Wikipedia:Reference desk/Archives/Science/2024 October 20

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October 20

Soap bubbles and flatulence

Let's say that someone ate a whole can of beans for lunch and had a piece of steak, and some milk to wash it all down. A couple of hours later, they're suffering from farting problems, as they have to fart a lot, and the gas doesn't smell good at all. They have filled a bathtub full of water and added a generous amount of soap, and in they go the bathtub. They fart in the there and those bubbles of soap caused by the release of gas travel up onto the surface. If they had a lighter nearby (for whatever reason) and tried to ignite those bubbles, would the bubbles catch on fire? Kurnahusa (talk) 05:16, 20 October 2024 (UTC)

I don't think the human digestive system works as fast as that, but leaving that aside, it's well known that human flatulance is inflammable – lighting one's farts is a widespread activity; I recall a story that one squad of British Army recruits managed to burn down their barracks hut while indulging in it; I would have liked to have been in the Colonel's office the following morning. {The poster formerly known as 87.81.230.195} 94.6.86.81 (talk) 05:40, 20 October 2024 (UTC)

I guess? Why wouldn't they? Is there a reason you were expecting them not to? Fun bio facts time the flammable stuff in human flatus is mostly hydrogen gas, made by your little belly buddies fermenting complex carbohydrates that your digestive system can't tackle. And they actually share some of the products with your cells and they're probably good for gut health. (Non-human primates consume buttloads of fiber, as did all humans pre-agriculture.)

The rest of it is mostly swallowed air which makes its way down there, along with small amounts of volatile sulfur compounds also produced by your flora, thiols, which your smeller is extremely sensitive to. That's the smelly stuff. Slowking Man (talk) 19:52, 20 October 2024 (UTC)

Haha, no reason why I wouldn't think the bubbles were not flammable; after all, the gases are basically trapped inside, but very interesting as well as informative - thank you! A while back I sprayed some gas from a nearly empty alcohol rub bottle into water and ignited it, and so I thought, if it was possible, the same stuff applies to farts. Kurnahusa (talk) 23:16, 20 October 2024 (UTC)

A bubble of flammable gas in water is an interesting apparatus where you can see inside the bubble before and during it popping. With an electronic igniter, could be fun to try to ignite the bubble itself to demonstate the effect of UEL. Are H2 and methane actually "flammable" when pure? DMacks (talk) 02:11, 21 October 2024 (UTC)

"Flame"/"fire" is a redox reaction, between at least 2 reactants, a fuel and an oxidizer. H2 and CH4 fill the "fuel" role while most commonly O2 takes the "oxidizer" one. If by "pure" you mean, "a volume of gas which is 100% eg H2 and no other substance", then no, no "flame" can occur without mixing w/ an oxidizer first. H2 is routinely used as a coolant in environments where lots of "sparky"/"arc-y" stuff is liable to happen; other stuff is simply kept out of said environment (already necessary anyway for things to work right). H2 is hard to beat in terms of drag coefficient!

This is why if you block an ICE's air intake, it's not going to be running much longer, and why jet engines don't work in space, and why rockets generally need two things, fuel and oxidizer. (The latter being often O2, sometimes even nastier stuff.) As do most chemical explosives—a rocket is in essence just a bomb that explodes more slowly, if everything goes right. (And when it doesn't...) --Slowking Man (talk) 22:28, 22 October 2024 (UTC)

Conventional jet engines may not work in space, but see Bussard ramjet.  --Lambiam 16:55, 27 October 2024 (UTC)

For pure hydrogen or methane, not by itself, no. The combustion of the gases inside the bubbles are using the air around it, which doesn't contain too much oxygen. Fun thing is if you managed to inject some oxygen into those fart bubbles, the mixture would be potentially explosive, and when ignited can create a small bang. People do, however, like to weld with fuel and oxidizer mixtures (see oxy-fuel welding); the flame of oxygen and acetylene can go up to 3,500 °C (6,330 °F), hot enough to melt a variety of metals. Kurnahusa (talk) 00:56, 23 October 2024 (UTC)

For the record, I was proposing experiments, not being ignorant of UEL that I had mentioned:) DMacks (talk) 12:01, 24 October 2024 (UTC)

Must every moving [accelerating] body lose energy, namely the energy of the gravitational waves emitted by that body while moving [accelerating]?

HOTmag (talk) 10:08, 20 October 2024 (UTC)

Can you accept as a counter example the body envisaged in Newton's first law of motion that remains at rest, or in motion at a constant speed in a straight line, except insofar as it is acted upon by a force? Philvoids (talk) 13:36, 20 October 2024 (UTC)

Given a body moving at a constant speed, there is a reference frame in which the body is at rest, from which it follows that it does not emit gravitational waves. Only a change in the gravitational interaction between massive bodies can stir up the gravitational field.  --Lambiam 15:31, 20 October 2024 (UTC)

OP's apology: Sorry for replacing the correct word "accelerating" by the wrong word "moving". I've just fixed that in the header [by brackets]. HOTmag (talk) 16:00, 20 October 2024 (UTC)

Responders can lose interest in freely helping a questioner who keeps changing their question. Can you accept that energy must always be added (or subtracted) to accelerate (or decelerate) a body? Reference: Kinetic energy. Philvoids (talk) 19:45, 20 October 2024 (UTC)

Responders can lose interest in freely helping a questioner who keeps changing their question. What do you mean by keeps changing my question? When did I change my question, excluding this single time (for which I have already apologized)?

Can you accept that energy must always be added (or subtracted) to accelerate (or decelerate) a body? Yes, I can, and I do accept. Still, I'm asking if, besides the energy your're talking about, one should also take into account another amount of energy that should actually be subtracted because of the gravitational waves which are always emitted by every accelerating body. HOTmag (talk) 22:59, 20 October 2024 (UTC)

The term Gravitational wave rather than gravity wave is used in article space.

Energy (luminosity) carried away by gravitational waves is purportedly given by Einstein's Quadrupole formula

${\frac {dE}{dt}}=\sum _{ij}{\frac {G}{5c^{5}}}\left({\frac {d^{3}I_{ij}^{T}}{dt^{3}}}\right)^{2}$

As yet this has been only partially confirmed by an observation of a binary star combination of a neutron star and a pulsar (earning the 1993 Nobel Prize in Physics). Research continues and I think we are far from the kind of laboratory demonstration that is needed to cement this theory to the same extent as, for example, the refined measurements of Gravitational constant G initiated (effectively but not deliberately) by Cavendish. Philvoids (talk) 02:11, 21 October 2024 (UTC)

The term Gravitational wave rather than gravity wave is used in article space. Of course. Has anyone ever used the term "gravity wave", in this thread?

Energy (luminosity) carried away by gravitational waves is purportedly given by Einstein's Quadrupole formula. Now let's assume that Einstein was correct. So, regardless of the other kind of energy mentioned in your previous response, must every accelerating body lose the kind of energy you're mentioning in your last response, namely the energy of the gravitational waves emitted by that body while accelerating? HOTmag (talk) 09:42, 21 October 2024 (UTC)

According to the theory, a spherically symmetric pulsing body wouldn't emit gravitational waves. NadVolum (talk) 10:46, 21 October 2024 (UTC)

Does it mean that spherically asymmetric pulsing bodies would? HOTmag (talk) 14:37, 21 October 2024 (UTC)

The opposite of "spherically symmetric" is "not spherically symmetric". The pulsation of body needs to respect certain symmetries in order to keep its centre of mass at rest. As the formula says, the quadrupole of the mass distribution needs to change. Think of a wobbling drop of water. A rotating barbell emits gravitational waves, as does (in a similar way) a pair of orbiting black holes. All the detected gravitational wave events are of that type, occasionally with a neutron star in place of a black hole. --Wrongfilter (talk) 16:12, 21 October 2024 (UTC)

I am content that the single counter-example provided by NadVolume answers the OP's question. Philvoids (talk) 16:41, 21 October 2024 (UTC)

Well, if you're content, I guess I'll just keep my mouth shut in the future. --Wrongfilter (talk) 16:45, 21 October 2024 (UTC)

Thank you for these examples. So, regardless of the kinetic energy added to an accelerating body, do the bodies in your examples also lose radiant energy - due to the emission of gravitational waves? HOTmag (talk) 19:30, 21 October 2024 (UTC)

The "bodies" as a whole (barbell, binary BH) are not accelerated (although parts of them are, e.g. the individual BHs), and no kinetic energy is added to them. Yet their rotation causes them to emit gravitational waves and they lose energy through them. The barbell's rotation slows down, the BHs approach each other and finally coalesce. Incidentally, I mentioned the barbell because Bernard Schutz uses that example (in Class. Quantum Grav. 16, A131 (1999)) to illustrate the strength of the emitted gravitational waves, which is tiny except for the most extreme cases. --Wrongfilter (talk) 05:52, 22 October 2024 (UTC)

If the rotating barbell is in vacuum, it's not supposed to slow down, is it? If it doesn't slow down, and it doesn't receive any kinetic energy, then this barbell will keep losing energy "for ever", or rather until it eventually "evaporates" (like a BH), am I right? HOTmag (talk) 12:49, 22 October 2024 (UTC)

Emitting gravitational waves slows its rotation. If the reason it emits gravitational waves is that it's rotating, it should cease emitting them (and thus cease losing energy) when it's lost enough energy to stop rotating. Its mass won't evaporate. -- Avocado (talk) 22:40, 22 October 2024 (UTC)

Why losing energy may make the body slow down or stop rotating? What about the conservation of angular momentum? HOTmag (talk) 23:32, 22 October 2024 (UTC)

You are not right, there is no reason for the barbell to evaporate. The rotation slows down because the GWs carry off energy (and angular momentum). If the thing is not in vacuum the GW effect is overwhelmed by friction. But this is a highly idealised example meant to illustrate the momentary emission of GWs. It does not occur as such in nature, and it is not worth thinking it through to the end. --Wrongfilter (talk) 13:02, 22 October 2024 (UTC)

I still think, Bernard schultz's example you've mentioned, is interesting, and worth thinking.

I've asked about vacuum, in which no friction exists.

Apparently, according to the conservation of angular momentum, this momentum is not supposed to change. So, I still wonder, what may prevent the rotating barbell from continuing to rotate "for ever". As long as it rotates, it emits GWs, thus loses energy, without changing the angular momentum, until the barbell loses all of its energy, i.e. until it "evaporates". I wonder where the mistake lies.

Is it possible, that I'm wrong with the conservation of angular momentum, so that what I was taught in school about this conservation in vacuum was not that accurate? HOTmag (talk) 14:10, 22 October 2024 (UTC)

Gravitational_wave#Energy,_momentum,_and_angular_momentum. --Wrongfilter (talk) 16:42, 22 October 2024 (UTC)

Thank you for this clarification. Consequently, all of the basic laws of conservation (i,.e. conservation of energy, of linear momentum and of angular momentum), are only true for spherical symmetric bodies, or bodies that don't rotate. All other bodies, emit GWs, so they can't satisfy those laws of conservation any more, at least according to the theory. Do you think we should mention this fact (an important one IMO) in the respective articles about those laws? HOTmag (talk) 17:36, 22 October 2024 (UTC)

No!!!!! Do you not understand how conservation laws work???? --Wrongfilter (talk) 18:09, 22 October 2024 (UTC)

Our article Angular momentum indicates "The symmetry associated with conservation of angular momentum is rotational invariance.". Does this reservation exclude every non-spherical symmetric body? If it does then all is fine. But if it doesn't then:

AFAIK, if no external forces act, then the angular momentum must be conserved. Correct? However, non-spherical symmetric bodies that rotate, emit GWs, so the angular momentum of those bodies is not conserved. Correct?

If I'm correct, then the conservation of angular momentum does not hold in all cases where no external forces act. What's wrong? HOTmag (talk) 18:26, 22 October 2024 (UTC)

The single counter-example provided by NadVolume, only answers the question in the header, but I was waiting for an answer to my follow-up question addressed to NadVolume. It seems that Wrongfilter gives a positive answer, by two theoretical examples: the wobbling drop of water, and the rotating barbell (besides the empirical example of a pair of orbiting black holes). HOTmag (talk) 19:21, 21 October 2024 (UTC)

How is the center of mass of the rotating barbell not at rest? The distribution of mass in the volume it rotates within changes, but if it's rotating around the center of mass, isn't the center of mass stationary? -- Avocado (talk) 22:28, 21 October 2024 (UTC)

There may be a misunderstanding here about what a gravitational wave is like. It does not push and pull in the direction from which it comes - it is polarized and squishes and stretches at right angles to its path. If what the observer sees is symmetric then they won't see gravitational waves. NadVolum (talk) 23:47, 21 October 2024 (UTC)

I did not say (or at least tried not to say) that the CM of the barbell is not at rest, quite the opposite. The rotating barbell is not spherically symmetric but still has some symmetry. My comment was triggered by the word "asymmetric", which I think is not entirely appropriate, and then tried to illustrate systems with a varying quadrupole moment. --Wrongfilter (talk) 05:52, 22 October 2024 (UTC)

Got it - thanks for the clarification, and sorry for misreading! -- Avocado (talk) 22:34, 22 October 2024 (UTC)

BTW, the term spherically asymmetic is found in our article GW: "a mass distribution will emit gravitational radiation only when there is spherically asymmetric motion among the masses". Do you think this should be reworded or rephrased? HOTmag (talk) 19:39, 2 November 2024 (UTC)

it is polarized and squishes and stretches at right angles to its path. Are the GWs emitted from both sides at right angles to the body's path? If they are then they have opposite directions, don't they, so the total momenta carried by them is zero, isn't it, so they don't have to carry momentum away from the body emitting them, do they? But our article about gravitational waves claims they do, so I'm probably missing something here... HOTmag (talk) 09:26, 1 November 2024 (UTC)