# Fantasy weapon physics - weaponising vacuum

If there is a magical weapon that can 'teleport away' a spherical volume of air/some fluid, basically creating a spherical volume of vacuum in that region, what kind would the resulting implosion be? (As the surrounding fluid rushes in). Will it be affected by whether the fluid formerly in the volume is 'pushed' to the side rather than teleported? Please give the case of air and water.

By 'what kind of implosion', I mean that how much energy would it release and, much more importantly (it is a weapon, after all), how much damage would it do to people/structures, both very close and a bit far away.

Also, assume that the initial teleportation/pushing is the only 'magical' part of the weapon; I want to analyse the results using hard(er) science.

To help in calculations, maybe this can be connected to real world phenomena in some way: In the case of water at least, this is sorta related to the phenomenon of cavitation, but I can find no analogue for air (sonic boom?)

The general use case for this weapon is at a distance of a few metres and having a vacuum-sphere radius of a few millimetres at most. This is what the user can typically handle, and the energy cost scales with distance and size both (and other, irrelevant-here factors). And though Ash's answer helps with the extreme use case, what I specifically want is:

Will the general use case (as mentioned above) be enough to do severe damage to soft tissue, when vacuuming air from inside the body? Damage to structures (wood, stone, rarely metals; note that vacuum is only formed on one side of something as solid as a building material i.e. 'bubble' cannot be on two sides of a wall at once )? And how does doing this in water perform vs air?

• How big a sphere? And how long can you hold it in vacuum?
– Ash
Commented Sep 10, 2020 at 12:14
• How long doesn't matter. Assuming there is a magical barrier that holds the vacuum, the pressure on it remains constant IF it occurs at a short enough time scale that the high pressure (relative) outside isn't dissipated somehow. Since this occurs at the timescale of milliseconds rather than hours or days, time doesn't matter. As for size, any size. To determine a reasonable size that creates reasonable pressure, I need a formula, or an approximation relating size to the implosion. Which is precisely why I asked the question Commented Sep 10, 2020 at 12:25
• This is substantially answered in the following question worldbuilding.stackexchange.com/questions/142167/… Commented Sep 10, 2020 at 22:05
• Does this answer your question? Weaponising Implosion instead of Explosion Commented Sep 10, 2020 at 22:05
• Thanks for the help, but I think the use case and size are substantially different. The answer deals with causing bubbles in blood rather than the force of the impact, which itself is on a larger area and affecting the body externally rather than internally. It also offers no answer in the case of damage to structures Commented Sep 11, 2020 at 3:51

### Precisely controlling this power could be devastating:

A nice intro to "what happens if a vacuum appears randomly" is What if the glass is really half empty?.

The glass on the right makes a loud shockwave, the glass on the left launches upwards, shatters when the water hits the glass, and sprays broken glass everywhere.

This little thought experiment should show that the size of the vacuum isn't as important as the ability to control where it goes. A precisely placed vacuum can wreck havoc.

This will spray broken glass to the left:

You could remove a sphere of water from one side of a water tower to knock the tower over. You could burst a dam by removing a chunk of water next to it.

### If you crank it up, this is very overpowered

So, a few issues we may have to deal with. Cherry picking your comment. "any size", "time doesn't matter"; this weapon could kill an entire town by suffocation. Just remove all the air for the entire village for 10 minutes. This isn't quite what you're after I'm guessing, so I'm going with "near-0 holding time" from here.

Removing the air around a person who is holding their breath will damage their lungs severely, probably fatally. Removing the air around a person who isn't holding their breath will give them ~6 seconds of useful consciousness before they pass out.

The air will rush back in at the speed of sound. So if you teleport out the air in a sphere of radius ~2km around someone, and release immediately (ie hold for 0 seconds), it will take about 6 seconds for the air to return. They'll be passing out when hit by the sonic boom. That doesn't sound pleasant.

A sphere of radius 80km will kill the average person even with immediate first aid after the air returns - it will take about 4 minutes for oxygen to become available.

### This can be a very effective area weapon

To the best of my knowledge, the sonic boom will be about 200db, and will hold 100 megawatts per square meter. This will cause permanent hearing damage.

The returning air will travel fast enough to knock anyone over, even if braced. It will be impossible to walk against the shock-wave. It will destroy most structures, but will not be fast enough to destroy reinforced concrete.

No matter how much water you remove, you will not be able to make massive waves which flatten cities. That report is about using nukes to vaporise parts of ocean but the physics is the same as if you teleported it to the edges or pushed it out. Vaporising a chunk of ocean near the surface makes waves which break very early. Vaporising a chunk down deep creates a massive (like 1km) bubble of steam which expands and shrinks 3 or 4 times.

Of course, your character needs to be out of the way of the area weapon. The best ear protection I could find lower the noise by ~30db, ie to 170db. 130db is a jet taking off, repeated exposure to 90db can cause hearing loss. Your character needs to create the sphere a long way away from them, as nothing will protect their ears from the repeat exposure.

### Teleport or push the fluid?

The resulting effect wont matter whether the air was pushed to the edges or teleported to it - it's still fluid returning to a vacuum. The surprise of the teleportation is a plus, but if you're able to suck the air out of your sphere at the same rate it flows back, then you've got a weapon twice as strong, eg two sonic booms, knocks people over twice. That may be a win too. A slow charge up will probably look cooler if this is going to a visual medium.

• +1 for the brilliant and well researched answer, and a metaphorical -1 to myself for not being specific enough in either my qn or comment (which I see now is pretty misleading). See, (in regular usage, your answer is still helpful for more extreme use cases, which are going to be rare), my plan is more like 'vacuum bubble in someone's throat/lungs/brain', causing mostly personal damage using smaller radii, though I did ask here for cases where one has to, say, bring down a tower. At least it seems plausible now. Also, since I realised this mistake, what should I do? Edit the qn? (new here) Commented Sep 10, 2020 at 13:52
• Put a section on the end of the question labelled: "Edit" with extra info.
– Ash
Commented Sep 10, 2020 at 13:56
• Ok, thanks. Also thanks for all the provided links. Commented Sep 10, 2020 at 14:07
• From your linked XKCD you can see that it takes mere μs for air that is removed to be replaced. So nobody would suffocate an lungs also don´t just explode in a vacuum, contrary to famous movie scenes. Also your glass example does not work, the water will rush in from the top and the side, clashing together and probably not even brake the glass. Commented Sep 10, 2020 at 16:47