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I started playing one of the Metro games, and one of the common enemies is an armor plated Spider the size of a small dog that your character's flashlight causes its armor to boil as if the light is acid. Eventually I took a break from this, walked past the TV and the movie Pitch Black was starting. It is set on a desert world inhabited by these bug like creatures larger than humans, and have the same trait of being physically injured by light. In both instances, too much exposure to light seems to be fatal to these creatures, with the Metro case being about 10-20 seconds being enough to kill them.

Having coincidentally seeing both of these instances in such a short time span I got to thinking. How realistic is the general idea of a creature which when exposed to light of even a flashlight causes its skin to burn as if it's on fire, leading to the creature's death in a short time span? If not a flashlight, then more powerful or special sources are still acceptable, such as maybe UV lights.

It would be preferable if a human were exposed to the same conditions that would be near certain death to the creature, the human would be mostly unscathed, but the line is drawn so long as a human is at least more likely to survive the said exposure than not. The ability to weaponize the light is of a high priority here, so we don't want to turn on a special flashlight and have it effectively become a suicide bomb unless that is the only way for this creature to exist.

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    $\begingroup$ Here's a word you need to Goolge: photodermatitis $\endgroup$ – cobaltduck Aug 8 '16 at 17:20
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    $\begingroup$ I can't see something as low powered as a simple flashlight directly burning them; but perhaps if their flesh contained compounds that broke down when exposed to light to form corrosives (or components of an exothermic reaction), then maybe indirectly. On a theoretically always dark planet that had life, the chemicals could perhaps be used to generate warmth safely, but when exposed to light.... $\endgroup$ – Uueerdo Aug 8 '16 at 18:54
  • $\begingroup$ @Uueerdo That is kind of what i was thinking. Chemical reactions are probably required here. Probably related to visible or UV light energizing the compounds just enough that it creates a second half of the chemicals for the reaction, and chain reacts. $\endgroup$ – Ryan Aug 8 '16 at 19:27
  • $\begingroup$ One word Phosphorus $\endgroup$ – jean Aug 8 '16 at 20:21
  • $\begingroup$ +1 for mentioning one of my favorite movies and one of favorite games! $\endgroup$ – vodolaz095 Aug 8 '16 at 23:38
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I'll give a conditional 'yes it is realistic' answer to this, but very conditional. The key part here is you don't actually need to fuel the reaction with the energy of light, you simply need light to initiate the reaction and the resulting compound to react with something else.

For example, hydrogen peroxide will degrade to water and oxygen when exposed to light (admittedly slower process). We know the reaction of some metals with water is relatively aggressive to outright explosive. Had this creature be composed of, lets say potassium and hydrogen peroxide, then the breakdown of hydrogen peroxide into water and the then reaction of water and potassium could give a boiling/acid texture to the outside of the creature. Mind you, in pitch black the giant rainstorm woulds produced some pretty fireworks if these creatures were composed of potassium. How exactly a creature would evolve to contain something so unstable is a bit of a mystery as well.

More research needed, but I do believe there is some feasibility in light initiating a reaction that cascades into something much greater from there.

edit: Try a second one. Silver iodide converts to metallic silver and releases the iodine in the presence of light (can be used in photography). Ammonia plus Iodine will create nitrogen triiodide, which gives off a purple smoke.

Ya I'm reaching. Kinda fun, but reaching.

second edit:

I think I might have the best option I can find here...silver bromine. Once again, you need to have a creature with a nearly silly mix of chemicals in it's body, but silver bromide reacts with light, releasing the bromine which could react with aluminum or a few others to cause a reaction. Why exactly a creature would be composed of something this unstable would be another question

I guess this post has become a very long winded way of saying theres less than 0.000001% chance of this feasibly happening.

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  • $\begingroup$ "Hydrogen peroxide is not a very stable compound, so, it is always decomposing to water and oxygen". That particular example wouldn't work because the chain reaction would be kicked off without any light at all. $\endgroup$ – Rob Watts Aug 8 '16 at 22:30
  • $\begingroup$ @RobWatts - one would have to assume the creature had another biological process going that kept the hydrogen peroxide from breaking down until the presence of light. Ya it's reaching. $\endgroup$ – Twelfth Aug 8 '16 at 22:35
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    $\begingroup$ Yes, it's reaching, but it's really top notch reaching! This kinda thing is why I love worldbuilding SE. Most other SEs poo-pooh such unlikely what-ifs, but we embrace them with glee! :D $\endgroup$ – Dewi Morgan Aug 9 '16 at 5:07
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    $\begingroup$ @Twelfth perhaps there's two processes, one that needs hydrogen peroxide, and one that produces it - like how plants photosynthesis (which produces oxygen) and also undergo respiration (which consumes oxygen). For whatever reason, a little hydrogen peroxide is needed, but it needs to be kept in balance. Light makes the decomposition happen faster than the replenishment process can keep up, and when it drops below a certain level it becomes harmful (because whatever process needs the hydrogen peroxide stops). $\endgroup$ – anaximander Aug 9 '16 at 12:38
  • $\begingroup$ I suppose the point of Reality check is not the .00000001% odds so much as that the odds are not 0, and in fact real life has such things that show it may be possible in some way, even if not to the extreme of those creatures described. $\endgroup$ – Ryan Aug 9 '16 at 17:33
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Let's consider the fact that the Sun is actually harmful to most creatures, given the right conditions. Humans around the world get sunburned every day, and millions contract skin cancer every year as a result of UV exposure.

What is a sunburn? Though we often don't think of it this way, it's actually a radiation burn that causes cellular damage. The Sun is constantly bombarding us with deadly ultraviolet radiation, and the Earth's ozone layer filters out most of this harmful light. But what about the radiation that makes it through? The vast majority of it is dissipated by melanin, the pigment that gives our skin color. People with darker skin tones - that is to say, more melanin - have been found to be able to handle sun exposure better than fair-skinned people, who tend to burn more easily. But what if someone didn't have any melanin at all?

That condition is known as albinism, a disorder that a person or animal is born with and is passed down genetically. In albinos, even the irises of the eyes lack pigment, which is why they appear red: you are seeing straight through to the retina. Normally, the iris contracts and expands to let more or less light in - this is why your pupils appear dilated in dim light, but much smaller in bright light. In the case of an albino's iris, since it is transparent, it cannot effectively block the light entering the eye. Because of this, albinos tend to have difficulty seeing in bright light, and many even experience discomfort or pain.

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    $\begingroup$ A bit more extreme case, but not different enough to put in separate answer, is Phytophotodermatitis. $\endgroup$ – Mołot Aug 8 '16 at 20:46
  • $\begingroup$ As an aside, while ozone (O3) does absorb several times as much UV radiation as regular oxygen (O2), if you brought the entire "ozone layer" down to sea-level, it would be about one to two millimeters thick. As opposed to ten miles of 20% oxygen troposphere. Entirely removing the ozone layer would have less of an effect on ground-level UV than the normal variation in solar output from year to year, plus the UV bombardment of the regular oxygen would re-form it in a short period of time. $\endgroup$ – Perkins Aug 8 '16 at 22:45
  • $\begingroup$ @Perkins why are people worried about the hole then? $\endgroup$ – Tim Aug 9 '16 at 0:39
  • $\begingroup$ @Tim it's not so much the hole, but more that the cause of the hole is persistent, meaning the hole isn't going away anytime soon $\endgroup$ – somebody Aug 9 '16 at 0:59
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    $\begingroup$ @Tim - people like finding things to fear...like the number of trans-gendered people in their washrooms. $\endgroup$ – Twelfth Aug 9 '16 at 17:07
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The lasers used in surgery (particularly eye surgery) uses a UV frequency that dissolves flesh efficiently without causing heat in the neighboring tissue.

The ultraviolet light from an excimer laser is well absorbed by biological matter and organic compounds. Rather than burning or cutting material, the excimer laser adds enough energy to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the air in a tightly controlled manner through ablation rather than burning.

The outer skin layer of a creature might be sensitive in this manner, so a flashlight-like device is especially distructive with surprisingly little power, if it puts all the energy into the target wavelength like lasers (and LEDs) do.

The thing is that UV is right on tge edge where photons have enough energy to affect chemical bonds directly at our Eartly temperature and composition. That's not universal though, as we manged to get photosensitive proteins for use in our eyes. So it’s at least chemically possible that the skin of some alien will have some protein in it that’s sensitive to what we call “visible” light.

In fact, one of my early (affordable) LED flashlights was a torquise LED. Emitting a monochromatic beam uses far less power, especially when it uses the color that our eyes are most sensitive too!

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    $\begingroup$ This seems like the right kind of idea, but I can't think of a flashlight that emits a narrow band of wavelengths, and I certainly haven't seen a movie where the protagonists use a single wavelength flashlight in this situation. I suppose that as long as the alien was sensitive to any wavelength the flashlight emits then it would work, but that divides the effective power output by a significant factor $\endgroup$ – Kevin Wells Aug 8 '16 at 22:21
  • $\begingroup$ @Kevin: well a white LED flashlight, if it's just a simple RGB combination, will emit three narrow wavelength bands. The alien could be sensitive to one of them in particular. $\endgroup$ – Steve Jessop Aug 8 '16 at 22:57
  • $\begingroup$ @SteveJessop That is a fair point. Another issue is that I would bet that the surgical laser is still significantly higher power than a flashlight, even if the flashlight has only between 1 and 3 wavelength outputs. The other advantage of a laser is of course that the waves are all in sync with each other so that they are much more efficient for transferring energy than incoherent light. $\endgroup$ – Kevin Wells Aug 8 '16 at 23:17
  • $\begingroup$ @kevin: actually, they are! Not so popular now, but were 15 years ago. Steve: no, white LEDs are florescent and produce a continuous spectrum, energised by a blue-nearUV LED rather than a mercury light as with the older tube style. The spreum is far better than a cheap florescent light; so little coating is used that they can use the good stuff even on cheap units. $\endgroup$ – JDługosz Aug 8 '16 at 23:45
  • $\begingroup$ FWIW, I also have a couple of UV LED lights. One is a tiny thing from PhotonLight co. and a cheaper one as part of my Dr.Who prop. $\endgroup$ – JDługosz Aug 8 '16 at 23:48
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For a creature to be hurt by any light, this is very unrealistic. Think about it - a large flashlight is typically powered by a couple batteries. If you had a flashlight that used 4 D-cell batteries in series, that gets you 6 Volts. You can (though I'd suggest watching a video of someone else doing it rather than trying it yourself) put a 9 volt battery on your tongue without serious harm. Why would transforming that energy into another form suddenly make it considerably more dangerous?

Given that not much energy is being delivered by such light, in order for the light to be harmful the creature's skin would have to be highly reactive to light. However, if it's highly reactive to any light that means the chemicals must be quite unstable, meaning they'd decay much faster and require a lot more energy to maintain. It's possible that you could contrive some situation in which this would actually be an evolutionary advantage, but it would have to be a very unusual situation.

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    $\begingroup$ Pacemakers use significantly less volts, and they can keep heart running - or stop a healthy heart if given the chance. Your answer has two misconceptions: 1) Volts =/= energy, and 2) It really, really matters how you apply energy. Laser pointer in the eye (typically 6V) can get you blind easily enough, can't it? $\endgroup$ – Mołot Aug 8 '16 at 20:30
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    $\begingroup$ Neither of your points is relevant to this situation. "1) Volts =/= energy": P=V^2/R power equals voltage squared divided by resistance, so it's proportional to the energy delivered. 2) yes, delivery method matters, but we're talking about a flashlight, which is quite a diffuse delivery method. $\endgroup$ – Rob Watts Aug 8 '16 at 20:39
  • $\begingroup$ @RobWatts Lightbulbs are also very diffuse delivery methods for light, but they are still strong enough to ruin undeveloped photographs, and I suspect even a weak flashlight would be as well. It isn't crazy to think that there could be a chemical reaction in which light plays a significant role that could be damaging to a creature that isn't adapted to exposure to light. $\endgroup$ – Kevin Wells Aug 8 '16 at 22:13
  • $\begingroup$ @KevinWells I already covered that in my answer. The chemicals in undeveloped film are fairly unstable. That's why they have to be kept away in cool (but not cold), dry conditions. It's not just exposure to light that can ruin undeveloped film. $\endgroup$ – Rob Watts Aug 8 '16 at 22:19
  • $\begingroup$ @RobWatts - you don't need the light itself to be harmful, you simply need the light to initiate a much more harmful reaction...the light itself does not need to be highly energetic if it's simply initiating a much larger reaction $\endgroup$ – Twelfth Aug 8 '16 at 22:27
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Extremely unrealistic.

To an extremely good first approximation all life on Earth is powered by the sun and it's very, very likely that this will be true of all life we ever find for the very simply reason that it's difficult to imagine another bio-available powersource that could take its place.

All plant life must exist in the sunlight, so it follows that for most animal life they would need to also be able to exist in sunlight or they're cut off from their food source for half of the time. This is especially true for earlier animal life since they will lack complex behavioural adaptations required to hide completely from the sunlight.

So we have life evolved from sunlight-capable life, and competing with sunlight-capable life, that not only adapts to living in the dark but somehow evolves to be harmed by light. To explode, as you suggest, this needs to go even further have a outer covering that is highly volatile in light but outer coverings benefit from being inert because this gives the most protection. I cannot conceive of any conditions where explody-reaction-to-light would be evolutionarily favoured over not-explody-reaction-to-light.

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  • $\begingroup$ I don't think this is necessarily true. There are places that have ecosystems not depending on sunlight for energy (hydrothermal vents), and there are places that depend on plant life but have near zero visible light (marine snow supports animal life at depths sufficient for almost complete darkness, and similarly cave life can depend on organic matter washed in from the surface by underground streams). $\endgroup$ – trichoplax Aug 9 '16 at 12:07
  • $\begingroup$ The point about earlier animal life is interesting, but it still seems feasible that single cell organisms could survive without complex behaviours, simply by spreading out as they reproduce. Those that spread into the light die, but those that spread through regions of darkness thrive. Over time there will be a selective pressure towards any trait that can increase the chances of avoiding the light (such as directly sensing light levels, or sensing heat or moisture levels which will give a clue). $\endgroup$ – trichoplax Aug 9 '16 at 12:10
  • $\begingroup$ @trichoplax: There's a reason I said to "an extremely good first approximation". Hydrothermal vents are, indeed, a separate possible source, but they're very localised, and won't support a planet-wide lifesource. Washed out matter into dark places is another but few of these are completely absent of life and life that made it from those places to the light would have a huge advantage. Combustive-in-light goes further than just harmed by light and would require an adaptive advantage for that combustibility, I don't see that as remotely plausible. $\endgroup$ – Jack Aidley Aug 9 '16 at 12:13
  • $\begingroup$ I agree there would need to be an adaptive advantage for having an explosive reaction to light, which does seem unlikely. Perhaps a creature that stores explosive chemicals just below its skin in glands adapted for defense would have no light shielding if it had evolved in a region with low/no light. Something like the bombadier beetle, but with glands all over the skin to burn any attacker when the chemicals are released. $\endgroup$ – trichoplax Aug 9 '16 at 12:21
  • $\begingroup$ @trichoplax: The bombardier beetle keeps the chemicals separate until the reaction is required and the reaction occurs in its natural environment. You're talking another level of dangerous chemical here, which not only is much more volatile but can actually destroy the creature if mis-activated. I'm still going with not remotely plausible. $\endgroup$ – Jack Aidley Aug 9 '16 at 12:26
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There are people who literally cannot go out into sunlight without receiving severe damage to their skin with severe sunburns or increased cancer risk, which can lead to death. The disease for this is called Xeroderma pigmentosum.

There have been several documentaries on this disease, as well as the focus of a few novels. Maybe not as severe as Metro's creatures, but still very severe.

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