I am a survivor of the United Nations Space Corp's 2nd Exploration Fleet. On our travel to the distant Epsilon Canares system, our scouts encountered a strange lifeform.

We call them 'shapeshifters'. At first contact, they looked like 6-legged crabs. After a few months, they started to mimic us and now, a year later, there is practically no visible difference between us and them. They can even speak our words now.

The only thing we know about them is that they are silicon-based lifeforms and instead of blood, they use ammonia to distribute nutrients to their cells. At first, we cut ourselves in front of each other to prove that we have blood in our veins instead of ammonia. But they copy us too fast now. I just checked my best friend two hours ago and now he's replaced with a shapeshifter. I've locked myself in the laboratory because I can't trust anyone else now.

Can anyone help me? How can I tell the difference between them and us? Is there a device I can build that scans bodies at a distance and tell me whether they're human or shapeshifter?

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    $\begingroup$ Liquid ammonia for blood? Or gaseous ammonia diffused into a primarily water-based fluid? $\endgroup$
    – Kromey
    Commented Oct 7, 2014 at 17:34
  • $\begingroup$ We discovered that while in its body, the ammonia is liquid. However, if a vein or section containing the liquid ammonia is cut, the ammonia turns into gas with the rancid smell of a cat's piss and seeps out of the wound. $\endgroup$ Commented Oct 7, 2014 at 17:37
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    $\begingroup$ Do they breath out sand (SiO2) instead of CO2? $\endgroup$
    – Philipp
    Commented Oct 7, 2014 at 18:46
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    $\begingroup$ Can we make this the required way to ask questions on this website? $\endgroup$
    – Sheraff
    Commented Oct 7, 2014 at 20:15
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    $\begingroup$ @Sheraff I've opened discussion of narrative question format on meta. $\endgroup$ Commented Oct 15, 2014 at 17:06

3 Answers 3


Stay calm, soldier. Our scientists have a few suggestions for you.

Liquid ammonia boils at a temperature of −28F (−33C), so any creature with this as its blood will be significantly colder on thermal imaging than a human being. (The alternative for maintaining liquid form, extreme pressure, would make them explode like a balloon from the tiniest of pinpricks, assuming any organic life could have developed to contain such pressures in the first place, so we find it extraordinarily unlikely that they've gone this route in their evolution.) The energy requirements to mask this — by keeping internal temperatures this cold but skin surface temperatures at the human-normal 98F (37C) — would be insane, and would likely drive their metabolism to such extremes that they could not stop eating — a distinctive behavior you could use to your advantage if they do try and mask their thermal signature to mimic yours.

Doing some rough back-of-the-envelope calculations, maintaining a skin temperature of 30C with an internal temperature of −30C (allowing for some pressure and/or additives to let their ammonia-blood stay liquid at slightly warmer temperature) is a difference of 60C. Maintaining this difference therefore requires approximately 250 Joules of energy per gram of mass, which is roughly 1,000 Calories. Now, accounting for insulation efficiency and ambient temperature (most likely around 20C), as well as efficiency of their metabolism to turn consumed calories into refrigeration/heating, they could easily require at least 10,000 Calories per hour, or almost a quarter of a million in a day! Per gram of mass! If they're around a human being's average mass of 65Kg, that's over 16 billion calories per day! Compare that to the USDA suggested intake of 2,000 calories per day, and you can see that your shapeshifters are many orders of magnitude higher just to maintain the temperature of their blood!

There's probably some issues with these calculations, especially taking the raw Joules up to a time-based figure. But this should give you a rough idea of the insane energy requirements your antagonists would have.

Edited to add: Since they have to maintain such a low internal temperature, if you can access the ship's environmental controls you can crank up the heat. You might be a little uncomfortable, sweating at 80F/27C, but those extra few degrees might be enough to literally make their blood boil — something that's pretty much universally fatal!

They're also very likely to have small leaks as their ammonia-blood evaporates minuscule amounts through their skin; this might be too little to detect by the woefully under-developed human sense of smell, but a simple gas detector could very likely detect what would amount to a faint aura of ammonia around the impostors.

Another possibility, if you have access to a spectrograph, is to check for high levels of silicon. It doesn't matter how good they are at mimicking the skin color to your eyes, atoms simply emit light at different wavelengths based on what they are, with silicon looking very different from carbon; even if they could somehow emit carbon's spectral fingerprint, it's extraordinarily unlikely that they could mask their silicon. Of course, this depends on how advanced your spectrography equipment is; if your stuff is little better than early 21st-century, they're bulky and slow, so unless you can convince one to give you a skin sample and wait an hour or two for the analysis results (during which time your legit compatriot could of course be replaced, invalidating the test results anyway!) this isn't likely to work, not unless you've got something far better miniaturized and far more sensitive to get faster readings.

Alternatively, if you could kill the lights and then emit specifically (and only) the wavelength of light that silicon reflects, the shapeshifters would practically glow while everything else was nigh invisible. (As a bonus this means you're essentially invisible to them!) You could play with this a little bit to find a combination of wavelengths that cause the shapeshifters to be visibly distinct while still lighting up everything else to a point that you can still work.

It's also highly probable that you could develop a toxin that specifically targets and breaks down the silicon bonds that make silicon-based life possible without affecting your own carbon-based bonds. Unfortunately, the specifics (and potential side effects) of such a route are unknown to us at this time, as our chemists are all on vacation at the moment.

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    $\begingroup$ Great idea on the blood temperature requiring them to be cooler. I felt a IR camera would work but wasn't exactly sure how. Please note, however, that if their blood is under pressure would help their situation significantly. $\endgroup$
    – kaine
    Commented Oct 7, 2014 at 18:06
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    $\begingroup$ @kaine True, but as noted any organic lifeform exerting this type of pressure on its internal fluids would have a host of problems maintaining it, to say nothing of the huge unlikelihood of such a system evolving in the first place. A combination of pressure and temperature would give them a few degrees of "fudge factor", but they'd still have to be vastly colder than humans. $\endgroup$
    – Kromey
    Commented Oct 7, 2014 at 18:08
  • $\begingroup$ I see, I wondered about that. They are indeed slightly colder. A crewmate remarked once that his bunkmate was freezing, but he looked fine. Assuming that their skin temperature is 30 degrees Celcius, how often will they need to eat to maintain that temperature? $\endgroup$ Commented Oct 7, 2014 at 18:09
  • $\begingroup$ Yes, they have the smell of ammonia about them, but they used some kind of deodorant to mask the smell. Truthfully, I think our sense of smell is shot, with so much ammonia in the air, even I smell like ammonia now. Is there anything that can be toxic to silicone-based lifeforms? Something like how arsenic is toxic to us. $\endgroup$ Commented Oct 7, 2014 at 18:12
  • $\begingroup$ I've edited in some (very) rough calculations on their dietary needs. It's insane. You would definitely notice the eating habits of these guys as being highly irregular! $\endgroup$
    – Kromey
    Commented Oct 7, 2014 at 18:31

For at least a little while you best bet will be a infrared camera to observe them from a distance. While they may work hard to mimic the characteristics you can see, they may not have tried to mimic thermal emmittance patterns yet. There are several reasons you probably already have one readily available.

Similarly, their density is likely to have changed as silicon has heavier nuclei than carbon. One could place a scale in thoroughfares or require them to step on it before meeting with you.

An Xray would likely work if you can convince them to walk in front of a source. I would not leave one out to give all your crew cancer needlessly. Use it to screen those entering from outside. The benefit here is you can screen their internal structure.

As I do not know exactly what molecules/polymers/etc they are made out of, however, I cannot predict how these properties would change. I can only know that they likely would have.

An IR spectrometer would work well if their skin is still made of a silicone matrix or their blood is still artificial. But that would only help when the start finding a way to mimic your blood. As you need a reliable sample which means you need to take it. The main thing there is to not let any of them know how you are testing them... kill them if the test is positive. If you could build a portable, long range, broad spectrum spectrometer you would tax their mimicking abilities to their extreme.

Passcodes work well unless they can extract them from your wounded/dead/tortured comrades.

I have one more risky suggestion for you friend. Microwaves ovens work well and hurt like hell because the molecules resonate with liquid water at 10 GHz while they run at 2.45GHz. Ammonia resonates at roughly 24 GHz and there is an ISM regulated frequency at roughly 24GHz. Aim it at a pet rat focused at him but at a low intensity. Point it at your silici conarts and see how they react... I deal with complex Silicon materials all they time that are annoying transparent to microwaves. If their already pressurized and cooled blood boils begins to heat up however... The issue is whether you can find an intensity low enough people will barely notice be hurt but shifters will show perceivable damage. We have no ammonia in our bodies but there are a lot of compounds which would resonate at different frequencies.

  • $\begingroup$ Good point on the mass, a sensitive (and portable) enough gravimeter might be useful to tell the difference -- though you'd likely need a baseline for each individual unless the mass difference is high enough, and even then drinking a quart of water would throw off your baseline, so again there has to be a significant mass difference for this to work... $\endgroup$
    – Kromey
    Commented Oct 7, 2014 at 18:01
  • $\begingroup$ Actually, now that I think a bit more about it, the gravimeter would be highly susceptible to everything around it -- take a reading on someone, then take a reading on the exact same person 20 feet away, and you've got two totally different results. (That's what they exist for, detecting differences in local gravitational fields!) So probably not effective, let alone practical, after all. The scale idea's still a go, though! $\endgroup$
    – Kromey
    Commented Oct 7, 2014 at 18:06
  • $\begingroup$ Yes indeed. They can copy our memories too whenever they shapeshifted into one of us. We once used passcodes too without knowing this. That was how so many of them infested our starship now. The rest of the fleet had abandoned us for dead because of this. Thank you for the weight suggestion. I wondered if they would be heavier or lighter than us. This can help. $\endgroup$ Commented Oct 7, 2014 at 18:06
  • $\begingroup$ If they can mimic your size, shape, and appearance, they may soon learn to mimic you density. Silicon nuclei are heavier than carbon but water in your body is dense enough that silicones (often low density) could be used to conteract this. Weight would work until they force themselves to copy that while you look for another way. $\endgroup$
    – kaine
    Commented Oct 7, 2014 at 18:10
  • $\begingroup$ Speaking of another way, do their bodies contain much water? We are largely transparent to microwaves besides our water. I don't know if there is a simular wavelength we can use for their ammonia. $\endgroup$
    – kaine
    Commented Oct 7, 2014 at 18:12

A laser spectroscope--the sort of thing we mounted on Curiosity.

Since you'll be using it on humans you will want to modify it a bit--all the energy should be delivered in one very quick pulse. You want to vaporize a tiny bit of the target while doing minimum harm to the surround. (Keep it small enough and you'll just be vaporizing dead skin, no harm done.) It's actually less harmful than the cutting technique but the equipment is bulkier.

  • $\begingroup$ Do you happen to know the rough modulation of the laser spectroscope needed for this, Ms. Pechtel? $\endgroup$ Commented Oct 8, 2014 at 15:13
  • $\begingroup$ @IfreeContractors No idea, I doubt anyone has actually built one. And I'm a Mr.--when I was born "Loren" was the male spelling of the name. $\endgroup$ Commented Oct 8, 2014 at 19:47
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    $\begingroup$ @IfreeContractors The modulation should be similar to that of FMCW radar. Where instead of delta per length, you'll get delta per density. $\endgroup$
    – Mast
    Commented Oct 12, 2014 at 15:38

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