Wikipedia:Reference desk/Archives/Science/2022 December 6

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December 6

If a woman instantly turned to vacuum in my arms would my eardrums break?

I know it's impossible but it should be a fairly simple physics problem. What would be peak decibels at the ear? How far away could the implosion be heard? Would I be physically injured and how bad? Sagittarian Milky Way (talk) 17:53, 6 December 2022 (UTC)

This is not necessarily a simple problem, and also, why are you asking this? Like, really, why do you want other people to take their time answering you a question you know to literally be impossible like if a woman instantly turned to vacuum in my arms would my eardrums break? Normally, I'm all for a fun thought experiment, but this is something you are asking other people to spend time figuring out. Can you at least tell us why first? If nothing else, it might help simplify the parameters of the question, which are anything but simple. --OuroborosCobra (talk) 18:11, 6 December 2022 (UTC)

https://what-if.xkcd.com/6/ and being interested in science fiction/fantasy including teleportation. I think I also once made a joke about this before teleporting in RuneScape (yo everyone clear a 20 meter radius for the implosion I'm gonna teleport) Sagittarian Milky Way (talk) 21:25, 6 December 2022 (UTC)

If you're going to invent a magic world where normal physics doesn't apply, it doesn't matter what would happen, because you've already thrown physics out the window. It's your magical invented world, you come up with an answer to your question. --Jayron32 18:17, 6 December 2022 (UTC)

That's a bit flip, Jayron. You could use that sort of rhetorical technique to dismiss pretty much any counterfactual. There are some extremist philosophers who do just that, but I don't think most of us would want to follow them; counterfactuals seem to be essential to human thought.

In this case it's pretty clear what it means: Set up a system that's like normal ambient conditions, with SMW standing in front of a woman-sized-and-shaped hole in the air, and evolve the system according to the usual equations.

The analysis below is in the right direction, but just computing the energy doesn't necessarily tell you much about the amount of damage it could do. 6 kJ of a high explosive will break a lot of stuff, but my intuition is that this situation won't have much brisance.

A detailed answer would be in the domain of computational fluid dynamics (which, by the way, is hard). --Trovatore (talk) 19:51, 7 December 2022 (UTC)

Yeah, I would've guessed at least a sphere in a sphere of air in a thin airtight sphere of kilogram prototype-material in an otherwise empty universe is determinable to plus or minus ≤10 decibels for a point not too close to the pressure vessel but what do I know, maybe even that's still too complex for a supercomputer to deterministically model. Sagittarian Milky Way (talk) 17:57, 8 December 2022 (UTC)

You could work out the energy involved, say 60 litres × 100000 Pascals ie 6 kiloJoules. Also exposing one side of a human body to a vacuum is not a good idea! There is going to be a force of body-exposed area × air pressure. perhaps 0.25×100000 =25000 Newtons (like a weight of 2.5 tons). Expect some damage. Graeme Bartlett (talk) 22:16, 6 December 2022 (UTC)

I didn't even realize you could multiply Pascals and 0.06 cubic meters like that. Metric is awesome! If asked for the Joules of 60 liters of STP imploding full vacuum on a test I'd bubble in 6 kJ and hope for the best but I would not be confident at all. Sagittarian Milky Way (talk) 17:57, 8 December 2022 (UTC)

The level of force means little, as it will only appear for a small time, before the vacuum is rapidly filled. For funsies (don’t judge me) I looked up this slow-motion video of a bouncing ball to estimate the force during the rebound. It turns out that it is roughly eight times your scary 2.5kN. Getting hit by a large rubber ball is certainly painful, but such incidents are routine during PE classes (see: football, handball, etc.) and rarely lead to serious injury.

Math: the video uploader said the ball is 85mm in diameter. It takes ~7ms to move a distance equal to its diameter, assuming a restitution coefficient of 1 for simplicity its vertical speed varies from -12 m/s to +12 m/s. It also takes roughly 7ms to rebound [from timestamps 12.3ms to 19.3ms] (this may not be a coincidence), so its average acceleration during the rebound is $a=\Delta V/\Delta t=24/0.007=8.10^{3}m/s^{2}$. Assuming a density of $\rho =1000kg/m^{3}$, its weight is $m={\frac {4}{3}}\pi R^{2}\rho =2.6kg$. Then by Newton’s second law $F=ma=21kN$.

The energy calculation is more meaningful. Assuming an implosion (air getting sucked in) has roughly the same detrimental effects as an explosion (air getting pushed out), one can compare the 60kJ of magical implosion with some sort of combustion. It’s about 15g of TNT equivalent, for instance, but I assume few people are familiar with explosives.

Here’s another comparison: a relatively common effect for stage magic is the rapid burning of nitrocellulose (which has a quick and ash-less combustion). For instance, the magician sets on fire one end of a rope, the rope burns quickly and turns into a solid cane (or a dove, or...). The "flash rope burning" effect is about 450kJ, or 7.5 times as much as the "real magic vanishing lady" one, and it certainly does not kill the magician.

Math: assume a "flash ribbon" of length 1m and cross-section 10mm^2, made out of pure nitrocellulose of density 0.77g/cm^3 so roughly 77g in weight. The enthalpy of combustion of nitrocellulose is surprisingly hard to find online, but this 1950 paper gives values between 1,400 calories per gram (5.9 kJ/g) and 4,100 calories per gram (17 kJ/g). Let’s take the lower value for simplicity, that is about 450kJ for my flash paper sheet.</math> Tigraan^{Click here for my talk page ("private" contact)} 15:27, 7 December 2022 (UTC)

-12 to +12 in 7 ms is ~3428.57143 meters per second per second. And a water density ball that fits in a liter cube can't be more than about half kilogram. Also the sphere volume formula has a 3 exponent not 2. And a 10x1x1000 ribbon of 0.77 density would weigh only 7.7 grams. Also 7.7 grams of modern gunpowder (chemically similar to flash paper) in a gun is a lot louder than 7.7 g of unconfined flash paper ribbon lit at one end (or 7.7 g's of candle, TNT actually has less Joules per gram than most fuels). A vacuum rising to ambient is also much fewer Pascals than peak gun pressure falling to ambient but Joules isn't the only factor. Sagittarian Milky Way (talk) 17:57, 8 December 2022 (UTC)

The creation of a true vacuum will trigger false vacuum decay. Poof lady, poof Milky Way, poof universe...  --Lambiam 20:41, 7 December 2022 (UTC)

Isn't that just a hypothesis? And would it even be possible for future technology to tell? If it kills everything in a sphere expanding at the speed of light no one would ever know (unless there's an afterlife) and if it hasn't happened yet is it possible to prove or disprove a lower state exists without causing it? Sagittarian Milky Way (talk) 17:57, 8 December 2022 (UTC)

As far as we know, and with our current theoretical understanding, it is possible that our current vacua are false. Yet we cannot quantify the likelihood of this possibility, nor the likelihood of spontaneous decay. And, finally, it is not even known whether the consequences of such decay will have a universal effect or will remain localized and be temporary.  --Lambiam 21:29, 8 December 2022 (UTC)

If that happened, wouldn't the surrounding air immediately fill the evacuated space? ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:15, 9 December 2022 (UTC)

Not immediately, a few (hydrogen-enriched) atoms or molecules extremely close to the edge will reach the centerish in less than 0.0001 seconds, the fuzzy edge of the void will reach there in about 0.0003 seconds and even light would take over 0.000000000003335 seconds to fill the outer millimeter of the evacuated space. Sagittarian Milky Way (talk) 19:36, 9 December 2022 (UTC)

Those numbers are sufficiently close to "immediate". ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:02, 9 December 2022 (UTC)

Useful links: Implosion (mechanical process), Misogyny, Toxic masculinity, Psychoanalysis. Philvoids (talk) 20:58, 9 December 2022 (UTC)

me standing in the back of a crowd who are confused about why you included toxic masculinity:... Allaoii ^talk 20:42, 13 December 2022 (UTC)