In the Asimov story Little Lost Robot, a special edition of the NS-2 series robot (this individual called Nestor-10) with a modified First Law has been sworn at and told to "get $&%@$#%&^ lost!". He proceeds to do so by hiding amongst a lot of other new and normal NS-2 robots on a shipment outbound for another location.

The robots were given the modified law because of the need for people to work in brief exposures to gamma-radiation, which "isn't harmful" to people, but destroys the robots' positronic brains (weak is OK according to the story). Despite being informed of this, the robots still try to help those exposed to the gamma radiation. To avoid losing the robots to the radiation, they have a modified First Law of "No robot may harm a human being" instead of the normal "No robot may harm a human being or through inaction allow a human to come to harm", which means they just let the people be when exposed potentially harmful situations. This weakens the first law and allows the second law of "A robot must obey the orders given it by human beings except where such orders would conflict with the First Law" to take some precedence and causes Nestor-10 to hide and try to escape the facility.

The NS-2 series robots have no innate knowledge of radiation and need to (and have the capacity to) learn the difference between infrared and gamma for example.

The great Susan Calvin is called in and sets to work to figure out which one is the hiding robot. She eventually does so by informing all robots that a human will be working in front of them but between the robots and the human (actually herself) will be a strong gamma field which will kill the robots instantly, so even if the person were in danger, the robot could do nothing about it and would be destroyed (to my mind this still conflicts with the 1st law; the 3rd law doesn't allow inaction and this was the problem in the first place). She sits there and has a weight dropped towards her head. Instead of gamma, she uses infrared, which Nestor-10 recognizes because he has learned it, but the others don't, so Nestor-10 comes out to rescue her, while the other new robots sit still because they don't know it is not gamma (and therefore think it is dangerous to them).

Now, aside from Asimov apparently getting confused in the story, onto the real question.

How would the robots be detecting the gamma radiation?

There's a field in place - but in order to not destroy all the robots it has to be limited in some way, perhaps LASER like? My understanding is that gamma is almost entirely non-reflective to most materials apart from some heavy metals, so they wouldn't see it like we would with visible light (i.e. no reflection off surfaces), and a scattered field would destroy all the robots, so it isn't radiating in a spread, it should be more like trying to see a LASER from the side.

Could it be through "absence" deduction: they are told there's a field IR, X-ray etc all scatter and reflect, so you can easily "see" those, but gamma you can't see. Except that wouldn't account for times when the gamma isn't present at all.

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    I think the most interesting question, rather than how their sensors work, is why the normal NS-2 sit still during Susan Calvin's test! I wonder why you didn't choose to ask this question instead, especially since -- like you stated -- their inability to "sit still" is what necessitated the modified First Law to begin with! In Asimov stories, robots put in an "impossible" situation like this usually have their positronic brains fried due to the contradiction...
    – Andres F.
    Commented May 17 at 13:20
  • @AndresF. I think because I was confused by the story and thought it must be something to do with the detection systems they use
    – bob1
    Commented May 18 at 0:08

3 Answers 3


Here’s my understanding. All the robots have sensors that can detect radiation across the electromagnetic spectrum, but are not hardwired to know how to interpret it.

Imagine telling a small child (who’s not colorblind) “Don’t eat the green berries!” The tyke is capable of seeing the color green, but someone would have to teach her what “green” is, or poison, or berries. She isn’t born knowing. If her mom, who she trusts, holds up a ripe red berry and tells her, “This is a poisonous green berry! Don’t eat it!” the child would believe her.

While I’m here, I’ll politely disagree with your other quibble. I don’t see how the Third Law is a problem with the premise: “A robot must protect its own existence as long as such protection does not conflict with the First or Second Law.” The puzzle here has nothing to do with the robots actively or passively protecting their own existence. They are compelled to save humans from even minor harm even if it means their own destruction—but not if they know that their attempt would fail, they would not save the human, and they would be destroyed for nothing.

The escaped robot isn’t actually compelled to try to save Susan Calvin. However, it’s trying not to get caught, so it was pretending to act like any other NS-2. Because NS-10 thought the other Nestors also knew that infrared is harmless, it expected them to try to save the human. If I recall correctly, it started to move forward, than as soon as it realized none of the other robots were, immediately halted.

Edit: This does not contradict Asimov’s other stories. Robots don’t die of a positronic aneurism if someone tells them that a human will be hurt and there is no way for them to prevent it. That only happens when the Rules create a paradox, and the robot is trapped in a situation where it must violate the Rules no matter what.

Many of the stories would not work at all if you could blow any robot up just by telling it that some human will suffer harm that is out of its power to prevent. One that comes to mind is the presidential candidate trying to prove he’s not a robot. The Positronic Man does, in a sense, destroy his own brain because of his discovery that every human being will die someday, but not in the way people are saying.

However, if I’m misremembering and there is another story where that’s the case, the NS-2 series is different from those other robots. In this story, it’s explicit: these robots don’t have to try to save a human if they believe it is impossible for them to succeed. They would have to do it if they believed the radiation would destroy them after they saved the human, but not if they believe it will destroy them before they can reach her.

  • I think NS-10's behavior is explained by your answer, but not the quibble about NS-2. In Asimov's stories, the 1st law takes precedence, and unsophisticated robots will have their positronic brains fried when put in an impossible situation; they simply cannot watch a human suffer harm or die with them doing nothing about it.
    – Andres F.
    Commented May 17 at 22:35
  • @AndresF. If the NS-2s worked that way, Susan Calvin would have fried all their positronic brains, except for NS-10. It’s been many years since I read this story.
    – Davislor
    Commented May 17 at 22:48
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    @Davislor I think Andres' interpretation is correct. And I think you are correct - based on the premise of the story, all the normal NS-2 should have jumped up as normal 1st law, including Nestor-10 who knew it was IR not gamma.
    – bob1
    Commented May 18 at 0:12
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    @AndresF. Yes, 1st law takes precedence; they can't ignore even if it means their destruction.
    – bob1
    Commented May 18 at 0:13
  • @Davislor exactly. My point is that this is a little inconsistency on Asimov's part. But that's ok, I believe this one was one of the early robot stories? His understanding of the Laws evolved; after all they are fictional and the Rule of Cool applies.
    – Andres F.
    Commented May 18 at 1:12

Exactly how the Nestor (NS-2) robots detected radiation is not mentioned in the story. The most we are told is in the final paragraph, when Dr Calvin explains:

the normal NS-2’s could detect radiation, but could not identify the type. That he himself (i.e. Nestor 10) could only identify wave lengths by virtue of the training he had received at Hyper Base, under mere human beings, was a little too humiliating to remember for just a moment

So it appears that the Nestors were equipped with some type of wide bandwidth EM-sensors, covering wavelengths from gamma rays to at least infra-red radiation. Exactly how these sensors functioned is not described in the story however. For for the purpose of the narrative it is sufficient that they existed, and that they worked as described.

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    It seems to be a plot hole - the rays can't be pointed at them (or only weak ones), so a "field" of gamma rays still allows scattered so they can detect and they can measure intensity/energy (implied, but not stated).
    – bob1
    Commented May 17 at 9:16
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    Yes, the Nestors must be able to measure the frequency of the radiation, but can only interpret it (to say if it is gamma or infra-red, for example) after some additional training. Commented May 17 at 10:33
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    @bob1 how can it be a "plot hole" if we don't know how their sensors work?
    – Andres F.
    Commented May 17 at 13:18
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    @bob1, all forms of electromagnetic radiation scatter. Gamma doesn't scatter very well, but it still scatters.
    – Mark
    Commented May 17 at 21:24
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    @bob1 my guess is that the Rule of Plot Necessity applies. There must be a way to detect low amounts of gamma radiation that are not enough to damage the robots. Hand wavy maybe, but Asimov's robot stories are not exactly hard scifi.
    – Andres F.
    Commented May 18 at 1:14

I do not recall the story details, but given the facts as you have stated them the field was actually infrared light (not gamma radiation). This is easily detectable; basically it's a heat lamp. One presumes that all the robots detected it but only Nestor-10 recognized that detecting it as heat meant it must be long wavelength, and hence infrared, rather than the undetectable gamma radiation (extremely short wavelength).

Of course there could have been gamma radiation also, but how is a poor robot to know that?

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    Thanks. I'm aware that Calvin uses the IR; that's in the question already. The question is how the robots are detecting when gamma is used at other points in the story, given that it is damaging to them (so you can't shine it on them) and it is almost non-reflective.
    – bob1
    Commented May 17 at 3:37
  • How could you detect that γ radiation may be present? Geiger counters, phosphor screens, warning signs, radiation alarms, failing positronic brains, ...
    – Ethan
    Commented May 17 at 3:54
  • The point is that Nestor-10 could detect and ID gamma and hence knew it was not gamma and was therefore safe to rescue Calvin. Presumably counters etc. weren't there or all the robots should have jumped up to help. Failing positronic brains isn't a good one as they (the people) are trying to avoid that hence the need for the modified 1st law.
    – bob1
    Commented May 17 at 4:11
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    @bob1 we know NS-10 can figure this out "after training". This means its reasoning abilities are involved. Which means it only has to detect IR, not actual gamma. Once it detects IR, it knows it cannot be gamma (Calvin didn't mention IR at all, therefore it must be a trick and NS-10 figured this). Regardless, we don't know how scifi sensors work, maybe they figured out an indirect way of detecting gamma radiation in the future of Asimov's stories.
    – Andres F.
    Commented May 17 at 13:24

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