Let us say that I have the following things:

  • a tube with a cross section of 1 cm²
  • a highly sophisticated moving forcefield generator
  • an energy source
  • a switch

Now I put these things together in a way that if I pull the trigger (the switch) I get a forcefield moving down my pipe at, say, Mach (1 Mach (I think) = speed of sound) 2. It keeps going until it has moved 1 cm. The resulting shockwave travels down the remaining 5 cms of pipe, and exits at the end. Let us say that there is an average person standing 1 m away, in the direction the gun is pointing. What happens to him?

If nothing happens, how much air do I need to move to E.G. knock him over/out?

I am adding the biology tag because of the knocking-out of a human.

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    $\begingroup$ I think a more interesting question is what happens to the user when the recoil hits him. $\endgroup$ – AndreiROM Jan 17 '17 at 15:21
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    $\begingroup$ @MarkGardner: You might want to get rid of the hard science tag then $\endgroup$ – nzaman Jan 17 '17 at 15:55
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    $\begingroup$ This is not a hard-science question. You can't include arbitrary force fields and magic physics and hope it floats on the hard-science sea - please, do a bit more of research on the topic before asking this type of question. $\endgroup$ – T. Sar - Reinstate Monica Jan 17 '17 at 17:34
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    $\begingroup$ hard science moving force field?.. Really?! $\endgroup$ – Mołot Jan 17 '17 at 18:06
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    $\begingroup$ Note to commenters and answerers: The question currently has the hard-science tag on it. People have complained, saying that such a device is impossible according to principles of hard science. The established procedure in this case is to write an answer according to the requirements of the tag explaining why this is impossible under hard science criteria. Ignoring the premise is not an acceptable excuse. $\endgroup$ – HDE 226868 Jan 17 '17 at 18:11

Nothing happens to him. You can get the same effect with a pistol firing blanks.

For comparison higher grade air rifles can achieve supersonic velocities.

as for knocking the person down, any gun that can do that will also knock down the person firing it. In fact the shockwave will risk collapsing the victim's lungs before it is strong enough to push them over. To knock them over you want slower moving air and a larger volume. something more like a huge potato gun.

To know someone out with a shockwave is both very risky and very unpredictable. A blow strong enough to risk a knockout can just as easily kill. You are basically trying to give the person a concussion.

several armies have researched vortex-guns and their conclusion it wouldn't work without making it excessively large and only works at very close range. You can see a PDF of the army report here. but the Wiki for a vortex gun will give more comprehensible information.

  • $\begingroup$ A gun firing blanks can do some serious damage at close range, just a note. Not as much as a bullet, but similar. $\endgroup$ – Riker Jan 17 '17 at 15:28
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    $\begingroup$ What about the second half of the question. Also I included the [hard-science] tag. $\endgroup$ – Mark Gardner Jan 17 '17 at 15:29
  • $\begingroup$ @RikerW at extremely close range not a meter away. $\endgroup$ – John Jan 17 '17 at 15:29
  • $\begingroup$ @John a meter away is still dangerous, and if not lethal still will cause serious damage. $\endgroup$ – Riker Jan 17 '17 at 15:31
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    $\begingroup$ Unless at a range in the order of a few centimetres, the shockwave from a blank round will not do any damage. What makes blank rounds dangerous at ranges in the order of several metres is the fact that they are sealed, and the sealing plug along with perhaps some unburnt propellant can be ejected at very high speed, and possibly damage delicate body parts such as the eyes. But to cause serious injury, you'd have to press the muzzle up against the target. $\endgroup$ – Matt Bowyer Jan 17 '17 at 15:34

Nothing will happen.

Air is a terrible choice for propagating shockwaves, because of its compressability. Military explosives designed for area-effect are built with a case, and it's the case that actually does the damage. Because the case doesn't compress (and there is no air between the explosive and the case) the case shatters with the shockwave, sending high-velocity fragments out in all directions. Without the casing, explosives become much less deadly. Some munitions (eg concussion grenades) are made without a casing with the specific intent of limiting their dangerous range.

All you've built is a 1cm² area (.45 acp) blank-firing pistol, and blanks are generally recognised as being safe at close range. Given the poor propagation qualities of air, the level of shock you'd need to affect someone even at a range of one or two metres would almost certainly hugely affect whoever was unlucky enough to be holding the gun.

If you can work out a way to focus the shockwaves however, it could be very useful in underwater combat. Someone better informed than me will have to tell me whether you could focus a shockwave in that fashion though.

  • $\begingroup$ Solution: Use a forcefield :D (sci-fi kind) $\endgroup$ – Mark Gardner Jan 17 '17 at 15:47
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    $\begingroup$ -1 This is not a hard science answer. Use 'equations, empirical evidence, scientific papers, [and] other citations.' $\endgroup$ – kingledion Jan 17 '17 at 16:34
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    $\begingroup$ @kingledion this is not a hard-science question to begin with. Magic force fields can't be HS - downvote the question, not the answers, for the lack of research effort. $\endgroup$ – T. Sar - Reinstate Monica Jan 17 '17 at 17:36
  • $\begingroup$ @MarkGardner - the question suggests that the force field doesn't leave the barrel; hence the only thing that transmits the force to the target is the shockwave. $\endgroup$ – Matt Bowyer Jan 17 '17 at 17:41
  • $\begingroup$ @MattBowyer I meant another force field. $\endgroup$ – Mark Gardner Jan 18 '17 at 14:48

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