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A character in my setting has the ability to, at any time, turn incorporeal and back. When she becomes a ghost, it's instantaneous, and when she turns back to flesh and blood, it takes about a tenth of a second. I want to get a sense of what that power would sound like. And since the magic itself makes no sound, I'm looking to specifically work out:

1: The sound of her body being replaced by a her-shaped vacuum that is then filled by air when she ghosts

2: The sound of a her-shaped pocket of air being shoved out of the way to make room for her body in one tenth of a second.

She is a 21-year-old woman, roughly 5 foot 2 and 115-ish pounds.

What sort of noise, if any, would the activation and de-activation of this power make?

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    $\begingroup$ Pretty sure this has been covered in the context of teleportation -- might be a dupe. $\endgroup$
    – Zeiss Ikon
    Jun 27 at 14:36
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    $\begingroup$ "BOOM!" worldbuilding.stackexchange.com/questions/207554/… $\endgroup$
    – Agent_L
    Jun 27 at 15:17
  • $\begingroup$ For the rephasing phase, it could be an almost instant teleportation thing, or a shift in the 4th dimension. The first will lead to the teleportation dupli' question, the second... Well, it depends on how much distance you move on the ana/kata direction. Like if you rephase 1cm back and over 0.1s, you're not exactly moving air that much, but if you move 1km... See it like making a tiny step forward vs shooting a gun. $\endgroup$
    – Tortliena
    Jun 27 at 16:14
  • $\begingroup$ Mandatory xkcd What If what-if.xkcd.com/6 $\endgroup$ Jun 27 at 23:06
  • $\begingroup$ @Tortliena, before you talk about the 4th dimension, you have to first identify how far the human body extends in the 4th, and what it's displacing when it moves there. Is everything the same thickness, and the body moves to vacuum? Are the bones thicker than the flesh due to density changes? Does the body get thicker closer to the middle of the 4th? Too many side questions to answer in a question like this. $\endgroup$ Jun 28 at 1:58

2 Answers 2

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1 is easy. It's similar to a sonic boom. The air collapsing into the space would create a shock wave that damages nearby fragile objects, deafen those in the room, and bust eardrums for anyone within about three feet. The magnitude of the shockwave would be proportional to the mass, and the pitch would be variable, but in the wavelength of the cross-section of the person's body. I did the rough calculations for this when I was trying to figure out how real The Flash-like movement would affect the world.

2 is harder. The force of the air getting pushed away would be like getting every part of her body slapped at once. The speed of sound is 13503.9 inches per second, and her expansion would only be in the order of 100 inches per second, so you wouldn't get a sonic boom. You'd still be rattling windows, but it wouldn't be deafening. Something on the order of a blank firing out of a gun, but much lower pitched.

Addendum: I disagree with the suggestion that it would be like a thunderclap. Anyone who has worked with a tesla coil knows the difference between an electrical snap and a whip crack. Electricity actually super-heats the air as it passes through, creating an explosion in the space instead of mere displacement.

Addendum 2: Randall Munroe seems to agree with me. He describes a half-glass of water collapsing like a loud bang. I think that a whole person disappearing would be louder and bangier. Munroe's kung fu is better than mine.

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Air movement causes no noise until it interacts with other objects. Sound is the vibration of air. Most of the sounds we associate with a sudden change of pressure are the result of the container, not the air movement. The "pop" from a balloon is caused by the balloon rubber moving quickly through the air as it contracts. Mixing vinegar and baking soda will cause a slight "fizzy" sound as the bubbles pop. Even though the baking soda and vinegar are creating gas which is displacing nearby air, there is no real sound from that movement. You can test this with many objects. Removing the powder from a firework and igniting it will not cause the "bang" associated with their typical sound. It is the rupturing container which causes it. The exact same energy is released, but one is much quieter than the other.

The air would fill the void based on two factors. The first is gravity, with an acceleration of 9.8 m/s/s. The second is atmospheric pressure of 14.7 pounds per square inch at sea level. Nearly 15 lbs of pressure is hardly enough to cause massive destruction, and the distance it would have to travel (only several inches at most from the outer layer of skin to the core of the body) would not allow for much acceleration. The mass of air is also not very large, as your example human body compressed into a uniform space would likely fit an area of 2 square feet. The air would rush in and collide with air from the other side, the pressure would equalize and that would be it. Air is not solid enough to make much in the way of sound as it hits itself at such a low speed.

That being said, there would be a slight sound if you were standing directly next to the person. Wave your hand quickly past your ear, or stand in front of a small fan. That slight "whoosh" is what you would likely hear as the nearby air moved to fill or vacate the area. Have a person stand several feet away and wave your hand in the air towards them. Ask them what they heard. The answer will likely be "nothing", unless it is a sound from the movement of your clothing or a creaking joint. There is very little air movement further away from the localized point. The mass volume equivalent of a human is insufficient to cause a loud noise simply by moving air a few inches. To further demonstrate, you can have a person walk forward. When they do so, they are pushing aside their surface equivalent worth of air. This air flows around them like a fluid and creates eddies behind them. Actually, if you walk through water in the shallow end of a pool you can see this visually. A slight bow wave is created in front of the person with a point of lower pressure behind them which pulls in more water to equalize the pressure. Even though they are moving their surface area equivalent worth of air mass, there is no real sound associated with the movement. If you were to wave your hand next to desk covered in papers, the rustle of the papers can provide you with your sounds, but waving your hand from across the room wouldn't do anything to the papers. It is also important to remember that the higher in elevation the person is, the less air is displaced as the air is thinner. At sea level the sound would be the loudest. High up in the mountains, the sound would be quieter.

Instead of sound, you could always use the sensation of a slight wind or breeze to indicate when your character teleports. Nearby objects would move slightly as the air moved past them. Dust, paper, trash, and other debris can easily stand in for sound. If it absolutely must have sound, then you could add an ionizing effect or something similar. Transitioning from a physical to a ghost form could create a localized energy field which is compressed by the vacuum and detonates in a snap of electrical discharge (mini lightning bolt). Ghosts are "known" to be detectable as energy so it would make sense for there to be an electrical effect. Reverting back to physical form could discharge it as more of a static type halo effect as the body would be pushing away charged air.

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    $\begingroup$ Hm... The question indicates that the person appears/disappears within a fraction of a second. I my intuition tells me this should be fairly energetic: more firework and less soda fizz. While I do not mind answers bucking my intuition, I think I need more evidence to be convinced. $\endgroup$
    – PipperChip
    Jun 27 at 17:42
  • $\begingroup$ have to agree with @PipperChip in the OPs question they teleport away essentially leaving a vacuum. to be filled in the shape of the body. As the air rushes in to fill the void (voids? from uneven shape and fill rate?) would there not be a more pronounced affect? $\endgroup$
    – Gillgamesh
    Jun 28 at 11:37
  • $\begingroup$ Actually, no. It is a fluid dynamics issue. As the gas pushes against some of the other gas rushing to fill uneven spaces, it causes eddies and vortexes which slow and spin the gas. A uniform shape like a sphere allows for maximum acceleration prior to impact. A humanoid shape causes an uneven gas flow which works against itself. These deflections prevent a clean collision of air and instead you have dozens of silent mini-twisters which vanish almost instantly once the pressure equalizes. Again, air movement but almost no sound. $\endgroup$ Jun 28 at 16:05

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