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I know so called ''invisible ink'' exists in the real world.

The most common types can be revealed either by applying heat to the paper it is written on (lightbulb, clothes iron, candle, lighter, etc.) or by holding a UV light over the paper.

I wanted to know if one could make a so-called ''invisible ink'' that would only become visible under moonlight (I know moonlight is just reflected sunlight, but in case these is any ever so subtle difference) and if that is possible if such an ink could also be made in such a way that when it is removed from moonlight it would turn invisible once again.

I fear this might be straight up impossible in real life, but I thought I might ask just to be sure.

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  • $\begingroup$ What if you added other conditions such as near the sea, at night, in the winter (high humidity, colder temperatures)? Maybe some ink will work for you under other conditions. $\endgroup$
    – mcbecker
    Commented Mar 3, 2022 at 15:44
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    $\begingroup$ Adding a condition as at night and at sea would work perfectly, the character who is to use this ink is supposed to be a naval officer with an affinity for night sailing/fighting $\endgroup$
    – Blue Devil
    Commented Mar 3, 2022 at 18:09
  • $\begingroup$ Perhaps the salt in the wind can also be a factor. $\endgroup$
    – mcbecker
    Commented Mar 3, 2022 at 18:52
  • $\begingroup$ Also, some writers give the advice that you shouldn't try to explain every little bit about how it works, but describe/show it working in a likely way. We dont have time travel but if the text read: for 45 minutes the whole US of A was without power. Only generator powered facilities or military complexes shyly braved the night. The heart of the machine was divided between several closely packed engineering facilities. The night was absolute and you could hear the beating of the machine come alive and die strangely fast. But it was enough. $\endgroup$
    – mcbecker
    Commented Mar 3, 2022 at 19:03
  • $\begingroup$ Does this have to be OUR moon, or can a different moon with exotic composition be included? Also, what tech level is allowed for the ink? $\endgroup$
    – DWKraus
    Commented Mar 4, 2022 at 1:21

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No

As said in other answers, moonlight is just attenuated sunlight, nothing more special about it.

But, actually yes...

Some materials exhibits a behavior called "emission quenching", meaning that the light that they emit reduces with an increasing stimulation, because the excited states in the molecule interfere with each other and make the relaxation (and subsequent light emission) more difficult.

Since sunlight is more intense than moonlight, if your ink has a threshold for "emission quenching" which is lower than the sunlight intensity, such an ink might work. However, it would work not only with moonlight, but also with cloudy sky or artificial illumination.

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    $\begingroup$ This could actually work. Moonlight is a lot dimmer than most artificial light you'd use for reading (a standard candle at one meter is brighter). $\endgroup$
    – Zeiss Ikon
    Commented Mar 3, 2022 at 14:40
  • $\begingroup$ except it will also be legible twice at day at sun rise and sun set as well as many unlint indoor spaces. $\endgroup$
    – John
    Commented Mar 4, 2022 at 21:21
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Short answer: no.

Moonlight is just sunlight, reflected off (effectively) asphalt pavement a quarter million miles away before it gets back to Earth. Magically, it's special in a number of ways, but scientifically, the only thing different from sunlight is it's not as bright.

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No

Any real property that moonlight has can be emulated with a suitable artificial light source. There's nothing special about moonlight or reflected light that applies outside a magical context.

Just enchant it, fairly simple magic like that doesn't require a full magic system that's any use beyond the light show.

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Frame Challenge: Real life is boring and frequently too limiting. What you really want is suspension-of-disbelief

And Tolkien's books, great as they are, require a lot of suspension-of-disbelief. But that's the point of fantasy, right? (I'm assuming you're pulling this question from the moon letters in The Hobbit.)

And when we open the door to suspension-of-disbelief what we get is an ink that will only be visible with limited light. Unfortunately, we can't limit it to just moonlight because moonlight and sunlight really are the same thing.[1] But we could limit the exposure to light the same spectra as sunlight, meaning if you have weak enough sunlight the ink would appear but light from a flashlight that wasn't specially designed to mimic sunlight wouldn't do it. And if you wanted to filter direct sunlight, you'd need to do it in a way that preserved the spectra (e.g., holding sunglasses above the letters wouldn't do it because common sunglasses only reduce a limited spectra of sunlight, not all of it).

So we're talking sunlight at 0.25 lux. How could we explain that (knowing that we can't do it at all in Real Life)?

The molecules of blue diablum 25, a fluid manufactured by the Azure Dybbuk Corp., align in an opaque lattice at approximately 0.25 lux. Below that the lattice relaxes into a random form that is effectively transparent to the human eye (but not necessarily to a microscope!). Above that the lattice changes to a wholly organized and completely translucent way. This isn't detectable by a microscope.

Of course the problem with using a microscope at less than 0.25 lux is that the human eye doesn't have enough light to see the result anyway. Very hard to detect without direct chemical processing, that blue diablum 25.

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  • $\begingroup$ I'm not sure I'm happy buying from a company named for an evil possessing spirit. I'll wait for a more harmless-sounding fictional corporation before I buy the stuff :P $\endgroup$
    – Corey
    Commented Mar 4, 2022 at 3:34
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Yes, one could manufacture a reason why this would work in a universe similar to ours (in non-laboratory settings). But, no, for the reasons that others have pointed out, it’s not possible in real life.

One thing others have overlooked is that moonlight is not merely dim sunlight; the sunlight has interacted with the moon rocks, and nonlinear or filtering processes could have taken place. So...

The mechanism is a mineral which occurs naturally on the moon’s surface, but on Earth it is confined to the core and inaccessible in any meaningful quantity. This mineral has a unique spectral emission line in the ultraviolet, labeled $\Xi$, which is remarkably narrow for a solid-state material. This material absorbs bright visible sunlight via multi-photon absorption and subsequently emits this narrow line of UV light with a purity and brightness unparalleled on Earth except in a synchrotron or a specialized laser lab.

Enterprising chemists have discovered/invented a molecule, $\Gamma$, which is unique in its complementary absorption line, $\Xi^*$, leading to broadband visible photoluminescence upon excitation by moonlight. But most remarkably, $\Gamma$ has a long-lived alternate electronic configuration, and the absorption of light with energy even a fraction above $\Xi$ is sufficient to bump $\Gamma$ into this quasi-stable state, which is dark (i.e. no photoluminescence). Over the course of a few microseconds, $\Gamma$ relaxes to its ground state by non-radiative means, ready for excitation once more. Thus, broadband excitation leads to orders of magnitude dimmer visible luminescence compared to excitation directly at $\Xi$. You could probably image a message written in $\Gamma$ in your lab using a bright UV lamp, some specialized filters, optics, and a fancy electron-multiplying CCD camera (but it won’t fit in your pocket!).

Then, as a practical matter, there are simply no easily accessible sources of light which have a narrow enough emission at the ideal wavelength to excite photoluminescence in $\Gamma$ without quenching it with higher-energy light. None, that is, except the moon.

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Not exactly what you thought, but maybe. Two types of fluorescent ink, one is very sensible and it is used to write the words. The other less sensible is spread on the page as a background.

If the text is exposed to the sun both inks become bright and the page turns into a bright white sheet with no discernible writing. If the text is exposed to the weak moonlight then only the sensible ink becomes visible.

If the text is exposed to a candle in a dark room it won't work, either the candle is too far to trigger some fluorescence or too close and works like under the sun. A candle with a translucent screen in front might work, but nobody thought about it .

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Yes, in principle

Apparently the spectrum of the moon is actually different from the sun - slightly shifted to the red. And that leaves open the option for a complicated molecule that changes color when light of several different intensities at different frequencies falls on it. Of course, this would still be foolable by something that could reproduce the same spectrum.

https://astronomy.stackexchange.com/questions/26780/why-is-moon-light-not-the-same-color-as-sunlight

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is it possible to build moonlight recognizer? (yes*)

Another way of looking at this is can you build a device that can give probability of 'is this light that is being observed from the moon'. That would be in terms of spectrum and magnitude.

That is this assumes the moon has a unique spectral fingerprint.

Imagine: A computer, a few optical sensors and a display. That with those items a person could then display a message upon pointing the sensors at the moon. But such a system is not close to invisible ink.

I would argue: yes such a device can be built. The problem: * There will be false positives and false negatives. To reduce the false positive and false negative rate it would essentially require a computing device. Or many small devices or nano ink for shorthand. Essentially small custom semiconductors that are designed to recognize moonlight.

Solution Nano-computer "ink"

Cover a substrate with the nano ink. The nano ink would consist of small semiconductors that could be charged by sunlight. Designed to recognize moon light via spectral fingerprint. Allow resetting by some signal. Programming a message via light of particular frequency through an optical mask.

The resulting device would something that would display writing upon exposure to moon light. Then you run into next problem of can the viewer actually read it... Since if the viewer then apply a lantern or flashlight that would deactivate the writing display. Moonlight isn't exactly great light to read by.

With very low utility for the cost to design, it is unlikely to ever be made.

Using more traditional ink: No.

If you are wanting strictly a pigment/ink that was completely passive with no computing aspect, no magic aspect? That only reveals a message in moonlight and no other means? Then no.

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Technically, probably yes. Let's use logic.

First, let's ignore variations of moonlight, see if we can solve it for one specific, phase of the moon/spectrum of moonlight, at all.

As you say, moonlight is reflected sunlight. So we can expect these differences:

  • Lower intensity (or missing entirely) at some frequencies due to reflection followed by atmosphere filtering of the modified reflection spectrum.
  • Some changes of frequency from absorbed/re emitted, etc, or Doppler effect (surely unnoticeable).
  • General lower intensity.

Can any of those cause a message to be differently visible?

  • lower and missing intensity suggests an ink that is invisible under high intensity, or when frequencies in sunlight but not moonlight are present. For example, suppose the ink fluoresces.... but not when exposed to some intensity/frequency in sunlight, because say, the process under moonlight involves steps that preferentially happen a different way under sunlight spectra. Or its simply swamped and can't shine. Or changes colour but the reaction is inhibited by sun spectra/intensity. Or its covered by a 2nd layer that is transparent, but can't transmit that fluorescent frequency if exposed to bright light.
  • Doppler is probably undetectable, ignore.
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You didn't specify what kind of time period your world is based in. My answer may be far fetched, but you can have ink that's actually e-ink that has some kind of logic or sensor that activates when moonlight is applied to it.

Even if your setting is medieval, if you want "realistic" magic you can always go for the "magical artifacts that are actually pieces of really advanced tech from an old technological era" approach.

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