale."? Human observer just fails to differ colours?

If yes, you can probably make visual sensory overload using bright stroboscopic lights, and it will tamper sight of anybody around this device so they cannot differ colours properly.

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You can make the follow experiment your self;

Replace your normal light bulb by a monochrome light bulb. I suggest red or green. Turn it on and it is near impossible to identify things original colors. Any thing is in monochrome-scale. So, if you replace the grayscale part by monochrome-scale you have a deal. I doubt there is a plausible way to turn things visible like watching them in an old (very old) black and white TV screen.

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The unit has a protective membrane, or a shield, that is meant to protect the unit from photon weapons (lasers and ionizing radiation). The membrane merges the wave functions of a group of photons and collapses them in such a way that in the output there are no high energy photons. The light spectrum of every group of photons shifts to a more evenly distribution. And the membrane works like this in both directions. This in effect turns the image grayscale for everyone looking through the membrane. It's not really grayscale, as you can still see a glimmer of color if you know how to look for it, but from afar it looks grayscale close enough.

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They're in the Matrix

Suppose your world is inside a simulation. The device you want takes advantage of this fact and exploits a bug that reduces the quality of its area's rendering. Among other things, it makes its immediate vicinity look like it's in grayscale.

How does it work exactly? That can be handwaved away, because the exact hack isn't necessary to know.

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Maybe it could involve slowing photons, something like how blueshift or redshift works?

Something like that, you'd need some sort of field which randomizes the frequency/energy of photons so that they have an appropriate spectral distribution to appear grey.

So, for example, you had something coloured blue. It's giving off a bunch of photons with a frequency distribution which peaks in the blue part of the spectrum. You red shift some of them to make it appear more grey, you also have to blue shift a proportionate amount so that energy is conserved.

You do that where ever there is a frequency distribution of photons that is not grey enough for you.

This is of course impossible under current physical laws, it contravenes the uncertainty principle. To do this you need to know the position and momentum of the individual photons, so you can adjust the momentum and leave the photon in the same place, on the same course.

You might be able to get away with that by having the result look "fuzzy", each photon has some random element changed to appease the uncertainty principle. I'm not immediately sure how fuzzy it'd have to be to make this work.

This is also of course, currently impossible, the technology is way beyond anything currently envisioned. But if your technology is sufficiently advanced it can look like magic.

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Adding this thought:

If you created a distortion in space-time, so that the light passing through it is red or blue-shifted, and you can control that shift. You could modulate the shift in a random manner, so that any light passing through it is "greyed".

The result would not be exactly greyscale, there would still be some colour there, but it would be very unsaturated. If you're not aware of it, "white" light is not one thing, it's a range of things, and the actual colour of "white" is described by its colour temperature. If you do photography, that's what "white balance" is all about.

So if something is blue, it would be seen as a "cool" blue-white through the field, if it's red it would be seen as a "warm" orange colour, greens would just seem white.

Even if you don't exactly get greyscale, that would look very weird, and probably be seen as greyscale. The advantage of this is that it has a more sound theoretical footing, and you don't need to be addressing individual photons, just bulk space. So no uncertainty principle problems.

I would guess that a theoretician could construct such a space-time distortion to do the right thing using general relativity. The difficult part would be realizing this distortion.

--

I mocked up the effect to see what it would look like.

I mocked up the effect.

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