# ­xkcd thread **URL:** \ **Category:** Cafe Society\ **Tags:** comic-strips\ **Created:** [May 28, 2020, 12:57am UTC](https://boards.straightdope.com/t/xkcd-thread/854574 "2020-05-28T00:57:03Z")\ **Posts on this page:** 20\ **Page:** 214
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [May 10, 2025, 7:25pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4263 "2025-05-10T19:25:24Z")
I’m not quite picturing how this works. How does a column of water exert more pressure than the denser rock that was removed to make the shaft? ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [May 10, 2025, 7:40pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4264 "2025-05-10T19:40:16Z")
As it was explained to me a long time ago water is a liquid and the pressure goes in all directions, rock is a solid and pressure is weight mostly downwards. I believed it, but did never try to blast a mountain that way myself (maybe it is not too late?). Pascal’s barrel would not work with sand, though powders have interesting properties as well. Now I’m wondering what happens at the bottom of skyscrapers with the water taps. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [May 10, 2025, 7:52pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4265 "2025-05-10T19:52:45Z")
I do know that water pressure is an important factor in the [collapse of ice dams](https://en.wikipedia.org/wiki/Glacial_lake_outburst_flood), since the water is not only denser than ice but can exert enough pressure to actually liquify the ice at the bottom on the dam, leading to self-accelerating expansion of cracks. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 10, 2025, 8:15pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4266 "2025-05-10T20:15:17Z")
As stated, I don’t quite get it either. Like you said, if you excavate a tunnel in a mountain and fill it with water, it exerts _less_ pressure than the rock did, because water is less dense. As best I can tell, it’s actually more dependent on water hammer effects than Pascal’s law, and also is more of a means of causing a controlled landslide than exploding a whole mountain. Excavate some tunnels in the side of a mountain (which also weakens the rock), build a reservoir higher up with a tunnel leading down, and let the water flow all at once. The pressurized air in the tunnels will also store energy and probably cause extra destruction once the walls give way. There’s a video here, though it’s in Spanish and I can’t evaluate its credibility: [![](https://img.youtube.com/vi/ubfjq2CCdZo/hqdefault.jpg "RUINA MONTIUM") ](https://www.youtube.com/watch?v=ubfjq2CCdZo) ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [May 10, 2025, 8:34pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4267 "2025-05-10T20:34:34Z")
Native Spanish speaker here: it sounds perfectly reasonable, despite the over-acting voices. The mountains in question are described as sedimentary, rather loose conglomerates. They dug them hollow with many wells and galleries, filled them with water, soaking the gold bearing conglomerate and thus making it softer, and then, as you described, blasting it dynamically with a lot of water at once from as high as they had it stored, which in itself was a crazy lot of work. Judging by the remains and the debris still in place the work must have taken a very long time. Rome’s version of fracking! I would not have survived one week as a slave there. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [May 11, 2025, 9:54pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4268 "2025-05-11T21:54:23Z")
OK, total coincidence, of course. Or maybe YouTube has access to my cookies. Yesterday I wrote for the first time in this board about Pascal’s barrel, a subject I had not thought about in ages and never in English, today I had this recommendation in my YouTube suggestions: [![](https://img.youtube.com/vi/EJHrr21UvY8/maxresdefault.jpg "Pascal's Blaising Barrel - Exploding Glass Barrel with Water Pressure") ](https://www.youtube.com/watch?v=EJHrr21UvY8) It is 6 minutes and a half long and nice to watch, actually. ---
**Author:** ![puzzlegal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/puzzlegal/32/3827_2.png) [@puzzlegal](https://boards.straightdope.com/u/puzzlegal)\ **Post date:** [May 11, 2025, 11:40pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4269 "2025-05-11T23:40:06Z")
That’s really well done. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 12, 2025, 1:04am UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4270 "2025-05-12T01:04:12Z")
> [@Pardel-Lux](#): > > Native Spanish speaker here: it sounds perfectly reasonable, despite the over-acting voices. Thanks for the report. Always hard to tell with that dodgy-looking style of computer graphics. Maybe from a legit university department, maybe from some garbage “science” channel that also dabbles in ancient aliens nonsense. Makes sense that the water would also soak in and weaken the soil. You can sorta see that in the animation. Looks like they dribble a small amount in first, wait for it to soak, then let the rest through in one blast. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 12, 2025, 1:11am UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4271 "2025-05-12T01:11:04Z")
> [@Pardel-Lux](#): > > Yesterday I wrote for the first time in this board about Pascal’s barrel, a subject I had not thought about in ages and never in English, today I had this recommendation in my YouTube suggestions: I’m mildly surprised that the bottle is that weak. The water at that height is less than 5 atmospheres. I’d expect a thick glass bottle like that to hold up better. It’s a neat demo, regardless. It’s counterintuitive that the pressure at the bottom is independent of the quantity of water. A thin tube supplies the same pressure as a wide one (as long as the cross-section is constant). The main issue you’d run into is that if the container at the bottom was somewhat elastic, it might stretch enough that you’d have to keep pouring water down the tube. Not an issue for glass, though. It won’t expand as you add pressure; it just has a brittle failure at some point. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 12, 2025, 1:22am UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4272 "2025-05-12T01:22:22Z")
Not exactly on point, but one of the very early xkcd what-ifs (#6!) explored some of the counterintuitive forces in situations like this. **[Glass Half Empty](https://what-if.xkcd.com/6/)**. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 12, 2025, 1:35am UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4273 "2025-05-12T01:35:47Z")
Disappointing. A vacuum is hardly _empty_. It’s a seething landscape of quantum fluctuations. What if half the glass was _truly_ empty? It would mean that the vacuum in that region has decayed to a ground state with zero energy or anything else. How much lower this ground state is than the vacuum we live in is unknown, but it’s undoubtedly huge. Quite possibly on the order of 10100 joules per cubic meter. Regardless, this bubble of _truly_ empty space would expand at the speed of light as nearby vacuum decays to the new energy state (and basically everything is a vacuum as far as this process is concerned). The sphere of expanding true vacuum would devour the Earth, the solar system, the galaxy, and eventually much of the visible universe in time. Matter as we know it could not be supported by the new vacuum. Only those regions of the universe far enough away that they are moving apart at more than the speed of light would be spared. That’s some consolation, at least. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [May 12, 2025, 2:12pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4274 "2025-05-12T14:12:25Z")
That sounds depressing enough to be the setting for a Kurt Vonnegut story. **ETA** provided that the vacuum collapse was the result of someone trying to achieve some incredibly trivial purpose. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 12, 2025, 9:10pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4275 "2025-05-12T21:10:11Z")
It’s not so different a concept than Ice-9. Metastability. Everything is in a happy _local_ minima until some perturbation happens, and then that catalyzes nearby stuff to fall down the energy well. Either freezing the oceans or converting space to a new vacuum. Greg Egan did write a novel about it, _Schild’s Ladder_. In that case the bubble expanded at _half_ the speed of light, and humans were advanced enough that they could outrun it, and even park ships right at the edge and probe the boundary. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 12, 2025, 9:15pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4276 "2025-05-12T21:15:21Z")
> [@Dr.Strangelove](#): > > Greg Egan did write a novel about it, _Schild’s Ladder_. In that case the bubble expanded at _half_ the speed of light, That’s… problematic. Half the speed of light in what reference frame? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [May 12, 2025, 9:25pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4277 "2025-05-12T21:25:17Z")
I don’t recall if he gave an explanation for that. The vacuum decay was triggered by a physics experiment in the Milky Way, so it was approximately in that reference frame. And it’s fast enough that you can ignore the movement relative to the Local Group or even the CMB. But, as you imply, if it’s eating the vacuum, how could it not be at the SoL? If there’s no matter nearby, then the only frame-invariant speed is the SoL, so it pretty much has to expand that fast. Hey, it’s only a novel… ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 12, 2025, 9:47pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4278 "2025-05-12T21:47:35Z")
Aah, yes, but the vacuum decay is expanding spherically. So for two points at antipodes relative to each other on the surface of the decay sphere to recede at 1.0_c_ relative to each other, they and the sphere have to be expanding at 0.5_c_ relative to the center = start point of the vacuum decay event. See, I’ve proven it. I’ve included the word “relativity”, or one of its derivative forms in my explanation. That’s how you know it’s genuine _Science!_, not mere treknobabble. 😉 ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [May 12, 2025, 10:23pm UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4279 "2025-05-12T22:23:13Z")
Maybe, as was postulated in a (later abandoned) storyline in Star Trek: The Next Generation, Alcubierre drives work but produce “denatured” space as the ultimate pollutant. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [May 13, 2025, 6:15am UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4280 "2025-05-13T06:15:26Z")
> **[Deposition](https://xkcd.com/3088/)** [![](https://imgs.xkcd.com/comics/deposition.png) ](https://imgs.xkcd.com/comics/deposition.png) “P.S. If you have time travel, come to my birthday party Saturday!” ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [May 13, 2025, 7:32am UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4281 "2025-05-13T07:32:22Z")
When the time comes, I’ll do my best to become a nice fossil. It is not as easy as one could naively assume. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [May 13, 2025, 9:40am UTC](https://boards.straightdope.com/t/xkcd-thread/854574/4282 "2025-05-13T09:40:19Z")
I wonder what forms of burial are most conductive to fossil making. The custom here is to be buried in shrouds, without a coffin or any embalming. Does that increase or decrease my fossilization chances? [Previous page](https://boards.straightdope.com/t/xkcd-thread/854574.md?page=213) [Next page](https://boards.straightdope.com/t/xkcd-thread/854574.md?page=215)