Suppose we have an indestructible and sharpest knife ever made, which can cut through everything at ease. If it cuts you in half, you're more likely to be dead. As the knife begins to be thinner and thinner it is more likely to become sharper and sharper. Is there a moment when, the knife is SO THIN that it will just cross your entire body without any harm? Like I don't know... a knife of just few atoms thick, can it cross you without harm? If not, what is the boundary between the "harmless" knife and the "killing" one?


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    $\begingroup$ I would reformulate this question without using biological setup. Just ask question like this "if i have some objet with given properties and a knife with given properties and i cut through the object in given way, what kind of way of cutting and what kind of properties must knife have to make only small structural damages to the object". If the object is human and the structural damages are small, than the human would probably survive. $\endgroup$ – Umaxo May 14 at 15:32
  • $\begingroup$ I'd agree with @Umaxo, maybe this can be rephrased to reduce the drama :) (It's fine to me but I guess it's not fine to several other picky users.) Also, it reminds me of science-fiction monomolecular string which was perfectly capable of cutting people in half. I look forward to an educated answer. $\endgroup$ – Helen May 14 at 16:01
  • $\begingroup$ Rather than cutting you want a blade that simply goes through. This is how the Q should be formulated, in my opinion. $\endgroup$ – Alchimista May 14 at 16:07
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    $\begingroup$ I think the way to answer the question is to consider the time the two sides are separated: how far will biomolecules diffuse in that time? If it is longer than their bond lengths, then they will be split and bad things happen. Same thing for elastic molecules under tension in tendons: do they have the time to separate enough that they would not rejoin when the blade passes? A rough guesstimate is a diffusion speed of 1500 m/s and 153pm bonds is 10^-13 s. That better be a very thin and fast blade. $\endgroup$ – Anders Sandberg May 14 at 22:17
  • $\begingroup$ @AndersSandberg what about making this into an answer? $\endgroup$ – Helen May 17 at 17:21

The thinnest blade is a beam of particles from an accelerator, as the particles themselves are of fermi dimensions ($10^{-15}$meters), and move in a straight line.

Actually there is a physicist who survived after a particle beam crossed his head,

Bugorski worked with the largest Soviet particle accelerator, the U-70 synchrotron.[2] On 13 July 1978, Bugorski was checking a malfunctioning piece of equipment when the safety mechanisms failed. Bugorski was leaning over the equipment when he stuck his head in the path of the 76 GeV proton beam. Reportedly, he saw a flash "brighter than a thousand suns" but did not feel any pain.

So an answer is that at the level of atomic distances, a "knife" made with a particle dimension beam will destroy tissue but will not separate as a cut. There was no blood. The chemical bonds may be randomly broken but the tissue remains in one piece.

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    $\begingroup$ I'm not sure if an electron beam is a good analog to a knife cut in this case. One being that the beam is extremely small in two dimensions, while the knife is only small in one. $\endgroup$ – JMac May 14 at 18:46
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    $\begingroup$ I am sitting here while muons and neutrinos pass right through me. The muons do damage, but so far, I have survived. Of course all this is totally irrelevant, as was the question. $\endgroup$ – Pieter May 14 at 19:02
  • $\begingroup$ @Pieter One could imagine moving thebeam as a sword in one plane. I just offer it as an experimental fact that tissue does not separate, no hole . If he had fallen down through the beam it would act like a knife but it would not cut him in two , . $\endgroup$ – anna v May 15 at 3:57
  • $\begingroup$ @JMac it was a proton beam, it could be thought as a thin blade, if he had fallen through it, for example. $\endgroup$ – anna v May 15 at 4:10
  • $\begingroup$ Great experimental evidence! :D Poor guy! $\endgroup$ – pela May 16 at 11:43

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