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I know this might be unusual since people usually ask the opposite question, but I simply have to ask this.

It's about this question: Can a human size object move so fast that it ceases to be observable?

I really think that this question belongs on https://cogsci.stackexchange.com/ (maybe https://biology.stackexchange.com/ since it also includes the neuroscience tag). Also I think it should have been put on hold in order to improve.

Here are my reasons:

1) It's about perception.

The author of the question wants to know whether "regular audiences" or a "normal person" can see it or not, the author isn't asking whether it's detectable by other physical means. To be fair, there are some physical considerations that we have to worry about here but I can't see how a physicist could conclude the issue, it has to come to the issue of human visual perception and that's not physics. The visual system is far from an idealized camera.

2) The questions is badly posed.

First of all, the title is very misleading, it's clearly not about the moving object, it's about the observer. And I don't see how it helps if the object is superhuman and if we have the exact dimensions of the space in which the superhuman is confined. The motion is practically arbitrary, only limited by special relativity and the object's only limitation is it's approximate size.

I think we can safely say that the question can be translated into:

Is it possible for an everyday-sized easily visible object to evade human visual perception by moving sufficiently fast in a confined space?

When you put it this way, I think it's even more clear that a physicist cannot give a final word here.

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  • $\begingroup$ To comment on the issue, the way you word it, no a physicist can't give a final word. But consider that a cognitive scientist or a biologist could talk our ears off about human vision and how the eye works, but similarly who are they to say that light doesn't reflect differently off faster objects and that this doesn't decrease visibility in a physics sense? How can one of them have the final word when they need to know whether a fast object even produces a visible image? $\endgroup$ – Jim Aug 26 '14 at 18:14
  • $\begingroup$ I wouldn't say that the situation is as symmetric as you just put it. Our eyes aren't very sensitive and the final product of vision is heavily processed on many different levels. If I had to bet, I'd say that this can be answered with a completely naive model of image production, without any physics. The limitations of the visual system will surely come into play long before physical limitations. Isn't it almost always like that with our senses? Regardless, I still think it's reasonable to present this question to those who are more probable to provide a conclusive answer. Don't we want it? $\endgroup$ – PhysSE is Cancer Aug 26 '14 at 18:24
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    $\begingroup$ Sooo...flag it for closing? There's no bounty or such on it, so it escaped closing because either not enough 3k users saw it, enough thought it to be good to take it out of the close queue, or no one ever VTC/flagged on it. I'm not sure what kind of answer you do expect except: There weren't 5 VTC at any time, apparently. $\endgroup$ – ACuriousMind Aug 26 '14 at 18:32
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    $\begingroup$ "The limitations of the visual system will surely come into play before physical limitations". That is an assumption. What if the object of the given size is rendered completely invisible at a particular speed as a purely physics effect? My point is not that cogscientists or biologists aren't applicable, it's that no matter who answers it, assumptions outside of their jurisdiction will have to be made. $\endgroup$ – Jim Aug 26 '14 at 18:32
  • $\begingroup$ The question was posted on physics.SE and the answer has physics to it. If it were originally posted on biology or cogsci, I would similarly be against its migration because it draws from all three and so it is applicable to each one. $\endgroup$ – Jim Aug 26 '14 at 18:34
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    $\begingroup$ @ACuriousMind it was flagged for closing, the first three reviewers voted to leave it open $\endgroup$ – Jim Aug 26 '14 at 18:35
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    $\begingroup$ @Jim: Then, well, the system has spoken, I guess? That's how it works, and there is nothing hindering it to enter the close queue again. If people are hesitant to cast VTC/flags on popular questions, that's another problem not specific to this question. (And I don't think we really need to debate the merit of this specific question as long as I still do not see a single VTC) $\endgroup$ – ACuriousMind Aug 26 '14 at 18:39
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    $\begingroup$ Yeah, the system has spoken, that is the problem. Why do I even hope for anything good on this SE... Who wants actual answers, we wouldn't want to advance science in any way, that would ruin everything. $\endgroup$ – PhysSE is Cancer Aug 26 '14 at 18:59
  • $\begingroup$ As ACuriousMind said, nothing prevents you from casting another flag on the question (perhaps flagging as VTC for reasons other than "unclear") $\endgroup$ – Jim Aug 26 '14 at 19:08
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    $\begingroup$ I already flagged it two times, but I guess another one won't hurt. Although it's probably pointless. EDIT: Nope, I'm all out of relevant flags. $\endgroup$ – PhysSE is Cancer Aug 26 '14 at 19:21
  • $\begingroup$ @Schlomo: if we were inundated with questions about superheroes, I might agree with your sentiments of advancing science here. As it stands, a popular question such as this one, every now and then, is actually good for the site because it draws new users who can contribute more. $\endgroup$ – Kyle Kanos Aug 27 '14 at 1:25
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    $\begingroup$ If this is people's idea of good contribution, I weep for the future of this SE. $\endgroup$ – PhysSE is Cancer Aug 27 '14 at 11:19
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The question wasn't moved or put on hold because the community consensus is that it is on topic and reasonably well written. (bearing in mind that "reasonably well written" in this context is not a particularly high bar)

But I think what you really want to know is why other people don't agree with you that the question is off topic and/or badly posed. For starters, it's not clear that the original poster was asking about the behavior of the human visual system. Sure, there are allusions to human sight, but that's not a clear indicator that the question is about perception. From the perspective of a layperson, there are (at least) two possible reasons a fast-moving object might be invisible: the amount of light it reflects may be diminished because of its motion, or the human eye might be incapable of picking up that reflected light. The latter of these is a matter of perception, sure, but the former is physics.

In cases like these where a question is partly about physics and partly not, it often takes just a minor edit to refocus the question primarily on the physical aspects, and that's preferable to going through the whole hold-edit-reopen cycle. You may notice that the title has been edited in exactly that manner.

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  • $\begingroup$ Maybe we should inform chemistry.SE, astronomy.SE, cogsci.SE, earthscience.SE and biology.SE that it's all basically physics. If you refocus the questions properly, it all comes down to quarks, leptons and bosons, why not move it all to physics.SE? $\endgroup$ – PhysSE is Cancer Aug 27 '14 at 11:24
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    $\begingroup$ @Schlomo And why not move everything here to math.SE? That's a really ridiculous argument. Yes, all of those things can be described by physics but each site provides a different perspective on things using different techniques and knowledge. There is nothing wrong with a question that is on-topic on multiple sites, and such a question would generate different, unique answers on each site from that site's unique perspective. $\endgroup$ – tpg2114 Aug 27 '14 at 15:45
  • $\begingroup$ This discussion was going way off topic - I've deleted the offending comments. $\endgroup$ – David Z Aug 27 '14 at 23:26

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